(a)(i) What is an electrolyte?
(ii) Classify each of the following as strong electrolyte/weak, electrolyte/non-electrolyte. Potassium chloride; sodium ethanoate, aqueous ammonia; cane sugar
(b)(i) Write half-cell equations for the reactions in the Daniel cell .
(ii) Why is the Daniel cell classified as an electrochemical cell?
(iii) Give two other examples of electrochemical cell.
(c) Explain the following observations.
(i) Graphite conducts electricity, unlike most non-metals
(ii) In the electrolysis of copper (II) tetraoxosulphate (VI) solution, the blue colour fades with platinum electrodes while the colour intensity is unaffected with copper electrodes (equations required)
(iii) A solution of dry hydrogen chloride in methylbenzene (toluene) does not conduct electricity whereas hydrochloric acid does.
(a)(i) An electrolyte is a substance which, in the molten state or in aqueous solution, conducts electricity and is chemically decomposed by it because it contains free mobile ions.
(ii)
- Potassium chloride - strong electrolyte
- Sodium ethanoate - strong electrolyte
- Aqueous ammonia - weak electrolyte
- Cane sugar - non-electrolyte
(b)(i) Daniell cell half-cell reactions
Anode (oxidation): \[Zn \rightarrow Zn^{2+} + 2e^-\]
Cathode (reduction): \[Cu^{2+} + 2e^- \rightarrow Cu\]
(ii) It is an electrochemical cell because it converts chemical energy (from the spontaneous redox reaction) into electrical energy.
(iii) Other examples: the dry (Leclanche) cell and the lead-acid accumulator.
(c)(i) In graphite, each carbon atom bonds to only three others, using three of its four valence electrons. The fourth electron of every atom is delocalized between the layers and is free to move, so graphite conducts electricity.
(ii) With platinum electrodes, Cu2+ ions are discharged at the cathode and removed from solution, so the blue colour fades:
\[Cu^{2+} + 2e^- \rightarrow Cu\]
at the anode oxygen is evolved: \[2H_2O \rightarrow O_2 + 4H^+ + 4e^-\]
With copper electrodes, the copper anode dissolves and replaces the Cu2+ removed at the cathode, so the concentration and blue colour stay the same:
\[Cu \rightarrow Cu^{2+} + 2e^-\]
(iii) In methylbenzene, hydrogen chloride exists as un-ionized covalent molecules, so there are no mobile ions and it does not conduct. In water, HCl ionizes into mobile H+ and Cl- ions, so hydrochloric acid conducts electricity.
(a)(i) An electrolyte is a substance which, in the molten state or in aqueous solution, conducts electricity and is chemically decomposed by it because it contains free mobile ions.
(ii)
- Potassium chloride - strong electrolyte
- Sodium ethanoate - strong electrolyte
- Aqueous ammonia - weak electrolyte
- Cane sugar - non-electrolyte
(b)(i) Daniell cell half-cell reactions
Anode (oxidation): \[Zn \rightarrow Zn^{2+} + 2e^-\]
Cathode (reduction): \[Cu^{2+} + 2e^- \rightarrow Cu\]
(ii) It is an electrochemical cell because it converts chemical energy (from the spontaneous redox reaction) into electrical energy.
(iii) Other examples: the dry (Leclanche) cell and the lead-acid accumulator.
(c)(i) In graphite, each carbon atom bonds to only three others, using three of its four valence electrons. The fourth electron of every atom is delocalized between the layers and is free to move, so graphite conducts electricity.
(ii) With platinum electrodes, Cu2+ ions are discharged at the cathode and removed from solution, so the blue colour fades:
\[Cu^{2+} + 2e^- \rightarrow Cu\]
at the anode oxygen is evolved: \[2H_2O \rightarrow O_2 + 4H^+ + 4e^-\]
With copper electrodes, the copper anode dissolves and replaces the Cu2+ removed at the cathode, so the concentration and blue colour stay the same:
\[Cu \rightarrow Cu^{2+} + 2e^-\]
(iii) In methylbenzene, hydrogen chloride exists as un-ionized covalent molecules, so there are no mobile ions and it does not conduct. In water, HCl ionizes into mobile H+ and Cl- ions, so hydrochloric acid conducts electricity.
