Question

what is the enthaply heat of rection

Answer 1

I think you mean "enthalpy (heat) of reaction" — here's a clear summary of what that is and how to find it.

Definition
- The enthalpy of reaction, ΔHrxn (often written ΔH or ΔH°rxn for standard conditions), is the heat absorbed or released by a chemical reaction at constant pressure.
- Sign convention: ΔH < 0 → exothermic (heat released); ΔH > 0 → endothermic (heat absorbed).
- Units: typically kJ per mole of reaction (kJ·mol⁻¹).

Ways to calculate ΔHrxn
1. Using standard enthalpies of formation (most accurate for tabulated data)
- ΔH°rxn = Σ ΔH°f(products) − Σ ΔH°f(reactants)
- Use stoichiometric coefficients. ΔH°f of elements in their standard state = 0.

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

2. Using bond enthalpies (approximate)
- ΔHrxn ≈ Σ(BDE of bonds broken) − Σ(BDE of bonds formed)
- Quick estimate; less accurate because bond energies are average gas-phase values and omit phase effects.

3. From calorimetry (experimental)
- Measure heat q absorbed/released by solution or calorimeter: q = m c ΔT (or use calorimeter constant).
- For a reaction in solution at constant pressure: ΔHrxn = −q / n (n = moles of limiting reagent or per reaction stoichiometry).

Thermochemical relations
- Relation to internal energy: ΔH = ΔU + Δ(PV). For ideal gases at constant T, ΔH = ΔU + Δn_g RT (Δn_g = change in moles of gas).
- Temperature dependence: ΔH at different T can be adjusted by integrating heat capacities (Kirchhoff’s law).

If you want, tell me a specific reaction (and whether you want a precise calculation using standard enthalpies of formation, an approximate bond-energy estimate, or how to get it from calorimetry) and I’ll calculate it.