Enthalpy - AP Chemistry

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

This problem requires a knowledge of the energy involved when bonds are broken versus when bonds are formed. Energy is released when bonds are formed, and energy is consumed when bonds are broken. Here, methane is being formed from C and H2. Since the carbon source, graphite, is monoatomic the carbon will be involved only in the formation of C–H bonds on it's way to forming methane. The H however exists as H2 so bonds will need to be broken to allow for C–H bonds to form. So first energy is consumed to break the H–H bonds, it is then released when the C–H bonds form.

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.

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.

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.

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.

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.

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.

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.

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.

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.

The change in enthalpy of a reaction is determined by the amount of heat gained when the products bonds form, versus the amount of heat lost to break the bonds in the reactants. The correct formula for the enthalpy of reaction is the difference between the enthalpy of the products and that of the reactants:

The reactions given show the heats of formation of all the reactants and products, and we know the heat of formation of is zero because it is a pure element. Formation of the product, gives us . For the reactants, gives us plus for . Recall that the overall reaction's change in enthalpy is:

Thus for this overall reaction:

When heat is added, enthalpy increases, ΔH refers to enthalpy

ΔS refers to entropy, which is disorder; disorder increases as it moves from solid to liquid to gas state.

As ice goes to liquid water, entropy increases

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

This problem requires a knowledge of the energy involved when bonds are broken versus when bonds are formed. Energy is released when bonds are formed, and energy is consumed when bonds are broken. Here, methane is being formed from C and H2. Since the carbon source, graphite, is monoatomic the carbon will be involved only in the formation of C–H bonds on it's way to forming methane. The H however exists as H2 so bonds will need to be broken to allow for C–H bonds to form. So first energy is consumed to break the H–H bonds, it is then released when the C–H bonds form.

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.

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.

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.