Reaction rates are reported as positive values, so the rate of disappearance of a reactant is written with a negative sign in front of its change in concentration.
Correct Answer
True
True. A reactant's concentration decreases, so its change is negative, and the minus sign makes the reported rate positive. Product rates need no minus sign because product concentrations increase.
Category: Reaction Rates
Question 2
For a reaction whose rate depends on reactant concentration, the rate increases as the reactants are used up.
Correct Answer
False
False. As reactants are consumed their concentrations fall, so a concentration-dependent rate decreases over time. This is why the instantaneous rate is usually highest at the start of the reaction.
Category: Reaction Rates
Question 3
For an endothermic reaction, the activation energy of the forward reaction is larger than the reaction's enthalpy change.
Correct Answer
True
True. In an endothermic reaction the products lie above the reactants, and the transition state lies above the products. The barrier from reactants to the transition state is therefore larger than the energy gap from reactants to products.
Category: Collision Theory
Question 4
Increasing the concentration of a reactant lowers the frequency of collisions between reactant molecules.
Correct Answer
False
False. More molecules per unit volume means collisions happen more often, which is why higher concentration usually increases the rate of a reaction.
Category: Factors Affecting Rates
Question 5
The exponents in a rate law must be found by experiment and generally cannot be read from the coefficients of the overall balanced equation.
Correct Answer
True
True. Reaction orders reflect the mechanism, which the overall equation does not show, so they are determined from data such as initial rates. Only for an elementary step do the coefficients give the exponents directly.
Category: Rate Laws
Question 6
The rate constant k of a reaction changes whenever the reactant concentrations change.
Correct Answer
False
False. The rate constant does not depend on concentration; the concentration terms in the rate law account for that. The value of k depends on the particular reaction and on the temperature.
Category: Rate Laws
Question 7
The rate constant of a first-order reaction has units of reciprocal time, such as s^-1.
Correct Answer
True
True. For rate = k[A], k = rate / [A] = (M/s) / M = s^-1. The units of k change with the overall order of the reaction.
Category: Rate Laws
Question 8
For a first-order reaction, a plot of ln[A] versus time gives a straight line with a slope equal to -k.
Correct Answer
True
True. The first-order integrated rate law is ln[A] = -kt + ln[A]0, which has the form y = mx + b. The slope is -k and the intercept is ln[A]0.
Category: Integrated Rate Laws
Question 9
The half-life of a zero-order reaction becomes longer as the initial concentration of the reactant decreases.
Correct Answer
False
False. For a zero-order reaction, t1/2 = [A]0/2k, so the half-life is directly proportional to the initial concentration. A smaller starting concentration gives a shorter half-life.
Category: Integrated Rate Laws
Question 10
The activated complex is a stable, long-lived species formed during a reaction.
Correct Answer
False
False. The activated complex, or transition state, is an unstable arrangement of atoms at the top of the energy barrier. It lasts so briefly that it usually cannot be detected.
Category: Collision Theory
Question 11
An exothermic reaction has products at lower energy than its reactants, giving a negative enthalpy change.
Correct Answer
True
True. When the products lie below the reactants on a reaction diagram, energy is released to the surroundings and the enthalpy change is negative.
Category: Collision Theory
Question 12
At the same temperature, a reaction with a large activation energy is generally faster than a reaction with a small activation energy.
Correct Answer
False
False. A larger activation energy means a smaller fraction of collisions have enough energy to react, so the reaction is generally slower. A small activation energy allows a faster reaction.
Category: Collision Theory
Question 13
In the Arrhenius equation, the frequency factor A reflects how often molecules collide and how often those collisions have a suitable orientation.
Correct Answer
True
True. The frequency factor in k = A e^(-Ea/RT) is related to the collision frequency and the orientation of the reacting molecules. A larger A means more effective collisions and a larger rate constant.
Category: Collision Theory
Question 14
A termolecular elementary step involves the simultaneous collision of two reactant particles.
Correct Answer
False
False. A termolecular step involves three particles colliding at the same time, which is why such steps are uncommon. A step with two colliding particles is bimolecular.
Category: Reaction Mechanisms
Question 15
The elementary steps of a valid reaction mechanism must add up to the overall balanced equation.
Correct Answer
True
True. When the steps are added, intermediates cancel and the overall equation remains. A proposed mechanism must also agree with the experimentally determined rate law.
Category: Reaction Mechanisms
Question 16
The overall rate of a multistep reaction is determined by its fastest step.
Correct Answer
False
False. A reaction can go no faster than its slowest step, called the rate-determining or rate-limiting step. The fast steps do not limit the overall rate.
Category: Reaction Mechanisms
Question 17
Chlorine atoms released from chlorofluorocarbons act as catalysts in the destruction of ozone in the stratosphere.
Correct Answer
True
True. A chlorine atom reacts with ozone and is then regenerated in a later step, so one atom can destroy many ozone molecules. This finding contributed to the international phase-out of chlorofluorocarbons.
Category: Catalysis
Question 18
Adding a catalyst to a reaction changes the reaction's enthalpy change.
Correct Answer
False
False. A catalyst does not change the energies of the reactants or products, so the enthalpy change is the same with or without it. It only lowers the activation energy.
Category: Catalysis
Question 19
In the induced fit model, an enzyme changes shape slightly as it binds its substrate.
Correct Answer
True
True. The induced fit model treats the enzyme as flexible, adjusting its shape to fit the substrate. The older lock-and-key model treats the enzyme and substrate as rigid complementary shapes.
Category: Catalysis
Question 20
In homogeneous catalysis, the catalyst is in a different phase from the reactants.
Correct Answer
False
False. In homogeneous catalysis the catalyst is in the same phase as the reactants. When the catalyst is in a different phase, usually a solid surface, the process is heterogeneous catalysis.
Category: Catalysis