ENTROPY

Spontaneous reactions Many spontaneous chemical reaction are exothermic e.g. burning methane to produce carbon dioxide and water Some endothermic reactions are also spontaneous e.g. dissolving ammonium nitrate in water or reaction baking soda with vinegar It is not sufficient to use the concept of energy as the sole judge of whether a reaction is spontaneous or not.

What else is there? Enter the concept of entropy! The entropy of a system is the measure of the number of ways that energy is distributed throughout the system or the extent of disorder of energy or matter The second law of thermodynamics states that “in any spontaneous reaction the entropy of the universe will increase”

Factors that affect entropy Entropy if affected by the physical state of a substance – In solids there is a high degree of order – In liquids the degree of order is reduced as the particles have more freedom to move – In gases the particles are free to move randomly and so this state has the highest entropy (high disorder)

Factors that affect entropy In gases, entropy increases with increasing volume – the particles have more space to move around in and this increases the number of possible positions where they can be found. Entropy is affected by temperature – increasing temperature increases the entropy as the particles move around faster and the randomness of their movement increases The number of particles in a system (number of moles) will affect the entropy as the more particles there are the more ways of arranging them there are.

Entropy change in chemical systems Entropy is given the symbol S. The standard entropy change for a chemical reaction is defined as the “difference in standard entropy between the reactants and products” ΔS o = ΣS products - ΣS reactants The units for entropy are Jmol -1 o C -1 Spontaneous reactions proceed towards maximum entropy i.e. ΔS o > 0

HSoo HSoo

How to predict spontaneity in a reaction The spontaneity of a reaction can be determined by considering a combination of the enthalpy and entropy of the reaction. When considering entropy, the temperature (in degrees Kelvin) at which the reaction is carried out, must be considered. For a reaction to be spontaneous ΔH o – TΔS o must be negative. ΔH o – TΔS o < 0 or ΔH o < TΔS o

How to predict spontaneity in a reaction There are 4 possible scenarios for combining enthalpy and entropy to predict spontaneity, remembering that for a reaction to be spontaneous ΔH o – TΔS o must be negative. ΔH o < 0 the reaction is exothermic ΔH o > 0 the reaction is endothermic In groups see if you can work out what the 4 scenarios are.

How well did you do? ΔS o > 0 ΔH o < 0 when this is the case ΔH o – TΔS o will always be negative – such reactions are always spontaneous ΔS o > 0 ΔH o > 0 in this case the reaction will only be spontaneous if ΔS o, at the temperature of the reaction, is large enough to compensate for the positive enthalpy value TΔS o > 0 ΔH o < 0 ΔH o > 0 TΔS o > 0

How well did you do? ΔS o 0 when this is the case a reaction will not be spontaneous. No matter what the temperature ΔH o will always be more than TΔS o ΔS o < 0 ΔH o < 0 in this case the reaction will only be spontaneous if ΔS o, at the temperature of the reaction, is small enough not to cancel out the negative enthalpy value When ΔH o = TΔS o the reaction is at equilibrium ΔH o > 0 TΔS o < 0 ΔH o < 0 TΔS o < 0