Calculate Enthalpy Change for The Following Reaction

What is Enthalpy Change?

Enthalpy change (ΔH) is a measure of the heat energy absorbed or released in a chemical reaction or physical process. It represents the total energy change, including the internal energy change and the work done by the system on its surroundings.

Enthalpy changes are crucial in chemistry because they help predict whether a reaction will be exothermic (releases heat) or endothermic (absorbs heat). This information is vital for designing chemical processes, understanding reaction mechanisms, and predicting reaction conditions.

How to Calculate Enthalpy Change

The enthalpy change for a reaction can be calculated using Hess's Law, which states that the enthalpy change for a reaction is the same regardless of the pathway taken. This means you can combine known enthalpy changes of other reactions to find the enthalpy change for your reaction of interest.

The general formula for calculating enthalpy change is:

Where:

• ΔH_reaction is the enthalpy change for the reaction you're interested in
• ΣΔH_products is the sum of the enthalpy changes of formation for all products
• ΣΔH_reactants is the sum of the enthalpy changes of formation for all reactants

Hess's Law

Hess's Law is a fundamental principle in thermochemistry that allows you to calculate the enthalpy change for a reaction by combining known enthalpy changes of other reactions. This is particularly useful when direct experimental data for your reaction is not available.

To apply Hess's Law:

1. Write the balanced chemical equation for the reaction you're interested in
2. Find the enthalpy changes for other reactions that can be combined to give your target reaction
3. Multiply each enthalpy change by the stoichiometric coefficient that relates it to your target reaction
4. Sum the enthalpy changes to find the total enthalpy change for your reaction

Example Calculation

Let's consider the reaction of methane (CH₄) with oxygen (O₂) to form carbon dioxide (CO₂) and water (H₂O):

CH₄ + 2O₂ → CO₂ + 2H₂O

We can calculate the enthalpy change for this reaction using Hess's Law by combining the following reactions:

1. CH₄ + 2O₂ → CO₂ + 2H₂O (ΔH₁ = -890.3 kJ/mol)
2. 2H₂ + O₂ → 2H₂O (ΔH₂ = -571.6 kJ/mol)

To find the enthalpy change for the first reaction, we can use the second reaction in reverse:

ΔH_reaction = ΔH₁ - 2 × ΔH₂

ΔH_reaction = -890.3 kJ/mol - 2 × (-571.6 kJ/mol)

ΔH_reaction = -890.3 kJ/mol + 1143.2 kJ/mol = 252.9 kJ/mol

This means the reaction releases 252.9 kJ of energy per mole of methane burned.

Interpreting Results

The sign of the enthalpy change tells you whether the reaction is exothermic or endothermic:

• If ΔH is negative, the reaction is exothermic (releases heat)
• If ΔH is positive, the reaction is endothermic (absorbs heat)

The magnitude of the enthalpy change indicates the amount of energy involved in the reaction. Larger absolute values mean more energy is involved.

Understanding enthalpy changes helps in:

• Predicting reaction spontaneity
• Designing energy-efficient chemical processes
• Understanding reaction mechanisms
• Selecting appropriate reaction conditions