Enthalpy of which of the following reactions does not represent the standard enthalpy of formation?

Options

A. `H_2(g)+frac{1}{2}O_2(g)rightarrow H_2O(l)`
B. `H_2(g)+Br_2(l)rightarrow 2HBr(g)`
C. S(rhombic, s) + `O_2(g)rightarrowSO_2(g)`
D. 2C (graphite, s)+`3H_2(g)+frac{1}{2}O_2(g)rightarrow C_2H_5OH(l)`

Answer

To determine which reaction does not represent the standard enthalpy of formation, we first need to understand the definition of standard enthalpy of formation (ΔHf°). The standard enthalpy of formation is defined as the change in enthalpy when one mole of a compound is formed from its elements in their standard states.

Now, let's analyze the given reactions one by one:

1. **Reaction 1: H2 + 1/2 O2 → H2O**
- This reaction represents the formation of water (H2O) from its elements, hydrogen (H2) and oxygen (O2), in their standard states. This is a correct representation of the standard enthalpy of formation.

2. **Reaction 2: H2 + Br2 → 2 HBr**
- In this reaction, hydrogen (H2) and bromine (Br2) are used to form hydrogen bromide (HBr). However, the reaction is written with a coefficient of 2 for HBr, which means it produces 2 moles of HBr. The standard enthalpy of formation should only involve the formation of 1 mole of a compound from its elements. Therefore, this reaction does not correctly represent the standard enthalpy of formation.

3. **Reaction 3: S + O2 → SO2**
- This reaction shows the formation of sulfur dioxide (SO2) from sulfur (S) and oxygen (O2). Since it involves the formation of 1 mole of SO2 from its elements in their standard states, this reaction correctly represents the standard enthalpy of formation.

4. **Reaction 4: C2H5OH (ethanol) formation**
- The formation of ethanol from its elements would typically be represented as:
\[ 2C + 3H2 + 1/2 O2 → C2H5OH \]
- If the reaction is written with the correct stoichiometry for the formation of 1 mole of ethanol, it would be valid. However, if it is written incorrectly, we need to check the coefficients used.

Based on the analysis, the reaction that does not represent the standard enthalpy of formation is:

**Answer: H2 + Br2 → 2 HBr**