What are some common mistakes students make with non-spontaneous processes?

1 Answer
Jul 4, 2014

First mistake is to think these transformations are impossible
Second mistake is to think that every process that is hindered is non-spontaneous.
Third mistake is to think that endothermic processes are non-spontaneous.

A non-spontaneous or endoergonic process is a process that can't occur by itself, without any external driving force.

But it is possible (first mistake) with external intervention (energy inputs, or coupling with other processes). For example, water decomposition is a non-spontaneous process. It can't occur without an external input of energy (very high temperature or electrical forces, as in electrolysis)

I we take hydrogen and oxygen and mix these two gaseous substances in a closed vessel, we can even wait for a long long time, but we don't get any water formation. The reaction should be spontaneous, being the opposite of the non-spontaneous reaction of the previous example, but it doesn't start.

We shouldn't conclude that this reaction is non-spontaneous (second mistake), as we see from the next example, that is very similar.

If we take a piece of paper, it should react spontaneously with oxygen, but this doesn't happen. Why don't? Are these processes non-spontaneous? Not really.

These reactions are simply too much slow at room temperature to be visible, but if they are triggered with a little spark or flame, they start quickly, producing enough thermal energy to be self-sustained and actually getting along spontaneously.

Endothermic processes are not always non-spontaneous (third mistake). Sometimes they occur at in a close system, without any external input. you can try putting a fizzing powder into water, and see that water temperature decreases, being the process spontaneous, and averything occurs spontaneously, even if you close the reacting mixture in a bottle.

What is true is that most of exothermic processes are also spontaneous.