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Use Hess's Law to Calculate Δh for The Following Reaction

Reviewed by Calculator Editorial Team

Hess's Law is a fundamental principle in thermochemistry that allows us to calculate the enthalpy change (ΔH) for a reaction by combining known enthalpy changes of other reactions. This guide explains how to apply Hess's Law to determine the enthalpy change for a specific chemical reaction.

What is Hess's Law?

Hess's Law states that the total enthalpy change for a chemical reaction is the same whether the reaction occurs in one step or in a series of steps. This principle allows chemists to calculate the enthalpy change for a reaction by combining the enthalpy changes of other reactions.

The law is based on the principle of conservation of energy and is expressed mathematically as:

ΔHreaction = ΣΔHsteps

Where ΔHreaction is the enthalpy change for the overall reaction, and ΣΔHsteps is the sum of the enthalpy changes for the individual steps that make up the overall reaction.

How to Use Hess's Law

To use Hess's Law, follow these steps:

  1. Write the balanced chemical equation for the reaction you want to study.
  2. Find the enthalpy changes for a series of reactions that can be combined to give the desired reaction.
  3. Multiply each enthalpy change by the appropriate stoichiometric coefficient to account for the number of moles of reactants and products.
  4. Sum the enthalpy changes to obtain the enthalpy change for the overall reaction.

It's important to ensure that the reactions you combine are thermodynamically consistent and that the signs of the enthalpy changes are correctly accounted for.

Example Calculation

Let's consider the following reaction:

C2H6(g) + 3O2(g) → 2CO2(g) + 3H2O(g)

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

  1. C2H6(g) + 4O2(g) → 2CO2(g) + 3H2O(g) (ΔH1 = -1560 kJ)
  2. H2(g) + ½O2(g) → H2O(g) (ΔH2 = -286 kJ)

The overall reaction can be obtained by combining these two reactions:

C2H6(g) + 3O2(g) → 2CO2(g) + 3H2O(g)

The enthalpy change for the overall reaction is calculated as:

ΔHreaction = ΔH1 + 3ΔH2 = -1560 kJ + 3(-286 kJ) = -2428 kJ

Therefore, the enthalpy change for the reaction is -2428 kJ.

Common Mistakes

When using Hess's Law, it's important to avoid the following common mistakes:

  • Using reactions that are not thermodynamically consistent with the desired reaction.
  • Forgetting to multiply enthalpy changes by the appropriate stoichiometric coefficients.
  • Ignoring the signs of the enthalpy changes when combining reactions.
  • Using enthalpy changes that are not measured under the same conditions (e.g., temperature and pressure).

By being aware of these potential pitfalls, you can ensure that your calculations using Hess's Law are accurate and reliable.

FAQ

What is the difference between Hess's Law and the First Law of Thermodynamics?

Hess's Law is a specific application of the First Law of Thermodynamics, which states that energy cannot be created or destroyed, only transformed. Hess's Law provides a method for calculating the enthalpy change for a reaction by combining known enthalpy changes of other reactions.

Can Hess's Law be used to calculate the entropy change for a reaction?

No, Hess's Law is specifically for calculating enthalpy changes. To calculate entropy changes, you would need to use the Second Law of Thermodynamics and other principles of thermodynamics.

What are the limitations of Hess's Law?

Hess's Law assumes that the enthalpy change for a reaction is independent of the path taken, which is not always true. Additionally, the law assumes that the enthalpy changes for the individual steps are known and accurate, which may not always be the case.