(a)(1) Define covalent bond.
(ii) Give two properties of covalent compounds
(ii) With the aid of a diagram, show how ammonia molecule is formed
(iv) Illustrate with a diagram the formation of ammonium ion?
(v) What type of bond(s) exist(s) in I. ammonia, H. ammonium ion? (\(_1\)H\(_7\)N)
(b)(i) Write three subatomic particles with their corresponding relative masses. CH\(_2\)OH
(ii) Name the possible states in which water can exist.
(c) (i) State Graham's law of diffusion
(ii) Arrange the following gases, He, CH\(_4\) and N\(_2\) in order of increasing rates of diffusion. Give a reason for the order. [ H = 1, He = 4, C = 12, N = 14 ]
(d) Draw the structures of the following compounds:
(i) 2,3-dimethylbutane;
(ii) 1,4-dibromocyclohexane.
(a) Covalent bonding
- (i) A covalent bond is a chemical bond formed by the mutual sharing of one or more pairs of electrons between two atoms (usually non-metals), so that each atom attains a stable noble-gas electron configuration.
- (ii) Two properties of covalent compounds: they generally have low melting and boiling points; they are usually non-conductors of electricity (no free ions or electrons); many are insoluble in water but soluble in organic solvents.
- (iii) Formation of ammonia (NH\(_3\)): nitrogen (\(1s^{2}2s^{2}2p^{3}\), 5 outer electrons) shares three of its electrons, one with each of three hydrogen atoms. Three shared pairs (three N-H single covalent bonds) form, and nitrogen keeps one lone pair. In a dot-and-cross diagram, nitrogen at the centre shows three N-H bonds (each a shared pair) plus one lone pair on nitrogen.
- (iv) Formation of ammonium ion (NH\(_4^{+}\)): the lone pair on the nitrogen of NH\(_3\) is donated to a proton (H\(^{+}\), which has no electron). This forms a fourth N-H bond called a dative (co-ordinate) bond, giving \(NH_4^{+}\) with an overall +1 charge. The dot-and-cross diagram shows four N-H bonds, one of which is dative (both electrons supplied by nitrogen), with the whole ion carrying a +1 charge.
- (v) Bonds present: I. in ammonia only ordinary covalent bonds (three N-H); II. in the ammonium ion covalent bonds plus one dative (co-ordinate) covalent bond. (Once formed, all four N-H bonds are identical.)
(b) Sub-atomic particles and states of water
- (i)
| Particle | Relative mass |
|---|
| Proton | 1 |
| Neutron | 1 |
| Electron | 1/1840 (approximately 0) |
- (ii) States of water: solid (ice), liquid (water) and gas (steam/water vapour).
(c) Graham's law of diffusion
- (i) At constant temperature and pressure, the rate of diffusion of a gas is inversely proportional to the square root of its density (or molar mass): \[\text{Rate} \propto \frac{1}{\sqrt{M}}\]
- (ii) Molar masses: He = 4, CH\(_4\) = 16, N\(_2\) = 28. The lighter the gas, the faster it diffuses. Order of increasing rate of diffusion: \[N_2 < CH_4 < He\] Reason: N\(_2\) is the heaviest (largest \(\sqrt{M}\)) so it diffuses slowest, while He is the lightest so it diffuses fastest.
(d) Structures
- (i) 2,3-dimethylbutane: a four-carbon chain with a methyl group on carbon 2 and on carbon 3, i.e. \((CH_3)_2CH{-}CH(CH_3)_2\).
- (ii) 1,4-dibromocyclohexane: a six-membered carbon ring (cyclohexane) with a bromine atom attached to carbon 1 and another to carbon 4 (directly opposite).
