Chemistry Notes

Chemistry Notes

Catalysis Chemistry Notes

Catalysis Chemistry Notes

Catalysis :

→ Berzelius in 1835, found that the rate of various chemical reactions are altered in the presence of some foreign substances. He called these foreign substances ‘catalyst’.

→ Thus “a substance which can change the rate of a chemical reaction without itself undergoing any change in mass and chemical composition at the completion of reaction is called catalyst. This phenomenon is called catalysis.”

Catalysis Chemistry Notes 1

Types of Catalysts :

The various types of catalysts are as follows:

→ Positive Catalyst: The catalyst which can increase the rate of a chemical reaction is called positive catalyst. Examples :
In the thermal decomposition of potassium chlorate, MnO2 behaves as positive catalyst.

Catalysis Chemistry Notes 2

→ In presence of positive catalyst, the value of activation energy decreases hence the rate of reaction increases.

Catalysis Chemistry Notes 3

→ Negative Catalyst: The catalyst which can decrease or retard the rate of a chemical reaction is called negative catalyst. Examples :
The rate of decomposition of hydrogen peroxide decreases in the presence of glycerol. That’s why, to store hydrogen peroxide, small amount of glycerol is added to it.

Catalysis Chemistry Notes 4

  • Auto oxidation of benzaldehyde is stopped in presence of small amount of sulphur compound.
  • To decrease the knocking of petrol, generally small amount of tetraethyl lead is added to it.
  • Oxidation of sodium sulphite in presence of air is stopped in presence of glycerine. Here glycerine behaves as negative catalyst.

Catalysis Chemistry Notes 5

→ In presence of negative catalyst, the value of activation energy increases hence the rate of reaction decreases.

Catalysis Chemistry Notes 6

→ Auto Catalyst: During a chemical reaction, when one of the products formed behave as a catalyst and increases the rate of reaction then such catalyst is known as auto catalyst and the phenomenon is known as autocatalysis. Example :
→ The hydrolysis of ester is slow in the begining but becomes fast after some times because acetic acid formed during the hydrolysis of ester behave as auto catalyst in this reaction.

Catalysis Chemistry Notes 7

→ During the oxidation of oxalic acid by acidified potassium permanganate, manganese ions (Mn2+) are formed. This manganese ions behave as autocatalyst. As soon as Mn2+ ions formed, the rate of reaction increases.

Catalysis Chemistry Notes 8

→ Induced Catalyst : There are some reactions in which any substance in the form of catalyst is not added but the rate of reaction increases due to the induction of any other reaction occurring simultaneously with it. This phenomenon is known as induced catalysis and the reaction which is inducing the other reaction is known as induced catalyst.

→ For example : Sodium sulphite (Na2SO3) is oxidised in presence of air but the oxidation of sodium arsenite (Na2 AsO3) is not possible. As both the reactants (Na2 SO3 and Na2 AsO3) are mixed then both (Na2 SO3 and Na2 AsO3) are oxidised by air because the oxidation of sodium sulphite (Na2 SO3) induces the oxidation of sodium arsenite (Na2 AsO3) Hence, in this reaction sodium sulphite behaves as induced catalyst.

Catalysis Chemistry Notes 9

Catalysis Chemistry Notes 9

Types of Catalysis :

Generally catalysis are of two types :

Homogeneous Catalysis :

→ If the reactanta, products and catalyst are in same phase then this type of catalysis is called homogeneous catalysis.
Example:
Oxidation of SO2 : In lead chamber process, the oxidation of SO2 is an example of homogeneous catalysis.

Catalysis Chemistry Notes 10

→ Hydrolysis of methyl acetate : Hydrolysis of methyl acetate by H ion produced from hydrochloric acid (HCl) is an example of homogeneous catalysis.

Catalysis Chemistry Notes 11

Here all the substances are in the liquid phase.

→ Hydrolysis of can sugar : Hydrolysis of cane sugar, catalysed by H+ ions produced from H2SO4 is an example of homogeneous catalysis.

Catalysis Chemistry Notes 12

Here all the substances are in same phase.

→ Oxidation of CO : Oxidation of CO by O2 takes place in presence of NO as catalyst.

Catalysis Chemistry Notes 13

→ Preparation of diethyl ether : Preparation of diethyl ether from ethyl alcohol using conc. H2SO4 is an example of homogeneous catalysis.

Catalysis Chemistry Notes 14

Heterogeneous Catalysis :

→ If the catalyst is not present in same phase as reactants and products in a chemical reaction then this type of catalysis is called heterogeneous catalysis.

Example :
Oxidation of SO2 : Oxidation of so, into SO3 in presence of Pt catalyst is an example of heterogeneous catalysis.

Catalysis Chemistry Notes 15

→ Haber’s Process : In this process, finely divided Fe is used as catalyst to produce ammonia from nitrogen and hydrogen gas.

Catalysis Chemistry Notes 16

→ Here all the reactants and products are in gas phase while catalyst iron is in solid phase.

→ Ostwald’s Process : In this process, oxidation of ammonia into nitric oxide takes place in presence of platinum gauze.

Mechanism of Catalysis :

Mechanism of catalysis are as follows:

→ Mechanism of Homogeneous Catalysis : Intermediate compounds formation theory According to this concept, catalyst forms an intermediate compound with one of the reactants. This intermediate compound is unstable which becomes free by reacting with other reactant to form product. A reaction A + B → AB takes place at very slow rate which takes place easily in the presence of catalyst X.
A + X → AX (Intermediate compound)
AX + B → AB + X

Low activation energy is required in the formation of intermediate AX. The reaction takes place at fast rate. Example:
Catalysis Chemistry Notes 17

Following facts can not be explained by intermediate compound theory:

  • Mechanism of heterogeneous catalysis.
  • Mechanism of catalyst promoter and catalyst poison.
  • Importance of active centres.

→ Mechanism of Heterogeneous catalysis : Adsorption Theory : Many gaseous reactions take place in the presence of solid catalyst. Active centres are present at the surface of solid catalyst due to free valencies. The molecules of reactants are adsorbed at the surface of solid catalyst by bonding with active molecules. Adsorbed molecules form activated complexes with catalyst; which are decomposed to form products and desorption starts at the surface of products.

Catalysis Chemistry Notes

Following points can be explained by this theory :

  • Adsorption and desorption take place continuously at the surface of catalyst. So, small amount of catalyst is required to catalyse high amount of reactants.
  • The surface of catalysts remains unchanged after desorption so, mass and composition of catalyst do not change at the end of reactions.
  • Reactants form chemical bonds at the surface of catalyst, in which free adsorption energy compensates activation energy. So, reaction takes place rapidly.
  • The molecules of catalyst poison are adsorbed rigidly on free valencies present at the surface of catalyst by which the molecules of reactants are not adsorbed.
  • Promoters are adsorbed at the surface of catalyst that number of active centres increased due to this reason adsorption capacity and reactivity increases.

Points to be remember :

→ Catalytic Poison or Inhibitor : The substances which destroy the activity of a catalyst by their presence is called catalytic poison or inhibitor. Example: In Haber’s process, if small amount of CO is present with hydrogen gas then it decreases the activity of iron catalyst i.e, CO behaves as catalytic poison for iron catalyst.

Catalysis Chemistry Notes 18

  • The platinum catalyst used in the oxidation of hydrogen is poisoned by CO.
  • The poisoning of the catalyst is probably due to adsorption of poison on active centres present at the surface of catalyst. Thus reducing the free surface for the adsorption of reacting molecules.
  • Due to this reason, the activity of catalyst either decreases or destroys and hence the rate of production of products also decreases.

→ Catalysis Promoters : The substances which can increase the efficiency of catalyst but themselves are not catalyst are known as catalytic promoters or activators.

Catalysis Chemistry Notes
Example :

→ In Haber’s process, for the synthesis of ammonia, iron powder behaves as catalyst but traces of molybdenum increases the activity of this catalyst. Hence Mo is catalytic promoter.

Catalysis Chemistry Notes 19

In the hydrogenation of vegetable oil, the activity of nickel catalyst can be increased by adding small amount of copper (Catalytic promoter).

Vegetable oil + H → Vegetable ghee

During the production of methyl alcohol from water gas (CO + H2),chromic oxide (Cr2O3) is used as catalytic promoter to increase the activity of zinc oxide (ZnO) catalyst.

Catalysis Chemistry Notes 20

Reason : Catalytic promoter increases the number of activity centres on the surface of catalyst due to which the rate of reaction increases.

Enzyme Catalysis :

→ Enzymes are complex nitrogenous organic compounds. They are obtained from living plants and animals. They are actually protein molecules having high molecular mass ranging from 15,000 to 1,000,000 g mol-1 and form colloidal solutions in water. Enzymes are very effective catalysts. Enzymes are effective generally for those chemical reactions which are related to natural processes.

→ They are also known as biochemical catalyst and the phenomenon of enzyme catalysis is known as biochemical catalysis Various enzymes are obtained from living cells. However, the first enzyme was synthesised in the laboratory in 1969. Enzymes are vital for the biological processes. Without enzyme, the life processes would be to slow to sustain life. The following are some of the examples of enzyme catalysis.

Inversion of can sugar : In the presence of enzyme invertase’ cane sugar is converted into glucose and fructose.

Catalysis Chemistry Notes 21

Conversion of glucose into ethyl alcohol : In presence of enzyme ‘zymase’, glucose is converted into ethyl alcohol and carbon dioxide.

Catalysis Chemistry Notes 22

Conversion of starch into maltose: In presence of enzyme ‘diastase’, starch is converted into maltose.

Catalysis Chemistry Notes 23

Conversion of maltose into glucose : In presence of enzyme ‘maltase’, maltose is converted into glucose.

Catalysis Chemistry Notes 24

Decomposition of urea into ammonia and carbon di oxide : In presence of enzyme ‘urease’, urea is converted into NH3 and CO2.

Catalysis Chemistry Notes 25

Conversion of proteins into peptides: In stomach, the enzyme ‘pepsin’ converts proteins into peptides.

Catalysis Chemistry Notes 26

Conversion of proteins into amino acids : In intestine, the pancreatic enzyme “trypsin converts proteins into amino acids by hydrolysis.

Catalysis Chemistry Notes 27

Conversion of milk into curd : The lacto bacilli’ which is present in curd is responsible to convert milk into curd by enzymatic action.

Catalysis Chemistry Notes 28

Conversion of ethyl alcohol into acetic acid : In presence of Mycoderma aceti’. dilute solution of ethyl alcohol is converted into acetic acid and water.

Catalysis Chemistry Notes 29

The summary of some important enzymatic reactions are given in table 5.2

Some enzymatic reactions

Enzyme Source Enzymatic reaction
Invertase Yeast Sucrose → Glucose and Fructose
Zymase Yeast Glucose → Ethyl alcohol and Carbon dioxide
Diastase Malt Starch → Maltose
Maltase Yeast Maltose → Glucose
Urease Soyabean Urea → Ammonia and C02
Pepsin Stomach Proteins → Peptides
Trypsin Intestine Proteins → Amino acids
Amylase Saliva Starch → Glucose
Lactobacilli Curd Fermentation of milk
Mycoderma Vinegar Ethyl alcohol → Acetic acid
Lipase Castor seed Fat → Glycerol
Ptyalin Saliva Starch → Sugar

Characteristics of Enzyme Catalysis :

Enzyme catalysis is quite similar to inorganic heterogeneous catalysis. It is unique in its efficiency and high degree of specificity. The main characteristic of enzyme catalysis are given below:

→ Highly Specific Nature : Enzymes are highly specific in their nature i.e., one enzyme can not catalyse more than one reaction.

For example:

  • Enzyme ‘urease’ can hydrolyse urea only. It can not hydrolyse the other amide.
  • Zymase converts only glucose into alcohol and CO2, It can not hydrolyse fructose.

→ High efficiency : Enzymes are highly effcient in their action. One molecule of an enzyme may catalyse one million molecules of the reactant per minute. It is due to the fact that the activation energy of chemical reaction is quite low in presence of enzyme.

Catalysis Chemistry Notes

→ Highly active at optimum temperature : The rate of an epzymatic reaction is maximum at a definite temperature, called the optimum temperature. Beyond the optimum temperature the activity of enzyme decreases and ultimately becomes zero. It is observed that the optimum temperature range for enzymatic activity is 298-310 K, which is human body temperature (i.e., 37°C). In fever (i.e., at higher temperature) the enzyme activity becomes slow.

→ Highly active at optimum pH : The rate of enzymatic reaction is maximum at a particular pH called optimum pH, which is in between 5-7. In human body, the optimum pH for enzymatic reaction is 7.4.

→ Effect on equilibrium state : Like catalyst, enzyme cannot disturb the final state of equilibrium of a reversible reaction. It only increases the rate of forward and backward reaction equal to extent so that equilibrium may achieve faster.

→ Colloidal nature : Enzymes form colloidal solutions in water. Their efficiency is decreased in presence of electrolytes.

→ Increasing activity in presence of activators and co-enzymes : Those substances, whose presence can increase the activity of enzyme is known as co-enzymes. For example : small amount of non-protein substance like vitamins, present along with enzyme may increase the catalytic activity of enzyme considerably.

→ Activators are generally metal ions such as Na+, Mn2+, CO2+, Cu2+ etc. These metal ions, when weakly bonded to enzyme molecules, increase their catalytic activity. For example, amylase in presence of sodium chloride i.e.. Na+ ions are catalytically very active.

→ Effect of Inhibitor or poisons : Like catalyst, enzymes are also poisoned in presence of certain substances. These substances are known as inhibitor or poisons. The inhibitors or poisons interact with the active functional groups on the suface of enzyme and often reduce or completely destroy the catalytic activity of the enzymes. For example, many drugs work as enzyme inhibitor and they destroy the activity of enzyme in the body.

Catalysis Chemistry Notes

Mechanism of Enzyme Catalysis :

The mechanism of enzyme catalysis involves the following steps:

→ Formation of Complex : There are various cavities on the surface of colloidal particles of enzymes. These cavities have characteristic shape and have active groups like-NH2, -COOH, -SH, -OH etc. on its surface. These active groups actually are active centers at the surface of enzyme particles. The reactants or substrate which have complementary substances, fit into these cavities just like key fits into a lock and form a activated complex

Catalysis Chemistry Notes 30

Formation of products : The complex is activated hence hase higher energy. So it decomposes easily into products.

Catalysis Chemistry Notes 31

Zeolite Catalysis or shape selective catalysis :

→ The catalysis process, which depends upon the pore structure of the catalyst and molecular size of reactants and products, is known as shape selective catalysis. Zeolites are good examples of shape selective catalysis due to its honey comb like structure. Zeolites are naturally occurring or synthetic microporous alumino silicates having general formula

Mx/n [(AlO2)x(SiO2)y].mH2O
where, M = Na+, K+, Ca2+ like metals
n = Valency of cation
m = Number of molecules of water of crystallisation Zeolites have three dimentional network structure of silicates in which some silicon atoms are replaced by aluminium atoms giving Al-O-Si frame work. Zeolites are formed in nature as well as synthesised artificially.

→ Zeolites, which are used as catalysts, first of all heated strongly in vacuum so that water of crystallization can be lost and it may become porous. The size of pores in zeolites varies between 260 pm to 740 pm. In this way only those molecules can be adsorbed whose size is small enough to enter in these pores and can also bo left catalyst easily.

→ Zeolites do not work as catalyst for those molecules which are too big to enter. Thus zeolites act as selective adsorbents and hence work as molecular sieves.

Example : (i) ZSM-5 (Zeolite sieve of molecular porosity 5) is used in petroleum industry. ZSM-5 converts alcohols directly into gasoline (petrol) by dehydrating the alcohol so that a mixture of hydrocarbons is obtained.

Catalysis Chemistry Notes 32

Zeolite is also used in softening of hard water in ion exchange method

Catalysis Chemistry Notes 33

Chemistry Notes

Corrosion Chemistry Notes

Corrosion Chemistry Notes

Corrosion :

→ When metals come in contact with atmosphere then gases and water vapour present in atmosphere attack at the surface of metals and some undesired compounds like metal oxide, carbonates, sulphides, sulphates etc., are formed. This process is known as corrosion.

OR

→ “The process in which oxides, carbonates, sulphides, sulphates etc., are formed by attack of atmospheric gases or water vapour on the surface of metals and metal corroded slowly, then this process is called corrosion.”

Example :

  • Tarnishing of silver.
  • Formation of green layer on copper surface.
  • Rusting of iron etc.

Corrosion Chemistry Notes

Types of Corrosion :

It is of following four types:

  • Soil Corrosion-Pipe lines, electric wires etc., which are embedded in soil undergo corrosion and such corrosion is called soil corrosion.
  • Microorganism Corrosion-When substances are corroded due to metabolic activity of microorganism then this type of corrosion is called microorganism corrosion.
  • Tearing Corrosion-It is generally a local corrosion which is generally produced from dust, cracking after paint and water.
  • Galvanic Corrosion-Most common corrosion is Galvanic corrosion. When two metals like Zn and Cu are attached electrochemically then that metal which lies above in electrochemical series corrodes in the presence of electrolyte.
  • Such type of corrosion is called Galvanic corrosion.

Factors Helping in Corrosion :

  • Metals form cell in the presence of impurities as a result corrosion takes place rapidly. Rust does not occur in pure iron while rust is formed in impure iron rapdily.
  • The rate of corrosion is high in the presence of electrolytes so iron corrodes rapidly in salty water than pure water.
  • If strains are present at surface of metal then that surface is corroded rapidly. It is the reason that the corners of boxes are broken rapidly.
  • Those metals like copper, iron etc., are more active, corrode rapidly and those metals like Au, Ag, Pt, Pd etc., are less reactive, undergo less corrosion.
  • The process of corrosion takes place rapidly in the presence of air and moisture. Corrosion takes place more rapidly in the presence of CO2 and SO2 in air.
  • Example : Rust occurs easily in iron in the presence of moist air while rusting of iron does not occur in dry air and vacuum.

Electrochemical Theory of Corrosion :

→ We can explain electrochemical theory of corrosion by example of rusting of iron. Oxygen and CO2 gas of atmosphere dissolved in water drops present at the surface of iron metal.
CO2 + H2O → H2CO3

Corrosion Chemistry Notes

→ Here, impure iron acts as cathode and pure iron acts as anode and aqueous solution present at surface in which O2 and CO2 are dissolved, acts as electrolyte. So; an electrochemical cell is formed at the surface of iron. The reactions take place in cell are as follows:

  • At anode : Iron undergoes oxidation and Fe2+ ions go in solution.
    Fe → Fe2+ + 2e (Oxidation)
  • At Cathode : H+ ions obtained by ionisation of H2CO3 and H2Oformed by adding H2O and CO2,accept electron.

Corrosion Chemistry Notes 1
Corrosion Chemistry Notes 2

Methods of Prevention of Corrosion :

Barrier Protection It is simplest method for protection of corrosion. In this method, iron surface can not in contact with atmospheric moisture, oxygen and CO2.

We can use following methods for barrier protection :

  • By painting at the surface of iron.
  • By using thin layer of oil or grease at the surface of iron.
  • By coating of a metal like Ni, Cr, Cu etc., which does not undergo corrosion, at the surface of iron by electroplating
  • By coating of Fe2 O3, iron phosphate or other chemicals at the surface of iron. These substances form insoluble layer at the surface of iron which acts as barrier.

→ Sacrificial Protection – In this method, the surface of iron is covered with a layer of more electropositive metal. So, oxidation of iron does not take place. Thus, iron prevents from corrosion due to sacrifice of more electropositive metal.

→ Its best example is galvanization, In this method, a thin layer of zine covers the surface of iron which protects rusting of iron. Since, zinc is placed above in electrochemical series. So, its oxidation takes place first and zinc is disintegrated and iron is protected.

Corrosion Chemistry Notes

→ Cathodic or Electrical Protection When underground iron pipes or tanks are used then we protect them from corrosion by this method. In this method, the bridges of pipe or tank or water etc., are attached with a block of more electropositive metals like Mg, Al, Zn etc. This metal block acts as anode and loses electrons.

→ The electrons obtained from anode move to cathode where the iron ions react with these electrons to convert into iron metal again and so here iron protects from corrosion. More electropositive metals are changed time to time so that iron is protected.

→ By Anti-rust Solution-Alkaline sodium phosphate and chromate solution act as anti-rust solutions. When iron metal objects are dipped in boiled phosphate solution then an invisible thin protective layer is formed on objects which protects iron from corrosion.

Chemistry Notes

Batteries Chemistry Notes

Batteries Chemistry Notes

Batteries :

→ When more than one calls are attached in series then battery is obtained. In fact, battery is an arrangement of Galvanic cells in series. Battery converts chemical energy of redox reaction into electric energy. There are of two types of batteries:

→ Primary Batteries – In primary batteries, reaction takes place only once and battery becomes inactive after some time and can not be reused again. These are of following types:

→ Dry Cell – It was discovered by Leclanche. So, it is called Leclanche cell. It is used generally in transistors and watches. This cell contains a zinc pot which acts as anode and the rod of carbon graphite, which is surrounded by powdered manganese dioxide and carbon, acts as cathode. The place between electrodes is filled with moist paste of ammonium chloride (NH4 Cl) and zinc chloride (ZnCl2). In this cell, electrode reactions are complex but these can be written simply as follows:

Batteries Chemistry Notes 1

Batteries Chemistry Notes 2

→ In cathode reaction, manganese is reduced from +4 to +3 oxidation state. Ammonia produced in the reaction forms a complex [Zn(NH3)4]2+ with Zn2+ ion. The cell potential is approximate 1.5 V The age of dry cell is not so high; since NH4Cl corrodes zinc apparatus due to acidic nature when the cell is not used.

→ Mercury Cell – Mercury cell is used in hearing devices, watches etc. which require less amount of electricity. In this cell, zinc-mercury amalgam acts as anode and HgO and paste of carbon acts as cathode. The paste of KOH and ZnO is electrolyte. The electrode reaction of cell is given below:

Batteries Chemistry Notes 3

  • Anode : Zn(Hg) + 2OH → ZnO(s) + H2O + 2e
  • Cathode : HgO + H2O + 2e → Hg(l) + 2OH

Complete cell reaction is represented as follows:

Zn(Hg) + HgO(s) → ZnO(s) + 2Hg(l)

→ Cell potential is approximate 1.35 V and constant in complete working, because no such ion is present in complet cell reaction whose cell potential can be changed in complete working of cell due to concentration present in solution.

Batteries Chemistry Notes

→ Secondary or Storage Batteries – These are reversible Galvanic cells. In these batteries, the substances having high energy produce electric current on rapid reaction and become inactive. These substances made active again by external sources and those substances are used again in cell. So, these cells can be recharged again. Storage battery can be discharged and charged many times. Main examples of these batteries are as follow:

→ Lead Storage Cell – In this cell, anode is made up of lead and cathode is lead grid filled with PbO2.
It contains 38% H2SO4 as electrolyte.

Following reactions take place in it:

Batteries Chemistry Notes 4

So, net reaction of cathode and anode is given below:

Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l) The reaction is reversible on charging of battery and anode and cathode change in PbSO4(s)). Pb and PbO2 respectively.

Reversed reaction :

2PbSO4(s) + 2H2O(l) →Pb(2) + PbO2 + 2H2SO4(aq)

Nickel Cadmium Storage Cell-It is an important storage cell whose working period is more than lead storage battery but its costing is more. Its discharge reaction is given below:

Batteries Chemistry Notes 5

Batteries Chemistry Notes 6

Difference between Primary and Secondary Cell :

Primary Cell Secondary Cell
Primary cells are irreversible. Secondary cells are reversible.
Chemical reaction takes place only in one direction. Chemical reaction takes p1ace in both directions.
These types of cells can not be charged. These types of cells can be charged.

Chemistry Notes

Salt Brldge and Its Eunctions Chemistry Notes

Salt Brldge and Its Eunctions Chemistry Notes

Salt Bridge and Its Functions :

→ It is a U-tube filled with concentrated solution of inert electrolyte like KNO, NH, Cl, KCl etc., which is kept inverted. Here, for electrolytes :

  • The movement of cation and anion should be nearly same.
  • Ions should not be chemically reactive with ions of cell.
  • Ions should not be included in chemical change.

→ Functions of salt Bridge

  • Electric circuit is completed by salt bridge.
  • Salt bridge mantains electrical neutrality of solutions of both half-cells.

→ As we know that when circuit is completed then electrons are transferred from anode to cathode as a result, positive charge is accumulated near anode and negative charge is accumulated near cathode. In above cell, the concentration of Zn2+ ions increases near anode and concentration of so ions increases near cathode.

Salt Brldge and Its Eunctions Chemistry Notes

→ Due to this reason, the flow of electron from anode and its receiving at cathode will be stopped. \(\mathrm{SO}_{4}^{2-}\) reaction will stop and flow of electric current will stop. In such condition, the anions of electrolyte present in salt bridge move at anode and cations move at cathode and maintain electric neutrality. Inert electrolyte present in salt bridge prevents accumulation of charges near electrodes. Porous pot may also be used in device in place of salt bridge.

Chemistry Notes

Electrochemical cell or Galvanic cell or Voltaic cell Chemistry Notes

Electrochemical cell or Galvanic cell or Voltaic cell Chemistry Notes

Electrochemical Cell or Galvanic Cell or Voltaic Cell :

→ Those apparatus or cells or electro-chemical devices by which we convert chemical energy into electric energy, are called Galvanic or voltaic or electrochemical cells. These were first discovered by L. Galvani and A. Volta. So, these were named as Galvanic or Voltaic cell on their names.

→ In these cells, oxidation or reduction reactions take place in different cells, which are called half-cells. Redox reaction takes place spontaneously here. Electrical energy is produced during these reactions. Here, spontaneous redox reactions takes place indirectly.

Electrochemical cell or Galvanic cell or Voltaic cell Chemistry Notes

→ Example: This redox reaction can be explained by working of an electro-chemical cell. This cell is made up of two electrodes, which is called Galvanic or Voltaic or Daniell cell. These two electrodes are attached with porous plate or salt bridge. The rod of Zn metal is dipped in 1.0 M ZnSO4 solution in one electrode and Cu rod is dipped in 1.0 M CusO4 solution in other electrode.

→ These metallic rods present in beaker are attached with ammeter by a connecting wire and a key is also attached in this path. Here, the solutions of both beakers are connected by salt bridge in which the saturated solution of KCl, KNO3 or NH4 NO3 is filled. This salt bridge is made up of U-tube. The saturated solution of KCl, KNO3 or NH4 NO3 does not represent any chemical reaction during the process. This electrolyte is called inert electrolyte.

→ Here, the saturated solution of inert electrolyte is generally formed in gelatin. U-tube is made up of glass. Both open ends of U-tube are closed by a porous substance like glass wool or cotton. It is found on completion of circuit by pressing key in circuit that electric current is passed through the circuit, which can be seen in ammeter.

Electrochemical cell or Galvanic cell or Voltaic cell Chemistry Notes 1

→ Following observations are seen during the reaction:

  • The weight of zinc rod decreases slowly.
  • The concentration of Zn2+ in ZnSO4 solution increases.
  • More amount of Cu is liberated on copper rod.
  • The concentration of Cu2+ in CuSO4 solution decreases.
  • Here, electron flows from zinc rod to copper rod in external circuit. So, electric current flows from copper
    rod to zinc rod. We can explain following on the basis of these observations:

→ Here, the oxidation of Zn takes place and Zn2+ ions go in the solution. As a result, the weight of rod decreases.
Zn → Zn2+ + 2e2- (Oxidation)

Electrochemical cell or Galvanic cell or Voltaic cell Chemistry Notes

→ When electrons are released from zinc electrode then these move towards Cu electrode. Here, Cu2+ ions of CuSO4 solution accept these electrons and liberated at Cu rod on reduction.
Cu2+ + 2e2- → Cu (Reduction)

→ The rod at which oxidation occurs, is called anode and where reduction occurs, that is called cathode. Since, electrons move from zinc rod to copper rod. So, zinc rod is called negative terminal and copper rod is called positive terminal. Apparatus in which oxidation and reduction takes place is called half-cell. Zinc rod dipped in ZnSO3 solution is called oxidation half-cell and copper rod dipped in CuSO4 solution is called reduction half-cell.

  • Trick Remember : LOAN
  • L = Left
  • O = Oxidation
  • A = Anode
  • N = Negative terminal

→ The reaction obtained by addition of oxidation reaction of Zn electrode and reduction reaction of Cu electrode is called cell reaction.
Zn + Cu2+ – Zn2+ + Cu (Redox Reaction)

Chemistry Notes

Kohlrausch’s Law Chemistry Notes

Kohlrausch’s Law Chemistry Notes

Kohlrausch’s Law :

On the basis of various observations Kohlrausch proposed a law of independent transport of ions. According to this law, “The limiting molar conductivity of an electrolyte can be expressed as sum of different contribution of its cation and anion.”
OR
“The value of molar conductivity is different for different electrolytes because each ion of electrolyte contributes certainly amount molar conductivity at infinite dilution of solution. This contribution does not depend on nature of other ion of electrolyte. This contribution of an individual ion is called molar ionic conductivity.”

Mathematical Representation of Kohlrausch’s Law :

Mathematically, for calculation of molar conductivity at infinite dilution of electrolytic solution, the number of cations present in its unit formula is multiplied by molar conductivity of cation and number of anion is multiplied by molar conductivity of anion. Then, both are added.

Kohlrausch’s Law Chemistry Notes 1

Applications of Kohlrausch’s Low :

There are following applications of this law:

  • With the help of this law, molar conductivity of weak electrolyte at inlinite dikition (\(\Lambda_{m}^{\infty}\))can be calculated.
  • For example : If we want to determine molar conductivity of acetic acid at infinite dilution then it can be determined with the help of strong electrolytes such as NaCl, HCl and CH3COONa, which is as follows:

Kohlrausch’s Law Chemistry Notes 2

With the help of above equation, we can determine the molar conductivity of CH,COOH at infinite dilution.

Degree of Dissociation of Weak Electrolyte can also be determined by this law.
We know that,

Kohlrausch’s Law Chemistry Notes 3

  • where, α = Degree of dissociation.
  • \(\Lambda_{m}^{c}\) = Molar conductivity at concentration ‘C’
  • \(\Lambda_{m}^{\infty}\) = Molar conductivity at infinite dilution

Dissociation Constant of Weak Electrolyte can also be determined by this law.

  • K = \(\frac{C \alpha^{2}}{(1-\alpha)}\)
  • Where, K -Dissociation constant
  • α = Degree of dissociation
  • C = Concentration

Kohlrausch’s Law Chemistry Notes

→ The Solubility of Partially soluble salts like AgCl, BaSO4. PbSO4, etc., can also be calculated by this law. Its formula is given as below:
\(\Lambda_{m}^{\infty}=\frac{\kappa \times 1000}{\text { Solubility }}\)
Where, k = Specific conductivity
\(\Lambda_{m}^{\infty}\) = Molar conductivity at infinite dilution

Ionic Product of Water can also be calculated with the help of this law.
The values of ionic molar conductivity of H+ and OH at infinite dilution are 349.8 ohm-1cm2 mol-1 and 198.5 ohm-1cm2mol-1 respectively. So,

Kohlrausch’s Law Chemistry Notes 4

→ Calculation of Transport Number of lons-The ratio of molar conductivity of ion at infinite dilution and molar conductivity of that electrolyte (in which that ion is present) at infinite dilution is called transport number of the ions. We know that,

Kohlrausch’s Law Chemistry Notes 5

This law of independent transport of Kohlrausch is applicable to all strong and weak electrolytes uniformly.

Chemistry Notes

Law of Electrolysis Chemistry Notes

Law of Electrolysis Chemistry Notes

→ Michael Faraday (1833-34) has done many experiments on electrolytic processes and obtained two important conclusions. The two main laws are given as below:

→ Faraday’s First Law of Electrolysis – “The amount of substance liberated at electrode is directly proportional to quantity of electric current passed in electrolytic solution.”

→ So, if mg substance is liberated on passing Q coulomb of electric current, then,
m ∝ Q
m = ZQ
Where, Z is proportionality constant and is called electro-chemical equivalent.
If I ampere electric current is passed through t second,
then,
Q = I × t
m = Z × Q
m = Z × I t
Now, if
Q = 1 C
or I = 1 A and t = 1 s
then,
m = Z × 1 × 1
or
M = Z

→ So, electrochemical equivalent of a substance is the mass of substance liberated, when one ampere current is passed in 1 second i.e. 1 coulomb quantity of electricity is passed.

Law of Electrolysis Chemistry Notes

Unit
Z = \(\frac{m}{I t}\) = g A-1 s-1

→ Faraday’s Second Law of Electrolysis According to this law, “When same quantity of electricity is passed throgh various electrolytic solutions arranged in series, then the weights of substances produced at electrodes are directly proportional to their chemical equivalent weights.” For exampleWhen same electric current is passed in two electrolytic solutions arranged in series i.e., copper sulphate (CuSO4) and silver nitrate (AgNO3), then the weight of copper and silver liberated will be as follows:

Law of Electrolysis Chemistry Notes 1

→ Although equivalent weight terms is not used in modern terms. Faraday’s law of electrolysis in terms of moles of electrons exchanged during electro-chemical change can be expressed as follows: “The moles of substance liberated i.e., quantity of chemical change is directly proportional to the number of moles of electrons exchanged during oxidation reduction reaction”.

Law of Electrolysis Chemistry Notes

→ When the same quantity of charge is passed through different electrolytes arranged in series then the quantities of substances deposited at electrodes of each electrolyte can be calculated easily. Consider the figure 3.3 in which electrolytic solutions of H2SO4, CuSO4 and AgNO3 are situated in equivalent amounts.

Law of Electrolysis Chemistry Notes 2

→ It is known to calculate the weight of H., Cu and Ag liberated at cathodes of three electrolytes, there weights are always obtained in the ratio of equivalent weights 1:13.78: 10.788
From Faraday’s second law

M ∝ E ……… (1)

→ If quantities of substances liberated from two electrolytic cells are m, and m, respectively, whose equivalent weights are E and E, respectively, then

Law of Electrolysis Chemistry Notes 3

→ from Faraday’s first law, m1 = Z1 It and m2= Z2 It in which w1 and w2 are chemical equivalents of both substances liberated.

Law of Electrolysis Chemistry Notes 4

→ So, electro-chemical equivalent of a substance is directly proportional to its equivalent weight. It is seen experimentally that if 1 ampere electric current is passed in 1 second i.e., on passing 1 coulomb electric charge, 0.0011180 g Ag, 0.0003296 g Cu and 0.00001036 × H2 are liberated. These quantities are called their electro-chemical equivalents respectively. So, that quantity (Q) of electricity which is necessary for liberation of one gram equivalent of Ag.

Law of Electrolysis Chemistry Notes 5

→ So, the quantity (charge) of electricity required for liberation of one gram equivalent weight of substance is equivalent to one Faraday. Hence, if I ampere electric current is passed in second for liberation of substance having E equivalent weight from electrolytic solution, then the quantity of substance liberated can be determined by following formula:

m = \(\frac{\text { I.t. } E}{F}\)

→ Both laws of Faraday are called laws of electrolysis. These laws are used in electroplating techniques. These laws are very helpful to understand behaviour of various electrolytes. 1 Faraday charge is equivalent to charge of one mole electrons.

Law of Electrolysis Chemistry Notes

Charge of one electron = 166 × 10-19 coulomb
So, charge of one mole electron
= 1.66 × 10-19 × 6.023 × 1023
= 96500 coulomb

Chemistry Notes

Electrotytlc cell Chemistry Notes

Electrotytlc cell Chemistry Notes

Electrolytic Cell :

The cell which can convert electric energy into chemical energy, is called electrolytic cell. Molten electrolyte or its aqueous solution filled in electrolytic cell is electrolysed on passing electric current. The process takes place here is a non-spontaneous redox process. This process is called electrolysis. It is clear that oxidation-reduction reactions takes place in this cell are of non-spontaneous nature. So, in the cell reaction, increase in free energy(∆G > 0) takes place.

Electrotytlc cell Chemistry Notes

Process of Electrolysis :

→ The process of electrolysis is completed by taking suitable amount of solution of an electrolyte in an apparatus. This apparatus is called electrolytic tank. It is made up of insulators like glass etc. Two metallic rods are dipped in this solution which are connected with terminals of battery with the help of wires.

→ Since these rods are responsible for flow of electric current so these are called electrodes. The electrode which is connected with positive terminal of battery, it is called anode and which is connected with negative terminal, is called cathode.

Electrotytlc cell Chemistry Notes 1

→ When electrolytic solution is formed then it dissociates in cations and anions. On passing electric current in solution, ions present in solution attract towards oppositely charged electrodes. As a result, cations move towards cathode and anions move towards anode. Anion gives its electron on reaching at anode and cation accepts its electron on reaching at cathode.

→ So, cations and anions are converted into neutral particles by neutralisation on their related electrodes which again form a new molecule or compound. Example 1. Electrolysis of Aqueous HCl Fig. 3.2 is used to explain mechanism of electrolysis in which dil. HCl solution is present. In this, two electrodes of platinum are connected with an electric source (battery).

Electrotytlc cell Chemistry Notes

→ Electrode connected with positive terminal of battery is called anode and electrode connected with negative terminal is called cathode. Anions are discharged at anode and cations are discharged at cathode. Hydrogen ions move towards cathode and chloride ions move towards anode on establishing circuit. Following type of reduction reaction takes place at cathode :
At cathode: H+ + e → H2(Reduction)

Similarly, chloride ion undergoes oxidation reaction at anode as follows:
At anode: Cl → \(\frac {1}{2}\)Cl2 + e (Oxidation)

Electrotytlc cell Chemistry Notes 2

→ It is clear that electrons move from anode to cathode (opposite in direction of flow of electric current) in external circuit for completing the circuit and ions move towards oppositely charged electrodes. In this process, oxidation-reduction process takes place with electric conduction. Chlorine gas is evolved from anode and hydrogen gas is evolved at cathode. Electrolyte remains electrically neutral at each moment because both ions are discharged together.

This process continues till:

  • Electric circuit is closed or
  • Complete HCl is not consumed.

Electrolysis of Molten Sodium Chloride :

Fused sodium chloride contains Na and Clions.

Nacl ↔ Na+ + Cl

Electrotytlc cell Chemistry Notes

→ During electrolysis, Na+ ions are attracted towards cathode and Cl ions are attracted towards anode. Here clo gives one electron to anode and converts into chlorine atom. Since chlorine atoms are unstable. So, these form chlorine molecules on combining in pairs.. Na+ ions move towards cathode and convert into sodium metal by taking an electron from each cathode.

Reactions can be expressed as follows:

At Cathode : Na+ + e → Na (Reduction)
At Anode : Cl – e – →Cl (Oxidation)
Cl + Cl → Cl2
Electrotytlc cell Chemistry Notes 3

Electrolysis of Aqueous Sodium Chloride :

Sodium chloride is ionized in water as follows:

Electrotytlc cell Chemistry Notes 4

→ Hence, the aqueous solution of sodium chloride contains Na+, H+, OH and Cl ions. When electric current is passed in solution then Na+, H+ ions move towards cathode and OH, Cl ions move towards anode. Here, is competition for reduction between Na+ and H+ and the ion whose ionization potential is more, will reduce first. Since reduction potential of H+ is high so it will reduce and H2 gas will be obtained.

Electrotytlc cell Chemistry Notes

→ OH and Cl will move towards anode. Here, also a competition for oxidation between OH and Cl and the ion whose oxidation potential is high will be oxidised first at anode. Here, the oxidation potential of Cl is high. So, it forms Cl2 gas on oxidation.
So, reactions will take place as follows:

Electrotytlc cell Chemistry Notes 5

Chemistry Notes

Ideal Solutions Chemistry Notes

Ideal Solutions Chemistry Notes

→ Those solutions which obey Raoult’s law at all ranges of temperature and concentration are called ideal solutions. An ideal solution of components A and B will satisfy the following conditions: (1) It obeys Raoult’s law i.e,

Ideal Solutions Chemistry Notes 1

Ideal Solutions Chemistry Notes 2

→ The volume of solution on mixing both components should be equal to volume of two components. i.e.,
∆V(mixing) = 0

→ There is no change in enthalpy on mixing both components.i.e.
∆H(mixing) = 0

→ The intermolecular forces of attraction between A – B in solution should be equal to intermolecular forces of attraction present in components (i.e, between A – A and B – B). Solute and solvent having same physical and chemical properties form ideal solution.

Ideal Solutions Chemistry Notes

Examples :

  • Benzene + Toluene (Aromatic hydrocarbons)
  • n – Hexane + n-Heptane (Alkanes)
  • Chlorobenzene + Bromobenzene (Aryl holides)
  • Carbon tetrachloride (CCl4) + Silicon tetrachloride (SiCl4) (Chlorides of group 14)
  • Ethylene chloride + Ethylene bromide (Alkyl halides)
  • Ethyl bromide + Ethyl iodide (Alkyl halides)
  • Methanol + Ethanol (Alcohols)
  • 1 – Chlorobutane +1-Bromobutane (Alkyl halides)

Chemistry Notes

Raoult’s Law Chemistry Notes

Raoult’s Law Chemistry Notes

Raoult’s Law :

First of all, French chemist F.M. Raoult (1886), established relationship between lowering of vapour pressure and concentration of solution.

Raoult’s Law for a solution containing volatile solute :

→ The partial vapour pressure of a volatile component present in solution at a certain temperature is equal to product of vapour pressure of that component in pure state and mole fraction of that component.

Raoult’s Law Chemistry Notes

→ Suppose two volatile components A and B are present in solution. Partial pressures in that solution are PA and PB respectively. Their vapour pressures in pure state are p°A and p°B respectively. Their mole fractions are xA and xB respectively then, Mole fraction of component A

Raoult’s Law Chemistry Notes 1

Raoult’s Law for a solution containing non volatile Solute :

The vapour pressure of solution having non-volatile solute (B) is equal to partial vapour pressure of volatile solvent (A) only because the partial vapour pressure of non-volatile solute is negligible. So, Vapour pressure of solution

Raoult’s Law Chemistry Notes 2

Raoult’s Law Chemistry Notes 3

Hence, according to Raoult’s law.” The relative lowering of vapour pressure for a solution having non-volatile solute at a certain temperature is equal to mole fraction of solute.”

Chemistry Notes