Skip to content
Dabour HubLEARN · PRACTICE · EXPLORESign inStudent

1.4.4Physical Chemistry

Applications of Hess’s Law — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 6 parts Lesson 25 of 63

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision
🩺
100% Curriculum Specification Match Lesson Doctor Verified
The only platform with 100% curriculum specification match and verified interactive question banks.
0%read

Part 1 of 6Hess's Law

1.4.4 Applications of Hess's Law
Statement
  • The total enthalpy change for a reaction is independent of the route taken, provided the initial and final conditions are the same.
Basis
  • It is an application of the conservation of energy, as energy cannot be created or destroyed.
Use
  • It allows enthalpy changes to be found for reactions that are too slow, dangerous or incomplete to measure.
Cycle
  • Draw the reactants and products with the elements or combustion products at the bottom, and add arrows for each route.

Part 2 of 6Using Formation Data

1
Write the balanced equation for the reaction, including state symbols.
2
Draw the elements in their standard states below the equation, with arrows from them to both reactants and products.
3
Apply the relationship ΔH = ΣΔfH°(products) − ΣΔfH°(reactants).
4
Multiply each value by its coefficient in the equation before adding.
5
Check the sign and give the answer in kJ mol⁻¹.

Part 3 of 6Using Combustion Data

Using Combustion Data
Cycle
  • Reactants and products both burn to give the same combustion products, forming the bottom of the cycle.
Formula
  • ΔH = ΣΔcH°(reactants) − ΣΔcH°(products), the reverse order of the formation relationship.
Example
  • Methane's formation from carbon and hydrogen cannot be measured directly but is found from combustion data.
Check
  • Elements in their standard states have ΔfH° = 0, so they do not need to be included in the sum.

Part 4 of 6Direction of Arrows

With the arrow
  • Add the enthalpy change when following an arrow in the direction it points.
Against the arrow
  • Subtract the enthalpy change, or reverse its sign, when going against an arrow.
Reversed reaction
  • Reversing a reaction changes the sign of its enthalpy change but not its size.
Scaling
  • Multiplying the equation by a number multiplies the enthalpy change by the same number.

Part 5 of 6Worked Example

Question Calculate ΔH° for CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), using ΔfH° values: CH₄(g) −74.8, CO₂(g) −393.5, H₂O(l) −285.8 kJ mol⁻¹.

  1. ΔH = ΣΔfH°(products) − ΣΔfH°(reactants).
  2. Products−393.5 + 2(−285.8) = −393.5 − 571.6 = −965.1 kJ mol⁻¹.
  3. Reactants−74.8 + 2(0) = −74.8 kJ mol⁻¹, since O₂ is an element.
  4. ΔH = −965.1 − (−74.8) = −890.3 kJ mol⁻¹.

Answer ΔcH°(CH₄) = −890.3 kJ mol⁻¹.

Part 6 of 6Exam Checklist & Tips

1.4.4 – Key Points for the Exam

  • Hess's law The enthalpy change of a reaction is independent of the route taken.
  • Enthalpy cycle A diagram linking reactants and products by alternative routes to apply Hess's law.
  • Standard state The physical form of a substance at 100 kPa and a stated temperature.
  • Formation The creation of one mole of a compound from its elements in their standard states.

Common Mistakes

  • Reversing the order of subtraction when switching between formation and combustion data.
  • Forgetting to multiply the enthalpy value by the number of moles in the equation.
  • Including the formation enthalpy of an element in its standard state as non-zero.
  • Using the wrong state, such as H₂O(g) instead of H₂O(l).

Last updated:

\n