What is the standard enthalpy change (ΔH°) for the formation of water from its e

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Question: What is the standard enthalpy change (ΔH°) for the formation of water from its elements? (2022)

Options:

Correct Answer: -285.83 kJ/mol

Exam Year: 2022

Solution:

The standard enthalpy change for the formation of water from its elements (H2 and O2) is -285.83 kJ/mol.

What is the standard enthalpy change (ΔH°) for the formation of water from its elements? (2022)

What is the standard enthalpy change (ΔH°) for the formation of water from its elements? (2022)

Step 1: Understand that the formation of water (H2O) involves combining hydrogen gas (H2) and oxygen gas (O2).
Step 2: Write the balanced chemical equation for the formation of water: 2 H2(g) + O2(g) → 2 H2O(l).
Step 3: Recognize that the standard enthalpy change (ΔH°) is the heat change that occurs at standard conditions (1 atm pressure and 25°C).
Step 4: Look up the standard enthalpy of formation values for H2O, H2, and O2. The standard enthalpy of formation for H2 and O2 is zero because they are in their elemental forms.
Step 5: Calculate the standard enthalpy change for the reaction using the formula: ΔH° = Σ(ΔH°f products) - Σ(ΔH°f reactants).
Step 6: Substitute the values: ΔH° = [2 * ΔH°f(H2O)] - [2 * ΔH°f(H2) + ΔH°f(O2)]. Since ΔH°f(H2) and ΔH°f(O2) are zero, it simplifies to ΔH° = 2 * ΔH°f(H2O).
Step 7: Use the known value of ΔH°f(H2O) which is -285.83 kJ/mol.
Step 8: Therefore, the standard enthalpy change for the formation of water from its elements is ΔH° = -285.83 kJ/mol.

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