3.2.1

Enthalpy changes

(a)→enthalpy [or heat content]: total chemical energy inside a substance

→enthalphy changes can be +ve [endothermic]: heat energy is absorbed by reaction from surroundings so system energy increases

→enthalpy changes can be -ve [exothermic]: heat energy is released by reaction to surroundings so system energy decreases

(b)→enthalpy profile diagrams: shows overall energy taken/given [image]

(c)→activation energy: minimun energy required for a reaction to take place

(d)(i)→standard conditions Ꝋ: 100kPa ; 298K ; standard physical state

(ii)→standard enthalpy change of reaction: EC associated with a stated equation under SC; [both exo and endo]

→standard enthalpy change of formation: formation of 1 mol of a compound from its elements under SC [both exo and endo]

→standard enthalpy change of combustion: complete combustion of 1 mol of a compound from its elements under SC [exo]

→standard enthalpy change of neutralisation: formation of 1 mol of water from neutralisation under SC [exo]

(e)→calorimetry: measurement enthalpy changes in endothermic reactions

→calorimeter: image

→q=mcΔT [image]

Bond enthalpies

(f)(i)→average bond enthalpy: breaking of 1 mol of bonds in gaseous molecules

→bond energies are affected by environments: so actual bond enthalpy may differ from average value

(ii)→endothermic enthalpy change: energy required to break bonds so bond breaking is endothermic; ΔH is positive

→exothermic enthalpy change: energy released to make bonds so bond forming is exothermic; ΔH is negative

→what determines whether overall reaction is exo or endo: difference between energy required for bond breaking and energy released by bond making

(iii)→how to find ΔH_r^ꝋ [enthalpy change of reaction] using bond energies: [image] [just add reactants and products]

Hess’ law and enthalpy cycles

(g)(i)→how are products formed from elements: directly ΔH₂ or indirectly ΔH₁ + ΔH_r [image]

→enthalpy change of combustion: [arrows down] reactants - products; as ΔH_c = ΔH₁ - ΔH₂ image

(ii)→enthalphy change of formation: [arrows up] products - reactants; as ΔH_r = ΔH₂ – ΔH_{1 image}