The question
Name: ______________________ Date: ____________ The enthalpy of formation of ethanol from combustion data (4 points)
Data table
Three combustion reactions
Carbon, hydrogen and ethanol each burn cleanly in oxygen, so their enthalpies of combustion can be measured.
| Reaction | Equation | ΔH° (kJ/mol) |
|---|---|---|
| 1 | C(graphite) + O₂(g) → CO₂(g) | −393.5 |
| 2 | H₂(g) + ½ O₂(g) → H₂O(l) | −285.8 |
| 3 | C₂H₅OH(l) + 3 O₂(g) → 2 CO₂(g) + 3 H₂O(l) | −1366.7 |
(a) The target reaction is 2 C(graphite) + 3 H₂(g) + ½ O₂(g) → C₂H₅OH(l). Identify which of the three reactions must be reversed to obtain the target, and explain why. [1 point]
(b) Calculate ΔH° for the target reaction. [2 points]
(c) The target reaction cannot be carried out in a lab, yet its ΔH° can be found from Reactions 1-3. Justify why this method gives the correct value. [1 point]