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

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Discussion Overview

The discussion revolves around the behavior of ammonia and sulfur dioxide as reducing agents in the presence of acidified potassium dichromate. Participants explore why sulfur dioxide can reduce potassium dichromate, causing a color change, while ammonia does not exhibit the same behavior. The conversation includes aspects of chemistry related to pH levels, neutralization reactions, and the properties of ammonia in acidic conditions.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant notes that sulfur dioxide can reduce potassium dichromate from orange to green, while ammonia cannot, and seeks an explanation for this difference.
  • Another participant suggests considering the pH of ammonia, indicating that it is alkaline.
  • A participant questions whether ammonia's inability to produce sufficient H+ ions affects its ability to participate in the reaction with potassium dichromate.
  • It is pointed out that the question specifically refers to acidified potassium dichromate, implying that the pH is low.
  • One participant proposes that neutralization occurs when ammonia is added to the acidic solution, preventing a color change.
  • Another participant argues that there is a significant excess of acid, suggesting that the pH remains low even after ammonia is added.
  • Questions arise about the behavior of ammonia in low pH conditions, including its protonation to form ammonium ions (NH4+).
  • Clarifications are made regarding the distinction between ammonia (NH3) and ammonium ions (NH4+), emphasizing that their properties differ.

Areas of Agreement / Disagreement

Participants express differing views on the behavior of ammonia in acidic conditions and its role as a reducing agent. There is no consensus on the exact reasons for the observed differences between ammonia and sulfur dioxide in their reactions with potassium dichromate.

Contextual Notes

Participants discuss the implications of pH levels and the nature of neutralization reactions, but the discussion does not resolve the underlying assumptions about the reactions involved or the specific conditions required for each reducing agent.

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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Think ammonia and low pH.
 
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?
 
Question asks about ACIDIFIED potassium dichromate, so pH is low, not high. Half reaction you wrote is correct, but irrelevant.
 
Potassium dichromate is acidic while ammonia is alkaline... neutralisation occurs so the potassium dichromate doesn't change colour?
 
No, there is a huge excess of acid, so pH doesn't change much after adding ammonia.

What happens to ammonia in low pH?
 
Ammonia has low pH?
 
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?
 
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.
 

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