3 (a) With reference to hydrogen combustion, explain what is meant by the terms chain branching reactions and chain termination reactions and therefore explain, with examples of relevant reactions, how the domination of chain branching reactions over termination reactions leads to the ignition of a hydrogen-air mixture.

(b) Briefly discuss the differences in the following properties between natural gas, gasoline and hydrogen:

(i) Minimum Ignition Energy (MIE)

(ii) Auto-Ignition Temperature (AIT) (iii) Quench Distance (QD) (iv) Upper and low flammability limits

(v) Vapour density relative to air [2 marks] (c) What is the reason most longer chain alkanes such as those contained in gasoline (e.g. n-heptane) have lower AITs than hydrogen or methane? In your answer you should refer to additional low temperature branching routes that occur for typical longer chain alkane fuels.

(d) Hydrogen is being discussed as a potential future low carbon fuel for various applications where it may displace natural gas or gasoline. Referring to the properties discussed in part (b), explain with reasons whether you would be concerned about the safety implications of: (i) a hydrogen filling station for transport applications when compared to the gasoline equivalent. (ii) the use of hydrogen in a domestic boiler when compared to natural gas. In your answers discuss design features that might be used to mitigate potential hazards.