Long free-response question

Methanol as a fuel

Units 3 and 6
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

A multipart question, often built around a lab setup or a data table, that mixes several topics. You calculate, explain and justify your answers across many lettered parts. On the exam: 3 questions (Questions 1–3), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

Methanol, CH₃OH, is a liquid fuel used in camping stoves and some racing engines. Its combustion is represented by the equation CH₃OH(l) + 3/2 O₂(g) → CO₂(g) + 2 H₂O(l).

Table 1. Thermochemical equations at 298 K

Equation	ΔH° (kJ/mol_rxn)
1: C(s) + O₂(g) → CO₂(g)	−393.5
2: H₂(g) + 1/2 O₂(g) → H₂O(l)	−285.8
3: C(s) + 2 H₂(g) + 1/2 O₂(g) → CH₃OH(l)	−238.7

Source: Standard enthalpies of formation, rounded

Calorimetry experiment

A student burns methanol in a small spirit burner placed under an aluminum can that holds 250.0 g of water.

Mass of the burner and methanol before burning: 152.36 g. Mass after burning: 151.16 g.

Initial temperature of the water: 20.0 °C. Final temperature of the water: 34.5 °C.

Assume the specific heat capacity of the water is 4.18 J/(g·°C), and ignore the heat absorbed by the can.

Source: Hypothetical data

Table 2. Methanol and chloromethane

Compound	Molar mass (g/mol)	Boiling point (°C)
Methanol, CH₃OH	32.04	64.7
Chloromethane, CH₃Cl	50.49	−24.2

Source: Standard reference values, rounded

Suggested time: 23 minutes

Your answers are saved in this browser as you type.

Part (a)

1 point

Use Hess's law and the equations in Table 1 to calculate ΔH° for the combustion of one mole of CH₃OH(l).

Your answer to part (a)

0 / 2,500 characters

Part (b)

2 points

Using the calorimetry data, (i) calculate the heat, in kJ, absorbed by the water, and (ii) calculate the experimental value of ΔH for the combustion of CH₃OH, in kJ/mol.

Your answer to part (b)

0 / 2,500 characters

Part (c)

1 point

The experimental value in part (b) is much less negative than the value calculated in part (a). Identify one specific feature of the experiment that accounts for this difference, and explain how it makes the experimental value less negative.

Your answer to part (c)

0 / 2,500 characters

Part (d)

1 point

Describe the energy diagram you would draw for the combustion of CH₃OH(l). Include what is plotted on the vertical axis, the relative positions of the reactants and products, and how ΔH° is shown.

Your answer to part (d)

0 / 2,500 characters

Part (e)

1 point

The enthalpy of vaporization of methanol is 35.2 kJ/mol at its boiling point. Calculate the heat needed to vaporize 10.0 g of CH₃OH(l) at its boiling point.

Your answer to part (e)

0 / 2,500 characters

Part (f)

1 point

Suppose the water formed in the combustion stayed as H₂O(g) instead of condensing to H₂O(l). Would the value of ΔH for the combustion of one mole of CH₃OH be more negative, less negative, or the same as the value in part (a)? Justify your answer.

Your answer to part (f)

0 / 2,500 characters

Part (g)

2 points

Using Table 2, explain why methanol has a much higher boiling point than chloromethane, even though chloromethane has the greater molar mass. Your answer must identify the strongest type of intermolecular force in each liquid and compare the strengths of those forces.

Your answer to part (g)

0 / 2,500 characters

Part (h)

1 point

A camper needs 1.00 × 10³ kJ of heat. Using your answer to part (a), calculate the minimum mass of CH₃OH that must be burned.

Your answer to part (h)

0 / 2,500 characters

Checking scoring…

Scoring it yourself shows you the rubric, examples and a model answer. Try writing your answer first.