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Date November 2017 Marks available 1 Reference code 17N.2.sl.TZ0.5
Level SL Paper 2 Time zone TZ0
Command term State Question number 5 Adapted from N/A

Question

Many reactions are in a state of equilibrium.

The equations for two acid-base reactions are given below.

HCO3 (aq) + H2O (l) \( \rightleftharpoons \) H2CO3 (aq) + OH (aq)
HCO3 (aq) + H2O (l) \( \rightleftharpoons \) CO32– (aq) + H3O+ (aq)

The following reaction was allowed to reach equilibrium at 761 K.

H2 (g) + I2 (g) \( \rightleftharpoons \) 2HI (g)               ΔHθ < 0

Outline the effect, if any, of each of the following changes on the position of equilibrium, giving a reason in each case.

[2]
a.

Identify two different amphiprotic species in the above reactions.

[1]
b.i.

State what is meant by the term conjugate base.

[1]
b.ii.

State the conjugate base of the hydroxide ion, OH.

[1]
b.iii.

A student working in the laboratory classified HNO3, H2SO4, H3PO4 and HClO4 as acids based on their pH. He hypothesized that “all acids contain oxygen and hydrogen”.

Evaluate his hypothesis.

[2]
c.

Markscheme

Award [1 max] if both effects are correct.

Reason for increasing volume:

Accept “concentration of all reagents reduced by an equal amount so cancels out in Kc expression”.

Accept “affects both forward and backward rates equally”.

a.

HCO3 AND H2O

b.i.

species that has one less proton/H+ ion «than its conjugate acid»

OR

species that forms its conjugate acid by accepting a proton

OR

species that is formed when an acid donates a proton

Do not accept “differs by one proton/H+ from conjugate acid”.

b.ii.

oxide ion/O2–

b.iii.

insufficient data to make generalization

OR

need to consider a «much» larger number of acids

OR

hypothesis will continue to be tested with new acids to see if it can stand the test of time

«hypothesis is false as» other acids/HCl/HBr/HCN/transition metal ion/BF3 do not contain oxygen

OR

other acids/HCl/HBr/HCN/transition metal ion/BF3 falsify hypothesis

correct inductive reasoning «based on limited sample»

«hypothesis not valid as» it contradicts current/accepted theories/Brønsted-Lowry/Lewis theory

[Max 2 Marks]

c.

Examiners report

[N/A]
a.
[N/A]
b.i.
[N/A]
b.ii.
[N/A]
b.iii.
[N/A]
c.

Syllabus sections

Core » Topic 8: Acids and bases » 8.1 Theories of acids and bases
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