Core Concept

Thermochemical equations combine chemical equations with energy changes, typically in terms of enthalpy (ΔH\Delta HΔH). Enthalpy changes tell us whether a reaction absorbs or releases heat and are often given in units of kilojoules (kJ).

Key Components of a Thermochemical Equation

1. Chemical Equation:

   • Shows the reactants and products of a reaction.

   • The reaction must be balanced to ensure accurate calculations.

2. Enthalpy Change (ΔH):

   • Represents the heat absorbed or released during the reaction at constant pressure.

   • ΔH < 0: Exothermic reaction (heat is released).

   • ΔH > 0: Endothermic reaction (heat is absorbed).

   • Enthalpy is typically expressed in kilojoules (kJ) per mole of reaction as written.

3. Thermochemical Equation Format:

   • A thermochemical equation includes the balanced chemical equation and the enthalpy change.

   • Example: $\text{CH}_4 \text{(g)} + 2\text{O}_2 \text{(g)} \rightarrow \text{CO}_2 \text{(g)} + 2\text{H}_2\text{O (g)} \quad \Delta H = -890 \$

   • This indicates that burning 1 mole of methane ($\text{CH}_4$​) releases 890 kJ of heat.

Writing Thermochemical Equations

1. Determine the Enthalpy of Reaction:

   • Use given data or calculate enthalpy based on the heats of formation of reactants and products.

2. Balance the Chemical Equation:

   • Ensure the equation is balanced in terms of moles of reactants and products.

3. Write the Enthalpy Change:

   • Add the enthalpy change (ΔH) next to the equation. Specify if the reaction is exothermic (negative ΔH) or endothermic (positive ΔH).

4. Include Physical States:

   • Indicate physical states of all reactants and products, as enthalpy values depend on whether substances are in their solid, liquid, or gaseous form.

Stoichiometry in Thermochemical Equations

Thermochemical equations allow us to calculate the heat change in a reaction based on the amount of reactants or products. Stoichiometry lets us relate moles of substances in a reaction to the enthalpy change.

1. Identify the Given and Desired Quantities:

   • Determine the mass or moles of a substance involved in the reaction.

2. Convert Mass to Moles (if necessary):

   • Use the molar mass to convert grams of a substance to moles.

3. Use the Mole Ratio and ΔH\Delta HΔH:

   • Set up a proportion using the mole ratio from the balanced equation to find the heat change based on the enthalpy given.

4. Calculate Heat Change:

   • Multiply the moles of the substance by the enthalpy per mole from the thermochemical equation.

Example Problem: Heat Released in an Exothermic Reaction

Problem: Given the thermochemical equation:

$\text{H}_2 \text{(g)} + \text{O}_2 \text{(g)} \rightarrow 2\text{H}_2\text{O (g)} \quad \Delta H = -483.6 \$

How much heat is released when 5.00 moles of $\text{H}_2$​ react with excess $\text{O}_2$​?

Solution:

1. Identify the Mole Ratio:

   • The equation shows that 222 moles of $\text{H}_2$​ release −483.6 kJ of heat.

2. Set Up a Proportion:

   $\frac{-483.6 \, \text{kJ}}{2 \, \text{moles}} = x \, \text{kJ} \text{ for } 5.00 \, \text{moles}$

3. Solve for x:

   $x = \frac{-483.6 \, \text{kJ}}{2} \times 5.00 = -1209 \, \text{kJ}$

Answer: 1209 kJ of heat is released.