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Q3.5:
Mention the factors that affect the rate of a chemical reaction.

Solution :

Initial Theoretical Foundation

The rate of a chemical reaction is defined as the change in the concentration of reactants or products per unit time ($Rate = \pm \frac{d[C]}{dt}$). This rate is governed by the molecular dynamics of the system, specifically the frequency and energy of collisions between reactant molecules [Per Collision Theory].

Step 1: Influence of Concentration of Reactants

According to the Rate Law expression, the rate of a reaction is directly proportional to the molar concentrations of the reactants raised to their respective stoichiometric powers: $Rate = k[A]^x[B]^y$.

  • An increase in concentration increases the number of effective collisions per unit volume [By Law of Mass Action].
  • For zero-order reactions, rate is independent of concentration; for positive-order reactions, rate increases as concentration increases.

Step 2: Influence of Temperature

The rate constant ($k$) is sensitive to temperature variations, expressed by the Arrhenius Equation: $k = Ae^{-E_a/RT}$.

  • As temperature ($T$) increases, the kinetic energy of molecules increases.
  • A larger fraction of molecules possesses energy equal to or greater than the activation energy ($E_a$), leading to a higher frequency of successful collisions.
  • As a general rule of thumb, a $10^\circ C$ rise in temperature approximately doubles the rate constant.

Step 3: Role of Catalyst

A catalyst is a substance that alters the reaction rate without being consumed. It functions by providing an alternative reaction pathway with a lower activation energy ($E_a'$).

Energy Reaction Coordinate Uncatalyzed Path Catalyzed Path

Step 4: Surface Area of Reactants

For heterogeneous reactions (involving solids), the rate is dependent on the total surface area exposed.

  • Increasing the surface area (e.g., using a powder instead of a block) provides more active sites for adsorption and subsequent reaction [Per Langmuir Adsorption Isotherm principles].

Step 5: Nature of Reactants and Presence of Light

Factor Effect
Nature of Reactants Ionic reactions are generally faster than covalent reactions due to the absence of bond-breaking requirements.
Presence of Light Photochemical reactions (e.g., $H_2 + Cl_2 \xrightarrow{h\nu} 2HCl$) require photons to provide the energy necessary to overcome $E_a$.

Final Solution: The factors affecting the rate of a chemical reaction are the concentration of reactants, temperature, presence of a catalyst, surface area of reactants, the physical state/nature of the reactants, and exposure to radiation (light).


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