Entropy, S By C Hughes

From the specification: understand that ∆H, whilst important, is not sufficient to explain spontaneous change (e.g. spontaneous endothermic reactions). understand that the concept of increasing disorder (entropy change ∆S) accounts for the above deficiency, illustrated by physical change (e.g. melting, evaporation) and chemical change (e.g. dissolution, evolution of CO2 from hydrogencarbonates with acid). understand that the balance between entropy and enthalpy determines the feasibility of a reaction; know that this is given by the relationship ∆G = ∆H. T∆S (derivation not required). be able to calculate entropy changes from absolute entropy values.

Entropy is a quantitative measurement of the disorder, or randomness, of the substances involved in a reaction. Eg, If Sodium chloride, which has a solid regular lattice is dissolved in water it becomes more disordered. This means that the entropy has increased. NaCl (s) + H 2 O  NaCl (aq)

Another example is ice melting. It becomes more disordered because liquids are more randomly distributed than solids. Therefore ENTROPY has increased H 2 O (s)  H 2 O (l) When water evaporates to become water vapour it becomes more disordered as gas particles are more randomly distributed than liquids and move quicker. Therefore ENTROPY has increased. H 2 O (l)  H 2 O (g)

This can be shown in the following diagram: SOLID MELTING LIQUID BOILING GAS ENTROPY

The symbol for entropy is S. The units are JK -1 mol -1 S values are always at standard temperature and pressure. The reason we talk about entropy is because some reactions don’t simply happen because they are heated and the only energy change that occurs in a system reacting is not such heat (enthalpy). Some reactions occur spontaneously

A spontaneous occurance might be a ball rolling down a hill. You don’t have to apply energy for it to happen. Another example is the endothermic reaction of ammonium nitrate dissolving in water. Without any heat being applied the reaction occurs and take in heat from the surroundings. NH 4 NO 3(s) + H 2 O (l)  NH 4 + (aq) + NO 3 - (aq) The highly ordered solid crystal has been dissolved in water resulting in a large increase in entropy.

Entropy values, unlike enthalpy can not be directly measured. But because it is possible to determine the actual entropy for many substances  S (entropy change) for a reaction can be determined using  S = ΣS PRODUCTS - Σ S REACTANTS

When S products > S reactants, the change in entropy is positive. Insuch reactions the entropy increases and the products have ore disorder than the reactants. Hen S products < S reactants, the change in entropy is negative. An example of this would be condensation or freezing

You have used the same kind of equation when determining enthalpy so we will go straight into some questions. Li 2 CO 3(s) Li 2 O (s) H 2 S (g) CO 2(g) S (s) SO 2(g) H 2 O (g) S/ JK -1 mol )Li 2 CO 3(S)  Li 2 O (s) + CO 2(g) Determine  S 2)SO 2(g) + 2H 2 S (g)  3S (S) + 2H 2 O (g) Determine  S

Complete the following table for the reactions listed on the next slide ExptIs ΔH + or -Entropy of reactants & products Surroundings 1 2 3

1) Place 5cm 3 water in a test tube, add spatula of sodium chloride, record temperature change. 2) Put 4cm 3 of ethanoic acid in a test tube, add 2.5g of solid ammonium carbonate and measure temp change. 3) Take a small length of Magnesium ribbon and clean it thoroughly with emery paper. Hold the ribbon in some tongs and observe if there is any sogn of reaction without heat. How hold in bunsen. Observe what happens. Write a balanced equation for each reaction

Temperature and entropy. It makes sense that entropy must be related to temperature because we know that as temperature increases particles get more energy to move more randomly and a change of state happens. They can be connected via the following equation: Entropy change,  S = Heat energy transferred Temperature, K ie,  S =  H T

Entropy leads to the second law of thermodynamics – that all chemical and physical changes results in an increase in entropy. All spontaneous reactions must results in an increase in S.