# Reaction rates problems and solutions

1. What is the rate of reaction? Solution: The rate of reaction measures how the concentration of a reactant or a product changes over time. It is usually expressed in terms of change in concentration per unit time.

2. How does temperature affect the rate of a chemical reaction? Solution: Generally, an increase in temperature increases the rate of a chemical reaction. This is because higher temperature provides the reactant molecules with more kinetic energy, making them move faster and collide more frequently and more violently, thus increasing the reaction rate.
3. What is a rate law, and how can it be determined experimentally? Solution: The rate law is a mathematical relationship obtained by comparing reaction rates with reactant concentrations. It can be determined experimentally by monitoring the changes in concentration of reactants or products over time.
4. What is meant by the term “order” in the context of reaction rates? Solution: The order of a reaction in respect to a certain reactant refers to the power to which its concentration in the rate equation is raised. It indicates the relationship between the rate of reaction and the concentration of the reactant.
5. What does the rate constant (k) in a reaction’s rate law signify? Solution: The rate constant (k) in a reaction’s rate law is a proportionality constant that links the reaction rate with the concentrations of the reactants. Its value depends on factors like temperature and catalyst presence.
6. Define a first-order reaction. Solution: A first-order reaction is a reaction whose rate is directly proportional to the concentration of one reactant. The rate law for a first-order reaction is usually written as Rate = k[A], where [A] is the concentration of the reactant and k is the rate constant.
7. How does the presence of a catalyst affect the rate of reaction? Solution: A catalyst speeds up the rate of a reaction by lowering the activation energy barrier, thus increasing the number of reactant molecules that have enough energy to react.
8. What is a half-life in the context of reaction rates? Solution: The half-life of a reaction is the time it takes for the concentration of a reactant to decrease to half of its initial value.
9. What is meant by the activation energy (Ea) of a reaction? Solution: The activation energy of a reaction is the minimum energy required for reactants to undergo a chemical reaction. It’s the energy barrier that must be overcome for the reaction to proceed.
10. How does concentration affect the rate of a reaction? Solution: Generally, an increase in the concentration of the reactants increases the rate of the reaction. This is because higher concentration means more reactant molecules are available to collide and react, thus increasing the reaction rate.
11. What is an elementary reaction? Solution: An elementary reaction is a single step reaction that describes a chemical change involving one or more molecules.
12. What is a rate-determining step in a reaction? Solution: The rate-determining step is the slowest step in a reaction mechanism, and it determines the overall rate of the reaction.
13. What is the Arrhenius equation and what does it describe? Solution: The Arrhenius equation describes how the rate constant (k) of a reaction varies with temperature. It shows that the rate constant increases exponentially with increasing temperature.
14. What is the difference between a homogeneous and a heterogeneous reaction in terms of reaction rate? Solution: A homogeneous reaction involves reactants in the same phase, while a heterogeneous reaction involves reactants in different phases. The reaction rates in heterogeneous reactions can be affected by factors such as surface area and physical state that do not affect homogeneous reactions.
15. What is the collision theory of reaction rates? Solution: The collision theory states that for a reaction to occur, particles must collide with each other with sufficient energy (greater than the activation energy) and in a proper orientation.
16. Define a zero-order reaction. Solution: A zero-order reaction is a reaction whose rate is independent of the concentration of the reactants. The rate law for a zero-order reaction is Rate = k.
17. How is reaction order determined experimentally? Solution: Reaction order is determined experimentally by observing how changes in reactant concentrations affect the reaction rate. For example, if doubling the concentration of a reactant doubles the rate of the reaction, the reaction is first order with respect to that reactant.
18. What does it mean when a reaction is in equilibrium? Solution: A reaction is in equilibrium when the rate of the forward reaction equals the rate of the reverse reaction, resulting in no net change in the concentrations of reactants and products.
19. What is the difference between reaction rate and reaction speed? Solution: The terms “reaction rate” and “reaction speed” essentially refer to the same concept—the change in concentration of a reactant or product per unit of time. However, “reaction rate” is the more commonly used term in chemistry.
20. What is the transition state theory? Solution: The transition state theory, also known as the activated complex theory, proposes that molecules form a short-lived, high-energy, unstable intermediate complex during a reaction. This transition state then breaks apart to form the products of the reaction.