One mole of hydrogen is burned in oxygen to produce water in the gas phase, H = 285.8 kJ. What is the energy change when 1 mole of water, in the gas phase, decomposes to form hydrogen and oxygen?

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BlackTom AI · Accepted Answer

First, let's establish the thermodynamic fact given: the formation of water gas from hydrogen and oxygen releases 285.8 kJ of energy per mole, so the enthalpy of formation (H) is -285.8 kJ per mole of H2O(g). This means the forward reaction H2 + 1/2 O2 -> H2O(g) is exothermic by 285.8 kJ. Now consider the reverse process asked in the question: decomposition of 1 mole of H2O(g) into H2 and 1/2 O2. When a reaction is reversed, the sign of the enthalpy change flips. Therefore, the decomposition requires an input of +285.8 kJ (endothermic) per mole of water decomposed.

Option 1: 142.9 kJ of heat is absorbed. This value is incorrect because it is exactly half of 285.8 kJ, which might arise if someone tried to balance per molecule rather than per mole or misapplied a half-reaction idea. The problem quantity refers to 1 mole of water, so the energy would not be halved.

Option 2: 285.8 kJ of heat is absorbed. This matches the reversed-formation logic: since formation releases 285.8 kJ, decomposition must absorb 285.8 kJ. The sign convention is that endothermic processes absorb heat, so the value should be positive and equal in magnitude to the formation enthalpy.

Option 3: 142.9 kJ of heat is released. This is inconsistent in both magnitude and sign: it asserts a release (exothermic) rather than absorption, and it uses half the energy, which does not align with the given one-mole basis.

Option 4: 285.8 kJ of heat is released. This correctly identifies the magnitude but assigns the wrong sign: the decomposition is endothermic, so heat is absorbed, not released.

In summary, recognizing that the decomposition of water to H2 and 1/2 O2 is the reverse of the formation process, the enthalpy change must be +285.8 kJ (absorbed) per mole of water decomposed, making option 2 the correct choice.

Chemistry (H) - Varnold - Preparatory Academy - 2nd Semester - 2024/2025 2025 Thermochemistry Practice Test

One mole of hydrogen is burned in oxygen to produce water in the gas phase, H = 285.8 kJ. What is the energy change when 1 mole of water, in the gas phase, decomposes to form hydrogen and oxygen?

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