# AP Chemistry : Enthalpy

## Example Questions

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### Example Question #1 : Enthalpy

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

Exothermic reactions are always spontaneous

A catalyst reduces the enthalpy change for the reaction

Endothermic reactions have lower activation energies than exothermic reactions

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

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?

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 #1 : 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?

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

-604.5 kJ/mol

604.5 kJ/mol

-890.3 kJ/mol

889.7 kJ/mol

not enough information

-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?

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?

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 #7 : Enthalpy

What is the enthalpy of the following reaction?

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. For this particular reaction, since there are two moles of product, the enthalpy of formation for must be multiplied by two.

### Example Question #3 : Enthalpy

What is the change in enthalpy for the following reaction?

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 #1 : Enthalpy

What is the change in enthalpy for the following reaction?

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 #2 : Enthalpy

Consider the following combustion reaction:

The following list is the enthalpies of formation for the compounds:

If one mole of propane is burned, what is the enthalpy of the reaction?

Explanation:

Since we have the enthalpies of formation, we can find the enthalpy of the reaction using the following equation:

We will need to use the coefficients from the balanced equation to calculate the enthalpy.

Since oxygen gas is elemental, it does not have an enthalpy of formation, and is omitted from the equation.

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