Calculate Δh for Each of The Following Reactions

What is Δh in Chemistry?

Δh (delta h) represents the change in enthalpy during a chemical reaction. Enthalpy is a measure of the total heat content of a system, including the internal energy and the product of pressure and volume. A positive Δh indicates an endothermic reaction, while a negative Δh indicates an exothermic reaction.

In chemistry, Δh is calculated using the standard enthalpies of formation (ΔH°f) of the reactants and products. The standard enthalpy of formation is the change in enthalpy when one mole of a compound is formed from its constituent elements in their standard states.

How to Calculate Δh for Reactions

To calculate Δh for a reaction, follow these steps:

1. Write the balanced chemical equation for the reaction.
2. Determine the standard enthalpies of formation (ΔH°f) for all reactants and products. These values can be found in chemistry reference books or databases.
3. Calculate the total enthalpy of the reactants by multiplying each reactant's ΔH°f by its stoichiometric coefficient and summing the results.
4. Calculate the total enthalpy of the products in the same way.
5. Subtract the total enthalpy of the reactants from the total enthalpy of the products to find Δh for the reaction.

The result will be in kilojoules per mole (kJ/mol). A positive value indicates an endothermic reaction, while a negative value indicates an exothermic reaction.

Example Calculations

Let's calculate Δh for the following reaction:

2H₂(g) + O₂(g) → 2H₂O(l)

Standard enthalpies of formation:

• H₂(g): 0 kJ/mol
• O₂(g): 0 kJ/mol
• H₂O(l): -285.8 kJ/mol

Calculation:

Δh = [2 × (-285.8 kJ/mol)] - [2 × 0 kJ/mol + 1 × 0 kJ/mol] = -571.6 kJ/mol

This is an exothermic reaction, releasing 571.6 kJ of energy per mole of water produced.