ΔH Nomenclature     

Each of the reactions represented by the equations below has an enthalpy of reaction Δ)H or ΔH_rxn) associated with it.

Which enthalpy of reaction can also be labeled as a

• standard enthalpy (or heat) of formation?
• enthalpy (or heat) of dissociation?
• enthalpy (or heat) of vaporization?
• enthalpy (or heat) of neutralization?

Explain your choices.

1/2 N₂(g) + O₂(g)  NO₂(g)

N₂(g) + O₂(g)  2 NO(g)

H₂O  H₂O(g)

2 NO(g) + O₂(g)  2 NO₂(g)

NO₂(g)  1/2 N₂(g) + O₂(g)

H⁺(aq) + OH^-(aq)  H₂O(?)

H₂(g) + 1/2 O₂(g)  H₂O(?)

The gas used in a propane torch or grill is essentially

pure propane. Explain why the heat produced by combustion of one mole of propane in such a torch or grill is not equal to the enthalpy of combustion of propane found in tables of enthalpies (heats) of combustion.

In a first experiment a student takes 2 g of solid NaOH, dissolves it in 100 mL of water in a calorimeter, and observes a temperature increase. In a second experiment the student uses 4 g of the NaOH and 200 mL of water. What should we expect for the second temperature change?

(a) Both temperature changes will be the same.
(b) The second temperature change will be approximately twice the first.
(c) The second temperature change will be approximately four times the first.
(d) The second temperature change will be approximately one-half of the first.
(e) The second temperature change will be approximately one-fourth of the first.