Topic 12 · page 1 of 17
Enthalpy is not enough
Some reactions take in heat and still happen by themselves. So the sign of ΔH cannot decide whether a reaction occurs, and we need a second quantity.
Endothermic, yet it happens 12.1
In Topic 6 you met exothermic reactions (ΔH negative) and endothermic reactions (ΔH positive). It is tempting to think that reactions simply roll downhill in energy, so only exothermic ones can happen. Many exothermic reactions do happen. But the rule fails.
Stir solid barium hydroxide octahydrate with solid ammonium chloride in a beaker standing on a wet wooden block. No heating is needed:
The two solids turn into a liquid, you smell ammonia, and the temperature falls so far below 0 °C that the water under the beaker freezes and the beaker sticks to the block. The reaction has taken heat from its surroundings, so ΔH is positive. It happened at room temperature, by itself.
Everyday examples point the same way:
- Instant cold packs contain ammonium nitrate. Dissolving it in water is endothermic, with ΔH = +25.7 kJ mol⁻¹, and it needs no energy supply to start.
- Water left in an open dish evaporates at room temperature. Evaporation is endothermic (about +44 kJ mol⁻¹).
What "feasible" means
Two words are used for the same idea. A change is feasible or spontaneous if it can take place on its own.
What is missing
Look again at the barium hydroxide reaction. Three moles of solid turn into ten moles of liquid water, two moles of ammonia gas and dissolved barium chloride. The particles end up far more free to move and spread out. Something other than energy is changing: how spread out the particles and the energy are.
That quantity is called entropy. The pages that follow build it in steps.
| Pages | Question they answer |
|---|---|
| 2 to 4 | What is entropy, and when does it go up or down? |
| 5 and 6 | How do you calculate entropy changes, for the chemicals and for their surroundings? |
| 7 and 8 | How do these combine, and how does temperature change the answer? |
| 9 | Feasible is not the same as fast. |
Check yourself
- A student says: "A reaction can only happen by itself if it gives out heat." Give evidence that the statement is wrong.
- What is meant by a feasible (spontaneous) change?
- When barium hydroxide octahydrate and ammonium chloride are mixed, the temperature falls. State the sign of ΔH and say how the temperature change shows it.
- Suggest what kind of change happens to the particles in the barium hydroxide reaction that could make it go even though it is endothermic.
Answers cover these until you have tried the questions
- Some endothermic changes happen spontaneously at room temperature, for example ammonium nitrate dissolving in water (ΔH = +25.7 kJ mol⁻¹) or barium hydroxide octahydrate reacting with ammonium chloride. They take in heat, yet they occur without any continuous energy input. So ΔH alone does not decide whether a change occurs.
- A change that can occur on its own, without a continuous supply of energy from outside. It does not tell you how fast it happens.
- ΔH is positive (endothermic). The temperature falls because the reaction takes in heat energy from the surroundings, which cools them.
- The particles become far more free to move and spread out: 3 mol of solid turn into liquid water and ammonia gas. The disorder (entropy) of the chemicals increases, which can drive a reaction even when it is endothermic.