Entropy and Spontaneity (Chemistry)

Lesson notes

Overview

A reaction is spontaneous when ΔG = ΔH − TΔS is negative. This topic is rated advanced and is number 91 of 110 in Chemistry. Deeper models, formal results and harder reasoning.

What you are seeing

The panel above shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. The animation is drawn live from the model, so everything you see responds to the sliders. Press Space to pause, use Restart to replay from the start, and change the speed to slow down quick events.

In depth

Whether a reaction is spontaneous depends on both enthalpy and entropy, summed up by the Gibbs free energy, ΔG = ΔH − TΔS. A reaction proceeds spontaneously when ΔG is negative. Exothermic reactions that increase disorder always qualify. An endothermic reaction can still occur if the entropy increase is large and the temperature high enough, like ice melting at room temperature. ΔG says whether a reaction can occur, not how fast.

Reading the relationship

The expression "ΔG = ΔH − TΔS" is a compact summary of the idea. It uses the symbols Δ, G, H, T, S; name what each one stands for before you start. Then use the sliders to see how changing one quantity changes the others.

A guided exploration

1. Set every slider to its middle value and note the shape of the curve. 2. Move one slider at a time and describe whether the curve shifts, stretches or turns. 3. Find the value where the behaviour changes sharply, then connect it to the formula.

Try this

Try to explain why the pattern holds, not just what it is. Look for limiting cases and what happens at the edges.

Common pitfalls

Keep the particle picture in mind. Mistakes often come from treating a rate, an amount and an equilibrium position as the same thing, or from ignoring units and conditions.

How it connects

It builds on “Faraday’s Law of Electrolysis”, the topic before it in Chemistry. Once you are comfortable, continue with “Gibbs Energy and Equilibrium Constant”.

Why it matters

Chemistry links the tiny scale of atoms to the materials, medicines and energy we use every day.

Where you might use it

Revising “Entropy and Spontaneity” before a class, exam or certification
Predicting how fast a reaction runs in a lab or an industrial plant
Choosing materials, medicines and fuels based on how their atoms bond and react
Reading the labels and data sheets of cleaning products, batteries and food

Frequently asked questions

What is Entropy and Spontaneity?

A reaction is spontaneous when ΔG = ΔH − TΔS is negative. It is an advanced topic in Chemistry.

How do I use the Entropy and Spontaneity simulation?

The panel at the top shows a graph of the function. Each slider changes one parameter, so you can see exactly how the curve bends, shifts or stretches. Move the sliders one at a time, then press Space to pause the animation and look closer.

What does the formula mean?

The expression ΔG = ΔH − TΔS packs the idea into symbols. Name each symbol first, then change one quantity with the sliders and see which others respond.

Who is this topic for?

Deeper models, formal results and harder reasoning. If the quiz feels hard, reread the lesson notes and try the previous topic first.

What should I learn before this?

Start with “Faraday’s Law of Electrolysis”, the previous topic in Chemistry. Each topic in a subject builds on the ones before it.

How do I track my progress?

Press “Mark as learned” on this page. Progress is stored only in this browser and is never sent anywhere. The Browse page can show topics you have learned or still need to learn.