1. ENERGETICS Δ G = Δ H - T Δ S

3. 15.3 Entropy (1.5h) 15.3.1 State and explain the factors that increase the entropy of a system 15.3.2 Predict whether the entropy change (  S) for a given reaction or process is positive or negative.15.3.3 Calculate the standard entropy change for a reaction (  S) using standard entropy values ( S Θ ) 15.4 Spontaneity (2.5h) 15.4.1 Predict whether a reaction or process will be spontaneous by using the sign of ∆ G Θ . 15.4.2 Calculate ∆ G Θ for a reaction using the equation ∆ G Θ = ∆ H Θ  T ∆ S Θ or by using values of the standard free energy change of formation, ∆ G f Θ 15.4.3 Predict the effect of a change in temperature on the spontaneity of a reaction using standard entropy and enthalpy changes and the equation ∆ G Θ = ∆ H Θ  T ∆ S Θ .

5. What is Entropy Δ S ? Entropy may be defined as : ‘ The trend in a chemical reaction is from order to disorder ’ ‘ order’ means few arrangements of energy ‘ disorder’ means many arrangements of energy For the states of matter Solids (least entropy) < liquids < gases

11. When you carry out this reaction in the lab, is it spontaneous? Look up the standard enthalpy change of combustion for methane and use it to calculate the standard Gibb’s Free Energy for this reaction using the mathematical equation After a class discussion, write down how you would calculate the standard Gibb’s free energy change of the combustion of methane using the standard Gibbs’ free energies of formation of the chemicals involved. Δ G = Δ H - T Δ S

14. Δ G = Δ H - T Δ S

15. A REAL QUESTION! (a) Explain in terms of Δ G Θ , why a reaction for which both Δ H Θ and positive is sometimes spontaneous and sometimes not. [4] (b) Consider the following reaction. N 2 (g) 3H 2 (g) -> 2NH 3 (g) (i) Using the average bond enthalpy values in Table 10 of the Data Booklet, calculate the standard enthalpy change for this reaction. [3] (ii) The absolute entropy values, S , at 300 K for are 193, 131 and 192 J K −1 mol −1 respectively for N 2 (g), H 2 (g) and NH 3 (g). Calculate for ΔS Θ the reaction and explain the sign of . Δ S Θ [2] (iii) Calculate Δ G Θ for the reaction at 300 K. [1]

16. QUESTION CONTINUED (iv) If the ammonia were produced as a liquid and not as a gas, state and explain the effect this would have on the value of Δ HΘ for the reaction. [5] (c) Define the term standard enthalpy of formation , and write the equation for the standard enthalpy of formation of ethanol. [2] (d) Bond enthalpies are tabulated as average bond enthalpies . Explain what this term means. [4] (e) Enthalpies of reactions, for example combustion, can be calculated using average bond enthalpies or enthalpies of formation. The two methods give closer results for cyclohexane than they do for benzene. Explain this difference. [1]