# Reaction rates concept questions and answers

1. What is the rate of a chemical reaction?

The rate of a chemical reaction is the speed at which reactants are converted into products. It’s usually measured in terms of the concentration of a substance that is consumed or produced in a unit of time.

2. What factors influence the rate of a chemical reaction?

The rate of a chemical reaction can be influenced by several factors including concentration of reactants, temperature, presence of a catalyst, and the surface area of solid reactants.

3. What is the collision theory in the context of reaction rates?

The collision theory states that in order for a reaction to occur, particles must collide with the correct orientation and with sufficient energy (equal to or greater than the activation energy).

4. How does the concentration of reactants affect the reaction rate?

As the concentration of reactants increases, the rate of reaction usually increases as well. This is because there are more particles in the same space, leading to more collisions and hence, more successful reactions.

5. How does temperature affect the reaction rate?

Increasing temperature typically increases the reaction rate. Higher temperatures mean more kinetic energy for particles, which leads to more collisions and thus more successful reactions.

6. What is the role of a catalyst in a chemical reaction?

A catalyst is a substance that speeds up a chemical reaction by lowering the activation energy, but it’s not consumed in the process, meaning it can be used repeatedly.

7. What is activation energy (Ea) in a chemical reaction?

Activation energy is the minimum energy that reactant molecules must have in order to successfully react and form products.

8. What is the Arrhenius equation and how is it related to reaction rates?

The Arrhenius equation is k = Ae^(-Ea/RT), where k is the reaction rate coefficient, A is the pre-exponential factor (also known as frequency factor), Ea is the activation energy, R is the gas constant, and T is the temperature in Kelvin. It describes how the reaction rate constant k changes with temperature.

9. What is the rate law of a chemical reaction?

The rate law is a mathematical relationship obtained by comparing reaction rates with concentrations of reactants. It typically has the form: rate = k[A]^m[B]^n, where [A] and [B] are the concentrations of the reactants, k is the rate constant, and m and n are the reaction orders.

10. How is the rate constant (k) related to temperature and activation energy?

The rate constant (k) increases with temperature and decreases with activation energy, as described by the Arrhenius equation.

1. What does it mean when a reaction is first order with respect to a certain reactant?

It means that the rate of reaction is directly proportional to the concentration of that reactant. If the concentration of the reactant is doubled, the rate of reaction also doubles.

1. What does a zero-order reaction imply?

A zero-order reaction is one where the rate of reaction does not depend on the concentration of the reactant(s). The rate remains constant.

1. What is the half-life of a reaction?

The half-life of a reaction is the time it takes for the concentration of a reactant to decrease by half.

1. How is the half-life related to the rate constant in a first-order reaction?

For a first-order reaction, the half-life is inversely proportional to the rate constant, and it is given by the formula t½ = 0.693/k.

1. What is meant by the term ‘rate-determining step’ in a chemical reaction?

The rate-determining step is the slowest step in a reaction mechanism. It determines the overall rate of the reaction because the reaction cannot proceed faster than this slowest step.

1. What is the difference between a homogeneous and a heterogeneous catalyst?

A homogeneous catalyst is in the same phase as the reactants, while a heterogeneous catalyst is in a different phase from the reactants.

1. What is the difference between endothermic and exothermic reactions in terms of energy change?

In an exothermic reaction, energy is released, usually in the form of heat. In an endothermic reaction, energy is absorbed from the surroundings.

1. How does pressure affect the rate of a gas reaction?

Increasing pressure typically increases the rate of a gas reaction because it effectively increases the concentration of the gas molecules, leading to more frequent collisions.

1. What does a reaction mechanism tell you about a reaction?

A reaction mechanism describes the sequence of elementary steps that make up an overall reaction. It gives detailed insight into the pathway of a reaction, including the formation and consumption of intermediates.

1. What does a negative sign in a rate of reaction equation represent?

A negative sign in a rate of reaction equation indicates that a reactant is being consumed. It’s used to show that the concentration of a reactant decreases over time.