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AQA · GCSE · Chemistry · Higher

Explaining copper chromate ion migration

Q05.5 3 marks

A student investigated the electrolysis of green copper chromate solution.

Figure 4 shows the apparatus.

Figure 4
Figure 4

Figure 4

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Figure 4

Figure 5 shows the results.

Figure 5
Figure 5

Figure 5

Enlarged question image

Figure 5

Copper chromate solution contains the ions Cu²⁺ and CrO₄²⁻.

Explain the results shown in Figure 5.

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Marking points

  1. 1 Identifies both Cu²⁺ ions as blue and CrO₄²⁻ ions as yellow.
  2. 2 Explains that positive Cu²⁺ ions move towards the negative electrode/cathode.
  3. 3 Explains that negative CrO₄²⁻ ions move towards the positive electrode/anode.

Full-mark answer

The blue colour is due to Cu²⁺ ions and the yellow colour is due to CrO₄²⁻ ions. The positively charged Cu²⁺ ions are attracted to and move towards the negative electrode, while the negatively charged CrO₄²⁻ ions move towards the positive electrode.

Why this answer loses marks

Both coloured ions move to the same electrode.

The colours identify oppositely charged ions, so each moves towards the opposite electrode polarity.

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