In vehicles, the combustion of H₂ rather than a hydrocarbon-based fuel reduces both CO₂ emissions and a dependence on fossil fuels. 

(a) Using data from Appendix 12, show that the combustion of H₂ releases 120 kJ g⁻¹. 

(b) The chart in Box 10.2 shows that the stored energy per unit mass of compressed (at 35 MPa) H₂ gas and of liquid H₂ are the same, but that liquid H₂ stores more energy per unit volume than compressed H₂. Rationalize these data. 

(c) 3 kg of gasoline (petrol) is equivalent to 1kg of H₂ in terms of stored energy. Comment on this fact in terms of the practical application of H₂ as a fuel in a family saloon car.

Data from Appendix 12

Chart from Box 10.2

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Bond H-H C-C C=C C=C Si-Si Ge-Ge Sn-Sn N-N N=N N=N P-P P=P As-As Bond enthalpy / kJ mol-¹ 436 346 598 813 226 186 151 160 400 946 209 490 177 0-0 0=0 S-S S=S Se-Se Te-Te 149 146 498 266 425 193 Bond F-F CI-CI Br-Br 1-1 C-H Si-H Ge-H Sn-H N-H P-H As-H O-H S-H Se-H Te-H F-H CI-H Br-H 1-H Bond enthalpy / kJ mol-¹ 159 242 193 151 416 326 289 251 391 322 247 464 366 276 238 570 432 366 298 Bond C-F C-Cl C-Br C-I C-O C=0 C-N C=N C-S Si-O Si=0 N-F N-CI N-O P-F P-CI P-O As-O S-F Bond enthalpy / kJ mol-¹ 485 327 285 213 359 806 285 866 272 466 642 272 193 201 490 319 340 330 326