Question

what is the enthalpy heat of reaction

Answer 1

The enthalpy (heat) of reaction, usually written ΔH_rxn, is the heat absorbed or released by a chemical reaction at constant pressure. Key points and ways to get it:

Definition and sign
- ΔH_rxn = H_products − H_reactants (per the stoichiometry of the balanced equation).
- ΔH < 0: exothermic (heat released). ΔH > 0: endothermic (heat absorbed).
- Standard enthalpy change is written ΔH° and refers to reactants and products in their standard states (usually 1 bar, specified temperature).

Common calculation methods
1) Using standard enthalpies of formation:
- ΔH°_rxn = Σ ν_i ΔH°_f(products) − Σ ν_j ΔH°_f(reactants)
- ν are stoichiometric coefficients. Use tabulated ΔH°_f values (units typically kJ/mol).

2) Using Hess’s law:
- Combine known reaction enthalpies (or formation enthalpies) algebraically to obtain the target reaction enthalpy.

3) Approximate from bond enthalpies:
- ΔH_rxn ≈ Σ (bond energies of bonds broken) − Σ (bond energies of bonds formed).
- This is approximate because bond energies are average values and depend on molecular environment.

4) Experimental (calorimetry):
- At constant pressure, q_p = ΔH. For example, for a calorimeter: q = m c ΔT; convert heat absorbed/released to per mole of reactant to get ΔH (kJ/mol).

Example (combustion of methane)
CH4(g) + 2 O2(g) → CO2(g) + 2 H2O(l)
Using typical ΔH°_f (kJ/mol): CH4 = −74.8, O2 = 0, CO2 = −393.5, H2O(l) = −285.8
ΔH°_rxn = [−393.5 + 2(−285.8)] − [−74.8 + 0] = −965.1 − (−74.8) = −890.3 kJ per mol CH4 (exothermic).

Units and practical notes
- Units: usually kJ per mole of reaction as written (kJ/mol).
- Always use the balanced equation and correct physical states (g, l, aq) because ΔH°_f depends on state.
- If you want, I can compute ΔH for a specific reaction if you provide the balanced equation (and whether you want standard values or experimental/calorimetric data).