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 : Enzymes And Catalysts

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

Possible Answers:

Making the reactants less energetically stable

Making the products less energetically stable

Elevating the temperature of the reaction

Making the products more energetically stable

Lowering the activation energy

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 are regenerated in the reaction.

They increase reaction rate.

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

They cannot be in a separate phase than the reactants.

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.

Example Question #1 : Kinetics And Energy

How does adding a catalyst affect a reaction?

Possible Answers:

It increases the kinetic energy of the reactants, resulting in more collisions

It increases the amount of product created in a reaction

It lowers the internal energy of the products, resulting in more stable bonds

It lowers the energy required to have reactants react with one another

Correct answer:

It lowers the energy required to have reactants react with one another

Explanation:

A catalyst is a substance that increases the rate of a reaction, typically by lowering the activation energy required to initiate a reaction. The catalyst does not affect the equilibrium of a reaction, and is not consumed during the reaction.

Example Question #1 : Enzymes And Catalysts

Which of the following is true of catalysts?

Possible Answers:

They raise the equilibrium product concentration

They are always in the same phase as the reactants

They increase reaction rate

They increase reaction activation energy

They are changed during the course of the reaction

Correct answer:

They increase reaction rate

Explanation:

Catalysts are substances that increase reaction rates without being consumed in the reaction. They decrease the activation energy needed, and they do not always need to be in the same phase as the reactants. In heterogeneous catalysis, the catalyst is in a different phase than the reactants. Equilibrium concentrations of both reactants and products are unchanged by the addition of a catalyst.

Example Question #1 : Enzymes And Catalysts

Name the catalyst in each reaction. 

I. 

II. 

Possible Answers:

I: Z

II: B

I: Z

II: A

I: Z

II: Z

I: A

II: A

I: A

II: C

Correct answer:

I: Z

II: A

Explanation:

A catalyst will not be consumed during a reaction, so the catalyst will be whichever chemical is found both on the reactant side of the equation and on the product side.

For equation 1 that is compound Z; for equation 2 that is compound A.

Example Question #1 : Enzymes And Catalysts

What is the function of a catalyst in a chemical reaction?

Possible Answers:

Catalysts speed up the reaction without being consumed

Catalysts slow down the reaction without being consumed

Catalysts increase the amount of product produced by the reaction

Catalysts increase the amount of reactant available

Catalysts decrease the amount of product produced by the reaction

Correct answer:

Catalysts speed up the reaction without being consumed

Explanation:

Catalysts increase the reaction rate without being consumed during the reaction. They don't cause the reaction to make more product, but since the catalyst won't be used up in the reaction.

Example Question #2 : Kinetics And Energy

Enzymes are very physiologically and industrially important molecules. From cheese-making to the metabolism of toxins, we need enzymes for life and society as we know it.

Which of the following is not true of enzymes?

Possible Answers:

Enzymes interact with substrates through active sites

Enzymes form complexes with their substrates

Enzymes increase the equilibrium product concentration

None of these

Enzymes decrease the activation energy of a reaction

Correct answer:

Enzymes increase the equilibrium product concentration

Explanation:

Enzymes function by forming complexes with their substrates at active sites. This interaction is often thought of as a lock and key mechanism, in the sense that the active site is shaped to fit a substrate. 

Enzymes are biological catalysts and therefore they work to lower the energy barrier or activation energy that prevents a reaction from proceeding to equilibrium. In other words: enzymes and catalysts in general make a reaction reach equilibrium faster.

Therefore enzymes do not change the equilibrium product concentration, just the time it takes to get to equilibrium.

Example Question #1 : Enzymes And Catalysts

Which of the following statements regarding catalysts is true?

Possible Answers:

Catalysts decrease the  of the reaction

Catalysts decrease the activation energy of the fast step of a reaction

Catalysts are included in the overall net reaction equation

Catalysts decrease the activation energy of both forward and reverse reactions

Correct answer:

Catalysts decrease the activation energy of both forward and reverse reactions

Explanation:

Energy diagram 

Note that when catalyst decreases the activation energy (Ea),  will not be affected. The step in a reaction with the largest activation energy usually is the slow step, which catalysts facilitate. Catalysts do not affect the thermodynamic quantities . Since catalysts are not consumed in the reaction, they do not appear in the net equation of the reaction.

Example Question #1 : Kinetic Molecular Theory

Two moles of nitrogen gas are kept in a glass container. The temperature of the gas is 400K.

What is the kinetic energy of the gas?

Possible Answers:

Correct answer:

Explanation:

Using the equation , n being the number of moles, R being 8.314, and T being the temperature in Kelvin, we can find the kinetic energy of two moles of gas in a container.

Example Question #1 : Kinetic Molecular Theory

Which of the following is not an assumption of the kinetic molecular theory of gases?

Possible Answers:

Gas particles do not experience attractions/repulsions to other particles

The kinetic energy of gas particles are different for all gases at a certain temperature

Gas particles are in random, continuous motion

Collisions between gas particles are elastic

The volumes of the particles of a gas are negligible

Correct answer:

The kinetic energy of gas particles are different for all gases at a certain temperature

Explanation:

The kinetic molecular theory of gases states that the average kinetic energy of gas particles is proportional to temperature, and it is the same for all gases at a given temperature. This is the opposite of what is stated in the answer choice.

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