AP Chemistry : Thermochemistry and Kinetics

Study concepts, example questions & explanations for AP Chemistry

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Example Questions

Example Question #5 : Endothermic And Exothermic Reactions

Consider the following reversible, exothermic reaction:

The temperature of the vessel in which the reaction is taking place is increased from 300K to 350K. This leads the rate of the reverse reaction to __________.

Possible Answers:

increase

decrease

stay the same

The change cannot be determined

Correct answer:

increase

Explanation:

In exothermic and endothermic reactions, heat can be considered to be either a product or reactant, respectively. Increasing the temperature of the an exothermic reaction is effectively the same as adding heat as a product, causing the reaction to shift to the left. This will cause the reverse reaction to increase in rate. 

Example Question #6 : Endothermic And Exothermic Reactions

If the decomposition of 2 moles of gaseous  into 1 mole each of gaseous  and gaseous  at  absorbs  of heat energy, then what is the standard heat of formation of  in kilojoules per mole?

Possible Answers:

The answer cannot be determined without additional information

Correct answer:

Explanation:

From the question stem, we are told that the breakdown of 2 moles of gaseous  requires an absorption of  of energy. For simplification, we can write out the reaction as:

By definition, the heat of formation of a compound is the enthalpy change that occurs for the formation () of that compound from its elements in their standard state. Since  and  are in their standard state, we know that they have a heat of formation of zero, thus simplifying the calculation. Thus, if we reverse the above reaction, we see that:

Notice that by reversing the reaction, we are also reversing the sign for the enthalpy of the reaction. Now, to calculate the  of  as kilojoules per mole, we need to multiply the reaction by one-half to obtain:

Example Question #7 : Endothermic And Exothermic Reactions

The  of individual products and reactants can be utilized to determine if the reaction is exothermic or endothermic.

Consider the following equation:

Use a standard enthalpy of formation table to calculate the , and determine if the reaction is endothermic or exothermic.

Possible Answers:

, the reaction is exothermic

, the reaction is endothermic

, the reaction is exothermic

, the reaction is exothermic

, the reaction is endothermic

Correct answer:

, the reaction is endothermic

Explanation:

We can calculate the standard change in enthalpy for the reaction using the following equation:

 of   

 of 

 of 

Plug in known values and solve.

A negative  indicates that the reaction is exothermic. This means that the reaction will release energy (usually in the form of heat) to the surroundings.

A positive  indicates that the reaction is endothermic. This means that the reaction will absorb energy (usually in the form of heat) from the surroundings.

Therefore, this overall reaction is endothermic.

Example Question #8 : Endothermic And Exothermic Reactions

  

The reaction shown is __________, and heat is __________ by the reaction.

Possible Answers:

endothermic . . . absorbed

endothermic . . . released

exothermic . . . absorbed

exothermic . . . released

Correct answer:

exothermic . . . released

Explanation:

Negative enthalpy change() indicates that heat is on the product side of the reaction, or, is released by the reaction. This is also the definition of an exothermic reaction.

Example Question #1 : Enthalpy

Which of the following statements is true concerning a chemical reaction?

Possible Answers:

Endothermic reactions have lower activation energies than exothermic reactions

Exothermic reactions are always spontaneous

A catalyst reduces the enthalpy change for the reaction

The  value for the forward reaction is negative the value of the reverse reaction

Correct answer:

The  value for the forward reaction is negative the value of the reverse reaction

Explanation:

When a chemical reaction is represented graphically, we see that the enthalpy change is reversed between the forward and reverse reactions. If a reaction produces energy in a forward process, it will require an input of energy in the reverse process, and vice versa.

A catalyst only affects the rate of a chemical reaction; it does not affect the equilibrium. Finally, exothermic reactions are not always spontaneous, but will have lower activation of energies compared to endothermic reactions.

Example Question #2 : Enthalpy

                   

                     

               

What is the change in enthalpy for the following reaction?

Possible Answers:

Correct answer:

Explanation:

The change in enthalpy is calculated by:

When  cannot be measured, it can be calculated from known enthalpies of formation.

                   

                     

               

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in enthalpy of a reaction.

Example Question #3 : Enthalpy

The formation of nitrogen dioxide is a two step process.

       

  

The net reaction is .

What is the change in enthalpy when creating four moles of nitrogen dioxide?

Possible Answers:

Correct answer:

Explanation:

Hess's law states that the change in enthalpy for a total reaction can be considered equal to the sum of the enthalpy changes for every step involved in the reaction. In other words, we can determine the enthalpy change for nitrogen dioxide by adding the enthalpy changes for both steps involved in its formation.

This gives us the total change in enthalpy for the listed reaction, . Because the question asks for the enthalpy change for four moles of nitrogen dioxide, the value must be doubled. The reaction only produces two moles of nitrogen dioxide.

Example Question #4 : Enthalpy

Calculate ΔH for the following reaction:

CH4 (g) + O2 (g)  ⇌  CO2 (g) + H2O (l)

Compound                ΔH

CH4 (g)             -74.8 kJ/mol

H2O (l)             -285.8 kJ/mol

CO(g)            -393.5 kJ/mol

Possible Answers:

889.7 kJ/mol

not enough information

-604.5 kJ/mol

-890.3 kJ/mol

604.5 kJ/mol

Correct answer:

-890.3 kJ/mol

Explanation:

ΔH = Σ ΔHf products - Σ ΔHf reactants

ΔHf O2 or any element is 0

First step is to balance the equation:

CH4 (g) + O2 (g)  ⇌  CO2 (g) + 2H2O (l)

ΔH = Σ ΔHf products - Σ ΔHf reactants

= [-393.5 kJ/mol + 2(-285.8) kJ/mol] - (-74.8 kJ/mol)

= -890.3 kJ/mol

Example Question #5 : Enthalpy

                    

           

                    

What is the change in enthalpy for the given reaction?

Possible Answers:

Correct answer:

Explanation:

The change in enthalpy is calculated by:

When  cannot be measured, it can be calculated from known reactions. In this case the known reactions are given.

                    

           

                    

Since the reactions are in the correct order, adding all the  values together can be used to calculate the  of the reaction.

Example Question #6 : Enthalpy

              

              

              

What is the change in enthalpy for the following reaction?

Possible Answers:

Correct answer:

Explanation:

The change in enthalpy is calculated by:

When  cannot be measured, it can be calculated from known enthalpies of formation.

              

              

              

It is important to first balance the reaction before performing calculations. The coefficients are important in determining the change in enthalpy of a reaction.

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