I'm faced a problem in form 4 chemistry...

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[JTSL_yvone]

To all Chemistry teachers, i'm faced a problem while i'm doing exercise.This question is from my subject teacher.

• When excess ammonia solution is added into copper(II)sulphate solution, a dark blue colour is formed. Explain.

my answer is,

CuSO₄₊4NH₃→(NH₄)₂SO₄+Cu
A dark blue colour is formed because that is reaction occured.

I konw the equation is wrong. Because i can't balenced the equation. Help me pls...

[Habsah Ibrahim]

Hello Yvone,

Please introduce yourself first if you really need help. See what you have posted here? Please take note. Thank you.

I'm student from SMK St.Augustine ^^
Form 4 student...
Dont ask me who I am==

[JTSL_yvone]

Hello Yvone,

Please introduce yourself first if you really need help. See what you have posted here? Please take note. Thank you.

Ops! Very sorry!
This acc is from my friend,
my firend register it for me.
Sorry. I will introduce myself.
Help me pls...
sorry...

[Tay Guan Piang]

Ops! Very sorry!
This acc is from my friend,
my firend register it for me.
Sorry. I will introduce myself.
Help me pls...
sorry...

Hi, Yvonne,

This is actually qualitative analysis of salts. The addition of ammonia to CuSO₄ is to prove the presence of copper (II) ion.

That's why it produced dark blue colour.

Of course your equation and your concept is totally wrong.

[Liew Hui Lee]

Ammonia solution is same as ammonium hydroxide, NH₄OH.
In this test, the equation will be more correctly if we use the following equation : -
CuSO₄ +  2NH₄)OH -> Cu(OH)₂ + (NH₄ )₂SO₄
Or the ionic equation :
Cu²⁺ + 2OH^- -> Cu(OH)₂
So, the dark blue precipitate is actually the solid copper(II) hydroxide.
How about the other teachers' opinion?

[Tay Guan Piang]

Thanks Cgu Liew for the explanation behind this reaction. i suppose this would be a better explanation for the students.

[Jong Kah Yin]

Adding an aqueous solution of sodium hydroxide will form a blue precipitate of copper(II) hydroxide. The ionic equation is:

    Cu²⁺ (aq) + 2 OH⁻ (aq) → Cu(OH)₂ (s)

The full equation shows that the reaction is due to hydroxide ions deprotonating the hexaaquacopper(II) complex:

    [Cu(H2O)₆]²⁺ (aq) + 2 OH⁻(aq) → Cu(H₂O)₄(OH)₂ (s) + 2 H₂O (l)

Adding ammonium hydroxide (aqueous ammonia) causes the same precipitate to form. Upon adding excess ammonia, the precipitate dissolves, forming a dark blue ammonia complex, tetraamminecopper(II):

    Cu(H₂O)₄(OH)₂ (s) + 4 NH₃ (aq) → [Cu(H₂O)₂(NH₃)₄]₂+ (aq) + 2 H₂O (l) + 2 OH⁻ (aq)

This is for teachers' knowledge to explain the observation

[Tay Guan Piang]

Thanks a lot, Mr. Jong for this further clarification. it really makes me understand more.

[Choo Yih Teen]

Complex ion, Tetraamminediaquacopper(II), [Cu(NH3)₄(H2O)₂]²⁺, a characteristic dark blue complex formed by adding excess ammonia to solution of copper(II) salts.
 

[Mohd Rizuan Mohd Nor]

Adding an aqueous solution of sodium hydroxide will form a blue precipitate of copper(II) hydroxide. The ionic equation is:

    Cu²⁺ (aq) + 2 OH⁻ (aq) → Cu(OH)₂ (s)

The full equation shows that the reaction is due to hydroxide ions deprotonating the hexaaquacopper(II) complex:

    [Cu(H2O)₆]²⁺ (aq) + 2 OH⁻(aq) → Cu(H₂O)₄(OH)₂ (s) + 2 H₂O (l)

Adding ammonium hydroxide (aqueous ammonia) causes the same precipitate to form. Upon adding excess ammonia, the precipitate dissolves, forming a dark blue ammonia complex, tetraamminecopper(II):

    Cu(H₂O)₄(OH)₂ (s) + 4 NH₃ (aq) → [Cu(H₂O)₂(NH₃)₄]₂+ (aq) + 2 H₂O (l) + 2 OH⁻ (aq)

This is for teachers' knowledge to explain the observation

I need to flashback my memory in University to understand this... quit interesting if our students can understand this...

[Chia Sia Hock]

For form 4 level, what you need to understand is copper(II) ions insoluble in excess NaOH but soluble in excess aqueous ammonia solution. (form blue precipitate if NaOH and ammonia are not in excess)
I will answer the question in this way: Copper(II) sulphate form dark blue solution in excess ammonia solution because copper(II) ions in the solution are soluble in excess ammonia solution. (I think the question tests students in the way that why no precipitate is formed in excess ammonia solution) So, answer should be something to do with the solubility.
Let's check:
1) Mark given for understanding the solubility of copper(II) ions in excess ammonia solution (qualitative analysis)
2) Mark is not given to equations, this topic does not test students to write equations. Even though teachers are able to explain with the equations, they are too complicated to be understand by students.
So, I think the answer I suggested above enough to answer the question....
Any opinion from other teachers?

[Patrick Chai Teck Khiong]

I want to share opinion here,
As i know ammonium hydroxide cannot be used because there is no ammonium hydroxide.
The correct writing is add ammonia solution into copper(II) sulphate solution.(Is not add ammonium hydroxide)
Any opinion from other teacher?

[Ting Tin Chung]

I want to share opinion here,
As i know ammonium hydroxide cannot be used because there is no ammonium hydroxide.
The correct writing is add ammonia solution into copper(II) sulphate solution.(Is not add ammonium hydroxide)
Any opinion from other teacher?

Ya... i agree with Mr Patrick....... we only have ammonia solution....
NH₃ + H₂O  -->  NH₄⁺ + OH^-

because the present of OH^- ion, so when ammonia dissolve into water, it shows alkaline properties.

[Chia Sia Hock]

I want to share opinion here,
As i know ammonium hydroxide cannot be used because there is no ammonium hydroxide.
The correct writing is add ammonia solution into copper(II) sulphate solution.(Is not add ammonium hydroxide)
Any opinion from other teacher?

Aqueous ammonia solution contains ammonium hydroxide. Ammonia is a gas that dissolves partially in the water to produce low concentration of ammonium hydroxide.
Basically, when we use aqueous ammonia solution to test cation Cu²⁺, the reaction is betwen Cu²⁺ and hydroxide ions. It forms dark blue precipitate which is copper(II) hydroxide. Where the hydroxide ions come from? They are from ammonium hydroxide which produced when ammonia gas dissolved partially in the water.

So, I think we can say ammonium hydroxide can be used to test copper(II) ions because it is same with ammonia solution.

[Patrick Chai Teck Khiong]

Try to share my opinion here:
Aqueous ammonia contain mostly ammonia molecules, a portion of ammonium ion and hydroxide ion..
If u say ammonium hydroxide, then the equation will be like this
NH₄OH --> NH₄⁺ +OH^-
This equation only contain ammonium ion and hydroxide ion, then where is the ammonia molecules? Thus, there is no ammonium hydroxide.

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