How to Find Delta H of a Reaction

Enthalpy change (Delta H) is a measure of the heat energy absorbed or released during a chemical reaction. It is an important thermodynamic property that can help us understand the energetics of a reaction and predict its spontaneity. In this article, we will provide a step-by-step guide on how to find Delta H of a reaction.

Step 1: Identify the Enthalpy of Reaction

The enthalpy of reaction is the difference between the total enthalpy of the products and the total enthalpy of the reactants. It can be expressed as follows:

“`
Delta H = H(products) – H(reactants)
“`

Step 2: Determine the Enthalpy of Formation for Each Reactant and Product

The enthalpy of formation is the enthalpy change when one mole of a compound is formed from its constituent elements. This value can be found in thermodynamic tables or databases.

Step 3: Calculate the Total Enthalpy of the Reactants and Products

To calculate the total enthalpy of the reactants and products, multiply the enthalpy of formation of each compound by its stoichiometric coefficient in the balanced chemical equation. Then, sum up the values for all the reactants and products.

Step 4: Determine Delta H

Once you have the total enthalpy of the reactants and products, simply subtract the total enthalpy of the reactants from the total enthalpy of the products to obtain Delta H.

Example

Let’s consider the following reaction:

“`
2H2 + O2 -> 2H2O
“`

The enthalpy of formation of H2 is 0 kJ/mol, the enthalpy of formation of O2 is 0 kJ/mol, and the enthalpy of formation of H2O is -286 kJ/mol.

The total enthalpy of the reactants is:

“`
H(reactants) = (2 x 0 kJ/mol) + (1 x 0 kJ/mol) = 0 kJ/mol
“`

The total enthalpy of the products is:

“`
H(products) = (2 x -286 kJ/mol) = -572 kJ/mol
“`

Therefore, Delta H for the reaction is:

“`
Delta H = H(products) – H(reactants) = -572 kJ/mol – 0 kJ/mol = -572 kJ/mol
“`

The negative value of Delta H indicates that the reaction is exothermic, meaning that it releases heat energy into the surroundings.

Conclusion

Finding Delta H of a reaction is a straightforward process that involves identifying the enthalpy of formation for each reactant and product, calculating the total enthalpy of the reactants and products, and then subtracting the total enthalpy of the reactants from the total enthalpy of the products. This information can be used to understand the energetics of a reaction and predict its spontaneity.

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