1.6.6 Applications of Hess's Law

Using Hess' Law: Calculation Methods

There are two main methods for applying Hess' Law in calculations: using enthalpies of formation or enthalpies of combustion.

1. Calculations Using Enthalpies of Formation

The standard enthalpy change of formation ( $Δ_fH^\circ$ ) is the enthalpy change when 1 mole of a compound is formed from its elements in their standard states.

ΔH_{\text{reaction}} = \sum (\Delta_fH^\circ \, \text{of products}) - \sum (\Delta_fH^\circ \, \text{of reactants})

Note: The $Δ_fH^\circ$ of an element in its standard state is zero because the element is formed from itself.

2. Calculations Using Enthalpies of Combustion

The standard enthalpy change of combustion ( $Δ_cH^\circ$ ) is the enthalpy change when 1 mole of a substance is completely burned in oxygen.

ΔH_{\text{reaction}} = \sum (\Delta_cH^\circ \, \text{of reactants}) - \sum (\Delta_cH^\circ \, \text{of products})

Note: For combustion reactions, the enthalpy of oxygen is typically ignored because you cannot burn oxygen in oxygen.

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Example: How to Perform a Hess' Law Calculation

Step 1: Identify the relevant reactions and the enthalpy values (either formation or combustion).

Step 2: Set up a Hess Cycle, showing all the different routes for the reaction.

Step 3: Apply Hess' Law by equating the total enthalpy change of different routes, ensuring all arrows are in the correct direction.

Step 4: Perform the calculation by adding or subtracting the enthalpy values, treating them like vectors.

Applications of Hess' Law

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Example: Thermal Decomposition of Sodium Hydrogen Carbonate ( $NaHCO₃$ )

Hess' Law can be applied to the thermal decomposition of sodium hydrogen carbonate, which cannot be measured directly.

The reaction is:

2 \, \text{NaHCO}_3 (s) \rightarrow \text{Na}_2\text{CO}_3 (s) + \text{CO}_2 (g) + \text{H}_2\text{O} (g)

Using Hess' Law, we can calculate the enthalpy change for this reaction by considering the formation enthalpies of sodium carbonate, carbon dioxide, and water, as well as sodium hydrogen carbonate.

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Example: Hydration of Magnesium Sulfate ( $MgSO₄$ )

In the hydration of magnesium sulfate:

\text{MgSO}_4 (s) + 7 \, \text{H}_2\text{O} (l) \rightarrow \text{MgSO}_4 \cdot 7 \, \text{H}_2\text{O} (s)

The enthalpy change for this process can be determined using Hess' Law by combining the enthalpy changes of formation for anhydrous magnesium sulfate, water, and hydrated magnesium sulfate.

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Example: Hydration of Copper(II) Sulfate ( $CuSO₄$ )

For the hydration of copper(II) sulfate:

\text{CuSO}_4 (s) + 5 \, \text{H}_2\text{O} (l) \rightarrow \text{CuSO}_4 \cdot 5 \, \text{H}_2\text{O} (s)

By applying Hess' Law, we can calculate the enthalpy change using the enthalpy of formation values for anhydrous copper sulfate, water, and hydrated copper sulfate.

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Summary

Hess' Law states that the enthalpy change of a reaction is independent of the route taken.
This law is used to calculate enthalpy changes for reactions that cannot be directly measured.
You can apply Hess' Law using either enthalpy of formation or enthalpy of combustion values.
Common applications include the thermal decomposition of $NaHCO₃$ and the hydration of $MgSO₄$ and $CuSO₄$ .

Applications of Hess’s Law Simplified Revision Notes for A-Level AQA Chemistry

1.6.6 Applications of Hess's Law

Using Hess' Law: Calculation Methods

1. Calculations Using Enthalpies of Formation

2. Calculations Using Enthalpies of Combustion

Example: How to Perform a Hess' Law Calculation

Applications of Hess' Law

Example: Thermal Decomposition of Sodium Hydrogen Carbonate ( $NaHCO₃$ )

Example: Hydration of Magnesium Sulfate ( $MgSO₄$ )

Example: Hydration of Copper(II) Sulfate ( $CuSO₄$ )

Summary

500K+ Students Use These Powerful Tools to Master Applications of Hess’s Law For their A-Level Exams.

Other Revision Notes related to Applications of Hess’s Law you should explore

Applications of Hess’s Law Simplified Revision Notes for A-Level AQA Chemistry

1.6.6 Applications of Hess's Law

Using Hess' Law: Calculation Methods

1. Calculations Using Enthalpies of Formation

2. Calculations Using Enthalpies of Combustion

Example: How to Perform a Hess' Law Calculation

Applications of Hess' Law

Example: Thermal Decomposition of Sodium Hydrogen Carbonate (NaHCO3NaHCO₃NaHCO3​)

Example: Hydration of Magnesium Sulfate (MgSO4MgSO₄MgSO4​)

Example: Hydration of Copper(II) Sulfate (CuSO4CuSO₄CuSO4​)

Summary

500K+ Students Use These Powerful Tools to Master Applications of Hess’s Law For their A-Level Exams.

Other Revision Notes related to Applications of Hess’s Law you should explore

Example: Thermal Decomposition of Sodium Hydrogen Carbonate ( $NaHCO₃$ )

Example: Hydration of Magnesium Sulfate ( $MgSO₄$ )

Example: Hydration of Copper(II) Sulfate ( $CuSO₄$ )