Csec Chemistry Notes 7

Structure and bonding

In this lesson, we will attempt a few questions on the topic. QUESTION 1 (a) Answer the questions concerning the solid substances A to D, the identities of which are given below: A – graphite, B – diamond, C – carbon dioxide, D – sodium chloride (b) Name the type(s) of bonding to be found in A. (1 mark) (c) Explain why A conducts electricity but B and D do not. (3 marks) (d) Why does B have a higher melting point than C? (2 marks) (e) Compare the hardness of A and B. (2 marks) ANSWERS (a) Graphite is made up of carbon atoms bonded together by strong covalent bonds. These bonds extend throughout the whole structure. (b) Graphite conducts electricity because electrons can move about in each layer. These electrons are said to be delocalised. Diamond, on the other hand, has no ‘free’ electrons while sodium chloride has no ‘free’ or mobile ions. Comment: For substances to conduct electricity, ions or electrons must be able to move so that the charge can be carried throughout the entire substance. All the valence electrons on carbon are involved in bonding. In sodium chloride, the ions are held in position and are free to move only if molten or in solution. (c) The carbon atoms in B are held by strong covalent bonds which are hard to break and require vast amounts of energy. In C, solid carbon dioxide molecules of CO2 are joined together by weak forces of attraction between the molecules. B, therefore, has a higher melting point than C. (d) A (graphite) consists of carbon atoms arranged hexagonally in layers, with weak forces of attraction between layers. This makes it soft, as the layers can slide over each other. The carbon atoms in B are arranged tetrahedrally and are held by very strong covalent bonds throughout. QUESTION 2 2. Compare calcium and carbon (include allotropes) by referring to: (a) their physical properties

(b) their electrical properties (c) the chemical properties of their oxides (d) the types of bonds present in their chlorides (5 marks) ANSWERS (a) Calcium has high melting and boiling points due to the strong bonds formed between cations and the ‘sea’ of mobile electrons. Carbon, in the form of graphite and diamond, has strong bonds which also result in high melting and boiling points. These substances are also solids at room temperature, owing to these strong bonds. (b) The mobile or delocalised electrons in calcium and graphite make them good conductors of heat and electricity. Diamond will not conduct heat or electricity since it has no free electrons. (c) Calcium oxide (CaO) is a basic oxide and will dissolve in water to form an alkaline solution (calcium hydroxide). It also reacts with acids to form salt and water. CaO(s) + H2O(l) = Ca(OH)2 (aq) (Alkaline solution). CaO (s) + H2SO4(aq) = CaSO4(aq) + H2O (l) Carbon dioxide is an acidic oxide and reacts with water to form acids. CO2(g) + H2O(l) = H2CO3 (aq) Carbonic acid carbon monoxide is a neutral oxide and is a good reducing agent, reducing zinc and metals below it in the reactivity series. CuO (s) + CO (g) = Cu (s) + CO2 (Reducing agent). (d) Calcium chloride (CaCl2) has ionic bonds present in its structure, while covalent bonds are present in carbon tetrachloride (CCl4).