How can values of heats of formation found in tables be used to calculate heats of reactions? I am having a really hard time understanding this.

reactants ==> products

(dHformation products) - (dHformation reactants) = dH reaction.

Calculating the heat of a reaction using values of heats of formation requires an understanding of Hess's law. Hess's law states that the overall heat change of a reaction is the same regardless of the pathway taken. Here's how you can use values of heats of formation to calculate heats of reactions:

1. Determine the balanced chemical equation for the reaction you are interested in.

2. Identify all the substances involved in the reaction and their respective heats of formation. These values can be found in tables, such as standard enthalpy of formation tables.

3. Calculate the heat of reaction using the following formula:

Heat of reaction = Σ(n * ΔHf)

In this formula, Σ represents the sum of all the terms, n represents the stoichiometric coefficients of each substance in the balanced equation, and ΔHf represents the heat of formation.

4. Substitute the values of stoichiometric coefficients and heats of formation into the formula. Make sure to multiply the heats of formation by their respective stoichiometric coefficients.

5. Calculate the sum of all these terms, taking into account the sign (positive or negative) of each heat of formation. If a substance is a reactant, its heat of formation will have a negative sign, while if it is a product, its heat of formation will have a positive sign.

6. The resulting value is the heat of reaction. If the value is positive, the reaction is endothermic (absorbs heat); if the value is negative, the reaction is exothermic (releases heat).

It's important to note that this method assumes that the reactants and products are in their standard states. If the substances are not in their standard states, additional terms may need to be considered in the calculations.