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| Comparison between Conductor and Electrolytes |
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Conductor
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Electrolyte
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- Substances that conduct electricity in a solid or molten state, but do not undergo chemical changes.
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- Substances that conduct electricity in molten state or aqueous solution, and undergo chemical changes.
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- Substances that conduct electricity without undergoing decomposition.
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- Substances that conduct electricity and undergo decomposition into their constituent elements.
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- Can conduct electricity due to the presence of free moving electrons.
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- Can conduct electricity due to the presence of free moving ions.
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- Electrical conductivity decreases as temperature increases.
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- Electrical conductivity increases as temperature increases.
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- Examples of conductors are metals and graphite.
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- Examples of electrolytes are ionic compounds, acids and alkalis.
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| Electrolysis of Molten Compounds |
| Definition of Electrolysis |
- A process whereby compounds in the molten state or an aqueous solution decompose into their constituent elements by passing electricity through them.
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| Apparatus Set Up for Electrolysis of Molten Lead(II) Bromide |
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| Electrolysis of Molten Lead(II) Bromide |
| Ion Present |
\(Pb^{2+},\, Br^-\) |
| Terminal |
Anode |
Cathode |
| Ions Move to the Terminal |
\(Br^-\) |
\(Pb^{2+}\) |
| Hal Equation |
\(2Br^- \rightarrow Br_2 + 2e^-\) |
\(Pb^{2+} + 2e^- \rightarrow Pb\) |
| Observation |
Brown gas released |
Grey solid deposited |
| Name of Product |
Bromine gas |
Solid lead |
| Type of Reaction |
Oxidation |
Reduction |
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| Factors that Affect the Electrolysis of An Aqueous Solution |
- The Eº value mentioned in the table is the Eº value in the standard electrode potential series.
| Factors Affecting the Electrolysis of An Aqueous Solution |
| E° value |
| Concentration of solution |
| Type of electrode |
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| Electrode |
Ion Chosen to be Discharged |
| Anode |
Anion with a more negative or less positive E° value will be easier to be discharged and oxidised. |
| Ctahode |
Cation with a more positive or less negative E° value will be easier to be discharged and reduced. |
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| Concentration of Solution |
| Electrode |
Ion Chosen to be Discharged |
| Anode |
- This factor is only considered for the selection of ions at the anode if the aqueous solution contains halide ions.
- Halide ions with a higher concentration in the electrolytes will be discharged at the anode, even though the E° value of the halide ions are more positive.
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| Cathode |
Cation with a more positive or less negative E° value will be easier to be discharged. |
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| Electrode |
Ion Chosen to be Discharged |
| Anode |
- For active electrodes (e.g. copper and silver).
- No anions are discharged.
- Metal atoms at the anode releases electrons to form metal ions.
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| Cathode |
Cation with a more positive or less negative E° value will be easier to be discharged. |
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| Comparison between An Electrolytic Cell and Chemical Cell |
| Category |
Electrolytic Cell |
Chemical Cell |
| Electric sources |
Electrodes are connected to a battery or any electrical source. |
Electrodes are not connected to any electrical sources. |
| Electrolyte |
Both electrodes are dipped in the electrolyte. |
| Type of metal for electrodes |
Usually carbon electrodes. |
Different metals if being dipped in the same electrolyte. |
| The same metal if being dipped in a different electrolyte. |
| Negatively charged electrode |
Cathode. |
Positive terminal (less electropositive metal). |
| Positively charged electrode |
Anode. |
Negative terminal (more electropositive metal). |
| Energy conversion |
Electric energy → chemical energy. |
Chemical energy → electric energy. |
| Electron transfer |
Anion releases electrons at the anode. |
The atom at the negative terminal releases electrons. |
| Cation receives electrons at the cathode. |
Ions in the electrolyte receive electrons. |
| Oxidation |
At anode. |
At the negative terminal. |
| Reduction |
At cathode. |
At the positive terminal. |
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| Electroplating of Metals |
- Electroplating of metals through electrolysis is done by making the object being electroplated as the cathode, the electroplating metal as the anode, and an aqueous solution containing the ions of the electroplating metal as the electrolyte.
- For example, to electroplate an iron ring with copper, Cu, the copper anode ionises to become copper(II) ions, \(Cu^{2+}\).
- Anode: \(Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-\)
- Copper(II) ions, \(Cu^{2+}\) move to the cathode, are discharged and deposited as a thin layer of copper, Cu on the iron ring.
- Cathode: \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \)
- The blue colour of copper(II) sulphate, \(CuSO_4\) solution does not change because the concentration of copper(II) ions, \(Cu^{2+}\) remains the same.
- The rate of ionisation of copper, Cu at the anode is the same as the rate of discharged copper(II) ions, \(Cu^{2+}\) at the cathode.
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| Purification of Metals |
- Copper is an important mineral and element in our daily life.
- It is an important industrial metal due to its ductility, malleability, electrical conductivity and resistance towards corrosion.
- Copper used in electrical wiring must have a 99.99% purity.
- The purity of copper extracted by the process of melting is about 99.5%.
- Even a slight difference in copper purity will negatively impact its conductivity.
- To determine whether a copper metal is pure, one must conduct the purification of metals through electrolysis.
- The purification of copper by electrolysis is carried out with a piece of pure, thin copper as the cathode; impure copper as the anode; and an aqueous salt solution of copper, such as copper(II) nitrate, \(Cu(NO_3)_2\) as electrolyte.
- Impure copper anode ionises to form copper(II) ions, \(Cu^{2+}\).
- \(Cu(s) \rightarrow Cu^{2+}(aq) + 2e^-\)
- Copper dissolves to become copper(II) ions, \(Cu^{2+}\) and impurities accumulate below the impure copper anode.
- The anode becomes thinner.
- At the pure copper cathode, copper(II) ions, \(Cu^{2+}\) are discharged to form copper atoms, Cu.
- \(Cu^{2+}(aq) + 2e^- \rightarrow Cu(s) \)
- Solid copper is deposited and the copper cathode becomes thicker.
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