In which of the compounds the oxidation state of hydrogen is -1?

Options

A. `H_2O`
B. `CaH_2`
C. `HBr`
D. `H_2S`

Answer

To determine in which of the compounds the oxidation state of hydrogen is -1, we will analyze the oxidation states of hydrogen in various compounds.

### Step-by-Step Solution:

1. **Understanding Oxidation States**:
- The oxidation state of an element in a compound indicates its degree of oxidation or reduction. For hydrogen, the common oxidation states are +1 and -1.

2. **Analyzing H2O (Water)**:
- In water (H2O), oxygen is more electronegative than hydrogen. Therefore, hydrogen has an oxidation state of +1.
- **Calculation**:
- Oxygen (O) has an oxidation state of -2.
- For two hydrogens: 2(+1) + (-2) = 0 (neutral molecule).
- **Conclusion**: In H2O, the oxidation state of hydrogen is +1.

3. **Analyzing CaH2 (Calcium Hydride)**:
- In calcium hydride (CaH2), calcium is a metal and less electronegative than hydrogen.
- Therefore, hydrogen takes on an oxidation state of -1.
- **Calculation**:
- Calcium (Ca) has an oxidation state of +2.
- For two hydrogens: 2(-1) + (+2) = 0 (neutral molecule).
- **Conclusion**: In CaH2, the oxidation state of hydrogen is -1.

4. **Analyzing HBr (Hydrobromic Acid)**:
- In HBr, bromine is more electronegative than hydrogen.
- Therefore, hydrogen has an oxidation state of +1.
- **Calculation**:
- Bromine (Br) has an oxidation state of -1.
- For one hydrogen: +1 + (-1) = 0 (neutral molecule).
- **Conclusion**: In HBr, the oxidation state of hydrogen is +1.

5. **Analyzing H2S (Hydrogen Sulfide)**:
- In H2S, sulfur is more electronegative than hydrogen.
- Therefore, hydrogen has an oxidation state of +1.
- **Calculation**:
- Sulfur (S) has an oxidation state of -2.
- For two hydrogens: 2(+1) + (-2) = 0 (neutral molecule).
- **Conclusion**: In H2S, the oxidation state of hydrogen is +1.

### Final Answer:
The compound in which the oxidation state of hydrogen is -1 is **Calcium Hydride (CaH2)**.