Chemical Tests

Question 0

What does tollens reagent do?

Answer 0

Weak oxidising agent that oxidises aldehydes but not ketones.

Ammoniacal silver nitrate.

Forms a silver mirror.

Ag+ is reduced to silver metal.
Ag+ (aq) + e- —> Ag(s)

Question 1

What does Brady’s reagent do?

Answer 1

Confirms presence of a carbonyl compound, only aldehyde or ketone.

Forms yellow/orange precipitate.

2,4-diphenylhydrazine.

Question 2

How can you identify the exact carbonyl compound?

Answer 2

2,4-DNP derivative is purified and recrystallised.

The melting point is measured and recorded, then compared to a database.

Question 3

How is tollens reagent made?

Answer 3

Aqueous silver nitrate is added to aqueous sodium hydroxide until a brown precipitate of silver oxide forms.

Dilute aqueous ammonia is added until the precipitate dissolves.

Question 4

How do these metals react with aqueous sodium hydroxide?
Cu2+
Co2+
Fe2+
Fe3+

Answer 4

Cu2+ + 2OH- —> Cu(OH)2
Pale blue —> pale blue precipitate.
Copper (II) hydroxide

Co2+ + 2OH- —> Co(OH)2
Pink —> blue, beige in air.
Colbalt (II) hydroxide

Fe2+ + 2OH- —> Fe(OH)2
Pale green —> green, rusty brown when exposed to air (oxidised)

Fe3+ + 3OH- —> Fe(OH)3
Pale yellow —> rusty brown

Question 5

Describe the reaction if aqueous copper (II) ions and ammonia.

Answer 5

[Cu(H2O)6]2+ + 4NH3 <><> [Cu(NH3)4(H2O2)]2+ + 4H2O

Octahedral product

Pale blue —> deep blue solution

Blue precipitate of Cu(OH)2 initially forms.

Question 6

Describe the reaction of copper (II) ions and hydrochloric acid.

Answer 6

[Cu(H2O)6]2+ + 4Cl- <><>
[CuCl4]2- + 6H2O

Pale blue —> green —> yellow

Chlorine larger than water, stronger repulsions.
Tetrahedral shape, like CH4.

Question 7

Describe the reaction of cobalt (II) ions and conc HCl.

Answer 7

[Co(H2O)6]2+ + 4Cl- <><>
[CoCl4]2- + 6H2O

Pink —> blue