Q.Why in the redox titration of $KMnO_4$ vs oxalic acid, we heat oxalic acid solution before starting the titration?

The key idea is the Arrhenius Equation: $k = A e^{-E_a/RT}$. The reaction between $KMnO_4$ and oxalic acid has a high activation energy ($E_a$). At room temperature, the reaction is too slow to give a sharp endpoint.

Reasoning:

1. The titration relies on the rapid decolourisation of $KMnO_4$ by oxalic acid in acidic medium.
2. At room temperature, the rate is sluggish — the first few drops of $KMnO_4$ take time to decolourise, making the endpoint detection difficult and inaccurate.
3. Heating (to about 60–70°C) increases the kinetic energy of molecules, effectively raising the fraction of molecules with energy ≥ $E_a$, thus dramatically increasing the reaction rate. …

The Arrhenius equation is the key to understanding this. It tells us that the rate constant $k$ of a reaction depends exponentially on temperature:

Here, $E_a$ is the activation energy — the minimum energy colliding molecules must have for a reaction to occur. For the reaction between permanganate ions ($MnO_4^-$) and oxalic acid ($H_2C_2O_4$), $E_a$ is quite large. At room temperature, very few collisions have enough energy, so the reaction is painfully slow — you'd be waiting minutes for a single drop of $KMnO_4$ to decolourise. Heating dramatically increases the fraction of molecules with energy $\ge E_a$, speeding up the reaction to a practical rate.

Let's walk through the titration logic step by step.

1. The reaction itself is autocatalytic. The balanced equation in acidic medium is:

The $Mn^{2+}$ ions produced act as a catalyst for the further reaction. So initially, even at room temperature, the reaction is extremely slow. Once a little $Mn^{2+}$ forms, it speeds up — but that initial lag makes the endpoint unclear and the titration inaccurate.

2. Heating solves the initial lag. By warming the oxalic acid solution to about 60–70 °C (never boiling, as oxalic acid can decompose), we supply enough thermal energy to overcome the activation barrier right from the start. The first few drops of $KMnO_4$ react quickly, producing $Mn^{2+}$ immediately. The autocatalytic effect then kicks in, and the reaction proceeds briskly throughout the titration.

3. Why not heat the $KMnO_4$ solution? $KMnO_4$ is thermally unstable. Heating it would cause it to decompose:

This changes the concentration of the titrant and introduces $MnO_2$ (a brown precipitate), ruining the titration. So we only heat the oxalic acid solution. …

Concept: Kinetics of Redox Reactions

The reaction between potassium permanganate ($KMnO_4$) and oxalic acid ($H_2C_2O_4$) is a redox titration. The relevant concept here is reaction rate — specifically, why the reaction is slow at room temperature and how we speed it up.

Method: Activation Energy & Temperature Effect

Method name: Temperature-controlled kinetic acceleration

Steps:

1. Identify the reaction The balanced redox reaction is:

2. Recognize the rate-limiting step

   At room temperature, the reaction between $MnO_4^-$ and $C_2O_4^{2-}$ is very slow. The purple colour of $KMnO_4$ takes a long time to disappear, making endpoint detection impractical.

3. Apply the concept of activation energy

   The reaction has a high activation energy barrier. According to the Arrhenius equation:

Increasing temperature ($T$) increases the rate constant ($k$), speeding up the reaction.

4. Heat the oxalic acid solution

   We heat the oxalic acid solution to about 60–70°C (not boiling). This provides enough energy for molecules to overcome the activation barrier.

5. Observe the autocatalytic effect …

Here are the common mistakes students make when answering why oxalic acid is heated before titration with $KMnO_4$, along with how to avoid each.

Mistake 1: Saying "to increase the rate of reaction" without specifying the slow step

Why it’s wrong:

This is too vague. Many reactions speed up on heating — the examiner wants the specific chemical reason.

How to avoid:

Always mention that the reaction between $KMnO_4$ and oxalic acid is autocatalytic (catalysed by $Mn^{2+}$ ions produced). Initially, the reaction is very slow because $Mn^{2+}$ is absent. Heating provides the activation energy to start the reaction, generating the first $Mn^{2+}$ ions, which then catalyse the rest.

Key point: Heat is needed to initiate the autocatalytic cycle, not just to speed up a fast reaction.

Mistake 2: Confusing the role of heat with decomposition of oxalic acid

Why it’s wrong:

Oxalic acid decomposes on strong heating (above ~150°C) to give $CO_2$, $CO$, and water. But in titration, we only warm (50–60°C), not boil. Students often write that we heat to "decompose oxalic acid" — which would ruin the titration.

How to avoid:

Clearly state: We warm the solution to around 50–60°C (not boiling). This is enough to overcome the activation energy barrier without decomposing the oxalic acid.

Mistake 3: Forgetting the autocatalytic role of $Mn^{2+}$

Why it’s wrong:

Some students say "heat increases kinetic energy of molecules" — which is true but incomplete. The real mechanism involves $Mn^{2+}$ as a catalyst.

How to avoid:

Write the two-step mechanism in your answer:

1. Slow step (without catalyst):

   $2MnO_4^- + 5C_2O_4^{2-} + 16H^+ \rightarrow 2Mn^{2+} + 10CO_2 + 8H_2O$

   (very slow initially)

2. Fast step (catalysed by $Mn^{2+}$):

   $Mn^{2+}$ formed in step 1 catalyses further reaction, making it rapid.

Heat is needed to initiate step 1 so that step 2 can take over.

Mistake 4: Thinking heat is needed throughout the titration

Why it’s wrong:

Once the reaction starts and $Mn^{2+}$ builds up, the reaction becomes fast even without continued heating. Students sometimes keep the flask on a hot plate the whole time.

How to avoid:

Explain that heating is only required at the start. After the first few drops of $KMnO_4$ decolourise, the autocatalysis sustains the rate. In fact, continued heating might cause oxalic acid decomposition or loss of volume by evaporation.

Mistake 5: Not mentioning the colour change as evidence

Why it’s wrong: …