DP Chemistry (last assessment 2024)

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Question 19M.2.hl.TZ2.5e

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Date May 2019 Marks available [Maximum mark: 2] Reference code 19M.2.hl.TZ2.5e
Level hl Paper 2 Time zone TZ2
Command term Discuss Question number e Adapted from N/A
e.
[Maximum mark: 2]
19M.2.hl.TZ2.5e

Carbonated water is produced when carbon dioxide is dissolved in water under pressure. The following equilibria are established.

Equilibrium (1)  CO2 (g)  CO2 (aq)

Equilibrium (2)  CO2 (aq) + H2O (l) H+ (aq) + HCO3− (aq)

(e)

The reaction of the hydroxide ion with carbon dioxide and with the hydrogencarbonate ion can be represented by Equations 3 and 4.

Equation (3)     OH− (aq) + CO2 (g) → HCO3− (aq)
Equation (4)     OH− (aq) + HCO3− (aq) → H2O (l) + CO32− (aq)

Discuss how these equations show the difference between a Lewis base and a Brønsted–Lowry base.

 

Equation (3):

Equation (4):

[2]

Markscheme

Equation (3):
OH- donates an electron pair AND acts as a Lewis base     [✔]

Equation (4):
OH- accepts a proton/H+/hydrogen ion AND acts as a Brønsted–Lowry base     [✔]

Examiners report

This was a good way to test this topic because answers showed that, while candidates usually knew the topic in theory, they could not apply this to identify the Lewis and Bronsted-Lowry bases in the context of a reaction that was given to them. In some cases, they failed to specify the base, OH- or also lost marks referring just to electrons, an electron or H instead of hydrogen ions or H+ for example.

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