The complete lecture — energy changes come alive in the live panel as you read. Scroll down; the animation keeps pace, and you can run a calorimetry experiment yourself.
1 — System & surroundings
| System | Exchanges |
|---|
| open | energy + matter |
| closed | energy only |
| isolated | neither |
2 — Internal energy, enthalpy & state functions
- State function — depends only on the present state, not the path (U, H, T, P).
3 — Exothermic & endothermic reactions
| Exo | Endo |
|---|
| ΔH | negative | positive |
| temp | rises | falls |
4 — Enthalpy diagrams
The arrow from reactants to products is the enthalpy change ΔH.
5 — Standard enthalpy changes
| ΔH° | per mole when… |
|---|
| formation | a compound forms from elements |
| combustion | it burns in O₂ |
| neutralisation | water forms (acid+base, ≈ −57 kJ) |
6 — Hess's law
Adding stepsΔH(overall) = ΔH₁ + ΔH₂ + …
7 — Calorimetry
Heatq = m c ΔT · c(water) = 4.18 J g⁻¹ °C⁻¹
Drag the sliders in the live panel to compute the heat from a temperature change.
8 — Bond energy & ΔH from bonds
From bondsΔH = Σ(bonds broken) − Σ(bonds formed)
9 — First law of thermodynamics
Conservation of energyΔU = q + w
10 — Worked numericals (calorimetry)
heat
250 g water cools 8 °C: q = 250×4.18×8 = 8.36 kJ
11 — Applications & worked
Hess
A→B −100, B→C −50 ⇒ A→C = −150 kJ
bonds
broken 680, formed 800 ⇒ ΔH = −120 kJ (exo)
12 — Exam recap
- System/surroundings; open/closed/isolated.
- Internal energy, enthalpy, ΔH.
- Exo (−ΔH) vs endo (+ΔH); enthalpy diagrams.
- Standard enthalpy changes; Hess's law.
- Calorimetry q = mcΔT; ΔH from bonds.
- First law ΔU = q + w.