Which of the following best describes how the total reaction energy (enthalpy change) is calculated for a chemical reaction?

• A. By dividing the total energy of the products by the total energy of the reactants
• B. By multiplying the enthalpy of each reactant by its coefficient and adding the results together
• C. By subtracting the sum of the enthalpies of the reactants from the sum of the enthalpies of the products: ΔH = ΣH(products) - ΣH(reactants)
• D. By adding the enthalpy of the reactants and the products together

Answer: (C)

Verified step by step guidance

1. Understand that the total reaction energy, or enthalpy change (ΔH), represents the difference in enthalpy between products and reactants in a chemical reaction.
2. Recall that each substance in the reaction has an associated enthalpy value, often given as standard enthalpy of formation (H), which must be considered along with its stoichiometric coefficient.
3. Calculate the sum of the enthalpies of all products, each multiplied by its respective coefficient from the balanced chemical equation: \(\sum H(\text{products}) = \sum (\text{coefficient} \times H_{\text{product}})\).
4. Calculate the sum of the enthalpies of all reactants, each multiplied by its respective coefficient: \(\sum H(\text{reactants}) = \sum (\text{coefficient} \times H_{\text{reactant}})\).
5. Find the total reaction enthalpy change by subtracting the sum of the reactants' enthalpies from the sum of the products' enthalpies: \(\Delta H = \sum H(\text{products}) - \sum H(\text{reactants})\).