Before you start

Physical chemistry asks how much energy a reaction gives or takes, whether it will go, how far, and how fast. This lesson recaps what it uses from General chemistry and the maths, so you can start here.

A mole is 6.022 × 10²³ particles; molar mass is grams per mole. A balanced equation like N₂ + 3H₂ → 2NH₃ gives mole ratios. Concentration in mol/L is written in square brackets: [NH₃]. Temperature in these formulas is always in kelvin: K = °C + 273.15. R, the gas constant, is 8.314 J/(mol·K).

Energy is measured in joules (J); 1 kJ = 1000 J. A watt is one joule per second. Food calories are kilocalories: 1 kcal = 4.184 kJ.

Powers of ten: 10³ = 1000 and 10⁻³ = 0.001. The logarithm undoes a power: log₁₀(1000) = 3. The number e ≈ 2.718 turns up whenever something changes in proportion to how much there is, and the natural log, ln, undoes it: ln(eˣ) = x. Two handy values: ln 2 ≈ 0.693 and ln 10 ≈ 2.303. Any calculator has ln and eˣ keys.

The Greek letter Δ (delta) means "change in": ΔT is the final temperature minus the starting one. A positive Δ means it went up.