Question

Identify the oxidizing and reducing agents in the following reactions: (a) \(8 \mathrm{H}^{+}(a q)+\mathrm{Cr}_{2} \mathrm{O}_{7}^{2-}(a q)+3 \mathrm{SO}_{3}^{2-}(a q) \longrightarrow\) \(2 \mathrm{Cr}^{3+}(a q)+3 \mathrm{SO}_{4}^{2-}(a q)+4 \mathrm{H}_{2} \mathrm{O}(l)\) (b) \(\mathrm{NO}_{3}^{-}(a q)+4 \mathrm{Zn}(s)+7 \mathrm{OH}^{-}(a q)+6 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow\) \(4 Z \mathrm{n}(\mathrm{OH})_{4}^{2-}(a q)+\mathrm{NH}_{3}(a q)\)

Short answer

In reaction (a), \(\text{Cr}_{2}\text{O}_{7}^{2-}\) is the oxidizing agent and \(\text{SO}_{3}^{2-}\) is the reducing agent. In reaction (b), \(\text{NO}_{3}^{-}\) is the oxidizing agent and \(\text{Zn}\) is the reducing agent.

Step-by-step solution

- Identify oxidation states in Reaction (a)

Determine the oxidation states of each element in the reactants and products for the given reaction (a):Reactants:- \(\text{H}^{+}\): +1- \(\text{Cr}_{2}\text{O}_{7}^{2-}\): Cr = +6, O = -2- \(\text{SO}_{3}^{2-}\): S = +4, O = -2Products:- \(\text{Cr}^{3+}\): +3- \(\text{SO}_{4}^{2-}\): S = +6, O = -2- \(\text{H}_{2}\text{O}\): H = +1, O = -2

- Identify changes in oxidation states for Reaction (a)

Compare the oxidation states of elements in reactants and products to identify which are being oxidized and reduced:- Chromium (Cr) changes from +6 in \(\text{Cr}_{2}\text{O}_{7}^{2-}\) to +3 in \(\text{Cr}^{3+}\). This is a reduction.- Sulfur (S) changes from +4 in \(\text{SO}_{3}^{2-}\) to +6 in \(\text{SO}_{4}^{2-}\). This is an oxidation.

- Identify oxidizing and reducing agents in Reaction (a)

The substance that is reduced (undergoes reduction) is the oxidizing agent and the substance that is oxidized (undergoes oxidation) is the reducing agent:- \(\text{Cr}_{2}\text{O}_{7}^{2-}\), which contains Cr undergoing reduction, is the oxidizing agent.- \(\text{SO}_{3}^{2-}\), which contains S undergoing oxidation, is the reducing agent.

- Identify oxidation states in Reaction (b)

Determine the oxidation states of each element in the reactants and products for the given reaction (b):Reactants:- \(\text{NO}_{3}^{-}\): N = +5, O = -2- \(\text{Zn}\): 0 (elemental)- \(\text{OH}^{-}\): H = +1, O = -2Products:- \(\text{Zn}(\text{OH})_{4}^{2-}\): Zn = +2, H = +1, O = -2- \(\text{NH}_{3}\): N = -3, H = +1

- Identify changes in oxidation states for Reaction (b)

Compare the oxidation states of elements in reactants and products to identify which are being oxidized and reduced:- Nitrogen (N) changes from +5 in \(\text{NO}_{3}^{-}\) to -3 in \(\text{NH}_{3}\). This is a reduction.- Zinc (Zn) changes from 0 (elemental Zn) to +2 in \(\text{Zn(OH)}_{4}^{2-}\). This is an oxidation.

- Identify oxidizing and reducing agents in Reaction (b)

The substance that is reduced (undergoes reduction) is the oxidizing agent and the substance that is oxidized (undergoes oxidation) is the reducing agent:- \(\text{NO}_{3}^{-}\), which contains N undergoing reduction, is the oxidizing agent.- \(\text{Zn}\), which undergoes oxidation, is the reducing agent.

Key concepts

Oxidizing Agents

In any chemical reaction, the oxidizing agent is the substance that gains electrons and thus gets reduced. A helpful way to remember this is by thinking of the oxidizing agent as the 'electron acceptor'.
For instance, in reaction (a), \(\text{Cr}_{2}\text{O}_{7}^{2-}\), containing chromium (Cr) which changes from +6 to +3, is the oxidizing agent because it undergoes reduction. Similarly, in reaction (b), \(\text{NO}_{3}^{-}\) is the oxidizing agent as nitrogen (N) within it changes its oxidation state from +5 to -3.
Key points to remember about oxidizing agents:

• They gain electrons (are reduced).
• They cause the oxidation of another substance.
• Look for the substance whose oxidation state decreases.

Reducing Agents

Conversely, the reducing agent in a chemical reaction is the substance that loses electrons and gets oxidized. Think of the reducing agent as the 'electron donor'.
In reaction (a), \(\text{SO}_{3}^{2-}\), containing sulfur (S) which changes from +4 to +6, is the reducing agent because it undergoes oxidation. Likewise, in reaction (b), zinc (Zn) is the reducing agent as it changes from 0 to +2 in oxidation state.
Important aspects of reducing agents:

• They lose electrons (are oxidized).
• They cause the reduction of another substance.
• Look for the substance whose oxidation state increases.

Oxidation States

Oxidation states, or oxidation numbers, are a convenient way to keep track of electron transfers in oxidation-reduction reactions. They help identify which atoms are oxidized and which are reduced.
The general rules for assigning oxidation states include:

• Elements in their elemental form have an oxidation state of 0 (e.g., \(H_{2}\), \(O_{2}\)).
• For ions, the oxidation state is equal to the charge of the ion (e.g., \(Na^{+}\) has an oxidation state of +1).
• Oxygen usually has an oxidation state of -2, and hydrogen is usually +1.
• The sum of oxidation states in a neutral compound is 0; in a polyatomic ion, it equals the ion's charge.

Chemical Reactions

Chemical reactions involve the transformation of reactants into products. Oxidation-reduction (redox) reactions are a specific subset where electrons are transferred between species. Such reactions encapsulate a wide range of phenomena, including combustion, corrosion, respiration, and photosynthesis.
In redox reactions, identifying the changes in oxidation states helps to pinpoint which substances are oxidized and which are reduced. Reactants like \(Zn\) turning into \(ZnO\) or \(Fe\) rusting into \(Fe_{2}O_{3}\) are classic examples of redox processes.

Redox Reaction

A redox (reduction-oxidation) reaction is a chemical reaction involving the transfer of electrons between two species. It includes both oxidation (loss of electrons) and reduction (gain of electrons) processes.
Consider the example reactions given:

• In reaction (a), \(8 \text{H}^{+} + \text{Cr}_{2}\text{O}_{7}^{2-} + 3 \text{SO}_{3}^{2-} \to 2 \text{Cr}^{3+} + 3 \text{SO}_{4}^{2-} + 4 \text{H}_{2}\text{O}\), chromium is reduced and sulfur is oxidized.
• In reaction (b), \( \text{NO}_{3}^{-} + 4 \text{Zn} + 7 \text{OH}^{-} + 6 \text{H}_{2}\text{O} \to 4 \text{Zn}(\text{OH})_{4}^{2-} + \text{NH}_{3}\), nitrogen is reduced and zinc is oxidized.

Here are key takeaways:

• Oxidation equals loss of electrons (OIL).
• Reduction equals gain of electrons (RIG).
• Oxidizing agents are reduced, and reducing agents are oxidized.