| Date | November 2013 | Marks available | 2 | Reference code | 13N.2.sl.TZ0.5 |
| Level | SL | Paper | 2 | Time zone | TZ0 |
| Command term | Justify and Predict | Question number | 5 | Adapted from | N/A |
Question
In acidic solution, ions containing titanium can react according to the half-equation below.
\[{\text{Ti}}{{\text{O}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{H}}^ + }{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{T}}{{\text{i}}^{3 + }}{\text{(aq)}} + {{\text{H}}_2}{\text{O(l)}}\]
A reactivity series comparing titanium, cadmium and europium is given below.
Least reactive Cd \( < \) Ti \( < \) Eu Most reactive
The half-equations corresponding to these metals are:
\({\text{E}}{{\text{u}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Eu(s)}}\)
\({\text{T}}{{\text{i}}^{3 + }}{\text{(aq)}} + {\text{3}}{{\text{e}}^ - } \rightleftharpoons {\text{Ti(s)}}\)
\({\text{C}}{{\text{d}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Cd(s)}}\)
Some students were provided with a \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) solution of a monobasic acid, HQ, and given the problem of determining whether HQ was a weak acid or a strong acid.
State the initial and final oxidation numbers of titanium and hence deduce whether it is oxidized or reduced in this change.
Considering the above equilibrium, predict, giving a reason, how adding more acid would affect the strength of the \({\text{Ti}}{{\text{O}}^{2 + }}\) ion as an oxidizing agent.
Deduce which of the species would react with titanium metal.
Deduce the balanced equation for this reaction.
Deduce which of the six species is the strongest oxidizing agent.
A voltaic cell can be constructed using cadmium and europium half-cells. State how the two solutions involved should be connected and outline how this connection works.
Define a Brønsted–Lowry acid.
Distinguish between the terms strong acid and weak acid.
Neelu and Charles decided to solve the problem by determining the volume of \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) sodium hydroxide solution needed to neutralize \({\text{25.0 c}}{{\text{m}}^{\text{3}}}\) of the acid. Outline whether this was a good choice.
Neelu and Charles decided to compare the volume of sodium hydroxide solution needed with those required by known \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) strong and weak acids. Unfortunately they chose sulfuric acid as the strong acid. Outline why this was an unsuitable choice.
State a suitable choice for both the strong acid and the weak acid.
Strong acid:
Weak acid:
Francisco and Shamiso decided to measure the pH of the initial solution, HQ, and they found that its pH was 3.7. Deduce, giving a reason, the strength (weak or strong) of the acid HQ.
Suggest a method, other than those mentioned above, that could be used to solve the problem and outline how the results would distinguish between a strong acid and a weak acid.
Markscheme
+ sign must be present. Do not award mark for incorrect notation 4, 4+, 3, 3+ etc.
Do not award M2 if inconsistent with M1.
increases / makes it stronger;
(more \({{\text{H}}^ + }\) would) drive/shift equilibrium to the right/towards products
(accepting more electrons);
\({\text{C}}{{\text{d}}^{2 + }}\);
Do not allow incorrect notation such as Cd, Cd(II), or Cd+2.
\({\text{2Ti(s)}} + {\text{3C}}{{\text{d}}^{2 + }}{\text{(aq)}} \to {\text{2T}}{{\text{i}}^{3 + }}{\text{(aq)}} + {\text{3Cd(s)}}\);
Ignore state symbols.
Allow ECF from (b)(i) for a correct equation.
\({\text{C}}{{\text{d}}^{2 + }}\);
Charge must be given.
Do not allow incorrect notation such as Cd, Cd(II), or Cd+2 but penalize
only once in b(i) and b(iii) .
Allow ECF, if Eu2+ is written both in part (i) and part (iii).
salt bridge;
Accept specific examples of salt bridges, such as filter paper dipped in aqueous KNO3.
allows the movement of ions (between the two solutions) / completes the circuit / maintains electrical neutrality;
Accept movement of charges/negative ions/positive ions.
donates \({{\text{H}}^ + }\)/protons;
strong acid completely/100%/fully dissociated/ionized and weak acid partially/slightly dissociated/ionized;
not a good choice / poor choice;
requires same volume of the base / the amount/volume to react/for neutralization does not depend on the acid strength;
sulfuric acid is diprotic/dibasic/liberates two protons/\({{\text{H}}^ + }\);
Accept “reacts with 2 moles of alkali/base”.
Strong acid: hydrochloric acid/HCl / nitric acid/\({\text{HN}}{{\text{O}}_{\text{3}}}\);
Weak acid: ethanoic acid/\({\text{C}}{{\text{H}}_{\text{3}}}{\text{COOH}}\);
Allow acetic acid for weak acid.
Accept any other strong/weak monobasic acids as appropriate.
Do not accept non-monobasic acids, such as phosphoric acid and carbonic acid.
weak;
strong \({\text{0.100 mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\) acid has a pH of 1/lower than that observed;
Accept “pH value of 3.7 means that it produces only 10–3.7/2.0 \( \times \) 10–4 [H+] in water”.
measure the rate of reaction with reactive metal/(metal) carbonate/metal oxide;
strong acid would react faster/more vigorously / weak acid would react slower/less vigorously;
Accept specific substances, such as Mg and CaCO3, which react with acids.
OR
measure conductivity;
higher for strong acid / lower for weak acid;
OR
measure heat/enthalpy of neutralization;
greater for strong acid / lower for weak acid;
Do not accept pH/universal indicator paper.
Examiners report
In part (a) (i), most candidates scored full marks although some candidates continue to write incorrect notation (4, 4+) for oxidation states.
In part (ii), some candidates missed the word equilibrium in the question and hence could not state that equilibrium will shift towards right and strength of oxidizing agent will increase.
In part (b) (i), (iii), the correct answer was \({\text{C}}{{\text{d}}^{2 + }}\) but many candidates wrote Cd, Eu or Ti.
In part (ii), the better candidates wrote the correct balanced chemical equation. Some included electrons in the equation which was surprising and some did not read the question where the reaction with Ti metal was asked.
In part (b) (i), (iii), the correct answer was \({\text{C}}{{\text{d}}^{2 + }}\) but many candidates wrote Cd, Eu or Ti.
In part (iv), many candidates identified the salt bridge but some missed the reference to the movement of ions.
In part (c), most candidates were able to define a Bronsted-Lowry acid. The difference between strong and weak was usually correctly stated although only better candidates stated that strong acid is assumed to be 100% dissociated. Part (iii) proved to be difficult where very few candidates stated correctly that it is not a good choice because it requires the same volume of the base. Many candidates, however, knew the fact that sulfuric acid is diprotic in part (iv). In part (v), majority of candidates correctly identified the strong and weak acid whereas weaker candidates stated NaOH as a weak acid. Part (vi) was poorly done with many candidates stating pH 3.7 as strong acid. In part (vii), many candidates scored full marks but universal indicator paper was often suggested, which of course, scored no marks.
In part (c), most candidates were able to define a Bronsted-Lowry acid. The difference between strong and weak was usually correctly stated although only better candidates stated that strong acid is assumed to be 100% dissociated. Part (iii) proved to be difficult where very few candidates stated correctly that it is not a good choice because it requires the same volume of the base. Many candidates, however, knew the fact that sulfuric acid is diprotic in part (iv). In part (v), majority of candidates correctly identified the strong and weak acid whereas weaker candidates stated NaOH as a weak acid. Part (vi) was poorly done with many candidates stating pH 3.7 as strong acid. In part (vii), many candidates scored full marks but universal indicator paper was often suggested, which of course, scored no marks.
In part (c), most candidates were able to define a Bronsted-Lowry acid. The difference between strong and weak was usually correctly stated although only better candidates stated that strong acid is assumed to be 100% dissociated. Part (iii) proved to be difficult where very few candidates stated correctly that it is not a good choice because it requires the same volume of the base. Many candidates, however, knew the fact that sulfuric acid is diprotic in part (iv). In part (v), majority of candidates correctly identified the strong and weak acid whereas weaker candidates stated NaOH as a weak acid. Part (vi) was poorly done with many candidates stating pH 3.7 as strong acid. In part (vii), many candidates scored full marks but universal indicator paper was often suggested, which of course, scored no marks.
In part (c), most candidates were able to define a Bronsted-Lowry acid. The difference between strong and weak was usually correctly stated although only better candidates stated that strong acid is assumed to be 100% dissociated. Part (iii) proved to be difficult where very few candidates stated correctly that it is not a good choice because it requires the same volume of the base. Many candidates, however, knew the fact that sulfuric acid is diprotic in part (iv). In part (v), majority of candidates correctly identified the strong and weak acid whereas weaker candidates stated NaOH as a weak acid. Part (vi) was poorly done with many candidates stating pH 3.7 as strong acid. In part (vii), many candidates scored full marks but universal indicator paper was often suggested, which of course, scored no marks.
In part (c), most candidates were able to define a Bronsted-Lowry acid. The difference between strong and weak was usually correctly stated although only better candidates stated that strong acid is assumed to be 100% dissociated. Part (iii) proved to be difficult where very few candidates stated correctly that it is not a good choice because it requires the same volume of the base. Many candidates, however, knew the fact that sulfuric acid is diprotic in part (iv). In part (v), majority of candidates correctly identified the strong and weak acid whereas weaker candidates stated NaOH as a weak acid. Part (vi) was poorly done with many candidates stating pH 3.7 as strong acid. In part (vii), many candidates scored full marks but universal indicator paper was often suggested, which of course, scored no marks.
In part (c), most candidates were able to define a Bronsted-Lowry acid. The difference between strong and weak was usually correctly stated although only better candidates stated that strong acid is assumed to be 100% dissociated. Part (iii) proved to be difficult where very few candidates stated correctly that it is not a good choice because it requires the same volume of the base. Many candidates, however, knew the fact that sulfuric acid is diprotic in part (iv). In part (v), majority of candidates correctly identified the strong and weak acid whereas weaker candidates stated NaOH as a weak acid. Part (vi) was poorly done with many candidates stating pH 3.7 as strong acid. In part (vii), many candidates scored full marks but universal indicator paper was often suggested, which of course, scored no marks.
In part (c), most candidates were able to define a Bronsted-Lowry acid. The difference between strong and weak was usually correctly stated although only better candidates stated that strong acid is assumed to be 100% dissociated. Part (iii) proved to be difficult where very few candidates stated correctly that it is not a good choice because it requires the same volume of the base. Many candidates, however, knew the fact that sulfuric acid is diprotic in part (iv). In part (v), majority of candidates correctly identified the strong and weak acid whereas weaker candidates stated NaOH as a weak acid. Part (vi) was poorly done with many candidates stating pH 3.7 as strong acid. In part (vii), many candidates scored full marks but universal indicator paper was often suggested, which of course, scored no marks.
