← Thermochemical principles and the properties of particles and substances

1 Reading the notation

Thermochemistry is almost entirely symbols. Each one below appears on this page. None is difficult, but none is obvious either.

📖 Symbols used in thermochemistry

8 terms

Cover the right column and check you can say what each one means.

Δ (capital delta)

Means “change in”. ΔH is the change in enthalpy, ΔS the change in entropy, ΔG the change in free energy. Always final minus initial. A negative Δ means the quantity went DOWN.

H (enthalpy)

Heat content at constant pressure. You never measure H itself, only the CHANGE, ΔH. ΔH negative = exothermic (heat released); positive = endothermic (heat absorbed).

S (entropy)

A measure of how many ways the energy and particles of a system can be arranged: loosely, disorder. Units J K⁻¹ mol⁻¹. Note JOULES, while ΔH is in kilojoules. Mismatched units are the commonest error in ΔG calculations.

G (Gibbs free energy)

The quantity that decides spontaneity: ΔG = ΔH − TΔS. Negative ΔG means spontaneous.

° (the degree symbol, e.g. ΔH°)

Means standard conditions: 100 kPa, a stated temperature (usually 298 K), all solutions 1 mol L⁻¹. A superscript circle. Not the same as °C, which is a temperature unit.

Subscripts f, c, r

f = formation (from elements), c = combustion (burned in oxygen), r = reaction (the overall reaction).

Σ (capital sigma)

Greek capital S, meaning “the sum of”. Σ ΔH_f°(products) means add up every product’s formation enthalpy.

T

Absolute temperature in kelvin (K), not °C. K = °C + 273. TΔS is meaningless in Celsius. The equation requires kelvin.

✓

Where the small letters go

A subscript after ΔH says WHICH enthalpy change (f = formation, c = combustion). A superscript ° says under what CONDITIONS. So ΔH_f° reads: “standard enthalpy change of formation”.

Practice for this standard covers the whole topic — flashcards and quiz.