Thermodynamics (A-level only) common mistakes

Common mistakes

• Confusing lattice enthalpy with enthalpy of formation

  Students often confuse lattice enthalpy with the overall enthalpy change of formation of the ionic compound.

  Fix itLattice enthalpy specifically refers to the energy associated with the formation of the ionic solid from gaseous ions, while enthalpy of formation refers to the overall energy change when forming the compound from its elements.

• Incorrect sign for lattice enthalpy

  Students often confuse the sign of lattice enthalpy, thinking it is always positive.

  Fix itRemember that lattice enthalpy is typically negative for the formation of ionic solids, as energy is released.

• Confusing lattice enthalpy with enthalpy of formation

  Students often confuse lattice enthalpy with the enthalpy change for the formation of a compound from its elements.

  Fix itLattice enthalpy specifically refers to the formation of an ionic solid from gaseous ions, while enthalpy of formation refers to the formation from standard states.

• Confusing lattice enthalpy with hydration enthalpy

  Students often confuse lattice enthalpy with hydration enthalpy, thinking they are the same.

  Fix itLattice enthalpy refers to the energy required to form a solid ionic compound from gaseous ions, while hydration enthalpy is the energy change when ions are surrounded by water molecules.

• Confusing entropy with energy

  Students often confuse entropy with the total energy of a system.

  Fix itRemember that entropy specifically measures disorder or energy dispersal, not total energy.

• Incorrectly calculating ΔS°

  Students often forget to multiply by the stoichiometric coefficients when calculating the total entropy change.

  Fix itAlways ensure to account for the coefficients from the balanced equation when summing the standard entropies.

• Incorrect Unit Conversion

  Failing to convert ΔS from J/(mol·K) to kJ/(mol·K) when calculating ΔG.

  Fix itAlways ensure units are consistent; convert J to kJ by dividing by 1000.

• Ignoring temperature units

  Students often forget to convert temperature to Kelvin when using the Gibbs free energy equation.

  Fix itAlways convert Celsius to Kelvin by adding 273.15 before substituting into equations.

• Confusing feasibility with reaction rate

  Students often think that if a reaction is feasible (ΔG < 0), it will occur quickly.

  Fix itRemember that feasibility does not imply speed; a high activation energy can slow down the reaction significantly.