Hydrogenation reactions are used to add hydrogen across double bonds in hydrocarbons and other organic compounds. Use average bond energies to calculate ΔH_rxn for the hydrogenation reaction. H₂C=CH₂(g) + H₂(g) → H₃C–CH₃(g)

Order these compounds in order of increasing carbon–carbon bond strength: HCCH, H₂CCH₂, H₃CCH₃.

Ethanol is a possible fuel. Use average bond energies to calculate ΔHrxn for the combustion of ethanol. CH₃CH₂OH(g) + 3 O2(g) → 2 CO₂(g) + 3 H₂O(g)

Order these compounds in order of decreasing carbon–carbon bond length: HCCH, H₂CCH₂, H₃CCH₃.

In the Chemistry and the Environment box on free radicals in this chapter, we discussed the importance of the hydroxyl radical in reacting with and eliminating many atmospheric pollutants. However, the hydroxyl radical does not clean up everything. For example, chlorofluorocarbons—which destroy stratospheric ozone—are not attacked by the hydroxyl radical. Consider the hypothetical reaction by which the hydroxyl radical might react with a chlorofluorocarbon: OH(g) + CF₂Cl₂(g) → HOF(g) + CFCl₂(g) Use bond energies to explain why this reaction is improbable. (The C–F bond energy is 552 kJ/mol.)

Ethane burns in air to form carbon dixode and water vapor.

2 H3C¬CH3( g) + 7 O2( g)¡4 CO2( g) + 6 H2O( g)

Use average bond energies to calculate ΔHrxn for the reaction.

Which of the two compounds, H₂NNH₂ and HNNH, has the strongest nitrogen-nitrogen bond, and which has the shorter nitrogen-nitrogen bond.