What is the standard half-cell potential of copper if the “zero potential reference electrode” is changed from the standard hydrogen electrode to a standard zinc electrode?

A.     –1.1

B.     –0.34

C.     +0.34

D.     +1.1

D

Four electrolytic cells are constructed. Which cell would produce the greatest mass of metal at the negative electrode (cathode)?

B

What are the major products of electrolysing concentrated aqueous potassium iodide, KI(aq)?

B

Which signs for both E^θ_cell and ΔG^θ result in a spontaneous redox reaction occurring under standard conditions?

D

C

An aqueous solution of a metal salt is electrolysed. Which factor will have no effect on the mass of the metal deposited on the negative electrode (cathode), if all other variables remain constant?

A.     Size of metal ion

B.     Relative atomic mass of metal

C.     Current

D.     Charge on metal ion

A

Four respondents stated in their G2 forms that they teach that time, current and charge will affect the mass of metal deposited during electrolysis, but nothing about size of the metal ion or the relative atomic mass of the metal. Although charge is important when considering the deposition of Na, Mg and Al when the same amount of current for the same amount of time is used, the relative atomic mass becomes important when considering the deposition of metal with ions of the same charge, Na and K for example.

This proved to be one of the fifth most difficult question in the paper with 37.54% choosing the correct answer A and 36.8% opting for answer B. The discrimination factor in this question was 0.41.

Consider these standard electrode potentials.

\[\begin{array}{*{20}{l}} {{\text{M}}{{\text{g}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Mg(s)}}}&{{E^\Theta } = - 2.36{\text{ V}}} \\ {{\text{Z}}{{\text{n}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Zn(s)}}}&{{E^\Theta } = - 0.76{\text{ V}}} \end{array}\]

What is the cell potential for the voltaic cell produced when the two half-cells are connected?

A.     –1.60 V

B.     +1.60 V

C.     –3.12 V

D.     +3.12 V

B

Which are necessary conditions for the standard hydrogen electrode to have an \({E^\Theta }\) of exactly zero?

I.     Temperature = 298 K

II.     \({\text{[}}{{\text{H}}^ + }{\text{]}} = 1{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\)

III.     \({\text{[}}{{\text{H}}_2}{\text{]}} = 1{\text{ mol}}\,{\text{d}}{{\text{m}}^{ - 3}}\)

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

A

D

A number of molten metal chlorides are electrolysed, using the same current for the same length of time. Which metal will be produced in the greatest amount, in mol?

A.     Mg

B.     Al

C.     K

D.     Ca

C

The standard electrode potentials of some half-reactions are given below.

     \({\text{S}}{{\text{n}}^{4 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}}\)     \({E^\Theta } =  + 0.15{\text{ V}}\)

    \(\frac{1}{2}{{\text{I}}_2}{\text{(s)}} + {{\text{e}}^ - } \rightleftharpoons {{\text{I}}^ - }{\text{(aq)}}\)     \({E^\Theta } =  + 0.54{\text{ V}}\)

     \({\text{F}}{{\text{e}}^{3 + }}{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{F}}{{\text{e}}^{2 + }}{\text{(aq)}}\)     \({E^\Theta } =  + 0.77{\text{ V}}\)

Which of the following reactions will occur spontaneously?

A.     Iodine reduces \({\text{F}}{{\text{e}}^{3 + }}\) to \({\text{F}}{{\text{e}}^{2 + }}\)

B.     Iodine reduces \({\text{S}}{{\text{n}}^{4 + }}\)to \({\text{S}}{{\text{n}}^{2 + }}\)

C.     Iodine oxidizes \({\text{F}}{{\text{e}}^{2 + }}\)to \({\text{F}}{{\text{e}}^{3 + }}\)

D.     Iodine oxidizes \({\text{S}}{{\text{n}}^{2 + }}\) to \({\text{S}}{{\text{n}}^{4 + }}\)

D

Two respondents stated in their G2 forms that the question was tricky and may have confused the candidates as they should apply the difference between Iodine and Iodide. If at Higher level candidates are not able to recognize that the symbol of Iodine is I2 and that of the Iodide ion is I- there is a severe problem in the candidates’ basic preparation in Chemistry. Indeed, 33.08% of the candidates chose A as the correct answer thinking Iodine is I-, with only 32.05% choosing the correct answer D. This proved to be the third most difficult question in the paper with a discrimination index of 0.38.

Which combination would electroplate an object with copper?

B

Two cells undergoing electrolysis are connected in series.

If \(x\) g of silver are deposited in cell 1, what volume of oxygen, in dm³ at STP, is given off in cell 2?

A_r(Ag) = 108; Molar volume of an ideal gas at STP = 22.7 dm³ mol⁻¹

A.    \(\frac{x}{{108}} \times \frac{1}{4} \times 22.7\)

B.    \(\frac{x}{{108}} \times 4 \times 22.7\)

C.    \(\frac{x}{{108}} \times \frac{1}{2} \times 22.7\)

D.    \(\frac{x}{{108}} \times 2 \times 22.7\)

A

The standard electrode potentials for two metals are given below.

\[\begin{array}{*{20}{l}} {{\text{A}}{{\text{l}}^{3 + }}{\text{(aq)}} + {\text{3}}{{\text{e}}^ - } \rightleftharpoons {\text{Al(s)}}}&{{E^\Theta } = - 1.66{\text{ V}}} \\ {{\text{N}}{{\text{i}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Ni(s)}}}&{{E^\Theta } = - 0.23{\text{ V}}} \end{array}\]

What is the equation and cell potential for the spontaneous reaction that occurs?

\(\begin{array}{*{20}{l}} {{\text{A.}}}&{2{\text{A}}{{\text{l}}^{3 + }}({\text{aq)}} + 3{\text{Ni(s)}} \to {\text{2Al(s)}} + 3{\text{N}}{{\text{i}}^{2 + }}({\text{aq)}}}&{{E^\Theta } = 1.89{\text{ V}}} \\ {{\text{B.}}}&{2{\text{Al(s)}} + 3{\text{N}}{{\text{i}}^{2 + }}({\text{aq)}} \to {\text{2A}}{{\text{l}}^{3 + }}({\text{aq)}} + 3{\text{Ni(s)}}}&{{E^\Theta } = 1.89{\text{ V}}} \\ {{\text{C.}}}&{2{\text{A}}{{\text{l}}^{3 + }}({\text{aq)}} + 3{\text{Ni(s)}} \to {\text{2Al(s)}} + 3{\text{N}}{{\text{i}}^{2 + }}({\text{aq)}}}&{{E^\Theta } = 1.43{\text{ V}}} \\ {{\text{D.}}}&{2{\text{Al(s)}} + 3{\text{N}}{{\text{i}}^{2 + }}({\text{aq)}} \to {\text{2A}}{{\text{l}}^{3 + }}({\text{aq)}} + 3{\text{Ni(s)}}}&{{E^\Theta } = 1.43{\text{ V}}} \end{array}\)

D

Consider the following standard electrode potentials.

     \({\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Sn(s)}}\)     \({E^\Theta } =  - 0.14{\text{ V}}\)

     \({{\text{H}}^ + }{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons \frac{{\text{1}}}{{\text{2}}}{{\text{H}}_{\text{2}}}{\text{(g)}}\)     \({E^\Theta } = 0.00{\text{ V}}\)

     \({\text{F}}{{\text{e}}^{3 + }}{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{F}}{{\text{e}}^{2 + }}{\text{(aq)}}\)     \({E^\Theta } =  + 0.77{\text{ V}}\)

Which species will reduce H\(^ + \)(aq) to H\(_2\) (g) under standard conditions?

A. Fe\(^{2 + }\)(aq)

B. Sn\(^{2 + }\)(aq)

C. Sn(s)

D. Fe\(^{3 + }\)(aq)

C

60.56% gave the correct answer with the others fairly evenly spread over the three distractors.

What happens during the electrolysis of concentrated aqueous potassium chloride?

I.     Reduction takes place at the negative electrode (cathode).

II.     Hydrogen gas is evolved at the negative electrode (cathode).

III.     The pH of the electrolyte increases.

A.     I and II only

B.     I and III only

C.     II and III only

D.     I, II and III

D

There were four comments on this question, more than the others and, indeed, it was the second most difficult question (just under 46% correct).

This was a tough but fair question. Students have studied the electrolysis of aqueous sodium chloride (19.2.1) so would be expected to transfer that understanding to aqueous potassium chloride. Chlorine gas is produced at the anode but much of this will bubble away as a gas rather than cause acidity to neutralize the hydroxide ions forming around the cathode.

The standard electrode potentials for three reactions involving copper and copper ions are:

     \({\text{C}}{{\text{u}}^{2 + }}{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{C}}{{\text{u}}^ + }{\text{(aq)     }}{E^\Theta } =  + 0.15{\text{ V}}\)

     \({\text{C}}{{\text{u}}^{{\text{2}} + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Cu(s)     }}{E^\Theta } =  + {\text{0.34 V}}\)

     \({\text{C}}{{\text{u}}^ + }{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{Cu(s)     }}{E^\Theta } =  + {\text{0.52 V}}\)

Which statement is correct?

A.     \({\text{C}}{{\text{u}}^{{\text{2}} + }}\) ions are a better oxidizing agent than \({\text{C}}{{\text{u}}^ + }\) ions.

B.     Copper metal is a better reducing agent than \({\text{C}}{{\text{u}}^ + }\) ions.

C.     \({\text{C}}{{\text{u}}^ + }\) ions will spontaneously form copper metal and \({\text{C}}{{\text{u}}^{{\text{2}} + }}\) ions in solution.

D.     Copper metal can be spontaneously oxidized by \({\text{C}}{{\text{u}}^{{\text{2}} + }}\) ions to form \({\text{C}}{{\text{u}}^ + }\) ions.

C

Candidates found this question tough – but it was fair, the data was set out in the conventional way and all candidates had to do was to apply the rules they had learnt and the understanding gained. Only 24% gave the correct answer whilst most (36%) chose B.

What are the relative volumes of gas given off at E and F during electrolysis of the two cells in series? Assume all electrodes are inert.

A.     1:1

B.     1:2

C.     2:1

D.     5:2

B

Two half-cells are connected via a salt bridge to make a voltaic cell. Which statement about this cell is correct?

A.     Oxidation occurs at the positive electrode (cathode).

B.     It is also known as an electrolytic cell.

C.     Ions flow through the salt bridge.

D.     It requires a power supply to operate.

C

Which signs are correct for a spontaneous redox reaction?

A

An iron rod is electroplated with silver. Which is a correct condition for this process?

A. The silver electrode is the positive electrode.

B. The iron rod is the positive electrode.

C. The electrolyte is iron(II) sulfate.

D. Oxidation occurs at the negative electrode.

A

What is the cell potential, in V, of the reaction below?

\[{{\text{I}}_2} + {\text{2}}{{\text{S}}_2}{\text{O}}_3^{2 - } \to {\text{2}}{{\text{I}}^ - } + {{\text{S}}_4}{\text{O}}_6^{2 - }\]

     \(\frac{1}{2}{{\text{S}}_4}{\text{O}}_6^{2 - }{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {{\text{S}}_2}{\text{O}}_3^{2 - }{\text{(aq)}}\)     \({E^\Theta } =  + 0.09{\text{ V}}\)

     \({{\text{I}}_2}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{2}}{{\text{I}}^ - }{\text{(aq)}}\)     \({E^\Theta } =  + 0.54{\text{ V}}\)

A.     \( + 0.63\)

B.     \( + 0.45\)

C.     \( - 0.45\)

D.     \( - 0.63\)

B

What are the products when an aqueous solution of copper(II) sulfate is electrolysed using inert graphite electrodes?

C

What is the cell potential, in V, for the reaction that occurs when the following two half-cells are connected?

\[\begin{array}{*{20}{l}} {{\text{F}}{{\text{e}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Fe(s)}}}&{{E^\Theta } = - 0.44{\text{ V}}} \\ {{\text{C}}{{\text{r}}_2}{\text{O}}_7^{2 - }({\text{aq)}} + 14{{\text{H}}^ + }({\text{aq)}} + 6{{\text{e}}^ - } \rightleftharpoons 2{\text{C}}{{\text{r}}^{3 + }}{\text{(aq)}} + {\text{7}}{{\text{H}}_2}{\text{O(l)}}}&{{E^\Theta } = + 1.33{\text{ V}}} \end{array}\]

A.     +0.01

B.     +0.89

C.     +1.77

D.     +2.65

C

In the electrolysis of aqueous potassium nitrate, KNO₃(aq), using inert electrodes, 0.1 mol of a gas was formed at the cathode (negative electrode).

Which is correct?

B

Consider the standard electrode potentials:

\[{\text{F}}{{\text{e}}^{{\text{2 + }}}}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Fe(s)     }}  {E^\Theta } =  - {\text{0.45 V}}\]

\[\frac{1}{2}{\text{C}}{{\text{l}}_{\text{2}}}{\text{(g)}} + {{\text{e}}^ - } \rightleftharpoons {\text{C}}{{\text{l}}^ - }{\text{(aq)     }}  {E^\Theta } = {\text{ }} + {\text{1.36 V}}\]

What is the standard cell potential, in V, for the reaction?

\[{\text{C}}{{\text{l}}_{\text{2}}}{\text{(g)}} + {\text{Fe(s)}} \to {\text{2C}}{{\text{l}}^ - }{\text{(aq)}} + {\text{F}}{{\text{e}}^{2 + }}{\text{(aq)}}\]

A.     \( + {\text{0.91}}\)

B.     \( + {\text{1.81}}\)

C.     \( + {\text{2.27}}\)

D.     \( + {\text{3.17}}\)

B

Which statement is correct for electroplating an object with gold?

A.     The object must be the negative electrode (cathode).

B.     The negative electrode (cathode) must be gold.

C.     The object must be the positive electrode (anode).

D.     The gold electrode must be pure.

A

Consider the following standard electrode potentials:

\[\begin{array}{*{20}{l}} {{\text{S}}{{\text{n}}^{4 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}}}&{{E^\Theta } = + 0.13{\text{ V}}} \\ {{\text{P}}{{\text{b}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Pb(s)}}}&{{E^\Theta } = - 0.13{\text{ V}}} \end{array}\]

What is the value of the cell potential, in V, for the spontaneous reaction that occurs when the two half-cells are connected together?

A.     \( - 0.26\)

B.     0.00

C.     \( + 0.13\)

D.     \( + 0.26\)

D

One respondent stated that it would have been better if the electrode potential half equations had equilibrium signs as in the Data Booklet. It is true that they could have been represented in this way (though not necessarily). This certainly did not have an impact on the question itself. 74.77% of candidates got the correct answer D.

The same quantity of electricity is passed through separate dilute aqueous solutions of sulfuric acid and copper(II) sulfate using platinum electrodes under the same conditions. Which statement is correct?

A.     The same volume of oxygen is obtained in both cases.

B.     The same volume of hydrogen is obtained in both cases.

C.     The amount of copper deposited at the negative electrode in the copper(II) sulfate solution is half the amount of hydrogen gas formed at the negative electrode in the sulfuric acid solution.

D.     The pH of both solutions increases as the electrolysis proceeds.

A

The most popular, but wrong, answer was C (42%) showing that candidates had not appreciated the formation of \({{\text{H}}_{\text{2}}}\). Many gave answer B but candidates should know that copper is deposited from aqueous solution. It is true that this question requires a lot of thought and it turned out to be the fourth most difficult question on the paper.

Which statement is correct for the overall reaction in a voltaic cell?

2AgNO₃(aq) + Ni(s) → 2Ag(s) + Ni(NO₃)₂(aq)         E ^θ= +1.06 V

A.     Electrons flow from Ag electrode to Ni electrode.

B.     Ni is oxidized to Ni²⁺ at the cathode (negative electrode).

C.     Ag⁺ is reduced to Ag at the anode (positive electrode).

D.     Ag has a more positive standard electrode potential value than Ni.

D

Two electrolytic cells are connected in series and the same current passes through each cell. The first cell contains silver electrodes in silver nitrate solution. The second cell contains copper electrodes in copper(II) sulfate solution. In one experiment 1.00 g of silver is deposited in the first cell. What mass of copper, in g, is deposited in the second cell?

A.     \(\frac{{1.00}}{{107.87}}\)

B.     \(\frac{{1.00}}{{63.55}}\)

C.     \(\frac{{1.00}}{{107.87}} \times \frac{{63.55}}{2}\)

D.     \(\frac{{1.00}}{{107.87}} \times 63.55\)

C

One respondent stated that mentioning the fact that the same current passes through each cell may have been better than using the term in series. This is a fair point. The question overall was reasonably challenging with 53.53% of candidates getting the correct answer, C.

Which components are used to make the standard hydrogen electrode?

A.     \({{\text{H}}_{\text{2}}}{\text{(g), }}{{\text{H}}^ + }{\text{(aq), Pt(s)}}\)

B.     \({{\text{H}}_{\text{2}}}{\text{(g), }}{{\text{H}}^ + }{\text{(aq), Ni(s)}}\)

C.     \({{\text{H}}_{\text{2}}}{\text{(g), H}}{{\text{O}}^ - }{\text{(aq), Pt(s)}}\)

D.     \({{\text{H}}_{\text{2}}}{\text{(g), H}}{{\text{O}}^ - }{\text{(aq), Ni(s)}}\)

A

A voltaic cell is made by connecting two half-cells represented by the half-equations below.

\[\begin{array}{*{20}{l}} {{\text{M}}{{\text{n}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \to {\text{Mn(s)}}}&{{E^\Theta } =  - 1.19{\text{ V}}} \\ {{\text{P}}{{\text{b}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \to {\text{Pb(s)}}}&{{E^\Theta } =  - 0.13{\text{ V}}} \end{array}\]

Which statement is correct about this voltaic cell?

A.     Mn is oxidized and the voltage of the cell is 1.06 V.

B.     Pb is oxidized and the voltage of the cell is 1.06 V.

C.     Mn is oxidized and the voltage of the cell is 1.32 V.

D.     Pb is oxidized and the voltage of the cell is 1.32 V.

A

The overall equation of a voltaic cell is:

\[{\text{Ni(s)}} + {\text{2A}}{{\text{g}}^ + }{\text{(aq)}} \rightleftharpoons {\text{N}}{{\text{i}}^{2 + }}{\text{(aq)}} + {\text{2Ag(s)}}\;\;\;\;\;{E^\Theta } = {\text{1.06 V}}\]

The standard electrode potential for \({\text{N}}{{\text{i}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Ni(s)}}\), is \( - 0.26{\text{ V}}\). What is the standard electrode potential for the silver half-cell, \({\text{A}}{{\text{g}}^ + }{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{Ag(s)}}\), in V?

A.     \( - 1.32\)

B.     \( - 0.80\)

C.     \( + 0.80\)

D.     \( + 1.32\)

C

The same quantity of electricity was passed through separate molten samples of sodium bromide, NaBr, and magnesium chloride, \({\text{MgC}}{{\text{l}}_{\text{2}}}\). Which statement is true about the amounts, in mol, that are formed?

A.     The amount of Mg formed is equal to the amount of Na formed.

B.     The amount of Mg formed is equal to the amount of \({\text{C}}{{\text{l}}_{\text{2}}}\) formed.

C.     The amount of Mg formed is twice the amount of \({\text{C}}{{\text{l}}_{\text{2}}}\) formed.

D.     The amount of Mg formed is twice the

B

What does not affect the mass of products formed in electrolysis of an aqueous solution?

A.     Current

B.     Duration of electrolysis

C.     Initial mass of cathode

D.     Charge on the ions

C

Consider the following two standard electrode potentials at 298 K.

     \({\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}} + {\text{2}}{{\text{e}}^ - } \rightleftharpoons {\text{Sn(s)}}\)     \({E^\Theta } =  - 0.14{\text{ V}}\)

     \({\text{F}}{{\text{e}}^{3 + }}{\text{(aq)}} + {{\text{e}}^ - } \rightleftharpoons {\text{F}}{{\text{e}}^{2 + }}{\text{(aq)}}\)     \({E^\Theta } =  + 0.77{\text{ V}}\)

What is the equation and cell potential for the spontaneous reaction that occurs?

A.     \({\text{2F}}{{\text{e}}^{2 + }}{\text{(aq)}} + {\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}} \to {\text{2F}}{{\text{e}}^{3 + }}{\text{(aq)}} + {\text{Sn(s)}}\)     \({E^\Theta } =  - 0.91{\text{ V}}\)

B.     \({\text{2F}}{{\text{e}}^{3 + }}{\text{(aq)}} + {\text{Sn(s)}} \to {\text{2F}}{{\text{e}}^{2 + }}{\text{(aq)}} + {\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}}\)     \({E^\Theta } =  + 0.91{\text{ V}}\)

C.     \({\text{2F}}{{\text{e}}^{2 + }}{\text{(aq)}} + {\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}} \to {\text{2F}}{{\text{e}}^{3 + }}{\text{(aq)}} + {\text{Sn(s)}}\)     \({E^\Theta } =  + 0.91{\text{ V}}\)

D.     \({\text{2F}}{{\text{e}}^{3 + }}{\text{(aq)}} + {\text{Sn(s)}} \to {\text{2F}}{{\text{e}}^{2 + }}{\text{(aq)}} + {\text{S}}{{\text{n}}^{2 + }}{\text{(aq)}}\)     \({E^\Theta } =  + 1.68{\text{ V}}\)

B

For the electrolysis of aqueous copper(II) sulfate, which of the following statements is correct?

A.     Cu and \({{\text{O}}_{\text{2}}}\) are produced in a mol ratio of 1:1

B.     \({{\text{H}}_{\text{2}}}\) and \({{\text{O}}_{\text{2}}}\) are produced in a mol ratio of 1:1

C.     Cu and \({{\text{O}}_{\text{2}}}\) are produced in a mol ratio of 2:1

D.     \({{\text{H}}_{\text{2}}}\) and \({{\text{O}}_{\text{2}}}\) are produced in a mol ratio of 2:1

C

There were three G2 comments on this particular question, all of which suggested that the question was ambiguous as the nature of the electrodes was not stated for the electrolysis of aqueous copper(II) sulphate. Also noting the fact that candidates do not have access to a Data Booklet for P1. As regards the first point, it is a valid comment that the nature of electrodes could have been specified in the question itself.

However, on close examination of each of the first choices, it should be obvious to the candidate that inert electrodes were used as \({{\text{O}}_{\text{2}}}\) is given as a product in all four cases. This would not be the case if copper electrodes were used. As regards the second point, it is also a fair comment that in the case of the electrolysis of aqueous copper(II) sulfate, candidates can work out the specific products using the standard electrode potentials from Table 14. of the Data Booklet. However, to do so, this would have made this question a clear Objective 3 type question which is outside the realm of P1. This was discussed at length at GA and it was felt that, realistically, candidates should have seen this electrolysis in the laboratory and, hence, as regards this particular electrolysis process candidates should have been able to determine the correct mol ratio of 2:1 for Cu:\({{\text{O}}_{\text{2}}}\) . Although the question was, in fact, the hardest question on the entire paper, 43% of candidates did get the correct answer, C. Yet again this should emphasise the inherent importance of laboratory work as an integral part of the overall IB Diploma Chemistry programme.

What are the products of electrolysis when concentrated calcium bromide solution is electrolysed using graphite electrodes?

A