Question 9.10: An engine generates 15.7 g of nitric oxide gas during a labo......
An engine generates 15.7 g of nitric oxide gas during a laboratory test. How much heat was absorbed in producing this NO?
Strategy The thermochemical equation for this reaction is shown above. This equation provides a link to convert the amount of NO formed to the energy absorbed. As in any other stoichiometry problem, we will work with moles of substance, which we can obtain from the given mass. Note that the stated ΔH° value is for the production of two moles of NO because two moles appear in the equation.
15.7\ {\mathrm{g}}\,{\mathrm{NO}}\times{\frac{\mathrm{l~mol~NO}}{30.0\,{\mathrm{g}}\,{\mathrm{NO}}}}=0.523\,{\mathrm{mol~NO}}
0.523~\mathrm{mol~NO}\times\frac{180.5~\mathrm{kJ}}{2~\mathrm{mol~NO}}=47.2\,\mathrm{kJ}
Analyze Your Answer The thermochemical equation gives us ΔH for the reaction as written. That means that 180 kJ are absorbed for every two moles of NO formed. The molar mass of NO is very close to 30 g/mol, so the amount formed in the problem is a little more than half a mole. That’s about a quarter of the amount formed in the thermochemical equation. Our answer is roughly a quarter of the ΔH value from the thermochemical equation as well, so it seems likely to be correct.
Check Your Understanding If 124 kJ of heat is absorbed in a reaction that forms nitric oxide from nitrogen and oxygen, what mass of NO must have been produced? What mass of N_2 was consumed?