Solving Thermochemistry Problems (∆H)

Steps - summary 1.Read Q + match number to subs. 2.Write & balance equation for each substance ---remember ∆H 3.Number each equation inside the margin (or in circles) 4.Write out the equation you “want” 5.Use simultaneous equations methods to solve problem 6.Double check that you have answered the Q asked and that you have one mole of “correct” substance. 7.Check signs and ∆H and UNITS

Steps – details 1.Read Q + match number to subs- and write in if it’s combustion or formation (or reaction) 2.Write equation for each substance &put in ∆ H kJ mol -1 a)if combustion – start with the subs + O 2  – hydrocarbons give CO 2 and H 2 O - elements gives “normal” oxide - CO 2 / H 2 O b)if formation – finish with the substance in Q -then make the substance from its ELEMENTS 3.Balance each equation – watch out for “strays” e.g. O or H in ethanol - check you have one mole of appropriate substance – Use fractions as needed 4.Number each equation inside the margin or put in circle 5.Write out the equation you “want” - again check you have one mole of appropriate substance Watch out for diatomic molecules H 2 etc

Steps – details 6.Now solve using simultaneous equation methods 7.Deal with each substance in turn -starting on left of “wanted” equation 8.If substance is in more than one equation -skip it and do next one -usually it solves itself -very often this applies to oxygen 9.Be neat  keep + under + and = under =. 10.Cancel on left and right of arrow – use pencil 11.Double check your maths and signs - reverse the equation – reverse the sign

Example Calculate the heat of formation of ethane given that the heat of combustion of ethane, hydrogen and carbon are kJ mol -1, -393 kJ mol -1, -286 kJ mol -1 respectively Steps 1. match subs + numbers ethane  combustion  hydrogen  comb  -393 carbon  comb  -286

H2 H2 + ½ O 2  H 2 O ∆H = -393 kJ mol Steps 2 Write and balance equations C2H6 C2H6 + O2 O2  CO 2 + H 2 O ∆H = kJ mol 233½3½ 7 oxygen C + O 2  CO 2 ∆H = -286 kJ mol 3. Number them Want 2 C + 3H23H2  C 2 H 6 ∆H = ? kJ mol solve 3 2x 2 C + 2O 2  2 CO 2 ∆H = -572 kJ mol 2 3x 3H2 3H2 + 1½O2 1½O2  3H 2 O ∆H = kJ mol 1 rev 2 CO H 2 O  C2H6 C2H6 + 3½ 3½ O 2 ∆H = kJ mol 2 C + 3H23H2  C 2 H 6 ∆H = -340 kJ mol -1 Double check – asked for heat of formation – one mole of ethane  6. Add

More hints 1.Leave a space after the + and the = when you are writing equations – room for balancing numbers 2.Do one step at a time – you can’t take short cuts or skip bits if you want to be sure to be correct but you should “see” the solution unfolding as you go along. 3.If you make a mistake don’t try to over write or scribble out bits – rub out or start again. 4.Turn page sideways if very long equations 5.Previous exam questions are given next – consult your exam papers if necessary 6.4 marks =1% so it is well worth your while to master this topic.

20028 (e) The combustion of butane is described by the following balanced equation. 2C 4 H 10(g) +13O 2(g) → 8CO 2(g) + 10H 2 O (l) Calculate the heat of combustion of butane given that the heats of formation of butane, carbon dioxide and water are –125, –394 and –286 kJ mol -1, respectively. (18) (a)Define heat of combustion. (7) Propane may be used in gas cylinders for cooking appliances. Propane burns according to the equation C 3 H 8 + 5O 2  3CO 2 + 4H 2 O (i)The heats of formation of propane, carbon dioxide and water are –104, –394 and –286 kJ mol–1 respectively. Calculate the heat of combustion of propane. (12) (ii) If 500 kJ of energy are needed to boil a kettle of water what mass of propane gas must be burned to generate this amount of heat? Express your answer to the nearest gram. (6) (b) The combustion of methane is described by the following balanced equation. CH 4(g) + 2O 2(g) CO 2(g) + 2H 2 O (l) ΔH = − kJ mol -1 The standard heats of formation of carbon dioxide and water are −394 and −286 kJ mol- 1 respectively. Calculate the heat of formation of methane. (12)

2005 6( c) The combustion of liquid benzene is described by the following equation : 2C 6 H 6(l) + 15O 2(g)  12 CO 2(g) + 6H 2 O (l) Given that the heats of formation of carbon dioxide gas, liquid water and liquid benzene are –394,–286 and 49 kJ mol -1 respectively, calculate the heat of combustion of liquid benzene. (12) (c) The combustion of cyclohexane may be described by the following balanced equation: C 6 H 12(l) + 9O 2(g)  6CO 2(g) + 6H 2 O (l) Given that the heats of formation of cyclohexane, carbon dioxide and water are –156, – 394 and –286 kJ mol –1, respectively, calculate the heat of combustion of cyclohexane. (12) Heat of neutralisation asked E=m.c. ∆T (b) Write a balanced chemical equation for the combustion of ethanol, C 2 H 5 OH. Given that the heats of formation of ethanol, carbon dioxide and water are –278, –394 and –286 kJ mol –1, respectively, calculate the heat of combustion of ethanol. (18) (e) The combustion of one of the C 4 H 8 isomers is described by the following balanced equation. C 4 H 8 + 6O 2  4CO 2 + 4H 2 O ΔH = –2710 kJ mol –1 The standard heats of formation of water and carbon dioxide are –286 and –394 kJ mol –1, respectively. Calculate the heat of formation of this C 4 H 8 isomer. (12)

(d) Define heat of combustion. Outline how the heat of combustion of X could be measured using a bomb calorimeter. (15) (e) In order to increase its octane rating, compound X is converted to compound Z in oil refineries by the following reforming (dehydrocyclisation) process : C 7 H 16 (l)  C 7 H 8 (l) + 4H 2 (g) Calculate the heat change for this reaction given that the heats of formation of C 7 H 16 (l), and C 7 H 8 (l) are –224.2 and 12.4 kJ mol –1, respectively. State one important industrial use for the hydrogen produced in this reaction. (12) ( e ) Steam reforming takes place according to the following balanced equation: CH 4 (g) + H 2 O (g)  CO (g) + 3H 2 (g) Calculate the heat of this steam reforming reaction given that the heats of formation of methane, steam and carbon monoxide are –74.6, –242 and –111 kJ mol –1 respectively. (12) 2012 ???