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Both the processes of ice melting at 10 oC and water freezing at -10 oC are spontaneous reactions they can occur.However, ice will melt at this temperature, as the entropy increases : At the higher temperature, the reaction is not feasible as the entropy of the reaction is decreasing.The feasibility of the reaction can then be determined at 10 oC:.The overall entropy increases, and so the reaction is feasible (in thermodynamic terms, the reaction would be described as being spontaneous however, this does not in any way describe how fast the reaction occurs).Using this equation for the freezing of water at -10 oC for example:.
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The total entropy change can then be used to predict whether a reaction is feasible or not at a given temperature.This relationship allows us to make a prediction about the entropy of the surroundings of a chemical process, whatever they are (even the whole universe!), using the measurements we can make on the chemical system.The amount by which the entropy of the surroundings has increased can be determined using the following principle: the entropy of the surroundings increases by an amount equal to the heat energy they gain divided by the temperature at which it happens, therefore:.Therefore, as the surroundings get hotter, they are gaining more energy and thus the entropy of the surroundings is increasing.Freezing is an exothermic process energy is lost from the water and dissipated to the surroundings.The following reaction shows the change in state from water to ice:.It is possible to predict what will happen by taking into account the entropy of the surroundings, in addition to the energy of the system.But ice has a lower energy than water and so energy favours freezing.Water has a greater entropy than ice and so entropy favours melting.Why does water freeze at temperatures below 0 oC? Entropy and the Second Law of Thermodynamics Entropy and the Second Law of Thermodynamics Entropy of the Surroundings