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Chemistry of the D-Block Transition Metals

Reactions of Cr³⁺, Fe²⁺, Fe³⁺ and Cu²⁺ with excess aqueous hydroxide

  • 3 things to know
  • Component 1

Overview

Recall questions on the precipitate formed by each of the four ions and its colour, which of them dissolves in excess hydroxide and why, the ionic equations, the darkening of the iron(II) precipitate in air, and using the pattern to identify an unknown ion.

What you need to know

Answer each one before you reveal it.

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Key term

Describe what is seen when a small amount of aqueous sodium hydroxide is added to a solution containing Cr³⁺ ions.

Key term

Write the ionic equation for the precipitation of chromium(III) hydroxide.

Key term

Describe what happens when aqueous sodium hydroxide is added to a chromium(III) solution until it is in excess.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Describe what is seen when a small amount of aqueous sodium hydroxide is added to a solution containing Cr³⁺ ions.

    Model answer

    A green precipitate of chromium(III) hydroxide is formed.

  2. 2

    Write the ionic equation for the precipitation of chromium(III) hydroxide.

    Model answer

    Cr³⁺(aq) + 3OH⁻(aq) → Cr(OH)₃(s).

  3. 3

    Describe what happens when aqueous sodium hydroxide is added to a chromium(III) solution until it is in excess.

    Model answer

    The green precipitate dissolves to give a green solution containing the [Cr(OH)₆]³⁻ ion.

  4. 4

    What does the behaviour of chromium(III) hydroxide in excess alkali show about it?

    Model answer

    It shows that chromium(III) hydroxide is amphoteric.

  5. 5

    Describe what is seen when aqueous sodium hydroxide is added to a solution containing Fe²⁺ ions.

    Model answer

    A green precipitate of iron(II) hydroxide is formed.

  6. 6

    Write the ionic equation for the precipitation of iron(II) hydroxide.

    Model answer

    Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s).

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