Q: What does H2 Chemistry Notes: CORE IDEA 3, Topic 7 - Chemical Energetics cover?
A: Integrate enthalpy changes, bond energies, Born-Haber cycles, entropy, and Gibbs free energy for Core Idea 3 (Chemical Energetics) in the 2026 H2 Chemistry syllabus.
Core Idea
CORE IDEA 3 - TRANSFORMATION, 7 CHEMICAL ENERGETICS: THERMOCHEMISTRY AND THERMODYNAMICS (GIBBS FREE ENERGY AND ENTROPY)
Energetics connects molecular behaviour to spontaneous change. Mastering enthalpy cycles, entropy arguments, and ΔG calculations is essential for structured questions across Papers 2 and 3. Use this note alongside worksheets stored at https://eclatinstitute.sg/blog/h2-chemistry-notes.
1 Enthalpy Changes
Enthalpy change
Definition
Example equation
Standard enthalpy of formation \(\Delta H_{\mathrm{f}}^{\circ}\)
Enthalpy change when one mole of compound forms from elements in standard states.
\(\ce{H2(g) + \tfrac{1}{2} O2(g) -> H2O(l)}\)
Standard enthalpy of combustion \(\Delta H_{\mathrm{c}}^{\circ}\)
Enthalpy change when one mole of substance burns completely in excess oxygen.
\(\ce{CH4(g) + 2O2(g) -> CO2(g) + 2H2O(l)}\)
Enthalpy of atomisation \(\Delta H_{\text{atom}}\)
Enthalpy required to form one mole of gaseous atoms from the element.
\(\ce{\tfrac{1}{2} Cl2(g) -> Cl(g)}\)
Bond dissociation enthalpy
Enthalpy to break one mole of a specified gaseous bond.
\(\ce{H2(g) -> 2H(g)}\)
State conditions: 298K, 1bar, substances in standard states.
2 Hess' Law and Enthalpy Cycles
Hess' law uses the state function property-route independent.
2.1 Formation/Combustion Cycle
Draw arrows from elements to compound (formation) or compounds to oxides (combustion).
Combine given ΔH∘ values; reverse sign if reversing a reaction; multiply when scaling.
2.2 Bond Energy Approach
Approximate reaction enthalpy:
ΔH≈∑(Ebroken)−∑(Eformed)
Ensure all species are gaseous; adjust for physical states if necessary (include enthalpy of vaporisation or atomisation).
3 Lattice Energy and Born-Haber Cycles
Born-Haber cycle for ionic compounds involves:
Atomisation of metal and non-metal.
First (and second) ionisation energies of metal.
Electron affinity of non-metal.
Lattice energy (formation of solid from gaseous ions).
Construct cycles carefully, labelling each step with values and sign convention (exothermic negative). Remember standard enthalpy of formation sits at bottom of cycle.
4 Entropy and Gibbs Free Energy
4.1 Entropy Change
Entropy measures disorder:
\[
\Delta S = \sum S^{\circ}{\text{products}} - \sum S^{\circ}{\text{reactants}}
\]
Temperature dependence: if both ΔH and ΔS are positive, spontaneity increases with temperature.
4.3 Equilibrium Connection
At equilibrium, ΔG=0, so T=ΔSΔH. Apply this to phase transitions or decomposition reactions.
5 Worked Example
Question:
Given the data below, calculate the lattice energy of NaCl.
ΔHf∘(NaCl)=−411kJmol−1
Enthalpy of atomisation of sodium: +108kJmol−1
Bond dissociation enthalpy of ClX2: +242kJmol−1
First ionisation energy of sodium: +496kJmol−1
Electron affinity of chlorine: −349kJmol−1
Solution:
Construct the cycle:
Na(s)Na(g): +108kJmol−1.
Na(g)NaX+(g)+eX−
21ClX2(g)Cl(g)
Cl(g)+eX−ClX−(g)
NaX+(g)+ClX−(g)NaCl(s)
Apply Hess' law:
ΔHf∘=108+496+121−349+U
Solve for U:
U=ΔHf∘−(108+496+121−349)=−787kJmol−1
State final answer: lattice energy of NaCl is −787kJmol−1.
6 Experimental Considerations
Calorimetry (Paper 4):
Use polystyrene cup to minimise heat loss.
Record initial and maximum temperatures; extrapolate to correction for heat loss if required.
Compute q=mcΔT with appropriate units c=4.18Jg−1K−1.
Error sources: heat exchange with environment, incomplete reaction, inaccurate mass/volume measurements. Suggest improvements (insulation, repeating and averaging, using lid).
7 Common Misconceptions
Forgetting to convert ΔS to kJ⋅mol−1⋅K−1 when combining with ΔH (divide by 1000).
Reversing sign for electron affinity (exothermic values negative).
Using bond energies from data booklet without adjusting for stoichiometric coefficients.
Assuming ΔH<0 guarantees spontaneity; emphasise the TΔS term.
8 Quick Drills
Determine ΔH for 2CX2HX6(g)+7OX2(g)4COX2(g)+6HX2O(l) using bond energies and compare with tabulated ΔHc∘ values.
For CaCOX3(s)CaO(s)+COX2(g)
Sketch a Born-Haber cycle for MgClX2 including the second ionisation energy of magnesium and the second electron affinity of chlorine (endothermic). Annotate each energy change.
Energy reasoning must blend quantitative data with conceptual arguments. Keep a data booklet handy, practise drawing cycles quickly, and revisit https://eclatinstitute.sg/blog/h2-chemistry-notes for aligned worksheets.
H2 Chemistry Notes: CORE IDEA 3, Topic 7 - Chemical Energetics
:
+496kJmol−1
.
:
+121kJmol−1
(half of 242).
:
−349kJmol−1
.
: lattice energy
U
(unknown).
, given
ΔH=178kJ⋅mol−1
and
ΔS=161J⋅mol−1⋅K−1
, compute the temperature above which decomposition is spontaneous.
