What is ΔH defined as in a chemical reaction?

• A. The energy change from reactants to products
• B. The activation energy
• C. The rate constant
• D. The entropy change

Answer: (A)

ΔH is the enthalpy change of a reaction, meaning the energy difference between products and reactants. It is defined as ΔH = H(products) − H(reactants). At constant pressure, this corresponds to the heat that flows into or out of the system during the reaction: negative ΔH means heat is released (exothermic), while positive ΔH means heat is absorbed (endothermic). This concept is distinct from activation energy (the energy barrier to start the reaction), the rate constant (how fast the reaction proceeds), and entropy change (ΔS, which measures disorder). In practice, you can also relate ΔHrxn to formation enthalpies: ΔHrxn = Σ νΔHf(products) − Σ νΔHf(reactants).