Enthalpy & Hess's Law Cycle Calculator

Enthalpy and Hess Law Guide

Hess Law

Hess Law states that the total enthalpy change of a reaction is independent of the pathway taken. From formation enthalpies: delta-H-rxn = sum delta-Hf(products) minus sum delta-Hf(reactants). Elements in their standard states have delta-Hf = 0. Example: CH4 + 2O2 to CO2 + 2H2O. delta-H = [delta-Hf(CO2) + 2 x delta-Hf(H2O)] minus [delta-Hf(CH4) + 2 x 0] = [-393.5 + 2x(-285.8)] minus [-74.8] = -890.3 kJ/mol.

Bond Energy Method

delta-H approx = sum E(bonds broken) minus sum E(bonds formed). Bonds broken in reactants are endothermic (positive). Bonds formed in products are exothermic (negative). This gives an approximation since bond energies are average values. Example CH4 + 2O2: bonds broken = 4xC-H (4x413) + 2xO=O (2x498) = 2648 kJ. Bonds formed = 2xC=O (2x805) + 4xO-H (4x463) = 3462 kJ. delta-H approx = 2648 - 3462 = -814 kJ/mol (actual -890).

Standard Enthalpy Values

Key standard enthalpies of formation (kJ/mol) at 298K: CO2(g) = -393.5. H2O(l) = -285.8. H2O(g) = -241.8. CH4(g) = -74.8. C2H5OH(l) = -277.7. NH3(g) = -46.1. Common combustion enthalpies: methane -890 kJ/mol. Ethanol -1367 kJ/mol. Hydrogen -286 kJ/mol. Carbon (graphite) -394 kJ/mol.

Calorimetry

Experimental measurement: q = mcDeltaT. Where m = mass of solution in grams, c = specific heat capacity approx 4.18 J per g per degree C for dilute aqueous solutions, and DeltaT = temperature change. Then delta-H = -q/n per mole. If temperature rises (exothermic) then q is positive and delta-H is negative. Sources of error: heat loss to surroundings, incomplete reaction, and wrong ratios. Use polystyrene cups and a lid to minimise heat loss.