The heat transferred during a chemical reaction is called reaction heat. Under constant-pressure conditions, the energy change is represented by the enthalpy change ΔH.
Basic Knowledge
The sign of reaction heat is expressed by the sign of the reaction enthalpy change ΔH. In an exothermic reaction, enthalpy decreases, so ΔH<0. In an endothermic reaction, ΔH>0. Main types of reaction heat:
① Enthalpy of formation (ΔHf): formation of 1 mol of a compound from its elements
② Enthalpy of combustion (ΔHc): complete combustion of 1 mol of a substance
③ Enthalpy of neutralization: approximately 56 kJ for the neutralization of a strong acid and strong base producing 1 mol of water
④ Enthalpy of solution: dissolving 1 mol of a substance in a large amount of solvent
A thermochemical equation lists ΔH to the right of the reaction equation (example: 2H2(g) + O2(g) → 2H2O(l)、ΔH = -572 kJ).
Enthalpy H（a state function equal to heat at constant pressure. H=U+PV）
ΔH（products - reactants. Negative for exothermic reactions and positive for endothermic reactions）
Standard enthalpy of formation ΔHf°（standard enthalpy of formation）
Enthalpy of combustion（heat released by complete combustion of 1 mol）
Enthalpy of neutralization（approximately -56 kJ/mol for a strong acid and strong base）
Calorimeter（a bomb calorimeter measures heat released）
Further Study (Principles and Applications)
The standard enthalpy of formation ΔHf° is the enthalpy value of each substance, based on the standard state of an element as the reference (0), and is listed in data tables. The ΔH of any reaction can be calculated using ΔHreaction = ΣΔHf°(products) - ΣΔHf°(reactants).
Food calorie labels are an application of enthalpy of combustion, with 1 kcal=4.18 kJ. Quantitative calculation of reaction energy is essential in rocket fuels, explosives, heating-system design, and energy policy.
- Exothermic: ΔH<0; endothermic: ΔH>0
- Always specify the state (g/l/s)
- Enthalpy of formation: elements → 1 mol of a compound
- Standard enthalpy of formation of an element = 0
- Enthalpy of neutralization for a strong acid and strong base is approximately 56 kJ/mol
- q = mcΔT (heat and specific heat)
- 1 kcal = 4.18 kJ
Points of Caution (Common Confusions and Frequent Mistakes)
① The old notation “reaction heat Q” and the newer notation “enthalpy change ΔH” have opposite signs (Q>0 ⇔ ΔH<0). ② The value changes with the state of matter (g/l/s); the enthalpies of formation of liquid water and gaseous water differ. ③ The enthalpy of neutralization of a weak acid and weak base is considerably smaller than 56 kJ because ionization is endothermic. ④ Enthalpy of combustion assumes complete combustion.
Practice
- Given C(s) + O2(g) → CO2(g), ΔH = -394 kJ, find the heat released when 12 g of carbon is burned.
- When the standard enthalpy of formation of H2O(l) is -286 kJ/mol, find ΔH for 2H2(g)+O2(g)→2H2O(l).
- Explain the essential reason why the enthalpy of neutralization of a strong acid and strong base is approximately -56 kJ/mol, including the chemical equation.