DP Chemistry (last assessment 2024)

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Question 18M.3.hl.TZ1.10

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Date May 2018 Marks available [Maximum mark: 5] Reference code 18M.3.hl.TZ1.10
Level hl Paper 3 Time zone TZ1
Command term Explain, Outline Question number 10 Adapted from N/A
10.
[Maximum mark: 5]
18M.3.hl.TZ1.10

Anthocyanins are naturally occurring plant pigments. Depending on the solution pH, they can exist as quinoidal bases or flavylium cations as shown in section 35 of the data booklet.

(a)

Outline why anthocyanins are coloured.

[2]

Markscheme

highly conjugated systems

OR

alternating single and double bonds

OR

many delocalized electrons

 

electron transitions occur when visible light is absorbed

 

[2 marks]

(b)

Explain why the blue colour of a quinoidal base changes to the red colour of a flavylium cation as pH decreases.

[3]

Markscheme

gaining protons

decreases electron density/extent of conjugation «in aromatic backbone»

increases energy of electron transitions

 

[3 marks]

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