Problem 17 - Chemistry: The Central Science

Question

(a) What is meant by the term reaction rate? (b) Name three factors that can affect the rate of a chemical reaction. (c) Is the rate of disappearance of reactants always the same as the rate of appearance of products?

Step-by-Step Solution

Answer

(a) Reaction rate is a measure of how fast a chemical reaction occurs, expressed as the change in concentration of a reactant or product over a given period of time, in units of moles per liter per second (M/s). (b) Three factors that can affect the rate of a chemical reaction are: (1) Concentration: higher concentration of reactants leads to increased reaction rates. (2) Temperature: higher temperature increases the kinetic energy of particles, leading to higher reaction rates. (3) Catalysts: these substances can lower the activation energy and increase the reaction rate without being consumed themselves. (c) In a balanced chemical equation, the rate of disappearance of reactants is equal to the rate of appearance of products, as the stoichiometry of the balanced equation dictates the necessary ratio of reactants to products. However, this applies to the overall reaction and not necessarily to each individual reactant or product; rates may differ for individual species but will still obey the stoichiometry of the balanced equation.

Step 1: (a) Definition of Reaction Rate

A reaction rate is a measure of how fast a chemical reaction occurs. It is usually expressed as the change in concentration of a reactant or product over a given period of time, typically in units of moles per liter per second (M/s).

Step 2: (b) Factors Affecting Reaction Rates

(1) Concentration: Increasing the concentration of reactants generally increases the rate of a chemical reaction, because there are more particles available to react, thus increasing the likelihood of collisions that lead to a reaction. (2) Temperature: Increasing the temperature generally increases the rate of a chemical reaction, as the increased kinetic energy of the particles allows them to collide with greater force and frequency. This leads to a higher chance of successful collisions (that is, collisions that result in a reaction). (3) Catalysts: Catalysts can affect the rate of a chemical reaction by lowering the activation energy, the minimum energy required for a reaction to occur. This allows for a greater number of successful collisions and increases the reaction rate. Note that a catalyst is not consumed in the reaction and can be recovered unchanged at the end of the process.

Step 3: (c) Rate of Disappearance of Reactants vs. Rate of Appearance of Products

In a balanced chemical equation, the rate of disappearance of reactants is indeed equal to the rate of appearance of products. This is because the stoichiometry of the balanced equation dictates the necessary ratio of reactants to products in the reaction, so when one reactant is consumed, a certain amount of product is formed according to the balanced equation. However, it is important to note that this applies to the overall reaction and not necessarily to each individual reactant or product; rates may differ for individual species within the reaction but will still obey the stoichiometry of the balanced equation.