AP Chemistry : Thermochemistry and Kinetics

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

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Example Question #1 : Thermochemistry And Kinetics

If the activation energy of the forward reaction is greater than the activatoin energy of the reverse reaction, then this reaction is __________.

Possible Answers:

uncatalyzed

spontaneous

exothermic

endothermic

Correct answer:

endothermic

Explanation:

If the activation energy of a forward reaction is greater than that of the reverse reaction, thenthe products must have a higher enthalpy than the reactants (draw a potential energy diagram to visualize).

Example Question #1 : Activation Energy

If the activation energy of a forward reaction is greater than the activation energy of a reverse reaction, what must be true of the reaction?

Possible Answers:

It is nonspontaneous

It is exothermic

It is at equilibrium

It is spontaneous

It is endothermic

Correct answer:

It is endothermic

Explanation:

If the activation energy of the forward reaction is greater than that of the reverse reaction, the products must have a higher enthalpy than the reactants. The net enthalpy change is therefore positive, meaning that it is endothermic.

Example Question #1 : Activation Energy

Which of the following is true regarding activation energy?

I. Activation energy depends on the pressure of the system

II. The reaction rate decreases as the activation energy increases

III. Catalysts increase the activation energy of a reaction

Possible Answers:

I only

II only

I and III

I and II

Correct answer:

II only

Explanation:

Activation energy is the energy barrier that needs to be overcome for a reaction to proceed; the higher the activation energy, the slower the reaction. Activation energy can only be altered via a catalyst. Catalysts are chemical substances that lower the activation energy, allowing reactions to proceed faster. Other physical quantities such as temperature and pressure don’t alter the activation energy.

Reaction rate is a direct measure of the speed of a reaction. As mentioned, increasing activation energy will increase the barrier and, therefore, slow down the reaction.

Catalysts are chemical substances that decrease the activation energy, thereby increasing the reaction rate. They are commonly used in chemical reactions to drastically speed up reactions that might otherwise take hours or days to complete. Enzymes are biological catalysts that facilitate most of the biological reactions happening in our bodies. 

Example Question #1 : Activation Energy

__________ the activation energy of a reaction will __________ the amount of products produced. 

Possible Answers:

Decreasing . . . increase

Decreasing . . . decrease

Increasing . . . not change

Increasing . . . increase

Correct answer:

Increasing . . . not change

Explanation:

This question is asking about the relationship between activation energy of a reaction (kinetics) and the amount of products produced (equilibrium). Remember that altering the speed of a reaction (kinetics) does not change the equilibrium of the reaction. Increasing or decreasing activation energy (which alters the speed of reaction) will simply allow for the reaction to proceed slower or faster, respectively. It will not change the amount of products produced at the end.

Example Question #1 : Kinetics And Energy

Which of the following is the correct way to calculate the activation energy of a reaction? 

Possible Answers:

Energy of transition state – Energy of product = Activation energy

Energy of product – Energy of transition state = Activation energy

Energy of reactant – Energy of transition state = Activation energy

Energy of transition state – Energy of reactant = Activation energy

Correct answer:

Energy of transition state – Energy of reactant = Activation energy

Explanation:

Activation energy is the difference between the energy of the transition state and the energy of the reactant. Recall that activation energy is the energy barrier that needs to be overcome by a reaction. The transition state is a higher-energy, intermediary molecule that lies in between the reactants and products. It is created when reactants are in the process of becoming products. Therefore, to get to products, it is necessary to go through this high-energy transition state.

Activation energy is the energy needed to climb this energy “hill” (energy needed to go from reactants to transition state); therefore, the activation energy is the energy of the transition state (top of the energy hill/barrier) minus the energy of the reactant.

Example Question #2 : Thermochemistry And Kinetics

Consider the following reaction:

The conversion of  to  releases  of energy and has an activation energy of .

What is the activation energy of the reverse reaction? 

Possible Answers:

Cannot be determined from the given information

Correct answer:

Explanation:

To answer this question, we have to consider an energy diagram for an exothermic reaction. We know the forward reaction is exothermic because the question states that energy is released. We are given the activation energy of the reaction (the energy difference between the activated complex, or transition state, and the reactants) and the energy released (the energy difference between the products and the reactants). In the reverse reaction we will be going from products to reactants as follows:

This means that the activation energy for reverse reaction will be higher (has to climb a higher hill from “products” to “activated complex”). Using the given information we can deduce that the activation energy () of the reverse reaction is the SUM of the activation energy of the forward reaction AND the energy released from the forward reaction.

The closest answer is . Note that the forward reaction is exothermic, whereas the reverse reaction is endothermic (energy is being consumed). The reverse reaction is endothermic because the reactant () has lower energy than the product ().

Example Question #1 : Thermochemistry And Kinetics

Which of the following is true regarding activation energy? 

Possible Answers:

A catalyst increases the energy of the reactants

Activation energy determines the equilibrium constant of a reaction

Enthalpy of a reaction has no effect on the activation energy

Increasing the temperature will decrease the activation energy

Correct answer:

Enthalpy of a reaction has no effect on the activation energy

Explanation:

Recall that the only thing that can alter the activation energy of a reaction is the addition of a catalyst. Factors such as enthalpy, entropy, temperature, and pressure don’t change the activation energy.

Catalysts decrease the activation energy and increase the rate of reaction. They have no effect on the energy of the reactants. As mentioned, temperature will not alter the activation energy; however, increasing the temperature will speed up the reaction. This occurs because adding energy in the form of heat will increase the energy of individual molecules in the reaction and will allow more molecules to overcome the energy barrier. Note, however, that temperature does not change the energy barrier (activation energy).  Recall that changing activation energy has no effect on the equilibrium of the reaction; therefore, equilibrium constant does not depend on the activation energy.

Example Question #2 : Thermochemistry And Kinetics

The following diagram shows how the energy changes as the chemical reaction

 progresses.

Fig 3

A chemical engineer wants to, both, increase yield and accelerate this reaction rate. He should __________.

Possible Answers:

Use a catalyst only

extract  continuously from the reactor only

 extract  continuously from the reactor and use a catalyst

increase the temperature only

Correct answer:

 extract  continuously from the reactor and use a catalyst

Explanation:

"Increase the temperature" is incorrect because we have an exothermic reaction and by increasing the temperature the equilibrium constant will become smaller. An increase in temperature will shift the equilibrium toward reagents. According to the Le Chatelier's principle, by extracting  continuously from the reactor the equilibrium shifts toward the product, . Furthermore, the use of a catalyst increases the reaction rate. The Haber process to synthesize ammonia is an example of this kind of reaction.

Example Question #771 : Ap Chemistry

A catalyst increases the rate of a reaction by which of the following mechanisms?

Possible Answers:

Making the reactants less energetically stable

Lowering the activation energy

Making the products more energetically stable

Elevating the temperature of the reaction

Making the products less energetically stable

Correct answer:

Lowering the activation energy

Explanation:

A catalyst has no effect on the relative stability of the reactants or products, nor does it effect the temperature of a reaction.

Instead, catalysts lower the energy of transition states, increasing their stability, to lower the overall activation energy of the reaction. When the reaction requires less energy, it proceeds at a faster rate.

Example Question #1 : Enzymes And Catalysts

Which of the following is not true of catalysts?

Possible Answers:

They lower the activation energy needed for the reaction to proceed.

They increase reaction rate.

They cannot be in a separate phase than the reactants.

They are regenerated in the reaction.

Correct answer:

They cannot be in a separate phase than the reactants.

Explanation:

All of the choices are true, except that catalysts can be in distinct phases than the reactants. These are known as heterogenous catalysts.

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