We often use the word, “lead”, to refer to the black core inside the wooden pencil. Yet, if we've studied Chemistry, we know that lead Pb is the grey, dense metal that shares the same Group as carbon C. The “pencil lead” is actually a mixture of graphite (an allotrope of C) and clay that had been heated to 180 ºC. If consumed, lead is poisonous but graphite is not. Why was graphite called lead? Graphite was thought to be a form of lead when a large amount of the pure substance was found in a valley in North West England in 1555.
Pencil "lead" is graphite mixed with clay...
The diagrams show circuits that use “pencil lead” as an electrical conductor. Graphite is unlike other non-metals in that it conducts electricity. You can imagine it to be made of vast flat layers of molecules stacked onto each other. The layers consists of only C atoms. You know the C has a valency of 4 meaning it can form 4 bonds with other atoms. Within the layer, however, each C atom forms 3 bonds in a trigonal plane. This extends to form a vast hexagonal layer of C atoms. There is one valence electron from one C atom not involved in bonding. This valence electron is able to move between the layers and thus conduct electricity.
Delocalised electrons between carbon layers help to conduct electricity...
Meanwhile if you are interested in knowing how pencils are made, here's the video.
http://www.youtube.com/watch?v=b5uUV9z7hfg
Try these questions before you check with the answers below.
1. (a) One other allotrope of carbon is diamond. What are allotropes?
(b) Why is graphite a conductor of electricity but diamond is not?
2. In the electrolysis of aqueous sodium sulfate,
(a) what ions migrate to the anode? What ion is preferentially discharged?
(b) what ions migrate to the cathode? What ion is preferentially discharged?
(c) what is the chemical equation for the overall reaction?
Answers: 1. (a) Allotropes are substances made of the same element but have different physical forms. (b) Graphite can conduct electricity because of the delocalised electrons between the hexagonal layers. All the valence electrons of the carbon atoms in diamond are involved in covalent bonding. There are no available mobile electrons in diamond to conduct electricity.
2. (a) The positively charged sodium ions and hydrogen ions migrate to the anode. Hydrogen is preferentially discharged. (b) The negatively charged sulfate ions and hydroxide ions migrate to the cathode. Hydroxide ions are preferentially discharged. (c) 2H2O(l) --> 2H2(g) + O2(g)
Try these questions before you check with the answers below.
1. (a) One other allotrope of carbon is diamond. What are allotropes?
(b) Why is graphite a conductor of electricity but diamond is not?
2. In the electrolysis of aqueous sodium sulfate,
(a) what ions migrate to the anode? What ion is preferentially discharged?
(b) what ions migrate to the cathode? What ion is preferentially discharged?
(c) what is the chemical equation for the overall reaction?
Answers: 1. (a) Allotropes are substances made of the same element but have different physical forms. (b) Graphite can conduct electricity because of the delocalised electrons between the hexagonal layers. All the valence electrons of the carbon atoms in diamond are involved in covalent bonding. There are no available mobile electrons in diamond to conduct electricity.
2. (a) The positively charged sodium ions and hydrogen ions migrate to the anode. Hydrogen is preferentially discharged. (b) The negatively charged sulfate ions and hydroxide ions migrate to the cathode. Hydroxide ions are preferentially discharged. (c) 2H2O(l) --> 2H2(g) + O2(g)
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