Bond energy: why reactions change heat
Atoms in a molecule are held by bonds. To pull two bonded atoms apart you must put energy in. When atoms join, energy comes out. The energy for one mole of a bond is called its bond energy (kJ/mol).
A reaction does both: break the bonds of the reactants (energy in), then make the bonds of the products (energy out).
ΔH = energy to break bonds − energy released making bonds
Example, H₂ + Cl₂ → 2HCl. Break H–H (436) and Cl–Cl (243) = 679 kJ in. Make 2 H–Cl (2 × 431) = 862 kJ out. ΔH = 679 − 862 = −183 kJ per mole of reaction. The minus sign means energy is released.
Exothermic and endothermic reactions
| Exothermic | Endothermic | |
|---|---|---|
| Heat | Released to surroundings | Taken from surroundings |
| Thermometer | Rises | Falls |
| Products vs reactants | Products have less energy | Products have more energy |
| ΔH | Negative | Positive |
| Examples | Burning fuel, rusting, acid + alkali | NH₄NO₃ in water, baking soda + citric acid, photosynthesis |
The total energy is never lost: it only moves between the substances and the surroundings.
Heat from a direct reaction: zinc in copper sulfate
Zn + CuSO₄ → ZnSO₄ + Cu. Zinc atoms push two electrons straight to copper ions. Red-brown copper builds up on the zinc and the beaker warms up. This is a redox reaction: electrons move from one substance to another.
Because the electrons jump over a tiny gap, all the energy appears as heat. We cannot use that energy to run a bulb.
Electricity from a reaction: the galvanic cell
If the two half-reactions are kept apart and the electrons must go through a wire, the energy comes out as electric current. That is a galvanic (voltaic) cell.
- Anode (−), zinc: Zn → Zn²⁺ + 2e⁻ (oxidation: zinc loses electrons).
- Cathode (+), copper: Cu²⁺ + 2e⁻ → Cu (reduction: copper ions gain electrons).
- Wire: electrons flow from Zn to Cu, so current is said to flow from Cu to Zn.
- Salt bridge: lets ions move so each beaker stays neutral. Without it the flow stops.
Voltage grows with the gap in reactivity between the two metals: Zn–Cu gives about 1.1 V, Mg–Cu about 2.7 V, Fe–Cu about 0.8 V. Two plates of the same metal give 0 V.
Batteries
A battery is cells joined in series (one after another). Their voltages add. Three 1.1 V cells give 3.3 V.
- Primary cell (dry cell in a torch): reaction cannot be reversed, use once.
- Secondary cell (lead-acid car battery, lithium-ion phone battery): an outside current can run the reaction backwards, so it can be recharged.
- Fuel cell: hydrogen and oxygen are fed in all the time and make water and electricity (2H₂ + O₂ → 2H₂O). It does not go flat while fuel is supplied.
Used batteries hold metals that harm the soil. Return them to collection bins and never throw them in a fire.
Try it: predict, then check
Before step 6 predict the voltage of 4 Zn–Cu cells in series (answer: 4.4 V), then check with the slider. At home with an adult: push a copper coin and a zinc-coated (galvanised) nail into a lemon, 2 cm apart, and touch both to your tongue. A faint tingle shows the cell. For endothermic feel: stir a spoon of baking soda in a little vinegar and hold the glass. It gets cold.
Key formulas and definitions
- ΔH = (energy to break bonds) − (energy released making bonds)
- Exothermic: ΔH < 0 (heat out). Endothermic: ΔH > 0 (heat in)
- Zn → Zn²⁺ + 2e⁻ (anode, oxidation); Cu²⁺ + 2e⁻ → Cu (cathode, reduction)
- Cells in series: V total = V of one cell × number of cells
- Fuel cell: 2H₂ + O₂ → 2H₂O + electricity
Worked examples
1. Bond energies: H–H 436, Cl–Cl 243, H–Cl 431 kJ/mol. Find ΔH for H₂ + Cl₂ → 2HCl.
In: 436 + 243 = 679 kJ. Out: 2 × 431 = 862 kJ. ΔH = 679 − 862 = −183 kJ. Exothermic.
2. N≡N is 945, H–H is 436 and N–H is 391 kJ/mol. Find ΔH for N₂ + 3H₂ → 2NH₃.
In: 945 + 3 × 436 = 2253 kJ. Out: 6 × 391 = 2346 kJ. ΔH = 2253 − 2346 = −93 kJ. Exothermic.
3. What is ΔH for 2HCl → H₂ + Cl₂, the reverse of the first reaction?
The sign flips: ΔH = +183 kJ. Energy must be put in, so it is endothermic.
4. A battery has 4 Zn–Cu cells in series (1.1 V each). What is its voltage?
V = 4 × 1.1 = 4.4 V.
5. In a Zn–Cu cell, which way do electrons flow in the wire, and which electrode loses mass?
Electrons flow from zinc to copper. Zinc (the anode) loses mass because it turns into Zn²⁺ ions, and copper gains mass as Cu is deposited.
Common mistakes
- Swapping the sign: energy released means ΔH is negative, not positive.
- Using energy of products minus reactants when using bond energies. With bond energies it is broken minus made.
- Saying electrons flow through the salt bridge. Electrons use the wire; ions use the salt bridge.
- Adding voltages of cells in parallel. Only series cells add up.