6$AgNO_{4(aq)} + PH_{3(g)} + 3H_2O_{(l)}$ → 6$Ag_{(s)} + H_3PO_{3(g)} + 6HNO_{3(aq)}$ In the above reaction, the reducing agent is?

Analysis of the Question

To identify the reducing agent in a redox reaction, we need to track the changes in the oxidation states of the elements involved. A reducing agent is the substance that gets oxidized (loses electrons or increases in oxidation number) during the reaction.

Concept Explanation

• Oxidation: An increase in oxidation number (loss of electrons).
• Reduction: A decrease in oxidation number (gain of electrons).
• Reducing Agent: The species that causes reduction by being oxidized itself.
• Oxidizing Agent: The species that causes oxidation by being reduced itself.

Step-by-Step Solution

1. Assign oxidation numbers to the reactants and products: • In $AgNO_3$: $Ag$ is $+1$, $N$ is $+5$, $O$ is $-2$. • In $PH_3$: $P$ is $-3$, $H$ is $+1$. • In $Ag_{(s)}$: The oxidation state of an element in its standard state is $0$. • In $H_3PO_3$: $H$ is $+1$, $O$ is $-2$. Let $P = x$. $3(1) + x + 3(-2) = 0 \implies 3 + x - 6 = 0 \implies x = +3$.

2. Track the changes: • Silver ($Ag$): Goes from $+1$ in $AgNO_3$ to $0$ in $Ag_{(s)}$. Since the oxidation number decreased, $AgNO_3$ is the oxidizing agent. • Phosphorus ($P$): Goes from $-3$ in $PH_3$ to $+3$ in $H_3PO_3$. Since the oxidation number increased (from $-3$ to $+3$), $PH_3$ has lost electrons.

3. Conclusion: Because $PH_3$ increased in oxidation state, it is the substance being oxidized. Therefore, $PH_3$ is the reducing agent.

Identifying the Answer

Comparing our findings with the given options: A. $HNO_3$ (Product) B. $H_2O$ (Reactant, but not the reducing agent) C. $PH_3$ (The species being oxidized) D. $AgNO_3$ (The oxidizing agent)

The correct option is C.

Exam Tip

Always remember the mnemonic OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). If you see an element's oxidation number increase from the reactant side to the product side, that substance is your reducing agent!