Chemical Kinetics Chemistry Notes
→ We know that in a chemical reaction, a substance having specific properties is converted into other substances whose properties are different. Chemists are always qurious to know about following details :
- A chemical reaction is possible or not in the given conditions. It is determined by thermodynamics. If the value of Gibbs free energy is negative (-4G) then reaction will be possible or not.
- How much part of reactants can be converted into products. It can not be answered by chemical equilibrium.
- How much time will be required to complete the reaction or reach to equilibrium?
→ Above facts can be determined with the help of given conditions like-pressure, temperature, concentration, catalyst etc. These conditions are also helpful for determining reaction mechanism.
→ Chemical reactions take place with various rates. So, the branch of chemistry which deals with the rates of chemical reactions, mechanism and factors affecting them is called chemical kinetics. The term ‘kinetics’ is derived from Greek word ‘kinesis’, which means rate. So, chemical kinetics determines rate of reaction.
Chapter in Brief and Glossary :
→ Ionic or Instantaneous Reactions : The reactions which take place between ions and possess high rate of reaction, are called ionic reactions.
Example : NaCl(aq) + AgNO3(aq) — AgCl(s) + NaNO3(aq)
→ Very Slow Reactions : These reactions occur between molecules. Their rate is very slow. So, there is no measurable change in these reactions.
Example : Formation of water by combination of hydrogen and oxygen in the absence of catalyst.
→ Molecular or Slow Reactions : Those reactions which take place between molecules, are called molecular reactions. Their rates can be determined.
→ Rate of Reaction: The change in concentration of a reactant or product in unit time is called rate of reaction. Its unit is mol L-1min-1
→ Average Rate : The change in concentration of reactant or product present in a chemical reaction per unit time is called average rate of reaction.
→ Instantaneous Rate : The actual rate of reaction at any time is called instantaneous rate.
→ Temperature Coefficient : The ratio of rate constants of reaction at 25°C and 35°C temperature or difference of 10°C temperature is called temperature coefficient. Its value is approximate 2 to 3.
→ Rate Law or Rate Expression : The equation which establishes relationship between rate of rection and concentration of reactants, is called rate law.
For example: aA + bB → cC + dD
According to rate law for reaction,
Reaction rate (r) = k[A]a [B]b
→ Molecularity of Reaction : The total number of molecules of reactants taking part in elementary step of reaction is called molecularity of reaction.
Simple Reactions : The reactions whose molecularity is equal to number of reactants, are called simple reactions.
→ Complex Reactions : The reactions, in which number of reactants is more than three, takes place generally in two or more steps. The slowest step in these reactions is called rate determining step. These reactions are called complex reactions. For example: HBr + O2 → 2H2O + 2Br2
Rate = k (HBr] [O2]
→ Order of Reaction : The number of reactant molecules taking part in a reaction which shows measurable change in concentration and determines the rate of reaction, is called order of reaction or the sum of power of concentration of reactants in rate law expression is known as order of reaction.
→ Half-life Period : The time required to change the half part of reactant in to product in a reaction is called half-life period (t1/2) of that reaction.
→ Pseudo Unimolecular Reactions : The reactions whose molecularity is two or more but order is one, are called pseudo unimolecular reactions, For example:
→ Activation Energy : The minimum extra amount of energy which is essentially provided to molecules for effective callision is called activation energy.
Ea = ET – ER
Where, Ea = Activation energy
ET = Threshold energy
ER = Average energy of reactants
→ Collision Frequency : The number of collisions taking place in a reaction mixture per unit volume per second is called collision frequency.