2. New Way Chemistry for Hong Kong A-Level Book 11 Chapter 6 Energetics 6.1 What is Energetics? 6.2 Ideal Enthalpy Changes Related to Breaking and Formation of Bonds 6.3 Standard Enthalpy Changes

3. New Way Chemistry for Hong Kong A-Level Book 12 6.4 Experimental Determination of Enthalpy Changes by Calorimetry 6.5 Hess’s Law 6.6 Calculations involving Enthalpy Changes of Reactions

4. New Way Chemistry for Hong Kong A-Level Book 13 What is Energetics? Energetics is the study of energy changes associated with chemical reactions. Thermochemistry is the study of heat changes associated with chemical reactions. Some terms Enthalpy(H) = heat content in a substance Enthalpy change(  H) = heat content of products - heat content of reactants = H p - H r 6.1 What is Energetics? (SB p.140)

5. New Way Chemistry for Hong Kong A-Level Book 14 Internal Energy and Enthalpy e.g. Zn(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 (g) Heat change at constant pressure = Change in internal energy + Work done on the surroundings (Heat change at constant volume) Enthalpy change 6.1 What is Energetics? (SB p.140)

6. New Way Chemistry for Hong Kong A-Level Book 15 Exothermic and Endothermic Reactions An exothermic reaction is a reaction that releases heat energy to the surroundings. (  H = -ve) 6.1 What is Energetics? (SB p.142)

7. New Way Chemistry for Hong Kong A-Level Book 16 Exothermic and Endothermic Reactions An endothermic reaction is a reaction that absorbs heat energy from the surroundings. (  H = +ve) 6.1 What is Energetics? (SB p.141)

8. New Way Chemistry for Hong Kong A-Level Book 17 Enthalpy Changes Related to Breaking and Forming of Bonds e.g. CH 4 + 2O 2  CO 2 + 2H 2 O 6.2 Enthalpy Changes Related to Breaking and Formation of Bonds? (SB p.143) In an exothermic reaction, the energy required in breaking the bonds in the reactants is less than the energy released in forming the bonds in the products (products contain stronger bonds).

9. New Way Chemistry for Hong Kong A-Level Book 18 Enthalpy Changes Related to Breaking and Forming of Bonds In an endothermic reaction, the energy required in breaking the bonds in the reactants is more than the energy released in forming the bonds in the products (reactants contain stronger bonds). 6.2 Enthalpy Changes Related to Breaking and Formation of Bonds? (SB p.143)

10. New Way Chemistry for Hong Kong A-Level Book 19 Bond Enthalpies To be discussed in later chapters. 6.2 Enthalpy Changes Related to Breaking and Formation of Bonds? (SB p.143) BondMean Bond Enthalpy (kJ mol -1 ) H – H C – C C≡C N – N N ═ N N≡ N 436 348 612 837 163 409 944

11. New Way Chemistry for Hong Kong A-Level Book 110 Standard Enthalpy Changes CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g)  H = -802 kJ mol -1 CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(l)  H = -890 kJ mol -1 As enthalpy changes depend on temperature and pressure, chemists find it convenient to report enthalpy changes based on an internationally agreed set standard conditions: 1. elements or compounds in their normal physical states; 2. a pressure of 1 atm (101325 Nm -2 ); and 3. a temperature of 25 0 C (298 K) 6.3 Standard Enthalpy Changes (SB p.144) Enthalpy change under standard conditions denoted by symbol:  H ø

12. New Way Chemistry for Hong Kong A-Level Book 111 Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Neutralization e.g. The standard enthalpy change of neutralization between HNO 3 and NaOH is -57.3 kJ mol -1 e.g. The standard enthalpy change of neutralization between HCl and NaOH is -57.1 kJ mol -1 6.3 Standard Enthalpy Changes (SB p.144) The standard enthalpy change of neutralization (  H neut ) is the enthalpy change when one mole of water is formed from the neutralization of an acid by an alkali under standard conditions. ø e.g. H + (aq) + OH - (aq)  H 2 O(l)  H neut = -57.3 kJ mol -1 ø

13. New Way Chemistry for Hong Kong A-Level Book 112 Standard Enthalpy Changes of neutralization H + (aq) + OH - (aq)  H 2 O(l) 6.3 Standard Enthalpy Changes (SB p.145) -57.1 -57.2 -52.2 -68.6 NaOH KOH NH 3 NaOH HCl HF  H neu AlkaliAcid ø

14. New Way Chemistry for Hong Kong A-Level Book 113 Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Solution Note that enthalpy changes of solution associate with physical changes. 6.3 Standard Enthalpy Changes (SB p.146) The standard enthalpy change of solution (  H soln ) is the enthalpy change when one mole of a solute is dissolved to form an infinitely dilute solution under standard conditions. ø e.g. NaCl(s) + water  Na + (aq)+Cl - (aq)  H soln =+3.9 kJ mol -1 ø e.g. LiCl(s) + water  Li + (aq) + Cl - (aq)  H soln =-37.2 kJ mol -1 ø

15. New Way Chemistry for Hong Kong A-Level Book 114 6.3 Standard Enthalpy Changes (SB p.146) Standard Enthalpy Changes of solution -44.7 +3.9 -57.8 +20.0 NaOH NaCl KOH KBr  H soln (kJ mol -1 ) Salt ø

16. New Way Chemistry for Hong Kong A-Level Book 115 Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Formation Standard enthalpy change of formation of NaCl is -411 kJ mol -1. 6.3 Standard Enthalpy Changes (SB p.147) The standard enthalpy change of formation (  H f ) is the enthalpy change of the reaction when one mole of the compound in its standard state is formed from its constituent elements under standard conditions. ø e.g. 2Na(s) + Cl 2 (g)  2NaCl(s)  H = -822 kJ mol -1 ø Na(s) + ½Cl 2 (g)  NaCl(s)  H f = -411 kJ mol -1 1 mole ø

17. New Way Chemistry for Hong Kong A-Level Book 116 Standard Enthalpy Changes of Reactions The enthalpy change of formation of an element is always zero. N 2 (g)  N 2 (g) 6.3 Standard Enthalpy Changes (SB p.147) What is  H f [N 2 (g)] ? ø  H f [N 2 (g)] = 0 ø

18. New Way Chemistry for Hong Kong A-Level Book 117 Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Combustion e.g. C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l)  H 1 = -2220 kJ 2C 3 H 8 (g) + 10O 2 (g)  6CO 2 (g) + 8H 2 O(l)  H 2 = ?  H 2 = -4440 kJ  It is more convenient to report enthalpy changes per mole of the main reactant reacted/product formed. 6.3 Standard Enthalpy Changes (SB p.149)

19. New Way Chemistry for Hong Kong A-Level Book 118 Standard Enthalpy Changes of Reactions Standard Enthalpy Change of Combustion The standard enthalpy change of combustion of propane is -2220 kJ mol -1 6.3 Standard Enthalpy Changes (SB p.149) The standard enthalpy change of combustion (  H c ) of a substance is the enthalpy change when one mole of the substance burns completely under standard conditions. ø e.g. C 3 H 8 (g) + 5O 2 (g)  3CO 2 (g) + 4H 2 O(l)  H c = -2220 kJ mol -1 1 mole ø

20. New Way Chemistry for Hong Kong A-Level Book 119 Standard Enthalpy Changes of Reactions 6.3 Standard Enthalpy Changes (SB p.150) -285.8 -395.4 -393.5 -283.0 -890.4 H 2 (g) C (diamond) C (graphite) CO(g) CH 4 (g)  Hc (kJ mol -1 ) Substance ø

21. New Way Chemistry for Hong Kong A-Level Book 120 Experimental Determination of Enthalpy Changes by Calorimetry Calorimeter = a container used for measuring the temperature change of solution Determination of Enthalpy Change of Neutralization 6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 151)

22. New Way Chemistry for Hong Kong A-Level Book 121 6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 151) Heat evolved = (m 1 c 1 + m 2 c 2 ) ΔT Where m 1 is the mass of the solution, m 2 is the mass of calorimeter, c 1 is the specific heat capacity of the solution, c 2 is the specific heat capacity of calorimeter, And Δ T is the temperature change of the reaction.

23. New Way Chemistry for Hong Kong A-Level Book 122 Determination of Enthalpy Change of Combustion 6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 153)

24. New Way Chemistry for Hong Kong A-Level Book 123 Heat evolved = (m 1 c 1 + m 2 c 2 ) ΔT Where m 1 is the mass of water in the calorimeter, m 2 is the mass of the calorimeter, c 1 is the specific heat capacity of the water, c 2 is the specific heat capacity of calorimeter, And Δ T is the temperature change of the reaction. 6.4 Experimental Determination of Enthalpy Changes by Calorimetry (p. 153)

25. New Way Chemistry for Hong Kong A-Level Book 124 Hess’s Law A + BC + D Route 1 H1H1 E H2H2 H3H3 Route 2  H 1 =  H 2 +  H 3 Hess’s Law states that the total enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction takes place. Why? 6.5 Hess’s Law (p. 157)

26. New Way Chemistry for Hong Kong A-Level Book 125 Importance of Hess’s Law the reactions cannot be performed in the laboratory the reaction rates are too slow the reactions may involve the formation of side products The enthalpy change of some chemical reactions cannot be determined directly because: But the enthalpy change of such reactions can be determined indirectly by applying Hess’s Law. 6.5 Hess’s Law (p. 158)

27. New Way Chemistry for Hong Kong A-Level Book 126 Enthalpy Change of Formation of CO(g) H2H2 + ½O 2 (g) CO 2 (g) H1H1 + ½O 2 (g) = -393.5 - (-283.0 ) = -110.5 kJ mol -1 6.5 Hess’s Law (p. 158) Given:  H f [CO 2 (g)] = -393.5 kJ mol -1 ;  H c [CO(g)] = -283.0 kJ mol -1 øø C(graphite) + ½O 2 (g)CO(g)  H f [CO(g)] ø  H f [CO(g)] +  H 2 =  H 1 ø  H f [CO(g)] =  H 1 -  H 2 ø

28. New Way Chemistry for Hong Kong A-Level Book 127 Enthalpy Change of Formation of CO(g) 6.5 Hess’s Law (p. 158)  H f CO(g)] +  H 2 =  H 1 ø

29. New Way Chemistry for Hong Kong A-Level Book 128 Enthalpy Change of Hydration of MgSO 4 (s) 6.5 Hess’s Law (p. 160) aq MgSO 4 (s) + 7H 2 O(l) MgSO 4 ·7H 2 O(s) Mg 2+ (aq) + SO 4 2- (aq) + 7H 2 O(l) ΔH ΔH 2 aq ΔH 1 ø ΔH = enthalpy of hydration of MgSO 4 (s) ΔH 1 = molar enthalpy change of solution of anhydrous magnesium sulphate(VI) ΔH 2 = molar enthalpy change of solution of magnesium sulphate(VI)-7-water ΔH = ΔH 1 - ΔH 2 ø ø

30. New Way Chemistry for Hong Kong A-Level Book 129 Calculations involving standard enthalpy changes of reactions reactants products  H reaction 6.6 Calculations involving Enthalphy Changes of Reactions (p. 161) elements -   H f [reactants] ø   H f [products] ø  H reaction =   H f [products] -   H f [reactants] øø

31. New Way Chemistry for Hong Kong A-Level Book 130 The END