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 + O2 
– hydrocarbons give CO2 and H2O
- elements gives “normal” oxide - CO2 / H2O
b)
if formation
– finish with the substance in Q
then make the substance from its ELEMENTS
3.
Watch out
for
diatomic
molecules
H2 etc
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
Steps – details
6.
7.
Now solve using simultaneous equation methods
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 -1411 kJ
mol-1, -393 kJ mol-1, -286 kJ mol-1
respectively.
--------------------------------------------------Steps
ethane  combustion  -1411
1. match subs + numbers
hydrogen  comb  -393
carbon  comb  -286
Steps
1
2 Write and
balance
equations
3. Number them
2
H2 + ½ O2 
3
C+
2x 3
3x 2
rev
6. Add
O2 
2 C + 3H2
4. Want
5. solve
C2H6 + 3½ O2  2 CO2 + 3 H2O
1
2C+
∆H = -1411 kJ mol -1
H27Ooxygen
∆H = -393 kJ mol -1
CO2
∆H = -286 kJ mol -1
 C2H6
2O2  2 CO2
3H2 + 1½O2  3H2O
∆H = ?
kJ mol -1
∆H = -572 kJ mol -1
∆H = -1179 kJ mol -1
2 CO2 + 3 H2O  C2H6 + 3½ O2 ∆H = +1411 kJ mol -1
2 C + 3H2
 C2H6
∆H = -340 kJ mol -1
Double check – asked for heat of formation – one mole of ethane

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.
2002 8
(e) The combustion of butane is described by the following balanced equation.
2C4H10(g) +13O2(g) → 8CO2(g) + 10H2O(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)
2003 10
(a) Define heat of combustion. (7)
Propane may be used in gas cylinders for cooking appliances. Propane burns according to the equation
C3H8 + 5O2 3CO2 + 4H2O
(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)
2004 6
(b) The combustion of methane is described by the following balanced equation.
CH4(g) + 2O2(g) CO2(g) + 2H2O(l) ΔH = − 890.4 kJ mol-1
The standard heats of formation of carbon dioxide and water are −394 and −286 kJ mol1 respectively.
Calculate the heat of formation of methane. (12)
2005
6
(c) The combustion of liquid benzene is described by the following equation:
2C6H6(l) + 15O2(g) = 12 CO2(g) + 6H2O(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)
2006 6
(c) The combustion of cyclohexane may be described by the following balanced equation:
C6H12(l) + 9O2(g)  6CO2(g) + 6H2O(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)
2007 6
Heat of neutralisation asked
E=m.c. ∆T
2008 6
(b) Write a balanced chemical equation for the combustion of ethanol, C2H5OH.
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)
2009 6
(e) The combustion of one of the C4H8 isomers is described by the following balanced
equation.
C4H8 + 6O2 4CO2 + 4H2O 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 C4H8 isomer. (12)
2010
6
(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:
C7H16 (l)  C7H8 (l) + 4H2 (g)
Calculate the heat change for this reaction given that the heats of formation of
C7H16 (l), and C7H8 (l) are –224.2 and 12.4 kJ mol–1, respectively.
State one important industrial use for the hydrogen produced in this reaction. (12)
2011 6
(e) Steam reforming takes place according to the following balanced equation:
CH4 (g) + H2O (g)  CO (g) + 3H2 (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
???