Developing Fuels Revision questions and suggested answers
1. Define ΔHfθ and ΔHcθ
ΔHfθ the enthalpy change when 1 mole of a compound is formed from its constituent elements in
their standard states measured under standard conditions
ΔHcθ the enthalpy change when 1 mole of a compound undergoes complete combustion measured
under standard conditions
2.
Write an equation for the enthalpy of formation of butane C4H10
3.
4C(s) + 5H2(g)  C4H10(g)
Write an equation for the enthalpy of combustion of butane C4H10
C4H10(g) + 6 ½ O2(g)
 4CO2(g)
+ 5 H2O (l)
4. Which has the most exothermic enthalpy of combustion methane or octane and why?
Octane as more energy released than when bonds are made than broken than with methane
5. Calculate the enthalpy change from an experiment in kJmol-1.
a. e.g. the temp of 100g of water rose by 12 degrees C when 1g of methanol was completely
combusted. The specific heat capacity of water is 4.2 Jg-1K-1
∆H = - 100 X 4.2 X 12 / 1/32 (/ moles methanol)
then / 1000 = - 161 kJ mol-1
6. State Hess’s law and use a Hess’s law cycle to find ΔHθf of CH4 if given the ΔHθc of C , ΔHθcH2.and
ΔHθc CH4
a. Hint : first write an equation for the enthalpy of formation of methane.
Enthalpy change is independent of the route taken
ΔHθf of CH4
C(s) + 2H2(g)  CH4(g)
ΔHθc of C
+ 2ΔHθcH2
ΔHθc CH4
CO2 (g) + 2H2O(l)
7. Which is exothermic bond making or bond breaking?
Bond making exo
8. Give an example of an
i. Alkane
ii. Alkene
iii. Cycloalkane
iv. Alcohol
v. Ether
Butane C4H10
Ethene C2H4
Cyclohexane C6H12
Ethanol C2H5OH
Methoxy methane CH3OCH3
9. What does an aromatic compound contain? Note all other compounds are aliphatic
A benzene ring
10. Give examples of all 3 types of structural isomerism
i. Chain
Butane and methyl propane
ii. Positional
2-methyl pentane and 3-methyl pentane
iii. Functional group Ethanol and methoxymethane
11.
Write equations for the complete combustion of ethanol C2H5OH and Propane C3H8
C2H5OH + 3O2  2 CO2 + 3 H2O
C3H8
+ 5O2  3 CO2 + 4 H2O
12. What does octane number indicate? Should it be high or low for better performance?
Tendency to autoignite
High is best
13. Draw the skeletal formula of the following and then decide which has the highest Octane number
i. Decane or heptane? Heptane
smaller molecule
ii. Octane or 2-methyl heptane (both C8H18) 2-methyl heptane
branched
14. Explain how the following pollutants are formed from car exhausts and explain what problems they
cause
i. CO incomplete combustion of fuel
toxic
ii. CO2 complete combustion of fuel
greenhouse gas
iii. NOx Nitrogen and oxygen from air react due to high T
acid rain/smog
iv. SOx S in fuel combusts
acid rain
v. Hydrocarbons CxHy incomplete combustion
smog
15. Explain how a heterogeneous catalyst works and how they can get poisoned
Molecules adsorb on the surface, bonds weaken and break, new bonds form , products desorb.
Poisons adsorb onto the surface and stay there and block the catalyst surface
16. Entropy is a measure of the number of ways of arranging molecules. Which has the higher entropy
i. Solid, liquid or gas gas
ii. Hexane or pentane hexane
17. What are the advantages and disadvantages of using Hydrogen as a fuel for cars?
Water only product of combustion but generate by electrolysis of water needs electricity and Hydrogen is
flammable and a gas so difficult to handle
18. Give the purpose of the following processes in oil processing and an example equation for each one
All boost octane no
Isomerisation hexane  methyl pentane Use Pt sieve out straight chains with zeolite / recycle
19.
Reforming
hexane  cyclohexane + H2 then  benzene + 3H2
Cracking
decane  propene + 2-methyl hexane shorter more useful
Use Pt
What is the volume of 0.44g of Carbon Dioxide at RTP. I mole of any gas occupies 24 dm3 at 25˚C and 1 atm
P (RTP). Moles CO2 = 0.44 / 44 = 0.01 vol = 0.01 x 24 = 0.24 dm 3
20.
What volume of Oxygen gas and then air is needed to completely combust 1kg of butane C4H10? First
write a balanced equation. Find moles of butane and use the equation to find moles of oxygen needed. Assume air is 20%
Oxygen.
C4H10(g) + 6 ½ O2(g)

4CO2(g)
moles butane = 1000/ 58 = 17.2
+ 5 H2O (l) 1kg = 1000g
moles O2 = 17.2 x 6.5 = 111.8
Volume of O2 = 111.8 x 24 = 2683 dm3
volume of air = 2683 x 100/ 20 = 13416 dm3