Enthalpy change, typically represented by ΔH, is a measure of the total energy exchanged during a chemical reaction. It plays a significant role in understanding the thermodynamics and feasibility of reactions, as well as predicting the energy released or consumed in chemical processes. In this article, we will discuss how to calculate the enthalpy change for a given reaction using three common methods: Hess’s Law, calorimetry, and bond enthalpy calculations.

Method 1: Hess’s Law

Hess’s law states that the enthalpy change of a reaction is the same regardless of the path taken to reach the final state. To apply Hess’s Law:

1. Identify the target reaction and find its formation reactions for each reactant and product.

2. Manipulate these formation reactions to match the target reaction’s stoichiometry.

3. Add up the enthalpies of each manipulated formation reactions to determine the overall ΔH.

Example:

Consider the combustion of ethane (C2H6) to produce carbon dioxide (CO2) and water (H2O).

C2H6 + 7/2 O2 → 2 CO2 + 3 H2O

Using standard enthalpies of formation for C2H6, CO2, and H2O, we can calculate ΔH:

ΔH = [2(ΔHf°[CO2]) + 3(ΔHf°[H2O])] – [(ΔHf°[C2H6]) + 7/2(ΔHf°[O2])]

ΔH = [(−393.5 kJ/mol) * 2 + (−285.8 kJ/mol) * 3] -[-83.8 kJ/mol]= -1426.4 kJ/mol

Method 2: Calorimetry

Calorimetry involves measuring the heat exchanged in a reaction to determine the enthalpy change. To perform calorimetry:

1. Set up an insulated container (calorimeter) that contains a known mass of reactants.

2. Measure the initial temperature before initiating the reaction.

3. Record the maximum or minimum temperature change after the reaction is complete.

4. Calculate ΔH using the formula: q = mcΔT, where q is heat exchanged, m is mass, c is specific heat capacity, and

ΔT is temperature change.

Method 3: Bond Enthalpy Calculations

Enthalpy change can be calculated based on bond dissociation energies. To calculate ΔH using bond enthalpies:

1. Write down the balanced chemical equation for the target reaction.

2. Calculate bond enthalpy changes by breaking all existing bonds in reactants and forming new bonds in products.

3. Determine ΔH by taking the difference between the total energy required to break bonds (ΔHb) and the total energy released by forming bonds (ΔHf).

ΔH = ΔHb – ΔHf

Conclusion:

Calculating enthalpy change is a significant step in understanding chemical reactions and their energy exchange processes, whether it be through Hess’s Law, calorimetry, or bond enthalpy calculations. Each method shines under different circumstances, with Hess’s Law being useful for obtaining accurate values from standard enthalpies, calorimetry suited for experimental determination of heat exchange, and bond enthalpy calculations providing estimates based on individual bond strengths.