90 total
By the end of these notes, you will be able to:
You have already learned about two important ideas:
You might think you could use just one of these — say, entropy change — to decide whether a reaction will happen. But that turns out to be inaccurate. A reaction's tendency to occur depends on both the enthalpy change and the entropy change together.
Gibbs free energy (G) is a value that combines both ΔH and ΔS into a single number. It tells you whether a reaction will happen on its own (is feasible) or not.
💡 Feasible simply means "likely to occur" or "able to proceed on its own."
The Gibbs equation is:
\boxed{ΔG^⦵ = ΔH^⦵ - TΔS^⦵}
Here is what each symbol means:
| Symbol | What it stands for | Units |
|---|---|---|
| ΔG⦵ | Gibbs free energy change | kJ mol⁻¹ |
| ΔH⦵ | Standard enthalpy change of the reaction | kJ mol⁻¹ |
| T | Temperature | K (Kelvin) |
| ΔS⦵ | Standard entropy change of the system | J K⁻¹ mol⁻¹ |
⚠️ Critical warning about units: ΔH is in kJ mol⁻¹, but ΔS is in J K⁻¹ mol⁻¹. These units do not match. Before you plug numbers into the equation, you must convert ΔS from J K⁻¹ mol⁻¹ into kJ K⁻¹ mol⁻¹ by dividing by 1000. This is the most common mistake students make.
Follow these steps every time:
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