Animated Solution for Chemistry - Redox Reactions: Which of the following is a redox reaction ?

When we talk about chemical reactions, we often categorize them based on what exactly is happening at the atomic level. Are ions simply swapping partners? Is an acid neutralizing a base? Or, is there a fundamental transfer of electrons from one species to another?

This last category is what we call a Redox Reaction (short for Reduction-Oxidation). The defining characteristic of a redox reaction is a change in the oxidation states of the participating elements. If an element loses electrons, its oxidation state increases, and we say it has been oxidized. Conversely, if an element gains electrons, its oxidation state decreases, and we say it has been reduced.

To solve this problem, our mission is straightforward: we must act as chemical detectives, assigning oxidation states to every element in the reactants and products for each given option. The reaction where we spot a change is our culprit!

Options (a), (b), and (c)

Let's start by examining the first three options.

Option (a): Here, we have Sodium Chloride reacting with Potassium Nitrate. If we assign oxidation states, we find $\text{Na}$ is $+1$, $\text{Cl}$ is $-1$, $\text{K}$ is $+1$, $\text{N}$ is $+5$, and $\text{O}$ is $-2$. Looking at the products, these values remain absolutely identical. This is a classic double displacement reaction where ions simply exchange their counter-ions. No electrons are transferred.

Option (b): In this reaction between Calcium Oxalate and Hydrochloric Acid, Calcium remains at $+2$, Carbon at $+3$, Oxygen at $-2$, Hydrogen at $+1$, and Chlorine at $-1$ throughout the process. Again, no change in oxidation states means no redox activity.

Option (c): This is an acid-base interaction. Calcium is $+2$, Oxygen is $-2$, Hydrogen is $+1$, Nitrogen is $-3$, and Chlorine is $-1$. Just like the previous examples, the oxidation states on the right side of the arrow are a perfect mirror of the left side.

Option (d)

Now, let's turn our attention to the final option, which looks a bit more complex:

Let's focus on the key players here: Silver ($\text{Ag}$) and Zinc ($\text{Zn}$).

1. Zinc's Journey: On the reactant side, Zinc is in its pure, elemental solid state. By definition, the oxidation state of any element in its free state is zero. On the product side, Zinc has formed a complex ion, $[{\text{Zn(CN)}}_4{]}^{2-}$. Since each Cyanide (${\text{CN}}^{-}$) ligand carries a $-1$ charge, Zinc must have an oxidation state of $+2$ to balance the overall charge. Zinc has gone from $0$ to $+2$, meaning it has lost two electrons. This is Oxidation.

2. Silver's Journey: In the reactant complex $[{\text{Ag(CN)}}_2{]}^{-}$, Silver has an oxidation state of $+1$. On the product side, Silver has been displaced and is now in its elemental solid state, meaning its oxidation state has dropped to zero. Silver has gone from $+1$ to $0$, meaning it has gained an electron. This is Reduction.

Because we have a clear, simultaneous transfer of electrons—Zinc losing them and Silver gaining them—we can definitively conclude that this is a Redox Reaction. Specifically, this is a metal displacement reaction, driven by the fact that Zinc is a more reactive metal than Silver, allowing it to force electrons onto the Silver ions and take their place in the coordination complex.