Chemical Kinetics - SS2 Chemistry Past Questions and Answers - page 3

Several factors influence the rate of chemical reactions:

a.    Concentration: The concentration of reactants affects the rate of reaction. An increase in reactant concentration leads to more frequent collisions between particles, resulting in a higher reaction rate. For example, in the reaction between hydrochloric acid (HCl) and sodium thiosulfate (Na2S2O3), increasing the concentration of HCl will result in a faster reaction rate.

b.    Temperature: Increasing the temperature generally increases the rate of reaction. Higher temperatures provide more energy to reactant particles, increasing their kinetic energy and collision frequency. As a result, more successful collisions occur, leading to a faster reaction rate. For instance, in the combustion of propane (C3H8), raising the temperature will accelerate the reaction.

c.     Surface area: The surface area of solid reactants influences the reaction rate. Finely divided or powdered substances provide more surface area for reactant particles to interact, leading to a higher reaction rate. This is because a larger surface area allows for more collisions to occur. An example is the reaction between solid calcium carbonate (CaCO3) and hydrochloric acid (HCl), where powdered CaCO3 will react faster than larger solid pieces.

d.    Catalysts: Catalysts are substances that increase the rate of reaction without being consumed in the process. They provide an alternative reaction pathway with lower activation energy, allowing reactants to undergo more successful collisions. For example, the enzyme amylase acts as a catalyst in the breakdown of starch into glucose during digestion.

Activation energy is the minimum energy required for a chemical reaction to occur. It is the energy needed to break the bonds in the reactant molecules, enabling the formation of new bonds and the conversion of reactants into products.

In a chemical reaction, reactant molecules need to overcome the energy barrier represented by the activation energy. Only when the reactants have sufficient energy to surpass this barrier can they proceed to form products. The activation energy determines the rate of reaction because it influences the frequency of successful collisions and the formation of activated complexes.

A higher activation energy generally results in a slower reaction rate since fewer reactant molecules possess the necessary energy to surpass the barrier. Conversely, a lower activation energy allows a larger fraction of reactant molecules to overcome the barrier and proceed to form products, leading to a faster reaction rate.

Catalysts play a crucial role in modifying the activation energy of a reaction. They lower the activation energy by providing an alternative reaction pathway with a lower energy barrier. This reduction in activation energy allows more reactant molecules to reach the activation energy threshold, resulting in an increased reaction rate.

Concentration is a key factor that influences the rate of chemical reactions. According to collision theory, for a reaction to occur, reactant particles must collide with sufficient energy and proper orientation.

An increase in reactant concentration leads to more frequent collisions between particles. With a higher concentration, there is a greater number of reactant particles within a given volume, increasing the probability of collisions. Consequently, the collision theory predicts a higher reaction rate when the reactant concentration is increased.

When the concentration of reactants is increased, the chances of effective collisions are enhanced. Effective collisions occur when particles collide with sufficient energy (exceeding the activation energy) and proper orientation for the reaction to proceed. By increasing the concentration, the number of effective collisions increases, resulting in a faster reaction rate.

Conversely, decreasing the reactant concentration decreases the collision frequency, reducing the number of effective collisions and thus lowering the reaction rate.

It's important to note that while concentration affects the reaction rate, it does not influence the activation energy. The activation energy remains constant, but the concentration alters the frequency of successful collisions required to surpass the activation energy barrier and initiate the reaction.