17.8: Thermochemical Equations

Heating a home is becoming more and more expensive. Do we use gas, oil, electricity, wood? Part of the decision is based on which fuel will provide the highest amount of energy release when burned. Studies of thermochemistry can be very useful in getting reliable information for making these important choices.

Thermochemical Equation

When methane gas is combusted, heat is released, making the reaction exothermic. Specifically, the combustion of $$1 \: \text{mol}$$ of methane releases 890.4 kilojoules of heat energy. This information can be shown as part of the balanced equation.

$\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + 2 \ce{H_2O} \left( l \right) + 890.4 \: \text{kJ}$

The equation tells us that $$1 \: \text{mol}$$ of methane combines with $$2 \: \text{mol}$$ of oxygen to produce $$1 \: \text{mol}$$ of carbon dioxide and $$2 \: \text{mol}$$ of water. In the process, $$890.4 \: \text{kJ}$$ is released and so it is written as a product of the reaction. A thermochemical equation is a chemical equation that includes the enthalpy change of the reaction. The process in the above thermochemical equation can be shown visually in the figure below.

Figure $$\PageIndex{1}$$: (A) As reactants are converted to products in an exothermic reaction, enthalpy is released into the surroundings. The enthalpy change of the reaction is negative. (B) As reactants are converted to products in an endothermic reaction, enthalpy is absorbed from the surroundings. The enthalpy change of the reaction is positive.

In the combustion of methane example, the enthalpy change is negative because heat is being released by the system. Therefore, the overall enthalpy of the system decreases. The heat of reaction is the enthalpy change for a chemical reaction. In the case above, the heat of reaction is $$-890.4 \: \text{kJ}$$. The thermochemical reaction can also be written in this way:

$\ce{CH_4} \left( g \right) + 2 \ce{O_2} \left( g \right) \rightarrow \ce{CO_2} \left( g \right) + 2 \ce{H_2O} \left( l \right) \: \: \: \: \: \Delta H = -890.4 \: \text{kJ}$

Heats of reaction are typically measured in kilojoules. It is important to include the physical states of the reactants and products in a thermochemical equation as the value of the $$\Delta H$$ depends on those states.

Endothermic reactions absorb energy from the surroundings as the reaction occurs. When $$1 \: \text{mol}$$ of calcium carbonate decomposes into $$1 \: \text{mol}$$ of calcium oxide and $$1 \: \text{mol}$$ of carbon dioxide, $$177.8 \: \text{kJ}$$ of heat is absorbed. The process is shown visually in the figure above (B). The thermochemical reaction is shown below.

$\ce{CaCO_3} \left( s \right) + 177.8 \: \text{kJ} \rightarrow \ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right)$

Because the heat is absorbed by the system, the $$177.8 \: \text{kJ}$$ is written as a reactant. The heat of reaction is positive for an endothermic reaction.

$\ce{CaCO_3} \left( s \right) \rightarrow \ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right) \: \: \: \: \: \Delta H = 177.8 \: \text{kJ}$

The way in which a reaction is written influences the value of the enthalpy change for the reaction. Many reactions are reversible, meaning that the product(s) of the reaction are capable of combining and reforming the reactant(s). If a reaction is written in the reverse direction, the sign of the $$\Delta H$$ changes. For example, we can write an equation for the reaction of calcium oxide with carbon dioxide to form calcium carbonate.

$\ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right) \rightarrow \ce{CaCO_3} \left( s \right) + 177.8 \: \text{kJ}$

The reaction is exothermic and thus the sign of the enthalpy change is negative.

$\ce{CaO} \left( s \right) + \ce{CO_2} \left( g \right) \rightarrow \ce{CaCO_3} \left( s \right) \: \: \: \: \: \Delta H = -177.8 \: \text{kJ}$

Summary

• The components of a thermochemical equation are described.

Contributors

• CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.