Thermochemistry
Thermochemistry studies the heat changes that accompany chemical reactions. Energy is measured in Joules (J) or kilojoules (kJ). Every chemical reaction either releases or absorbs energy.
A. Exothermic vs Endothermic Reactions
Comparison
🔥 Exothermic
ΔH = NEGATIVE (−)
- Releases heat to surroundings
- Temperature of surroundings INCREASES
- Products have LESS energy than reactants
- Energy profile: products lower than reactants
Examples: combustion, neutralisation, respiration, rusting, condensation
🧊 Endothermic
ΔH = POSITIVE (+)
- Absorbs heat from surroundings
- Temperature of surroundings DECREASES
- Products have MORE energy than reactants
- Energy profile: products higher than reactants
Examples: photosynthesis, dissolving ammonium nitrate, thermal decomposition, melting
⚡ MCQ Tip Exothermic = heat OUT = ΔH negative = surroundings get hotter. Endothermic = heat IN = ΔH positive = surroundings get colder. Combustion is always exothermic. Photosynthesis is always endothermic.
B. Key Thermochemical Terms
Definitions
| Term | Definition |
|---|---|
| Enthalpy (H) | Total heat content of a system at constant pressure |
| ΔH (Enthalpy Change) | Heat absorbed or released during a reaction at constant pressure. ΔH = H_products − H_reactants |
| Heat of Combustion | Heat released when exactly 1 mole of a substance burns completely in oxygen. Always exothermic (−ΔH). |
| Heat of Neutralisation | Heat released when exactly 1 mole of water forms from an acid-base reaction. Always exothermic. |
| Heat of Formation | Enthalpy change when exactly 1 mole of a compound forms from its elements in standard state |
| Bond Energy | Energy required to break 1 mole of a covalent bond in the gaseous phase. Breaking bonds = endothermic; forming bonds = exothermic. |
| Specific Heat Capacity (c) | Heat required to raise 1 g of substance by 1°C. Water = 4.18 J/g°C (highest common value — explains water's role as a coolant). |
C. Hess's Law
Hess's Law of Constant Heat Summation
- Statement: The total enthalpy change of a reaction is the same regardless of the route taken — only the initial and final states matter
- Enthalpy is a state function — it depends only on the current state, not the path taken
- Allows calculation of ΔH for reactions that cannot be measured directly
- ΔH_total = ΔH₁ + ΔH₂ + ΔH₃ + … (when reactions are added)
- If a reaction is reversed, the sign of ΔH changes (−ΔH becomes +ΔH)
Hess's Law
ΔH_reaction = Σ ΔH_f(products) − Σ ΔH_f(reactants)
D. Heat Energy Calculation (Q = mcΔT)
The Q = mcΔT Formula
Heat Energy
Q = m × c × ΔT
Q = heat energy (J) · m = mass (g) · c = specific heat capacity (J/g°C) · ΔT = temperature change (°C or K)
| Variable | Symbol | Unit | Note |
|---|---|---|---|
| Heat energy | Q | Joules (J) | Positive = heat absorbed; negative = heat released |
| Mass | m | grams (g) | Mass of substance being heated/cooled |
| Specific heat capacity | c | J/g°C | Water = 4.18 J/g°C; the most commonly used value |
| Temperature change | ΔT | °C or K | ΔT = T_final − T_initial |
Heat energy
SymbolQ
UnitJoules (J)
NotePositive = heat absorbed; negative = heat released
Mass
Symbolm
Unitgrams (g)
NoteMass of substance being heated/cooled
Specific heat capacity
Symbolc
UnitJ/g°C
NoteWater = 4.18 J/g°C; the most commonly used value
Temperature change
SymbolΔT
Unit°C or K
NoteΔT = T_final − T_initial
⚡ Worked Example How much heat to raise 100 g of water by 20°C?
Q = 100 × 4.18 × 20 = 8,360 J = 8.36 kJ
Q = 100 × 4.18 × 20 = 8,360 J = 8.36 kJ
Quick MCQ Revision
| Fact | Answer |
|---|---|
| Exothermic ΔH sign | Negative (−) — heat released to surroundings |
| Endothermic ΔH sign | Positive (+) — heat absorbed from surroundings |
| Combustion is always | Exothermic (releases heat) |
| Photosynthesis is always | Endothermic (absorbs light energy) |
| Hess's Law states | Total ΔH is same regardless of route taken (state function) |
| Q = mcΔT | Heat = mass × specific heat × temperature change |
| Specific heat of water | 4.18 J/g°C |
| Breaking bonds | Endothermic (energy absorbed) |
| Forming bonds | Exothermic (energy released) |