Explaining the Reaction of Reducing Agents: Ammonia vs. Sulfur Dioxide

[Kyoma]

Homework Statement

Ammonia and sulfur dioxide are reducing agents. Acidified potassium dichromate can be used to test for reducing agents. But sulfur dioxide can turn potassium dichromate from orange to green but ammonia cannot. Why?

2. The attempt at a solution
From what I know, potassium dichromate is an oxidising agent, thus when it meets with ammonia and sulfur dioxide, it will turn from orange to green. However, ammonia cannot. I just need some rough idea on why because I'm still studying chemistry according to the syllabus in the British GCE O level. Thanks.

 

[Borek]

Think ammonia and low pH.

 

[Kyoma]

Ammonia has a high pH, yes, 'cause it's alkaline.

I don't know whether this is correct or wrong:

Cr₂O_-7^2- + 14H⁺ + 6e -----> 2Cr³⁺ + 7H₂O... then is it because ammonia can't produce that 14 H⁺ for the reaction to proceed?

 

[Borek]

Question asks about ACIDIFIED potassium dichromate, so pH is low, not high. Half reaction you wrote is correct, but irrelevant.

 

[Kyoma]

Potassium dichromate is acidic while ammonia is alkaline... neutralisation occurs so the potassium dichromate doesn't change colour?

 

[Borek]

No, there is a huge excess of acid, so pH doesn't change much after adding ammonia.

What happens to ammonia in low pH?

 

[Kyoma]

Ammonia has low pH?

 

[Borek]

What happens to ammonia added to solution of low pH, if there is enough excess acid that the pH is still low after ammonia gets neutralized.

Alternatively: what happens to ammonia when it gets neutralized? How does ammonia react with acids? Is it still ammonia after the reaction?

 

[Kyoma]

So there will be a salt and water.. neutralisation occurs so no colour change?

 

[chemisttree]

NH3 is not the same as NH4+.

 

[Kyoma]

Isn't ammonia alkaline? I read wikipedia and it says that ammonia will self-ionise to form ammonium ions. I don't get it.

 

[Borek]

In low pH ammonia gets protonated to NH₄⁺ - and this form is dominating in the solution. NH₄⁺ has different properties from NH₃.

You are right that ammonia on itself is alkaline, but here it is not on itself - it is added to the acidic solution, and there is not not enough ammonia to neutralize the solution, so even after ammonia gets protonated pH stays low.