IB Chemistry HL 3.1 Proton Transfer Reactions Question Bank

Practise IB Chemistry HL 3.1 with SL and HL questions on proton transfer, pH, Ka/Kb, buffers, indicators and titration analysis.

Covered syllabus points

3.1.1—Brønsted-Lowry theory 3.1.2—Conjugate acid-base pairs 3.1.3—Amphiprotic species 3.1.4—pH scale 3.1.5—Ion product of water (Kw)3.1.6—Strong vs. weak acids/bases 3.1.7—Neutralization reactions 3.1.8—pH curves 3.1.9 (HL)—pOH scale 3.1.10 (HL)—Acid/base strength constants 3.1.11 (HL)—Conjugate pair relationship 3.1.12 (HL)—Salt hydrolysis 3.1.13 (HL)—pH curves for weak acids/bases 3.1.14 (HL)—Acid-base indicators 3.1.15 (HL)—Indicator selection 3.1.16 (HL)—Buffer solutions 3.1.17 (HL)—Buffer pH and composition

[Maximum number: 9]

Bleaches in which chlorine is the active ingredient are the most common, although some environmental groups have concerns about their use.

(a)

In aqueous chlorine the equilibrium below produces chloric(I) acid (hypochlorous acid), HOCl , the active bleach.

\mathrm{Cl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{HOCl}(\mathrm{aq})+\mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})

[ 9 ]

(i)

Chloric(I) acid is a weak acid, but hydrochloric acid is a strong acid. Outline how this is indicated in the equation above.

[ 1 ]

(ii)

State a balanced equation for the reaction of chloric(I) acid with water.

[ 1 ]

$\quad \mathrm{HOCl}(\mathrm{aq}) \rightleftharpoons \mathrm{H}^{+}(\mathrm{aq})+\mathrm{ClO}^{-}(\mathrm{aq}) / \mathrm{HOCl}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq})+\mathrm{ClO}^{-}(\mathrm{aq})$ ;

Equilibrium sign required for the mark.

Marking guidance:

Ignore state symbols.

(iii)

Partial neutralization of chloric(I) acid creates a buffer solution. Given that the $\mathrm{p} K_{\mathrm{a}}$ of chloric(I) acid is 7.53 , determine the pH of a solution that has $[\mathrm{HOCl}]=0.100 \mathrm{~mol} \mathrm{dm}^{-3}$ and $\left[\mathrm{ClO}^{-}\right]=0.0500 \mathrm{~mol} \mathrm{dm}{ }^{-3}$ .

[ 4 ]

$K_{\mathrm{a}}=10^{-7.53}=2.95 \times 10^{-8}\left(\mathrm{moldm}^{-3}\right)$ ;
$K_{\mathrm{a}}=\frac{\left[\mathrm{H}^{+}\right]\left[\mathrm{ClO}^{-}\right]}{[\mathrm{HOCl}]}=\frac{\left[\mathrm{H}^{+}\right](0.05)}{(0.1)} \approx \frac{\left[\mathrm{H}^{+}\right]}{2}=2.95 \times 10^{-8}\left(\mathrm{moldm}^{-3}\right) ;$ $\left[\mathrm{H}^{+}\right]=2 \times 2.95 \times 10^{-8}=5.9 \times 10^{-8}\left(\mathrm{moldm}^{-3}\right)$ ;
$\mathrm{pH}=-\log \left(5.9 \times 10^{-8}\right)=7.23$ ;

Marking guidance:

Accept other methods of carrying out the calculation.
Award [4] for correct final answer.

(iv)

Describe, using HIn to represent the indicator in its acid form, why an indicator changes colour when excess alkali is added.

[ 3 ]

$\mathrm{HIn} \rightleftharpoons \mathrm{H}^{+}+\mathrm{In}^{-}$ ;

Marking guidance:

Do not accept equation without equilibrium arrow.
(weak acid in which the) acid/HIn and conjugate base/In- have different colours / OWTTE;
excess alkali shifts the equilibrium to the RHS/towards the conjugate base;

[Maximum number: 5]

Nitrous acid, $\mathrm{HNO}_{2}$ , is a weak acid which can be used to make acidic buffers.

(a)

A $1.00 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of nitrous acid was prepared. This solution contains two conjugate acid-base pairs.

[ 5 ]

(i)

State the formulas of the conjugate acid and conjugate base in each pair.

Conjugate acid:
Conjugate base:
Conjugate acid:
Conjugate base:

[ 2 ]

（a）

（i）

conjugate acid $\mathrm{H}_{3} \mathrm{O}^{+}$ «（aq）» AND conjugate base $\mathrm{H}_{2} \mathrm{O}$ « $(\mathrm{l}) »$
conjugate acid $\mathrm{HNO}_{2}$ «（aq）» AND conjugate base $\mathrm{NO}_{2}{ }^{-}$ «（aq）»

(ii)

This $1.00 \mathrm{~mol} \mathrm{dm}^{-3}$ solution of nitrous acid was used to prepare a buffer with pH 3.00 .

Calculate the concentration of the conjugate base of nitrous acid required to make this buffer. The $\mathrm{pK}_{\mathrm{a}}$ of nitrous acid is 3.25.

Concentration of conjugate base:

[ 3 ]

（a）

（ii）

$K_{a}<10^{-3.25}>=5.62 \times 10^{-4}$

« $\left[\mathrm{H}^{+}\right]=10^{-3.00}$ » =0.001 «mol dm ${ }^{-3}$ »
$\text { « }\left[\mathrm{A}^{-}\right]=\left(5.62 \times 10^{-4} \times 1.00\right) / 0.001 »=0.562 \text { «mol } \mathrm{dm}^{-3} \text { » }$

Alternative solution：
$\mathrm{pH}=\mathrm{p} K_{a}+\log _{10}[\text { salt }] /[\text { acid }]$ $\text { « } \log _{10}[\text { salt }] /[\text { acid }] \text { 》 }=-0.25$

OR
«[salt]/[acid]» = 0.562、

《 $\left[\mathrm{A}^{-}\right]=0.562 / 1.00 »=0.562$ «mol $\mathrm{dm}^{-3}$ » ↓

Award［3］for correct final answer．

[Maximum number: 4]

Phosphine (IUPAC name phosphane) is a hydride of phosphorus, with the formula $\mathrm{PH}_{3}$ .

(a)

Ammonia acts as a weak Brønsted-Lowry base when dissolved in water.

\mathrm{NH}_{3}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightleftharpoons \mathrm{NH}_{4}^{+}(\mathrm{aq})+\mathrm{OH}^{-}(\mathrm{aq})

Outline what is meant by the terms "weak" and "Brønsted-Lowry base".

Weak:

Brønsted-Lowry base:

[ 2 ]

(b)

Phosphine is usually prepared by heating white phosphorus, one of the allotropes of phosphorus, with concentrated aqueous sodium hydroxide. The equation for the reaction is:

\mathrm{P}_{4}(\mathrm{~s})+3 \mathrm{OH}^{-}(\mathrm{aq})+3 \mathrm{H}_{2} \mathrm{O}(\mathrm{l}) \rightarrow \mathrm{PH}_{3}(\mathrm{~g})+3 \mathrm{H}_{2} \mathrm{PO}_{2}^{-}(\mathrm{aq})

[ 2 ]

(i)

The ion $\mathrm{H}_{2} \mathrm{PO}_{2}^{-}$ is amphiprotic. Outline what is meant by amphiprotic, giving the formulas of both species it is converted to when it behaves in this manner.

[ 2 ]

can act as both a «Brønsted-Lowry» acid and a «Brønsted-Lowry» base OR can accept and/or donate a hydrogen ion/proton/ $\mathrm{H}^{+}$ $\mathrm{HPO}_{2}{ }^{2-}$ AND $\mathrm{H}_{3} \mathrm{PO}_{2}$

All question bank results loaded

Covered syllabus points

3.1 Proton transfer reactions question 1

Question (a)

Question (i)

Question (ii)

Question (iii)

Question (iv)

3.1 Proton transfer reactions question 2

Question (a)

Question (i)

Question (ii)

3.1 Proton transfer reactions question 3

Question (a)

Question (b)

Question (i)