R1.4.3 – 
ΔG and spontaneity
📌 Predicting feasibility
- When carrying out calculations, we know that the value of T is always positive
- Using the Gibbs formula, we can also find the temperature at which any reaction will become spontaneous
ΔG = ΔH – TΔS system
We know a negative G gives a spontaneous reaction
Therefore when ΔH – TΔS system < 0
which then gives ΔH < TΔS system
Using this we can then calculate the temperature at which a particular reaction will be spontaneous
📌 Simplified understanding
| ΔH° | ΔS° | T | ΔG | Spontaneity |
|---|---|---|---|---|
| positive (endothermic) | positive | low | positive ≈ ΔH° | not spontaneous |
| positive (endothermic) | positive | high | negative ≈ –TΔS° | spontaneous |
| positive (endothermic) | negative | low | positive ≈ ΔH° | not spontaneous |
| positive (endothermic) | negative | high | positive ≈ –TΔS° | not spontaneous |
| negative (exothermic) | positive | low | negative ≈ ΔH° | spontaneous |
| negative (exothermic) | positive | high | negative ≈ –TΔS° | spontaneous |
| negative (exothermic) | negative | low | negative ≈ ΔH° | spontaneous |
| negative (exothermic) | negative | high | positive ≈ –TΔS° | not spontaneous |
- This table outlines the different possibilities for reactions and how we can make reasonable guesses as to whether or not they can occur spontaneously