Edexcel Jun 2012 Paper 2 Q20

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SECTION C Answer ALL the questions. Write your answers in the spaces provided. 20 Fuels of the Future Concerns about the future availability of fossil fuels, and the fact that their combustion produces greenhouse gases, have led to a search for alternative sources of energy. A great deal of attention has been directed at developing the use of hydrogen as a fuel. Since the only product of its combustion is water, hydrogen is considered to be a clean fuel. However, the use of hydrogen has major drawbacks. The small size of the hydrogen molecule means that it is difficult to prevent leaks and, to store enough to provide a reasonable amount of fuel for a car, hydrogen must be compressed to around 700 atmospheres. Furthermore, the main source of hydrogen is currently fossil fuels such as methane, which is combined with steam in a series of reactions to form carbon dioxide and hydrogen. One suggested alternative to hydrogen is ammonia. Ammonia, which is obtained by combining nitrogen and hydrogen at temperatures around 450C and pressures of about 150 atmospheres, also has serious disadvantages: it is a toxic, corrosive and pungent gas which is difficult to ignite. However, burning ammonia produces only nitrogen and water and it is relatively easy to liquefy, having a boiling temperature of just 33C. Furthermore, the technology works: ammonia was used as a fuel for Belgian buses in the Second World War and, in 2007, the NH3 Car project based in Ann Arbor, Michigan, used a mixture of ammonia and petrol to fuel a 2500 mile journey, from Detroit to San Francisco, in a modified pickup truck. (a) (i) Explain the term greenhouse gas. (2)16 *P39307A01620*<br /> *(ii) State and explain whether or not water (in the gaseous state) is a greenhouse gas. (2)(iii) Write the equation for the formation of hydrogen from methane and steam. State symbols are not required. (iv) Suggest why using methane to form hydrogen in this way is preferable to burning methane directly. (2) (1)(v) Storing hydrogen at a pressure of 700 atmospheres is a disadvantage to its use as a fuel because of the costs involved. Suggest why using such high pressures is so expensive. (1)(b) (i) Draw a dot and cross diagram for ammonia, showing the outer electrons only. (1) *P39307A01720* 17 Turn over<br /> *(ii) By considering the intermolecular forces involved, explain why methane has a boiling temperature of 109 K while ammonia has a boiling temperature of 240 K, although these two compounds have very similar molar masses. (4)(c) (i) Write the equation for the combustion of ammonia. State symbols are not required. (2) (ii) The enthalpy change of combustion of methane is 890.3 kJ mol1 and that of ammonia is 510.1 kJ mol1. Suggest two additional items of information, not connected with environmental factors or the fact that methane is non-renewable, which would be useful in comparing methane and ammonia as fuels. (2)(iii) The fact that ammonia has a pungent smell is listed as a disadvantage of its use as a fuel. Suggest why this might also be an advantage. (1)18 *P39307A01820*<br /> (iv) Suggest why ammonia was mixed with petrol in the NH3 Car project. (1)(v) State, with a reason, whether hydrogen or ammonia can currently be considered to be long term replacements for fossil fuels. (1)(Total for Question 20 = 20 marks) TOTAL FOR SECTION C = 20 MARKS TOTAL FOR PAPER = 80 MARKS *P39307A01920* 19<br />

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