IB DP Chemistry Topic 8.1 Theories of acids and bases SL Paper 2

Question

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

Reaction A     \({\text{N}}{{\text{H}}_{\text{3}}}({\text{aq)}} + {{\text{H}}_{\text{2}}}{\text{O(l)}} \rightleftharpoons \) \({\rm{NH}}_4^ + ({\rm{aq}}) + {\rm{O}}{{\rm{H}}^ – }({\rm{aq}})\)

The reaction mixture in A consists mainly of reactants because the equilibrium lies to the left.

Reaction B     \({\text{NH}}_2^ -({\text{aq)}} + {{\text{H}}_2}{\text{O(l)}} \rightleftharpoons \) \({\rm{NH}}_3^{}({\rm{aq}}) + {\rm{O}}{{\rm{H}}^ – }({\rm{aq}})\)

The reaction mixture in B consists mainly of products because the equilibrium lies to the right.

Two acidic solutions, X and Y, of equal concentrations have pH values of 2 and 6 respectively.

For each of the reactions A and B, deduce whether water is acting as an acid or a base and explain your answer.

[2]
a.i.

In reaction B, identify the stronger base, \({\text{NH}}_2^ – \) or \({\text{O}}{{\text{H}}^ – }\) and explain your answer.

[2]
a.ii.

In reactions A and B, identify the stronger acid, \({\text{NH}}_4^ + \) or \({\text{N}}{{\text{H}}_{\text{3}}}\) (underlined) and explain your answer.

[2]
a.iii.

Describe two different experimental methods to distinguish between aqueous solutions of a strong base and a weak base.

[5]
b.

Calculate the hydrogen ion concentrations in the two solutions and identify the stronger acid.

[2]
c.i.

Determine the ratio of the hydrogen ion concentrations in the two solutions X and Y.

[1]
c.ii.
Answer/Explanation

Markscheme

acid in both reactions;

because it loses a proton/hydrogen ion/\({{\text{H}}^ + }\) / proton/hydrogen ion/\({{\text{H}}^ + }\) donor;

Second mark can be scored if they do not identify it as an acid in both reactions.

a.i.

\({\text{NH}}_2^ – \);

more readily accepts a proton / equilibrium lies to the right / takes \({{\text{H}}^ + }\) from \({{\text{H}}_{\text{2}}}{\text{O}}\);

If OHchosen award [0]

a.ii.

\({\text{NH}}_4^ + \);

donates a proton more readily than \({\text{N}}{{\text{H}}_{\text{3}}}\) / equilibrium lies to the left;

If NH3 chosen award [0]

a.iii.

solutions of the same concentration;

pH meter;

strong base has a higher pH / weak base has lower pH;

indicator paper/U.I solution;

strong base has a higher pH/more purple / weak base has lower pH/blue not purple / OWTTE;

measuring conductivity (with conductivity meter);

strong base has a higher conductivity / weak base has lower conductivity;

comparing heat of neutralisation with acid;

strong base releases more heat / weak base releases less heat;

Award [4 max] for two correct methods with expected results.

b.

X;

\({\text{[X]}} = {10^{ – 2}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ – 3}}{\text{)}}\) and \({\text{[Y]}} = {10^{ – 6}}{\text{ (mol}}\,{\text{d}}{{\text{m}}^{ – 3}}{\text{)}}\);

c.i.

\(10\,000/{10^4}:1\);

Ratio should be in form above.

c.ii.

Examiners report

This was the second most popular question. In (a) many candidates scored marks for their understanding of acid-base behaviour in terms of proton transfer and correctly identified \({{\text{H}}_{\text{2}}}{\text{O}}\) as acting as an acid. Identifying and explaining \({\text{NH}}_2^ – \) as the strongest base and \({\text{NH}}_4^ + \) as the strongest acid proved more problematic.

a.i.

This was the second most popular question. In (a) many candidates scored marks for their understanding of acid-base behaviour in terms of proton transfer and correctly identified \({{\text{H}}_{\text{2}}}{\text{O}}\) as acting as an acid. Identifying and explaining \({\text{NH}}_2^ – \) as the strongest base and \({\text{NH}}_4^ + \) as the strongest acid proved more problematic.

a.ii.

This was the second most popular question. In (a) many candidates scored marks for their understanding of acid-base behaviour in terms of proton transfer and correctly identified \({{\text{H}}_{\text{2}}}{\text{O}}\) as acting as an acid. Identifying and explaining \({\text{NH}}_2^ – \) as the strongest base and \({\text{NH}}_4^ + \) as the strongest acid proved more problematic.

a.iii.

In (b), in spite of the wording in the question (“experimental methods”) many answers mentioned only a property, such as “a strong base has a higher pH than a weak base”, and several who chose an indicator to distinguish them picked one with only two colours, such as phenolphthalein. Most candidates omitted to mention that the solutions should be of the same concentration. Although most could describe one good method (either pH or conductivity), the second method often involved reaction rates or titrations and descriptions of how these were poor.

b.

In (c), although most were able to convert pH values into \({\text{[}}{{\text{H}}^ + }{\text{]}}\) values, fewer were able to compare them as a ratio in the correct form –10,000:1. Some candidates had difficulty identifying the stronger acid.

c.i.

In (c), although most were able to convert pH values into \({\text{[}}{{\text{H}}^ + }{\text{]}}\) values, fewer were able to compare them as a ratio in the correct form –10,000:1. Some candidates had difficulty identifying the stronger acid.

c.ii.

Question

Chloroethene, C2H3Cl, is an important organic compound used to manufacture the polymer poly(chloroethene).

Draw the Lewis structure for chloroethene and predict the H–C–Cl bond angle.

[2]
a.i.

Draw a section of poly(chloroethene) containing six carbon atoms.

[1]
a.ii.

Outline why the polymerization of alkenes is of economic importance and why the disposal of plastics is a problem.

[2]
a.iii.

Chloroethene can be converted to ethanol in two steps. For each step deduce an overall equation for the reaction taking place.

Step 1:

Step 2:

[2]
b.i.

State the reagents and conditions necessary to prepare ethanoic acid from ethanol in the laboratory.

[2]
b.ii.

State an equation, including state symbols, for the reaction of ethanoic acid with water. Identify a Brønsted-Lowry acid in the equation and its conjugate base.

[3]
b.iii.
Answer/Explanation

Markscheme

M10/4/CHEMI/SP2/ENG/TZ1/03.a.i/M ;

Accept lines, dots or crosses for electron pairs.

Lone pairs required on chlorine.

(approximately) 120°;

Accept any bond angle in the range 113–120°.

a.i.

M10/4/CHEMI/SP2/ENG/TZ1/03.a.ii/M ;

Brackets not required for mark.

Continuation bonds from each carbon are required.

Cl atoms can be above or below carbon spine or alternating above and below.

a.ii.

plastics are cheap/versatile/a large industry / plastics have many uses / OWTTE;

plastics are not biodegradeable / plastics take up large amounts of space in landfill / pollution caused by burning of plastics / OWTTE;

Do not accept plastics cause litter.

Allow plastics don’t decompose quickly / OWTTE.

a.iii.

(i)     Step 1:

\({\text{C}}{{\text{H}}_2}{\text{CHCl}} + {{\text{H}}_2} \to {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{Cl}}\);

Step 2:

\({\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{Cl}} + {\text{O}}{{\text{H}}^ – } \to {\text{C}}{{\text{H}}_3}{\text{C}}{{\text{H}}_2}{\text{OH}} + {\text{C}}{{\text{l}}^ – }\);

Allow NaOH or NaCl etc. instead of OHand Cl.

Allow abbreviated formulas C2H3Cl, C2H5Cl, C2H5OH.

b.i.

\({{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\)/\({{\text{H}}^ + }\)/acidified and \({\text{C}}{{\text{r}}_{\text{2}}}{\text{O}}_{_{\text{7}}}^{2 – }\)/(potassium/sodium) dichromate;

Accept suitable oxidizing agents (e.g. KMnO4 etc.) but only with acid.

Ignore missing or incorrect oxidation states in reagents.

(heat under) reflux;

Second mark can be scored even if reagent is incorrect.

b.ii.

\({\text{C}}{{\text{H}}_3}{\text{COOH(aq)}} + {{\text{H}}_2}{\text{O(l)}} \rightleftharpoons {\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ – }{\text{(aq)}} + {{\text{H}}_3}{{\text{O}}^ + }{\text{(aq)}}\)

OR

\({\text{C}}{{\text{H}}_3}{\text{COOH(l)}} + {{\text{H}}_2}{\text{O(l)}} \rightleftharpoons {\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ – }{\text{(aq)}} + {{\text{H}}_3}{{\text{O}}^ + }{\text{(aq)}}\)

OR

\({\text{C}}{{\text{H}}_3}{\text{COOH(aq)}} \rightleftharpoons {\text{C}}{{\text{H}}_3}{\text{CO}}{{\text{O}}^ – }{\text{(aq)}} + {{\text{H}}^ + }{\text{(aq)}}\)

correct equation;

state symbols and \( \rightleftharpoons \);

BL acid is \({\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}\) and cb is \({\text{C}}{{\text{H}}_{\text{3}}}{\text{CO}}{{\text{O}}^ – }\) / BL acid is \({{\text{H}}_{\text{3}}}{{\text{O}}^ + }\) and cb is \({{\text{H}}_{\text{2}}}{\text{O}}\);

b.iii.

Examiners report

The main G2 comments on this question related to the inclusion of organic chemistry in Section A. It should be noted that ANY Topic can be asked in Section A of P2, and there is no set-formula in relation to question setting. Organic chemistry is an integral part of the IB SL Chemistry programme, and is covered in Topic 10 of the guide (12 hours in total). Hence, candidates should be adequately prepared for questions on this topic, even in Section A. In 3(a), the Lewis structure of chlorethene was generally drawn correctly, though the weaker candidates often omitted the lone pairs on the chlorine. The bond angle was usually predicted, although right angles and 109.5° were often given. Even some of the better candidates explained their choice of bond angle, based on the fact that the double bond occupies more space causing the HCCl bond angle to drop less than 120°.

a.i.

Many candidates gave double bonds and some forgot to include continuation bonds.

a.ii.

The Aim 8 question in part (iii) was very well answered this session. Almost all candidates scored the disposal problem of plastics mark and many achieved the economics importance mark also.

a.iii.

In general (b) was very poorly answered, again showing a clear weakness in organic chemistry, which is an area of major concern. (i) was poorly done. Candidates who managed a correct reaction for the first step often used water instead of hydroxide ion for the second step.

b.i.

In general (b) was very poorly answered, again showing a clear weakness in organic chemistry, which is an area of major concern. In (ii), candidates who mentioned dichromate(VI) or permanganate(VIII) often omitted the acid. In addition, reflux was often missing.

b.ii.

In general (b) was very poorly answered, again showing a clear weakness in organic chemistry, which is an area of major concern. In (iii), very few candidates scored all three marks here, even though the question itself was easy. The equation was often correct, but the equilibrium arrow was rarely given. Some candidates did not know the formula for ethanoic acid which was surprising.

b.iii.
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