For which of the following substances is the heat of formation in the standard state zero ?

Options

A. Sucrose
B. Ethanol
C. Aluminium
D. Calcium chloride

Explanation

To determine which substance has a heat of formation in the standard state equal to zero, we need to understand the concept of standard enthalpy of formation (ΔH°f). The standard enthalpy of formation of a substance is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states.

Here’s a step-by-step solution:

### Step 1: Understand the Definition
The standard heat of formation (ΔH°f) is zero for elements in their most stable form at standard conditions (1 atm pressure and 25°C). This means that the heat change associated with the formation of the element from itself is zero.

### Step 2: Identify the Given Substances
We need to analyze the substances provided in the question. Let's assume the substances are:
1. Sucrose (C12H22O11)
2. Ethanol (C2H5OH)
3. Aluminum (Al)
4. Oxygen (O2)

### Step 3: Analyze Each Substance
- **Sucrose (C12H22O11)**: This is a compound made from carbon, hydrogen, and oxygen. Since it is not an element in its standard state, its ΔH°f is not zero.

- **Ethanol (C2H5OH)**: This is also a compound formed from elements (carbon, hydrogen, and oxygen). Its ΔH°f is not zero as it is not in its elemental form.

- **Aluminum (Al)**: Aluminum is a metal and is considered to be in its most stable elemental form when it is in solid state. Therefore, the ΔH°f for aluminum in its standard state is zero.

- **Oxygen (O2)**: Oxygen in its diatomic form (O2) is also an element in its most stable form at standard conditions, and thus its ΔH°f is zero as well.

### Step 4: Conclusion
From the analysis, both aluminum (Al) and oxygen (O2) have a standard heat of formation equal to zero. However, if the question specifically asks for a single substance that is commonly recognized, aluminum is often the answer given in such contexts.

### Final Answer
The correct answer is **Aluminum (Al)**.

---