Which of the following reaction is possible at anode ?
(a) F2 + 2e^- - 2F^-
(b) 2H^+ + 1/2 O2 + 2e^

Q: Does the anode always have a positive charge?

Not always! In an electrolytic cell, the anode is positive, but in a galvanic (voltaic) cell, the anode is negative. However, oxidation *always* occurs at the anode, regardless of the cell type.

Q: How can I quickly identify an oxidation reaction?

Look for electrons on the product side of the chemical equation. If a species is losing electrons, its oxidation state will increase.

Q: Why is option (a) incorrect?

In option (a), fluorine gas ($F_2$) is gaining electrons to become fluoride ions ($F^-$). Gaining electrons is reduction, which happens at the cathode.

Q: What is happening to the oxidation state of Chromium in option (c)?

Chromium starts with an oxidation state of $+3$ in $Cr^{3+}$. In the dichromate ion ($Cr_2O_7^{2-}$), each chromium atom has an oxidation state of $+6$. This increase from $+3$ to $+6$ confirms it's an oxidation process.

Q: Is water being oxidized or reduced in option (b)?

In option (b), oxygen gas ($O_2$) is being reduced to form water ($H_2O$). The oxidation state of oxygen goes from $0$ to $-2$.

Question

Which of the following reaction is possible at anode ?

Options:

(A) $F_2 + 2e^- \longrightarrow 2F^-$

(B) $2H^+ + \frac{1}{2}O_2 + 2e^- \longrightarrow H_2O$

(C) $2Cr^{3+} + 7H_2O \longrightarrow Cr_2O_7^{2-} + 14H^+ + 6e^-$

(D) None of the above

Sigma Percentile · Accepted Answer

### The Anode's Secret: Unveiling the Dance of Electrons

Electrochemistry can sometimes feel like a complex puzzle of moving parts, but at its core, it is simply the study of how electrons travel. To master this topic, you need to firmly anchor a few fundamental rules in your mind. One of the most unbreakable rules in all of chemistry is this: **Oxidation always occurs at the anode**.

Whether you are dealing with a galvanic cell that generates electricity or an electrolytic cell that consumes it, the anode is the stage where electrons are lost. A handy mnemonic to remember this is **"An Ox"** (Anode = Oxidation) and **"Red Cat"** (Reduction = Cathode).

### Decoding the Options

To find out which reaction is possible at the anode, we must act as electron detectives. We need to examine each given reaction and determine if electrons are being lost (oxidation) or gained (reduction).

Let's look at the first candidate:

$$F_2 + 2e^- \longrightarrow 2F^-$$

In this reaction, fluorine gas ($F_2$) is reacting with two electrons to form fluoride ions ($F^-$). Because the electrons are on the reactant side, they are being consumed or gained. Gaining electrons is the very definition of reduction. Therefore, this reaction would take place at the cathode, not the anode.

Now, let's evaluate the second option:

$$2H^+ + \frac{1}{2}O_2 + 2e^- \longrightarrow H_2O$$

Here, hydrogen ions and oxygen gas are combining with two electrons to produce water. Once again, electrons are acting as reactants. They are being gained by the oxygen atoms (whose oxidation state drops from $0$ to $-2$). This is another classic reduction reaction, disqualifying it from occurring at the anode.

### The Chromium Revelation

Finally, we arrive at the third option:

$$2Cr^{3+} + 7H_2O \longrightarrow Cr_2O_7^{2-} + 14H^+ + 6e^-$$

Take a close look at where the electrons are in this equation. They are on the product side! This means that as the reaction proceeds, six electrons are being released or lost.

If we want to be absolutely certain, we can check the oxidation states. The chromium ion starts with an oxidation state of $+3$. In the dichromate ion ($Cr_2O_7^{2-}$), each chromium atom has an oxidation state of $+6$. An increase in oxidation state is the mathematical proof of oxidation.

Because this reaction represents a clear loss of electrons, it is an **oxidation** process. And since oxidation is the exclusive domain of the anode, this is the only reaction among the choices that can possibly occur there.

Mastering these simple definitions of electron transfer will make navigating electrochemical cells a breeze!

Animated Solution for Chemistry - Electrochemistry: Which of the following reaction is possible at anode ?

Select Answer:

Visualized Solution

Anode \text{ and } Oxidation

Analyzing \text{ Option (a)}

Analyzing \text{ Option (b)}

Analyzing \text{ Option (c)}

Conclusion

The Sigma Insight: Electrochemical Cells

The Anode's Secret

Decoding the Options

The Chromium Revelation

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