Factors affecting the Rate of Reaction Chemistry Notes
Factors Affecting the Rate of Reaction :
The rate of reaction depends on experimental conditions. The factors which affect rate of chemical reaction are:
→ Concentration of Reactants : According to law of mass action, “rate of reaction is directly proportional to the product of concentration of reactants.” So, rate of reaction increases with increase in the concentration of reactants but concentration change in zero order reaction does not affect rate of reaction.
→ For example If concentration of PCl3 or Cl2 in the reaction PCl3 + C2 → PCl5 increases then the rate of reaction also increases.
→ Nature of Reactants : The rate of reaction depends on the nature of reactants. The bonds present in reactants are broken during the reaction, while bonds present in products are formed. If number of bonds to be broken in reaction is more, then this reaction takes place at a slow rate. Similarly, if the number of bonds to be broken is less, then the reaction takes place at fast rate.
→ For example, Ionic reaction takes place rapidly in aqueous medium because ions come in free state in aqueous medium and these react rapidly. On the other hand, the rate of molecular reactions is slow.
→ Surface Area of Solid Reactants : This factor is very important for a heterogeneous chemical reaction. The rate of reaction also increases with the increase in surface area of a solid reactant and rate of reaction decreases with the decrease in surface area of reactant.
→ Effect of Temperature : When temperature of reaction increases then kinetic energy of molecules of reactant increases by which rate of reaction increases. This is supposed that rate of reaction becomes twice from normal with the increase of 10°C temperature. The ratio of rate constants of reaction at 25°C and 35°C temperature is called temperature coefficient, whose value is approximate 2 to 3.
So, Temperature coefficient
→ Effect of Catalyst : Catalysts remain unchanged during reaction but their presence increases or decreases the rate of reaction which depends on nature of catalyst. Positive catalyst decreases the activation energy of reaction by which molecules of reactants present in low energy level also form products and he rate of reaction increases. Negative catalyst increases the value of activation energy for the reaction by which the rate of reaction decreases. So, catalysts give the alternative path for the reaction.
→ Effect of Physical State : If one of the reactant is found in solid state in the reaction then solid reactant should be finely divided as finely divided substance have more surface area. So the number of molecules undergo collision will be more and rate of reaction will increases.
→ Effect of Radiations : The rate of reaction for the reactions taking place in the presence of light depends upon presence or absence of light. Example : Photosynthesis reactions do not take place in dark but this reaction takes place rapidly in the presence of sun light. So, rate of such reactions increases in the presence of specific radiations (hv).