Standard Heats of Formation

Standard heats of formation

• changes in potential energy

• occur when compounds formed from their elements

• measure of stability

• elements that are not measured are zero

stable compounds

• more exothermic formation reaction = more stable compound

• release of more energy is more stable

elemental Standard Molar Enthalpies

Every element/allotrope has a standard molar enthalpy of formation equal to zero

Hess’ Law

difference between the standard heats of formation of the reactants and the products

Hess’ Law formula

• Δ_rH = ΣnΔ_fHº_(products) – ΣnΔ_fHº_(reactants)

• subtract energy released from initial energy

sig digs (addition and subtraction)

least number of decimal places

standard heat of combustion formula

1. (mol)(standard molar enthalpy) + (mol)(standard molar enthalpy) → (mol)(standard molar enthalpy)

2. product - reactant

graph

• exothermic

• formation

graph

• endothermic

• decomposition