Enthalpy Change of Chemical Reactions: Physical Chemistry

Standard enthalpy of formation (∆_fH˚)

is the reaction enthalpy of formation of 1 mole the substance from elements in the reference state.

∆_rH˚=∆_fH˚(products)-∆_fH˚(reactants)

Pay attention to state especially for water since there is a different ∆_fH˚ of (l) and (g) H₂O

if you change pressure ∆_fH˚ changes

Bond enthalpies

Bond enthalpy is the reaction enthalpy of breaking a bond:

always > 0 because it takes energy to break a bond

Note 1: every atom is brought to gas phase to break every bond

Note 2: bond enthalpy somewhat depends on the molecule therefore, we use average bond enthalpies.

Combustion enthalpies

∆_rH˚=∆_combH˚(reactants)-∆_combH˚(products)

Note 1: ∆_cH˚ increases as the number of C atoms increases. Specific enthalpy and enthalpy density are better figures.

Note 2: Hydrocarbons have the most negative ∆_cH˚, compared to other fuels with the same C atom count (compare CH₄ and CH₃OH).

Energy storage in organisms:

• Fats (tristearin, C₅₇H₁₁₀O₆): 38 kJ/g (9 kcal/g)

• Carbohydrates (C₆H₁₂O₆): 17 kJ/g (4 kcal/g)

Fats have a lower oxygen content per carbon and are more reduced than carbohydrates.

Fats offer the most compact way of energy storage.

Kirchhoff’s Law