Chemistry 2 Nov/Dec 2012

Chemistry Paper 2,Nov/Dec 2012

Questions:		1	2	3	4	7	8		Main

General Comments
Weakness/Remedies
Strength

Question 2
(Question 2 (a) (i) State Boyle’s law. Give the mathematical expression of Boyle’s law. Give two reasons why real gases deviate from ideal gas behavior. State: three postulates of the kinetic theory of gases; Dalton’s law of partial pressures (b) (i) Consider the figure as illustrated below: Liquid P Q Gas Solid R State the processes represented by P, Q and R respectively (ii) Some solids when heated change directly to the gaseous state. What name is given to this phenomenon? Name two substances which exhibit the phenomenon referred to in 2(b)(ii) I. (c) If 80 cm3 of methane (CH4) diffuses through a porous membrane in 20 seconds while 50 cm3 of gas Q diffuse through the same membrane in 25 seconds, calculate the molecular mass of gas Q. [ CH4 = 16.0 ] Explain briefly why at room temperature fluorine is a gas while bromine is a liquid. This question was attempted by majority of the candidates and the performance was fairly good. In (a), majority of the candidates gave correct statement of Boyle’s law as well as the mathematical expression, but could not give reasons why real gases deviate from ideal gas behaviour. In (b), processes of freezing, condensation and sublimation were easily identified by majority of the candidates. In (c) some candidates could not solve the problem, however few candidates were able to solve the problem using Graham’s law of diffusion. In (d) Some candidates could not explain correctly why bromine is a liquid and fluorine a gas at room temperature.
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OBSERVATION
PThe expected answers include: (a) (i) Boyle’s law states that the volume of a given mass of gas is inversely proportional to its pressure provided temperature remains constant. (ii) V a (1) at constant T. OR PV = K; OR V = OR P1V1 = P2V2 (iii) - There are attractive forces between molecules of real gases. - The volume of molecules of real gases is not negligible. (iv) I - Motion of gas molecules is random or chaotic. - Collision between molecules is perfectly elastic/no energy is lost on collision. - The volume of the molecules is negligible compared with the total volume of the container. - There are no attractive or repulsive forces between molecules of the gas. - The average kinetic energy of the molecules is proportional to the (absolute) temperature. - The pressure exerted by the gas results from the impact/collision of molecules upon the walls of the container. II - Dalton’ Law of partial pressures states that for a mixture of gases which do not react together the total pressure is the sum of the partial pressures of the gases present. Accept mathematical expression PT = Pa + Pb + Pc where the terms are explained and candidates must state that gases do not react together (b) (i) P - Condensation Q - Freezing/Solidification R - Sublimation (ii) I. Sublimation II. Iodine, camphor, naphthalene, aluminum chloride (c) = Þ= = 4 = = 2 = = = 4 = Þ = 4 x 16 = 64 (d) Fluorine and bromine are both held by weak van der Waals forces. The van der Waals forces increase with increasing number of electrons. Bromine having more electrons than fluorine has a stronger force which holds the molecules closer together hence bromine is a liquid. OR Fluorine having less number of electrons has weaker van der Waals forces hence the molecules are not tightly held together.