The enthalpies of elements in their standard states are taken as zero. The enthalpy of formation of a compound

Options

A. is always negative
B. is always positive
C. may be positive or negative
D. is never negative

Explanation

To solve the question regarding the enthalpy of formation of a compound, we need to understand the concept of enthalpy changes involved in the formation of a compound from its elements in their standard states.

### Step-by-Step Solution:

1. **Understanding Enthalpy of Formation**:
The enthalpy of formation (ΔH_f°) of a compound is defined as the change in enthalpy when one mole of the compound is formed from its elements in their standard states.

2. **Standard State Definition**:
The standard state of an element is its most stable form at 1 bar pressure and a specified temperature (usually 25°C). For example, the standard state of carbon is graphite, and that of oxygen is O₂ gas.

3. **Enthalpy of Elements**:
The enthalpy of elements in their standard states is taken as zero. This means that when calculating the enthalpy of formation of a compound, we start from a baseline of zero for all the reactants if they are in their standard states.

4. **Formation Reaction**:
Consider a hypothetical formation reaction for a compound \( AB \):
\[ A(s) + \frac{1}{2}B_2(g) \rightarrow AB(s) \]
In this reaction, \( A \) and \( B_2 \) are in their standard states.

5. **Calculating Enthalpy of Formation**:
The enthalpy change for this reaction (ΔH_f°) can be expressed as:
\[ ΔH_f° = H_{products} - H_{reactants} \]
Since the enthalpy of the elements in their standard states is zero, we have:
\[ ΔH_f° = H_{AB} - (0 + 0) = H_{AB} \]
Thus, the enthalpy of formation of the compound \( AB \) is simply the enthalpy of the compound itself.

6. **Conclusion**:
Therefore, the enthalpy of formation of a compound is the enthalpy associated with the formation of that compound from its elements in their standard states.

### Final Answer:
The enthalpy of formation of a compound is equal to the enthalpy of the compound itself, as the enthalpies of the elements in their standard states are taken as zero.