Enthalpy Change

Enthalpy change

H°, heat energy change of a reaction

Standard conditions

100 kPa pressure, usually 298 K temperature

Standard states

Physical states of reactants under standard conditions

Bond breaking vs making

Energy taken in to break bonds, energy released when making bonds

Endothermic reaction

More energy needed to break bonds than to make, ΔH positive, heat absorbed

Exothermic reaction

More energy released in making bonds than breaking, ΔH negative, heat released

Overall enthalpy change

ΔH = energy to break bonds (+) + energy released by making bonds (-)

Activation energy

Ea, minimum energy required for a reaction to occur between colliding particles

Enthalpy profile diagram

Shows energy changes during a reaction, with Ea as the peak above reactants

Enthalpy change of reaction

ΔH°, enthalpy change for a stated reaction under standard conditions

Enthalpy change of formation

ΔHf°, enthalpy change when 1 mole of a substance is formed from elements in standard states

Enthalpy change of combustion

ΔHc°, enthalpy change when 1 mole of a substance is completely burned in oxygen

Enthalpy change of neutralisation

ΔHneut°, enthalpy change when 1 mole of water is formed from acid and alkali under standard conditions

Calorimetry

Method to determine enthalpy change by measuring temperature change over time

Energy change formula

q = m × c × ΔT

Variables in calorimetry

q = energy change (J), m = mass (g), c = specific heat capacity (J g⁻¹ K⁻¹), ΔT = temperature change (K)

Specific heat capacity

Energy required to raise 1 g of substance by 1 K without change of state

Calculating ΔH per mole

ΔH = (q × 10⁻³) ÷ moles

Sources of error in calorimetry

Heat loss by conduction or convection, inaccurate temperature measurement

Minimising heat loss

Lid on calorimeter, insulation like polystyrene

Assumptions in calorimetry

Specific heat capacity taken as 4.18 J g⁻¹ K⁻¹, calorimeter heat capacity ignored