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6.3Thermochemistry

Enthalpy Changes — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 9 parts Lesson 8 of 14 Unit quiz

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Lesson summary

6.3 Enthalpy Changes — AP CHEMISTRY notes: Compare the ways of finding a reaction enthalpy change and the rules that link ΔH to the equation.Enthalpy change ΔH — heat transferred at constant pressure: ΔH = Hproducts − HreactantsNegative ΔH is exothermic; positive ΔH is endothermi…

  • Standard enthalpy change ΔH°
  • Breaking bonds
  • Forming bonds
  • Elements in standard states
  • Coefficients
  • Reversed equation

Part 1 of 7Enthalpy change

1Enthalpy change ΔH
  • Enthalpy change ΔH heat transferred at constant pressure: ΔH = Hproducts − Hreactants
  • Negative ΔH is exothermic; positive ΔH is endothermic.
2Standard enthalpy change ΔH°
  • Standard enthalpy change ΔH° measured at 100 kPa, 1 M and standard states, usually at 298 K

Part 2 of 7Methods of calculating ΔH°

MethodFormulaData needed
CalorimetryΔH = −mcΔT / nMass, ΔT, moles
Bond enthalpiesΔH ≈ Σ(bonds broken) − Σ(bonds formed)Average bond enthalpies
Enthalpies of formationΔH° = ΣΔH°f(products) − ΣΔH°f(reactants)ΔH°f values
Hess's lawAdd the ΔH values of equations that sum to the targetΔH of related reactions

Part 3 of 7Bond enthalpy method

Breaking bonds
  • Breaking bonds absorbs energy (+)
Forming bonds
  • Forming bonds releases energy (−)
Result
  • Result an estimate, because bond enthalpies are averages over many compounds

Part 4 of 7Formation method

1Elements in standard states
  • ΔH°f = 0
2Coefficients
  • Coefficients multiply each ΔH°f by the moles in the balanced equation

Part 5 of 7Rules linking ΔH to the equation

Reversed equation
  • ΔH has the same magnitude and the opposite sign
Multiplied equation
  • ΔH is multiplied by the same factor
Added equations
  • Added equations their ΔH values add (Hess's law)

Part 6 of 7Standard enthalpy types

Standard enthalpy types
ΔH°f
  • ΔH°f formation of one mole of compound from its elements
ΔH°c
  • ΔH°c complete combustion of one mole in excess oxygen, always exothermic
ΔH°neut
  • ΔH°neut formation of one mole of water from an acid and alkali, always exothermic

Part 7 of 7More from the specification

1Substance
  • A material with only one type of particle.
2Standard conditions
  • 298 K and 101 kPa.
3Haber process
  • Industrial manufacture of ammonia from nitrogen and hydrogen.

Key terms

TermMeaning
Enthalpy changeHeat transferred at constant pressure.
Standard enthalpy of formationΔH when one mole of compound forms from its elements in standard states.
Bond enthalpyEnergy to break one mole of a gaseous bond.
Hess's lawΔH is independent of the route.
CalorimetryMeasurement of heat transfer using q = mcΔT.

Avoid these mistakes

Avoid these mistakes
Using bond enthalpies as signed values for both bonds broken and formed — use positive values and subtract.
Using reactants minus products for formation data — it is products minus reactants.
Forgetting to scale ΔH when the equation is multiplied.

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