Question

The enthalpy of formation of \(\mathrm{HI}\) is \(30.4 \mathrm{~kJ}\). Which statement is false according to this observation? (1) HI is an endothermic compound. (2) For the reaction \(\mathrm{H}_{2(\mathrm{~g})}+\mathrm{I}_{2 \mathrm{ig}} \rightarrow 2 \mathrm{HI}_{(\mathrm{g}}, \Delta / I=60.8 \mathrm{~kJ}\) (3) HI is a stable compound. (4) HI is an unstable compound.

Short answer

Option (3) is false.

Step-by-step solution

Understand the given data

The enthalpy of formation of \(\text{HI}\) is given as \(30.4 \text{ kJ}\). This means the energy required to form 1 mole of \(\text{HI}\) from its elements in their standard states is \(30.4 \text{ kJ}\).

Analyze Options

Evaluate each statement based on the enthalpy of formation.

Option (1)

A compound with a positive enthalpy of formation means energy is absorbed during its formation, indicating an endothermic process. Therefore, \(\text{HI}\) is endothermic as mentioned.

Option (2)

The formation reaction of \(2 \text{ HI}_{(\text{g})}\) from \(\text{H}_2_{(\text{g})} + \text{I}_2_{(\text{g})}\) involves twice the amount of \(\text{HI}\), so the enthalpy change is \(2 \times 30.4 \text{ kJ} = 60.8 \text{ kJ}\). This statement is accurate.

Option (3)

A positive enthalpy of formation indicates that the compound is less stable than its constituent elements because it requires energy to form.

Option (4)

Based on the enthalpy of formation, \(\text{HI}\) being less stable translates to it being relatively unstable. This observation supports option (4).

Conclude the false statement

Since \(\text{HI}\) is less stable (or unstable) given its positive enthalpy of formation, option (3) claiming it is stable is incorrect.

Key concepts

Endothermic Process

An endothermic process is when a chemical reaction absorbs energy from its surroundings. This can be observed in the enthalpy of formation of \(\text{HI}\), which is given as \(30.4 \text{ kJ}\). A positive enthalpy of formation signifies that energy is absorbed to form the compound, indicating an endothermic reaction.
Understanding the concept of an endothermic process is crucial because it helps explain why some reactions require heat to occur. In the case of \(\text{HI}\), the absorption of energy means that it takes more energy to form \(\text{HI}\) from its elements, \(\text{H}_2\) and \(\text{I}_2\).
This absorption of energy can be visualized when you feel a reaction vessel cool down during an endothermic reaction, as heat from the surroundings is being used up in the formation process.

Chemical Stability

Chemical stability refers to how likely a compound is to remain in its current state without decomposing or reacting. A stable compound does not change easily, while an unstable one is more reactive.
The enthalpy of formation can indicate stability. A positive enthalpy of formation, like the \(\text{30.4 kJ/mol}\) for \(\text{HI}\), suggests that the compound is **less stable** than its constituent elements. This is because additional energy is required for the compound to form, making it less favorable energetically.
In the problem, \(\text{HI}\) being less stable (or unstable) translates to it being more reactive, contradicting option (3), which incorrectly states that \(\text{HI}\) is stable. Therefore, \(\text{HI}\) can be considered relatively unstable based on this enthalpy.

Enthalpy Change Calculation

When it comes to calculating the enthalpy change for a reaction, it is essential to understand the relationship between moles and enthalpy values. Given the formation reaction of \(2 \text{HI}_{(\text{g})}\) from \( \text{H}_2 (\text{g}) + \text{I}_2 (\text{g}) \), we need to account for the stoichiometry.

To break it down:

• The enthalpy of formation of \(\text{HI}\) is \(30.4 \text{ kJ/mol}\)
• The reaction produces 2 moles of \(\text{HI}\)
• Thus, the total enthalpy change for the reaction is \(2 \times 30.4 \text{ kJ} = 60.8 \text{ kJ}\)

This calculation aligns with option (2), confirming it to be correct.
In general, for any chemical reaction, the enthalpy change \(\text{ \Delta H}\) can be calculated using the formula:
\[ \Delta H = \text{sum of enthalpies of products} - \text{sum of enthalpies of reactants} \]This principle helps in predicting whether the reaction is endothermic (positive \(\text{ \Delta H}\)) or exothermic (negative \(\text{ \Delta H}\)).