gen-define-enthalpy-of-reaction-and-give-one-short-worked-exampl-122b

Define enthalpy of reaction and give one short worked example.

# Enthalpy of Reaction **Definition:** The enthalpy of reaction (ΔH_rxn) is the heat energy change that occurs when a chemical reaction takes place at constant pressure. It represents the difference between the enthalpy of the products and the enthalpy of the reactants. $$\Delta H_{rxn} = H_{products} - H_{reactants}$$ - If ΔH_rxn < 0: the reaction is **exothermic** (releases heat) - If ΔH_rxn > 0: the reaction is **endothermic** (absorbs heat) ## Worked Example **Calculate the enthalpy of reaction for the combustion of methane:** $$\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l)$$ **Given standard enthalpies of formation (ΔH°_f):** - CH₄(g) = -74.8 kJ/mol - O₂(g) = 0 kJ/mol (element in standard state) - CO₂(g) = -393.5 kJ/mol - H₂O(l) = -285.8 kJ/mol **Solution:** $$\Delta H_{rxn} = \sum \Delta H_f°(\text{products}) - \sum \Delta H_f°(\text{reactants})$$ $$\Delta H_{rxn} = [(-393.5) + 2(-285.8)] - [(-74.8) + 2(0)]$$ $$\Delta H_{rxn} = [-965.1] - [-74.8]$$ $$\Delta H_{rxn} = -890.3 \text{ kJ/mol}$$ The negative value indicates this is an **exothermic reaction** that releases 890.3 kJ of heat per mole of methane burned.