Aim for FiveChemistry

Short free-response question

Hydrogen and oxygen: favorable but slow

  • Unit 9
  • 4 points
  • About 9 minutes

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

A shorter multipart question that sticks to one or two topics. Each part usually asks for one calculation, claim or explanation. On the exam: 4 questions (Questions 4–7), part of the 1-hour-45-minute free-response section; calculator allowed.

The question and its sources

Hydrogen gas reacts with oxygen gas to form liquid water: 2 H₂(g) + O₂(g) → 2 H₂O(l), ΔH° = −571.6 kJ/mol_rxn.

Table 1. Standard molar entropies at 298 K

SubstanceS° (J/(mol·K))
H₂(g)130.7
O₂(g)205.2
H₂O(l)69.9

Source: Standard reference values, rounded

Suggested time: 9 minutes

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Part (a)

1 point

Without performing a calculation, predict the sign of ΔS° for the reaction. Justify your prediction.

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Part (b)

1 point

Calculate ΔG° for the reaction at 298 K.

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Part (c)

1 point

Is the reaction thermodynamically favorable at all temperatures? Explain using the signs of ΔH° and ΔS°.

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Part (d)

1 point

A mixture of H₂(g) and O₂(g) can be stored at 25 °C for years without forming a measurable amount of water. Reconcile this observation with your answer to part (b).

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