Chemical reactions - Redox (3)
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1.
The compound of iron(II) chloride is FeCl2. What is the oxidation number of iron (Fe) in this compound? What is the oxidation number of chlorine (Cl)?
Iron(II) chloride means the iron is in the +2 oxidation state. The oxidation number of chlorine is typically -1. Since there are two chloride ions, the total negative charge is -2 (2 x -1 = -2). The +2 oxidation state of iron balances the -2 charge from the two chloride ions, resulting in a neutral compound.
Answer: Iron (Fe) = +2, Chlorine (Cl) = -1
2.
Consider the following half-reaction:
NO2(g) → NO(g) + O2(g)
Identify the species that is oxidized and the species that is reduced. State the change in oxidation state for each species.
Oxidation: Nitrogen dioxide (NO2) is oxidized. The oxidation state of nitrogen changes from +4 in NO2 to +2 in NO. This represents a loss of 2 electrons.
Reduction: Oxygen (O2) is reduced. The oxidation state of oxygen changes from 0 in O2 to -2 in O. This represents a gain of 2 electrons.
In summary:
NO2(g) → NO(g) + O2(g)
Nitrogen: +4 → +2 (Oxidation - Loss of 2 electrons)
Oxygen: 0 → -2 (Reduction - Gain of 2 electrons)
3.
A student states: "Oxidation always involves the gain of electrons, and reduction always involves the loss of electrons." Explain why this statement is incorrect, using examples to support your answer.
The student's statement is incorrect. Oxidation and reduction are defined by the gain or loss of electrons, not oxygen.
Oxidation is defined as the loss of electrons.
Reduction is defined as the gain of electrons.
Here are some examples:
- Rusting of iron: Iron (Fe) loses electrons (is oxidized) to become iron ions (Fe2+ or Fe3+). This is oxidation. The electrons are gained by oxygen, which is reduced.
- The reaction of sodium with chlorine: Sodium (Na) loses an electron (is oxidized) to become a sodium ion (Na+). Chlorine gains an electron (is reduced) to become a chloride ion (Cl-).
- Photosynthesis: Carbon dioxide (CO2) gains electrons (is reduced) to form glucose (C6H12O6). Oxygen loses electrons (is oxidized).
Therefore, the terms oxidation and reduction are always defined in terms of electron transfer, regardless of whether oxygen is involved.