![]() If ΔG is positive, then the reaction isn't feasible - it can't happen.įor a reaction to be feasible, ΔG has to be negative. Whether or not a reaction (or other physical change) is feasible depends on the sign of ΔG. I am dropping the "standard" symbol after ΔG from now on, because most of the time we shall be using it at non-standard temperatures. Throughout this topic I shall use the word "feasible", but if your examiners expect you to use the word "spontaneous", that is what you should do. The energetics are right for a reaction to happen, but there is a huge activation energy.Ĭhemistry counts the reaction between carbon and oxygen as spontaneous! Personally, I think that is daft, and I prefer the word "feasible", which is often used in this topic. It is a spontaneous change.īut in chemistry, a spontaneous change doesn't have to be rapid in fact, it can be very, very, very slow indeed - even infinitely slow!įor example, carbon burns in oxygen to make carbon dioxide, but a piece of carbon will stay totally unchanged however long you keep it unless you first heat it. ![]() You don't need to do anything else - the reaction happens entirely of its own accord. If you drop marble chips (calcium carbonate) into dilute hydrochloric acid, there is an immediate fizzing. The same thing is true in chemistry, but there is one major difference which defies everyday common sense. In everyday life, something is spontaneous if it happens of its own accord, without any input from outside. It is easy as long as you remember to convert the entropy change value into kJ. So if you had to calculate the Gibbs free energy change at, say, 298 K, you can just slot the numbers in: But you should, of course, know how to calculate this from enthalpy changes of formation. This reaction is actually the combustion of methane, and so we can just take a value of this from a Data Book. On an earlier page in this section, we calculated the entropy change for the reactionīefore you do anything else, convert this to kJ by dividing by 1000. ![]() You must remember to change the entropy change value into kJ before you start, otherwise you will get the calculation completely wrong. There is an easy mistake to be made though! You have to remember that the entropy change is calculated in energy units of joules, but ΔG° and ΔH° are both measured in kJ.īecause you are calculating ΔG, your answer will be in kJ mol -1. If you know (or can work out) the enthalpy change for the reaction, and you know (or can work out) the entropy change, and you know the temperature (in kelvin), then it would seem to be really easy to work out ΔG°. This is how standard Gibbs free energy change is calculated: If you have already read the page about how to do this with total entropy changes, you will find a little bit of repetition on this page. This page introduces Gibbs free energy (often just called free energy), and shows how it can be used to predict the feasibility of reactions. ![]()
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