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The system and the surroundings
Here is the solution to the puzzle about the ammonia / hydrogen chloride reaction. This reaction is strongly exothermic (gives out a lot of heat to the surroundings). (In fact ∆H is –176 kJ mol-1). The key is the idea of the surroundings. For each mole of ammonium chloride that is formed, 176 kJ of heat energy is transferred to the surroundings. As we have seen, this increases the entropy of the surroundings because of the increased number of ways of arranging the quanta of energy. So, within the reaction itself (ie starting materials and products), entropy decreases but, because of the heat energy passed to the surroundings, the entropy of the surroundings increases, and more than compensates for the decrease in entropy in the reaction. In other words this increase in the entropy of the surroundings is more than the decrease in entropy of the reaction and thus there is an overall increase in entropy.
We call the reaction itself ‘the system’ and everything else ‘the surroundings’. In principle, ‘the surroundings’ is literally the rest of the Universe, but in practice it is the area immediately around the reaction vessel.
So, the Second Law of Thermodynamics is not broken by the ammonia / hydrogen chloride reaction; the problem was that we had forgotten the surroundings. This is because, as chemists, we are used to concentrating only on what happens inside our reaction vessel.
We can make a better statement of the Second Law of Thermodynamics:
In a spontaneous change, the entropy of the Universe increases, ie the sum of the entropy of the system and the entropy of the surroundings increases.
| The system and the surroundings | |
