In fact, the reaction does not simply ‘flip’ between feasibility and non-feasibility (or going in one direction or the other). Δ S system may change with temperature. For example, if one of the reactants or products changes state (melts or boils) this will affect Δ S system. This is an approximation only, but for most reactions the change with temperature is small. Throughout these calculations, we have assumed that Δ H does not change with temperature. This explains why some reactions go in one direction at one temperature and in the opposite direction at a different temperature.įor example, let us look at the reaction between calcium oxide and carbon dioxide (considered in the tutorial on chemical reactions and direction):įor this reaction, Δ H is –178 kJ mol -1 (ie –178 000 J mol -1) and Δ S system is –161 J K -1 mol -1. Sometimes, changing the temperature can change the sign of Δ G. We can see from the expression for the Gibbs free energy that the value of Δ G depends on temperature: The temperature dependence of the Gibbs free energy It is also an energy term, which is a concept more familiar to most chemists than entropy. This is what makes this quantity so useful to chemists. Notice also that all the terms in the expression relate to the system rather than the surroundings. Notice that if Δ G is negative, the reaction is feasible. We call the term ‘– TΔ S total’ the Gibbs free energy, after the American chemist Josiah Willard Gibbs. So it is necessary to convert the units, usually by dividing the entropy values by 1000 so that they are measured in kJ K -1 mol -1. Chemists normally measure energy (both enthalpy and Gibbs free energy) in kJ mol -1 (kilojoules per mole) but measure entropy in J K -1 mol -1 (joules per kelvin per mole). We must take care when using mathematical expressions that include both energy and entropy. RSC Yusuf Hamied Inspirational Science Programme.Introductory maths for higher education.The physics of restoration and conservation.
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