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Calculating Delta H Rxn From Delta H Use Following Reactions

Reviewed by Calculator Editorial Team

Calculating the enthalpy change of a reaction (ΔH rxn) from the enthalpy changes of constituent reactions is a fundamental concept in thermochemistry. This process involves using Hess's Law, which states that the total enthalpy change of a reaction is the sum of the enthalpy changes of the individual steps that make up the reaction.

Introduction

When studying chemical reactions, chemists often need to determine the enthalpy change of a reaction that isn't directly measurable. Instead, they can calculate it by combining the enthalpy changes of other reactions that make up the overall process. This method is based on Hess's Law, which provides a way to calculate the enthalpy change of a reaction by adding and subtracting the enthalpy changes of other reactions.

The key to this method is to find a series of reactions that can be combined to give the desired reaction. Each reaction in the series must have a known enthalpy change, and the sum of these enthalpy changes should equal the enthalpy change of the desired reaction.

How to Calculate Delta H Rxn

To calculate the enthalpy change of a reaction (ΔH rxn) using the enthalpy changes of other reactions, follow these steps:

  1. Write down the balanced chemical equation for the reaction whose enthalpy change you want to find.
  2. Find a series of reactions that can be combined to give the desired reaction. Each reaction in the series should have a known enthalpy change.
  3. Reverse any reactions in the series that need to be reversed to match the desired reaction.
  4. Multiply the enthalpy change of each reaction in the series by the appropriate coefficient to match the stoichiometry of the desired reaction.
  5. Sum the enthalpy changes of the reactions in the series to find the enthalpy change of the desired reaction.
ΔH_rxn = Σ(n × ΔH_reaction) where: ΔH_rxn = enthalpy change of the desired reaction n = stoichiometric coefficient of the reaction in the series ΔH_reaction = enthalpy change of the reaction in the series

This method allows chemists to calculate the enthalpy change of a reaction even when it cannot be measured directly. By using Hess's Law and combining the enthalpy changes of other reactions, chemists can determine the enthalpy change of any reaction.

Example Calculation

Let's consider an example to illustrate how to calculate the enthalpy change of a reaction using the enthalpy changes of other reactions.

Suppose we want to find the enthalpy change for the reaction:

C(s) + O₂(g) → CO₂(g) ΔH_rxn = ?

We can use the following reactions with known enthalpy changes:

  1. C(s) + ½O₂(g) → CO(g) ΔH₁ = -110.5 kJ/mol
  2. CO(g) + ½O₂(g) → CO₂(g) ΔH₂ = -283.0 kJ/mol

To get the desired reaction, we can add these two reactions together:

C(s) + ½O₂(g) → CO(g) ΔH₁ = -110.5 kJ/mol CO(g) + ½O₂(g) → CO₂(g) ΔH₂ = -283.0 kJ/mol -------------------------------------------- C(s) + O₂(g) → CO₂(g) ΔH_rxn = ΔH₁ + ΔH₂ = -110.5 + (-283.0) = -393.5 kJ/mol

Therefore, the enthalpy change for the reaction C(s) + O₂(g) → CO₂(g) is -393.5 kJ/mol.

Important Notes

When calculating the enthalpy change of a reaction using the enthalpy changes of other reactions, keep the following points in mind:

  • Ensure that all reactions in the series are balanced and that the stoichiometry matches the desired reaction.
  • Reverse any reactions in the series that need to be reversed to match the desired reaction. When reversing a reaction, the sign of the enthalpy change is also reversed.
  • Multiply the enthalpy change of each reaction in the series by the appropriate coefficient to match the stoichiometry of the desired reaction.
  • Sum the enthalpy changes of the reactions in the series to find the enthalpy change of the desired reaction.

Note

This method assumes that the enthalpy changes of the reactions in the series are known and accurate. Additionally, this method assumes that the enthalpy change of the desired reaction is independent of the path taken to reach it, which is a fundamental assumption of Hess's Law.

Frequently Asked Questions

What is Hess's Law?

Hess's Law states that the total enthalpy change of a reaction is the sum of the enthalpy changes of the individual steps that make up the reaction. This law allows chemists to calculate the enthalpy change of a reaction by combining the enthalpy changes of other reactions.

How do I find a series of reactions to calculate ΔH rxn?

To find a series of reactions, look for reactions that can be combined to give the desired reaction. Each reaction in the series should have a known enthalpy change, and the sum of these enthalpy changes should equal the enthalpy change of the desired reaction.

What if I can't find a series of reactions with known enthalpy changes?

If you can't find a series of reactions with known enthalpy changes, you may need to measure the enthalpy change of the desired reaction directly. Alternatively, you can use theoretical calculations or experimental methods to estimate the enthalpy change.

Is the enthalpy change of a reaction always the same, regardless of the path taken to reach it?

According to Hess's Law, the enthalpy change of a reaction is independent of the path taken to reach it. This means that the enthalpy change of a reaction can be calculated by combining the enthalpy changes of other reactions, regardless of the intermediate steps involved.