In the fourth case, when water evaporates, the association of water molecules is replaced by individual water vapor molecules moving independently in vapor phase with much higher kinetic energy. In the third case, when ice melts, the regular hydrogen bonded structure of ice is replaced by less associated fluid which is water. In the second case, when potassium chloride dissolves, the regular arrangement of the crystal structure is replaced by random distribution of mobile ions in solution. When there is combustion of methane and the reaction generates heat there is significant increase of kinetic energy of the product molecules of carbon dioxide and steam, which makes the individual gas molecules moving rapidly in all directions. If we look at all four reactions above, there is a common factor. Question: What makes a reaction spontaneous? Therefore, any enthalpy change in a particular direction, positive or negative, cannot be the only factor to decide if a chemical reaction would take place? There must be some additional factors. We saw in the above reactions that both exothermic and endothermic reactions could be spontaneous. Essentially, it depends on the difference in enthalpy between the products and reactants of a reaction. H2O(s) - > H2O(l), ΔcHo = 6.0 kj / moleĪbove examples show, both exothermic and endothermic reactions could be spontaneous.Įnthalpy (H) is a measure of how much energy is released or absorbed during a chemical reaction. KCl(s) + aq - > KCl(aq), ΔcHo = 19 kj / mole Similarly there are spontaneous reactions which are endothermic, like It is an exothermic reaction and it is a spontaneous reaction. When ΔG is negative, the reaction would go in the direction of reactants forming products.Ī reaction is called spontaneous when it takes place of its own accord without any external help.ĬH4(g) + 2 O2(g) - > CO2(g) + 2 H2O(g), ΔcHo = −891 kj / mole A spontaneous reaction may involve an increase or decrease in enthalpy, it may involve an increase or decrease in entropy, but it will always involve a decrease in free energy that is a negative ΔG. All spontaneous physical and chemical changes take place in the direction of a decrease in free energy.
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