Enthalpy Calculators

0 calculators tagged with “Enthalpy”

Enthalpy (H) is a thermodynamic state function representing total heat content at constant pressure: H = U + PV. The enthalpy change ΔH = H_products − H_reactants determines whether a reaction is exothermic (ΔH < 0) or endothermic (ΔH > 0). Enthalpy is measured by calorimetry and calculated using Hess's law or standard enthalpies of formation. Understanding ΔH is essential for predicting reaction spontaneity (via Gibbs free energy ΔG = ΔH − TΔS) and for industrial process design.

All Calculators

No calculators found for this topic.

Enthalpy Change

ΔH = H_products − H_reactants

  • ΔH < 0: exothermic — heat released; surroundings warm
  • ΔH > 0: endothermic — heat absorbed; surroundings cool

Units: kJ/mol. Standard conditions: 298 K, 1 bar (ΔH°).

Hess's Law

ΔH°_rxn = Σ ΔH°_f(products) − Σ ΔH°_f(reactants)

Enthalpy is a state function — ΔH depends only on initial and final states, not the path. Standard enthalpy of formation (ΔH°_f) is the enthalpy change when 1 mol of a compound forms from its elements in their standard states; ΔH°_f = 0 for all elements by definition.

Bond Enthalpy Approximation

ΔH_rxn ≈ Σ(bond energies broken) − Σ(bond energies formed). Exothermic when more energy is released forming bonds than is absorbed breaking them. Example: H₂ + Cl₂ → 2HCl: bonds broken = H-H (436) + Cl-Cl (242) = 678 kJ; bonds formed = 2×H-Cl (862 kJ); ΔH ≈ −184 kJ/mol.

Calorimetry

q = mcΔT (coffee cup, constant pressure). ΔH_rxn = −q/n. Bomb calorimeter (constant volume) measures ΔU; ΔH = ΔU + ΔngasRT.

Glossary

Enthalpy (H)
H = U + PV; ΔH = heat exchanged at constant pressure; ΔH < 0 = exothermic; ΔH > 0 = endothermic; units kJ/mol.
Hess's Law
ΔH_rxn = Σ ΔH°_f(products) − Σ ΔH°_f(reactants); enthalpy is path-independent (state function); allows calculation of ΔH from tabulated formation enthalpies.
Standard Enthalpy of Formation (ΔH°_f)
Enthalpy change forming 1 mol of compound from its elements in standard states at 298 K; ΔH°_f = 0 for pure elements; used in Hess's law calculations.

Frequently Asked Questions

Enthalpy H = U + PV is the total heat content of a system at constant pressure. ΔH = H_products − H_reactants. ΔH < 0: exothermic — products more stable; heat flows to surroundings (warms). ΔH > 0: endothermic — products less stable; heat absorbed from surroundings (cools). ΔH is measured by calorimetry or calculated from tabulated standard enthalpies of formation using Hess's law.

Hess's law: the total enthalpy change for a reaction equals the sum of enthalpy changes for any series of steps leading from the same reactants to the same products (enthalpy is a state function). Formula: ΔH°_rxn = Σ ΔH°_f(products) − Σ ΔH°_f(reactants). Standard enthalpy of formation ΔH°_f is the enthalpy change forming 1 mol of compound from its elements; ΔH°_f = 0 for all pure elements in their standard state by definition. This allows calculation of ΔH for reactions difficult to measure directly.

ΔH_rxn ≈ Σ(bond energies of bonds broken) − Σ(bond energies of bonds formed). Breaking bonds absorbs energy (+); forming bonds releases energy (−). If total energy released forming bonds > energy absorbed breaking bonds: exothermic. Bond enthalpy calculations are approximate because they use average values; more accurate results come from standard enthalpies of formation. Useful for estimating ΔH when tabulated data are unavailable.

Gibbs free energy: ΔG = ΔH − TΔS. ΔG < 0 = spontaneous; ΔG > 0 = non-spontaneous; ΔG = 0 = equilibrium. A reaction can be exothermic (ΔH < 0) but non-spontaneous if ΔS < 0 (large entropy decrease). Conversely, endothermic reactions (ΔH > 0) can be spontaneous if ΔS > 0 and temperature is high enough. Neither ΔH nor ΔS alone determines spontaneity — both contribute through ΔG = ΔH − TΔS.