predicting entropy change from an equation — HSC chemistry year 11, live

Predicting entropy change from an equation

Entropy (S) measures how spread out the particles and energy in a system are. Gas particles fill their whole container, so gases have far more entropy than liquids, and liquids more than solids. That means you can predict the sign of an entropy change straight from a balanced equation with state symbols: count the moles of gas on each side, then check for changes of state. This is the real knowscape from knowhere, not a picture of one. Drag it. Watch what actually changes.

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Spontaneity works like a winter sleepover.

Every reaction is a start, a finish and a hill in between. The energy change depends only on start and finish, which is why Hess's Law works and a catalyst can't touch it. The hill is all route: it sets how hard a reaction is to get going, not where it ends up.

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The house warms. Leaving the huddle still costs warmth, but a warmer house makes spreading out count for more.

Warm enough and spreading wins: people scatter through the house. Cool it again and they huddle back.

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Entropy is a measure of how spread out, or disordered, particles and their energy are: gas is much higher than liquid, and liquid is higher than solid.

To predict ΔS from a balanced equation, count only the moles of gas on each side: more gas in the products means ΔS > 0, fewer means ΔS < 0.

With no gas involved, read the states: solid to liquid to gas increases entropy, and an ionic solid dissolving into aqueous ions usually increases it.

When the gas moles and states are the same on both sides, the entropy change is small and its sign cannot be safely predicted from the equation.

A particle model of the container before and after the reaction shows why: products that spread through more space have more possible arrangements.

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this conceptpredicting entropy change from an equationA closed container of particles for six balanced equations.

sits under itbalancing equations atom by atombecause You count gas moles from the coefficients, so the equation has to be balanced first.

sits under itdissolution of ionic compoundsbecause Knowing an ionic solid breaks into spread-out aqueous ions explains why dissolving raises entropy.

they don't read it. they run it.

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Spontaneity works like a winter sleepover.

five things, not forty.

nothing here is a standalone fact.

1 more, same treatment.

they don't read it. they run it.