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

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In summary, ammonia and sulfur dioxide are reducing agents, and acidified potassium dichromate can be used to test for reducing agents. However, sulfur dioxide can turn potassium dichromate from orange to green, while ammonia cannot. This is because ammonia has a high pH and does not produce enough H+ ions to react with the potassium dichromate, which requires a low pH to change color. When ammonia is added to a low pH solution, it gets neutralized and forms NH4+ ions, which have different properties and cannot cause a color change in potassium dichromate.
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Kyoma
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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.
 
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  • #2
Think ammonia and low pH.
 
  • #3
Ammonia has a high pH, yes, 'cause it's alkaline.

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

Cr2O-72- + 14H+ + 6e -----> 2Cr3+ + 7H2O... then is it because ammonia can't produce that 14 H+ for the reaction to proceed?
 
  • #4
Question asks about ACIDIFIED potassium dichromate, so pH is low, not high. Half reaction you wrote is correct, but irrelevant.
 
  • #5
Potassium dichromate is acidic while ammonia is alkaline... neutralisation occurs so the potassium dichromate doesn't change colour?
 
  • #6
No, there is a huge excess of acid, so pH doesn't change much after adding ammonia.

What happens to ammonia in low pH?
 
  • #7
Ammonia has low pH?
 
  • #8
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?
 
  • #9
So there will be a salt and water.. neutralisation occurs so no colour change?
 
  • #10
NH3 is not the same as NH4+.
 
  • #11
Isn't ammonia alkaline? I read wikipedia and it says that ammonia will self-ionise to form ammonium ions. I don't get it.
 
  • #12
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.
 

1. What is a reducing agent?

A reducing agent is a substance that has the ability to donate electrons to another substance, causing it to be reduced. This means that the reducing agent itself becomes oxidized in the process.

2. Why is it important to test for reducing agents?

Testing for reducing agents is important because they play a crucial role in many chemical reactions, such as redox reactions. They can also be used as indicators for the presence of certain compounds or substances in a sample.

3. How do you test for reducing agents?

One common test for reducing agents is the Benedict's test, which involves mixing the sample with Benedict's reagent and heating it. The presence of a reducing agent will cause a color change in the solution, indicating a positive result.

4. What are some examples of reducing agents?

Some common examples of reducing agents include metals such as magnesium and zinc, as well as compounds like sodium borohydride and hydrogen gas.

5. Can reducing agents be harmful?

Yes, some reducing agents can be harmful if they are ingested, inhaled, or come into contact with skin or eyes. It is important to handle them with caution and follow proper safety protocols when working with them.

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