,CHEM 121 Module 6 | Foundations of General Chemistry
w/Lab Exam Q&A | Portage Learning
1. Which statement best defines the rate of a chemical reaction?
A) The time required for a reaction to reach completion
B) The change in concentration of a reactant or product per unit time
C) The total amount of product formed at equilibrium
D) The energy released when products are formed
Correct Answer: The change in concentration of a reactant or product per unit
time
Rationale: Reaction rate is defined as the change in concentration of a reactant
or product divided by the change in time. It describes how quickly a reaction
proceeds, not the total amount formed or the energy released. The time required
to reach completion is related but not the definition of rate itself.
2. In the rate law Rate = k[A]^m[B]^n, what does the sum m + n represent?
A) The rate constant
B) The overall reaction order
C) The activation energy
D) The equilibrium constant
Correct Answer: The overall reaction order
,Rationale: The exponents m and n are the individual reaction orders with
respect to A and B, and their sum (m + n) is the overall reaction order. The rate
constant is k, the activation energy is a separate concept, and the equilibrium
constant describes the position of equilibrium.
3. What are the units of the rate constant k for a second-order reaction overall?
A) s⁻¹
B) M⁻¹·s⁻¹
C) M⁻²·s⁻¹
D) M·s⁻¹
Correct Answer: M⁻¹·s⁻¹
Rationale: For an overall second-order reaction, Rate = k[A]². Rate has units of
M·s⁻¹ and [A]² has units of M², so k = Rate/[A]² = (M·s⁻¹)/M² = M⁻¹·s⁻¹. First-order
reactions have units s⁻¹, and zero-order reactions have units M·s⁻¹.
4. Which statement best describes the role of a catalyst in a chemical reaction?
A) It increases the equilibrium constant
B) It lowers the activation energy and speeds up the reaction
C) It is consumed during the reaction
D) It changes the enthalpy of the reaction
Correct Answer: It lowers the activation energy and speeds up the reaction
Rationale: A catalyst increases the reaction rate by providing an alternative
pathway with a lower activation energy. It is not consumed in the overall reaction
, and does not change the equilibrium constant or the overall enthalpy change. The
equilibrium position remains unchanged.
5. Which statement best describes activation energy?
A) The energy released when products form
B) The minimum energy barrier that reacting particles must overcome
C) The energy stored in the bonds of reactants
D) The difference in energy between reactants and products
Correct Answer: The minimum energy barrier that reacting particles must
overcome
Rationale: Activation energy (Ea) is the minimum energy that colliding particles
must possess for a reaction to occur. It is not the energy released, the stored bond
energy, or the enthalpy change. The difference between reactant and product
energies is the enthalpy change (ΔH).
6. How does increasing temperature generally affect the rate of a chemical
reaction?
A) It decreases the rate because particles move more slowly
B) It has no effect on the rate
C) It increases the rate because more collisions have energy at or above the
activation energy
D) It increases the activation energy
Correct Answer: It increases the rate because more collisions have energy at or
above the activation energy
w/Lab Exam Q&A | Portage Learning
1. Which statement best defines the rate of a chemical reaction?
A) The time required for a reaction to reach completion
B) The change in concentration of a reactant or product per unit time
C) The total amount of product formed at equilibrium
D) The energy released when products are formed
Correct Answer: The change in concentration of a reactant or product per unit
time
Rationale: Reaction rate is defined as the change in concentration of a reactant
or product divided by the change in time. It describes how quickly a reaction
proceeds, not the total amount formed or the energy released. The time required
to reach completion is related but not the definition of rate itself.
2. In the rate law Rate = k[A]^m[B]^n, what does the sum m + n represent?
A) The rate constant
B) The overall reaction order
C) The activation energy
D) The equilibrium constant
Correct Answer: The overall reaction order
,Rationale: The exponents m and n are the individual reaction orders with
respect to A and B, and their sum (m + n) is the overall reaction order. The rate
constant is k, the activation energy is a separate concept, and the equilibrium
constant describes the position of equilibrium.
3. What are the units of the rate constant k for a second-order reaction overall?
A) s⁻¹
B) M⁻¹·s⁻¹
C) M⁻²·s⁻¹
D) M·s⁻¹
Correct Answer: M⁻¹·s⁻¹
Rationale: For an overall second-order reaction, Rate = k[A]². Rate has units of
M·s⁻¹ and [A]² has units of M², so k = Rate/[A]² = (M·s⁻¹)/M² = M⁻¹·s⁻¹. First-order
reactions have units s⁻¹, and zero-order reactions have units M·s⁻¹.
4. Which statement best describes the role of a catalyst in a chemical reaction?
A) It increases the equilibrium constant
B) It lowers the activation energy and speeds up the reaction
C) It is consumed during the reaction
D) It changes the enthalpy of the reaction
Correct Answer: It lowers the activation energy and speeds up the reaction
Rationale: A catalyst increases the reaction rate by providing an alternative
pathway with a lower activation energy. It is not consumed in the overall reaction
, and does not change the equilibrium constant or the overall enthalpy change. The
equilibrium position remains unchanged.
5. Which statement best describes activation energy?
A) The energy released when products form
B) The minimum energy barrier that reacting particles must overcome
C) The energy stored in the bonds of reactants
D) The difference in energy between reactants and products
Correct Answer: The minimum energy barrier that reacting particles must
overcome
Rationale: Activation energy (Ea) is the minimum energy that colliding particles
must possess for a reaction to occur. It is not the energy released, the stored bond
energy, or the enthalpy change. The difference between reactant and product
energies is the enthalpy change (ΔH).
6. How does increasing temperature generally affect the rate of a chemical
reaction?
A) It decreases the rate because particles move more slowly
B) It has no effect on the rate
C) It increases the rate because more collisions have energy at or above the
activation energy
D) It increases the activation energy
Correct Answer: It increases the rate because more collisions have energy at or
above the activation energy
