In basic solution the chromate ion, CrO4^2-, can be converted to the chromite, CrO2- ion. The number of electrons gained or lost by the chromium atom is
A. 0
B. 3 electrons lost
C. 2 electrons lost
D. 3 electrons gained
E. 2 electrons gained
The correct answer and explanation is :
Correct Answer: D. 3 electrons gained
Explanation:
To determine how many electrons are gained or lost by the chromium atom, we must analyze the change in the oxidation state of chromium in the two ions: chromate ion (CrO₄²⁻) and chromite ion (CrO₂⁻).
Step 1: Determine the oxidation state of chromium in CrO₄²⁻.
The formula for the chromate ion is CrO₄²⁻.
- Oxygen typically has an oxidation state of –2.
- There are four oxygen atoms: 4 × (–2) = –8.
- The total charge of the ion is –2, so we can set up the equation:
Cr+(−8)=−2⇒Cr=+6\text{Cr} + (-8) = -2 \Rightarrow \text{Cr} = +6
So, chromium is in the +6 oxidation state in CrO₄²⁻.
Step 2: Determine the oxidation state of chromium in CrO₂⁻.
The formula for the chromite ion is CrO₂⁻.
- Again, oxygen has an oxidation state of –2.
- Two oxygen atoms: 2 × (–2) = –4.
- Total charge of the ion is –1, so:
Cr+(−4)=−1⇒Cr=+3\text{Cr} + (-4) = -1 \Rightarrow \text{Cr} = +3
So, chromium is in the +3 oxidation state in CrO₂⁻.
Step 3: Compare the oxidation states
Chromium changes from:
- +6 in CrO₄²⁻
- to +3 in CrO₂⁻
This is a reduction, because the oxidation number is decreasing. When an atom is reduced, it gains electrons. +6→+3⇒3 electrons gained+6 \rightarrow +3 \Rightarrow \text{3 electrons gained}
Conclusion:
The chromium atom gains 3 electrons during this process.
Thus, the correct answer is:
D. 3 electrons gained.