Understanding Energy Changes in Chemical Reactions
A chemical reaction that releases heat energy from the system to the surroundings. The reaction mixture and its surroundings get hotter.
A chemical reaction that takes in heat energy from the surroundings. The reaction container feels colder.
To describe the temperature changes involved in exothermic and endothermic reactions.
| Solution | Initial Temp (°C) | Final Temp (°C) | Temperature Change (°C) |
|---|---|---|---|
| Ammonium chloride (NH₄Cl) | _____ | _____ | _____ |
| Sodium hydroxide (NaOH) | _____ | _____ | _____ |
NH₄Cl: Temperature decreased, test tube felt cold
NaOH: Temperature increased, test tube felt hot
• NH₄Cl dissolution: Endothermic (ΔT negative)
• NaOH dissolution: Exothermic (ΔT positive)
A balanced equation that includes physical states of all reactants and products and the enthalpy change.
The energy stored in the bonds of a substance. Measured in kilojoules (kJ).
The change in energy from reactants to products. Also called heat of reaction.
Exothermic: ΔH is negative (-)
Endothermic: ΔH is positive (+)
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)
H₂O(l) → 2H₂(g) + O₂(g)
Energy level diagrams show energy changes involving reactants and products during chemical reactions.
Energy decreases → Heat released
Energy increases → Heat absorbed
H₂(g) + Cl₂(g) → 2HCl(aq), ΔH = -92 kJ/mol
Exothermic (ΔH negative)
NH₄NO₃(s) + H₂O(l) → NH₄⁺(aq) + NO₃⁻(aq), ΔH = +28.1 kJ/mol
Endothermic (ΔH positive)
Bond energy is the amount of energy required to break one mole of chemical bonds in a molecular element or compound. Unit: kJ/mol
| Chemical Bond | Bond Energy (kJ/mol) |
|---|---|
| H–H | 436 |
| O=O | 498 |
| O–H | 464 |
| C–H | 413 |
| C–C | 346 |
| Cl–Cl | 242 |
| C=O | 805 |
Requires energy to separate atoms against attractive forces. Bond breaking is endothermic.
Releases energy when new bonds are formed. Bond formation is exothermic.
ΔH = Energy to break bonds - Energy from bond formation
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
Breaking 4 C–H bonds in methane:
4 × 413 kJ = 1,652 kJ
Breaking 2 O=O bonds in oxygen:
2 × 498 kJ = 996 kJ
Total energy required:
(1,652 + 996) kJ = 2,648 kJ
Making 2 C=O bonds in CO₂:
2 × 805 kJ = 1,610 kJ
Making 4 O–H bonds in water:
4 × 464 kJ = 1,856 kJ
Total energy released:
(1,610 + 1,856) kJ = 3,466 kJ
ΔH = Energy to break bonds - Energy from bond formation
ΔH = 2,648 - 3,466 = -818 kJ
✅ The reaction is EXOTHERMIC (ΔH is negative)
Releases heat
ΔH negative
Temperature increases
Absorbs heat
ΔH positive
Temperature decreases
Requires energy
Endothermic process
Releases energy
Exothermic process