What a learner can do afterwards
- Ranks solid, liquid and gas by entropy and explains the order in terms of arrangements
- Predicts the sign of the entropy change for a reaction from the states and the number of particles
- Calculates an entropy change from standard entropy values
- Explains why ammonium nitrate dissolving cools the water and still happens on its own
1 · Read
Entropy, S, measures how spread out energy and particles are. More arrangements mean higher entropy. For one substance, gas beats liquid and liquid beats solid, because particles gain freedom at each step.
To predict the sign, count gas moles first. Making more gas, or turning solids and liquids into gas, raises entropy. Reactions that consume gas to make solids lower it.
Dissolving ammonium nitrate cools the water, so it is endothermic, yet it happens alone. The ions spreading through the solution give a big entropy gain that outweighs the unfavourable enthalpy.
To calculate, add the standard entropies of the products and subtract the reactants, scaling each by its moles. Units are joules per mole per kelvin, so divide by 1000 before mixing with enthalpy in kilojoules.
Rank the states, count gas moles for the sign, and let spreading explain why some endothermic changes run alone.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.