🔥 ENTHALPY, HESS’S LAW & FORMATION

ENTHALPY (H)

Heat content at constant pressure; defined H = E + PV; state function; absolute value unknown

ENTHALPY CHANGE (ΔH)

Heat flow at constant pressure; ΔH = q_p; ΔH = H_products − H_reactants

ENDOTHERMIC ΔH

Positive value; H_products > H_reactants; system absorbs heat

EXOTHERMIC ΔH

Negative value; H_products < H_reactants; system releases heat

THERMOCHEMICAL EQUATION

Balanced chemical equation including enthalpy change; coefficients correspond to moles; ΔH scales with amount

HESS’S LAW

Enthalpy change for overall reaction = sum of ΔH values for individual steps; ΔH independent of path; deduced by Germain Hess (1840)

REVERSAL RULE

Reverse reaction → ΔH same magnitude, opposite sign

MULTIPLIER RULE

Multiply equation by factor → multiply ΔH by same factor

STANDARD STATE

Reference condition; 1 atm pressure, specified temperature (usually 25°C); pure element in most stable form

STANDARD ENTHALPY OF FORMATION (ΔH°_f)

Enthalpy change forming 1 mole compound from its elements in standard states; elements in standard state have ΔH°_f = 0

STANDARD ENTHALPY OF REACTION (ΔH°_rxn)

Σ [n × ΔH°_f(products)] − Σ [n × ΔH°_f(reactants)]