Why are standard KMnO4 solutions are seldom used for the titrations of solutions containing HCl?

2. Why are standard solutions of reductants less often for titrations than standard solutions of oxidants?
3. When a solution of KMnO4 was left standing in a buret for 3 hours, a browning ring forms at the surface of the liquid. Write a balance net equation to account for this observation?

1. The chloride ion is oxidized by KMnO4. It's a slow reaction and can be reduced by a few tricks but it causes more KMnO4 to be used than should be used. I'm not sure I would say "seldom used" for there was a time when KMnO4 titrations were quite popular. The use of a Zimmerman-Reinhart solution reduces the effect of the oxidation of chloride ion.

2.I don't know the answer to this question.

3. The brown color is MnO2 which is one of the decay products of KMnO4 and the chief reason why KMnO4 must be standardized fairly often. That's just another way of saying that solutions of KMnO4 are not stable. So the half equation will be KMnO4 ==> MnO2 in neutral solution. I'll leave it for you to balance.

1. Standard KMnO4 solutions are seldom used for titrations of solutions containing HCl because HCl is a strong reducing agent that can react with KMnO4, reducing the Mn(VII) present in KMnO4 to Mn(II). This reaction causes the characteristic purple color of KMnO4 to fade, making it difficult to determine the endpoint of the titration accurately.

2. Standard solutions of oxidants are more commonly used for titrations than standard solutions of reductants because oxidants tend to be more stable and have a longer shelf life. Reductants, on the other hand, can easily undergo self-oxidation reactions in the presence of air or other oxidizing agents, which can alter the concentration and accuracy of the standard solution.

3. The formation of a browning ring when a solution of KMnO4 is left standing in a buret for 3 hours can be accounted for by the following balanced net equation:

2 MnO4- + 3 H2O → 2 MnO2 + 4 OH- + 3O2

In this equation, MnO4- is reduced to MnO2, and oxygen (O2) is released. The MnO2 is responsible for the browning color observed in the solution.

1. Standard KMnO4 solutions are seldom used for titrations of solutions containing HCl because KMnO4 is a powerful oxidizing agent, and HCl is a strong reducing agent. In the presence of HCl, KMnO4 can be reduced to Mn2+ ions before it can react with the substance being titrated. This reduction reaction can lead to inaccuracies in the titration results since the KMnO4 is being consumed by the HCl instead of reacting with the target substance.

2. Standard solutions of reductants are less often used for titrations compared to standard solutions of oxidants because reductants are typically more unstable and can be easily oxidized by air or other oxidizing agents present in the environment. This can cause the concentration of the reductant to decrease over time, leading to inaccurate titration results. In contrast, oxidants are generally more stable and less likely to undergo spontaneous reduction, making them more suitable for standard solutions in titrations.

3. The formation of a browning ring at the surface of liquid in a buret containing KMnO4 that has been left standing for 3 hours is due to the reaction of KMnO4 with atmospheric oxygen. This reaction can be represented by the following balanced net equation:

2 MnO4- + 3 O2 → 2 MnO2 + 3 O2-

In this reaction, MnO4- is reduced to MnO2 while oxygen (O2) is reduced to superoxide (O2-). The resulting MnO2 is responsible for the brownish color observed.