Which one of the following statements is correct?
Hydrogen can have more than one oxidation number
Understanding the Oxidation Number of Hydrogen
The oxidation number (also called oxidation state) is a number assigned to an element in a chemical compound or ion that represents the number of electrons gained or lost by an atom of that element when it forms the compound or ion from its pure elemental state. It's a useful concept for tracking electron transfer in redox reactions.
Analyzing the Oxidation Number of Hydrogen
Let's examine the statements provided regarding the oxidation number for hydrogen:
- Statement 1: The oxidation number for hydrogen is always zero.
- Statement 2: The oxidation number for hydrogen is always +1.
- Statement 3: The oxidation number for hydrogen is always -1.
- Statement 4: Hydrogen can have more than one oxidation number.
To determine the correct statement, we need to consider the different chemical environments in which hydrogen atoms exist.
Common Oxidation State of Hydrogen (+1)
Hydrogen is typically found with an oxidation number of +1 in most of its compounds. This occurs when hydrogen is bonded to elements that are more electronegative than itself. In these cases, the more electronegative atom pulls the electron density away from hydrogen, causing hydrogen to have a partial positive charge, represented by an oxidation number of +1.
Examples include:
- Water (\(\text{H}_2\text{O}\)): Oxygen is more electronegative than hydrogen. Oxygen has an oxidation number of -2, and each hydrogen has an oxidation number of +1. \(2(+1) + (-2) = 0\) (neutral molecule).
- Hydrogen Chloride (\(\text{HCl}\)): Chlorine is more electronegative than hydrogen. Chlorine has an oxidation number of -1, and hydrogen has an oxidation number of +1. \((+1) + (-1) = 0\) (neutral molecule).
- Sulfuric Acid (\(\text{H}_2\text{SO}_4\)): Oxygen is more electronegative than hydrogen and sulfur. The oxidation number of hydrogen is +1.
Less Common Oxidation State of Hydrogen (-1)
Hydrogen can also have an oxidation number of -1. This happens in metal hydrides, which are compounds where hydrogen is bonded to a metal. Metals are typically less electronegative than hydrogen. In these compounds, hydrogen gains or partially gains electron density from the metal, resulting in a partial negative charge and an oxidation number of -1.
Examples include:
- Sodium Hydride (\(\text{NaH}\)): Sodium is less electronegative than hydrogen. Sodium is a Group 1 metal, so its oxidation number is +1. For the compound to be neutral, hydrogen must have an oxidation number of -1. \((+1) + (-1) = 0\).
- Calcium Hydride (\(\text{CaH}_2\)): Calcium is less electronegative than hydrogen. Calcium is a Group 2 metal, so its oxidation number is +2. For the compound to be neutral, the two hydrogen atoms must collectively have an oxidation number of -2, meaning each hydrogen atom has an oxidation number of -1. \((+2) + 2(-1) = 0\).
Oxidation State of Hydrogen (0)
In its elemental form, diatomic hydrogen gas (\(\text{H}_2\)), the oxidation number of hydrogen is 0. This is true for any element in its elemental state.
Evaluating the Statements
Based on the examples:
- Hydrogen has an oxidation number of 0 in \(\text{H}_2\). So, statement 1 ("always zero") is incorrect.
- Hydrogen has an oxidation number of +1 in \(\text{H}_2\text{O}\), \(\text{HCl}\), etc. But it's -1 in metal hydrides and 0 in \(\text{H}_2\). So, statement 2 ("always +1") is incorrect.
- Hydrogen has an oxidation number of -1 in \(\text{NaH}\), \(\text{CaH}_2\), etc. But it's +1 in many compounds and 0 in \(\text{H}_2\). So, statement 3 ("always -1") is incorrect.
- Hydrogen can have oxidation numbers of 0, +1, and -1 depending on the compound. This means hydrogen can have more than one oxidation number. So, statement 4 is correct.
Therefore, the statement that hydrogen can have more than one oxidation number is the correct one.
Revision Table: Hydrogen Oxidation Numbers
| Chemical Species | Example Formula | Oxidation Number of Hydrogen | Explanation |
|---|---|---|---|
| Elemental Hydrogen | \(\text{H}_2\) | 0 | Element in its standard state. |
| Compounds with more electronegative elements | \(\text{H}_2\text{O}\), \(\text{HCl}\), \(\text{NH}_3\), \(\text{CH}_4\) | +1 | Bonded to O, Cl, N, C (more electronegative than H). |
| Metal Hydrides | \(\text{NaH}\), \(\text{CaH}_2\), \(\text{LiAlH}_4\) | -1 | Bonded to metals (less electronegative than H). |
Additional Information: Factors Affecting Hydrogen Oxidation State
The oxidation state of hydrogen in a compound is primarily determined by the electronegativity of the atom it is bonded to. Electronegativity is a measure of an atom's ability to attract bonding electrons.
- When hydrogen bonds with an atom that is more electronegative (like oxygen, chlorine, nitrogen, carbon, sulfur), hydrogen tends to lose electron density and gets a +1 oxidation state.
- When hydrogen bonds with an atom that is less electronegative (typically metals like sodium, calcium, lithium, aluminum), hydrogen tends to gain electron density and gets a -1 oxidation state.
Understanding electronegativity is key to predicting the oxidation number of hydrogen in various compounds.
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