welcome to mubula resource center of excellence: 1992 uace chemistry

Wednesday, 14 September 2016

1992 uace chemistry

INSTRUCTIONS TO CANDIDATES

Answer all questions section A and six questions in section
1. The diagram below shows the lattice structure of an ionic salt MX

(a) State the coordination number of M⁺ and X^- ions. Explain your answer

...............................................................................................................................................................................................................................

(b) Name the type of lattice structure shown in the diagram above

2. When 20g of a mixture containing anhydrous sodium carbonate and sodium hydrogen was heated and cooled, the mass of the mixture changed to 13.8g.

(a) Write equation for the reaction that took place during the heating

(b) Calculate the percentage of sodium hydrogen carbonate in the mixture

3. (a) Steam distillation is one of the methods used for the separation of a component from a liquid mixture.

State one requirement for the component to be separated by steam distillation

(b) A mixture containing a substance X was steam distilled at 760 mm Hg and 980C.the distillate contained 85% by mass of water. If the vapour pressure of water is 734 mm Hg at 980C. Calculate the molar mass of X.

4. Name reagent(s) that can be used to distinguish between each of the following pairs of compounds. In each case state what would be observed when the compounds are treated with reagent(s) you have named

(a) (CH₃)₃COH and CH₃CH₂CH₂OH

Reagent(s) ...............................................................................................................................................................................................................................

Observation

(b) CH₃CH₂CCH₂CH₃ and CH₃CH₂CH₂CCH₃

Observation

5. The energy diagram for the reaction between hydrogen and chlorine is given below

(a)Identify the energy changes

A..................................................................................................

B..................................................................................................

C.....................................................................................................

D......................................................................................................

(b) Calculate the enthalpy change for the reaction

(The H-H, Cl-Cl and H-Cl bond energies are 435.9,241.8 and 431.0 kj mol^-1)

6. (a)Write an ionic equation for the reaction between sodium hydroxide and

(i) Silicon (IV) oxide

(ii) Lead (IV) oxide

(b)When lead (IV) oxide is heated with sulphur dioxide a white solid is formed

Explain the observation

SECTION B

10. (a) A solution containing 15.0g of phenylamine hydrochloride in 100cm³ of water was shaken with 100cm³ of benzene. At equilibrium the benzene layer contained 0.12g of phenylamine

Calculate

(i)the molar concentration of phenylamine in the benzene layer.

(ii) the hydrolysis constant Kh, of phenylamine hydrochloride.

11. The first ionization energies and first electron affinities of the group VII elements are given in the table below

Table 1

Element	First ionization energy (kj mol -1)	First electron affinity (kj mol-1)
Fluorine Chlorine Bromine iodine	1681 1250 1139 1007	-328 -349 -325 -295

(a)State how the first electron affinity of the elements generally vary with their first ionization energies.

(b) Explain why first ionization affinities of these elements have negative signs.

(d)State the trend in the first ionization energies of the elements and give a reason for your answer

12. Complete each of the following equations and suggest a mechanism for the reaction in each case.

13. Hydrogen reacts with nitrogen to produce ammonia according to the following equation

3H₂(g) + N₂(g) ⇄ 2NH(g)

(a)Write an expression for the equilibrium constant Kc.

(b)State giving reasons what would happen to the value of the equilibrium constant when

(i) pressure is increased at constant temperature

(ii) argon is added to the reaction mixture at constant pressure

(iii) argon is added to the reaction mixture at constant volume

14. When a pale green solid P was heated, it decomposed to a green solid Q and a gas that turned lime water milky was evolved. Q dissolved in dilute nitric acid to give a green solution R. On adding dilute sodium hydroxide solution to R. a green precipitate S insoluble in excess alkali was obtained S dissolved in aqueous ammonia to give a purplish blue solution.

(a)Identify compounds P, Q. S and R

P..............................................................................................

Q............................................................................................

S.....................................................................................................

Solution R........................................................................................

(b)Write equation for the reaction between

(i)Q and nitric acid

(ii)R and sodium hydroxide

(iii)S and ammonia

15. (a) (i) State Kohlrausch's law of ionic conductivity at infinite dilution

(ii) Calculate the molar conductivity of methanoic acid at infinite dilution

(λo (HCOONa)=9.5X10^-2Sm² mol^-1;λo(NaCl)=1.26X10^-1Sm² mol^-1;λo(HCl)=4.26X10^-1Sm²mol^-1)

(b) Ionic conductivities of Ag⁺ and Cl^- at infinite dilution are 6.2x10^-2 and 7.6x10^-2Sm² mol^-1 respectively at 298K.The electrolytic conductivity of silver chloride at 298K is 1.22x10^-4Sm^-1. Calculate the solubility, in mol dm^-3 of silver chloride at 298K

16. (a) An alkyne, Z has molecular formula C₄H₆

Write the names and structural formulae of all possible isomers of Z

(b) Z reacts with a solution of copper (I) chloride in aqueous ammonia to give a red precipitate

(i)Identify Z

(ii)Write equation for the reaction between Z and excess hydrogen bromide.

(d) Write the structural formulae and names of two hydrocarbons that are not alkynes but are isomeric with Z.

17. Polystyrene and nylon are both synthetic polymers.

(a)In each case write the structural formulae of the monomer(s);state whether the polymer.

Is formed by addition or condensation polymerization and write the structural formula of the polymer.

polymer	Structural formulae of monomer	Method of formation of polymer	Structural formula of polymer
Polystyrene
Nylon

(b)Write equation(s) to show how styrene can be synthesized starting from 2-phenylethanol.

P525/2

CEHMISTRY

Paper 2

March 1992

Answer five questions including three questions from section A and any two from section B

SECTION A

1. (a) (i) State two common states of chromium.

(ii) Write the electronic configuration of chromium ions in the oxidation states you have named above in (i)(atomic number of chromium =24).

(iii) Write the formula of one compound in each case in which chromium shows the oxidation states you have stated in (i).(your answer should not include chrome alum and potassium chromate).

(b) A dilute solution of chrome alum, K₂Cr₂(SO₄)₄.2H₂O was prepared and divided into two portions.

(i) The first portion turned blue litmus paper red. Explain the observation and write equation to illustrate your answer.

(ii) To the second portion was added a few drops of sodium carbonate solution. State what was observed and explain your answer.

(c) When about 1cm3 of dilute sulphuric acid was added about 1cm3 of potassium chromate, an orange solution was observed. Addition of an excess dilute sodium hydroxide solution to the mixture gave a yellow solution

Write equations for the reactions that took place.

(d) Briefly describe a test that can be carried out in laboratory to confirm that potassium chromate contains chromium ions.

2. (a) The boiling point of water is 100⁰C and that of ethanol is 80⁰C .A mixture of the two liquids when distilled gives a constant boiling mixture at 78.2⁰C containing 95.6% ethanol.

(i) Draw a labelled diagram of boiling point against composition for the water ethanol mixture.

(ii) Describe what would happen if a mixture containing less than 95.6% of ethanol is fractionally distilled.

(b) (i) A solvent Y of molecular mass 62 has a vapour pressure of 1.0x10⁴Nm^-2 at 298K 23.3g of a non volatile solute of molecular mass 270 was added to 100g of Y at 298K.

Calculate the vapour pressure of the solution.

(ii) State and explain the effect of increasing the concentration of the solute on the boiling point of Y.

3. Write equations to show how each of the following compounds can be synthesized.

In each case indicate the reagents and conditions for the reactions.

(a) CH₃CO₂H (from ethene)

(d) CH₃CH₂OCH₂CH₃ (from ethanol)

(e) CH₃CO₂CH₂CH₂CH₃ (from 1-bromopropane)

4. (a)Explain what is meant by each of the following terms

(i) rate equation

(ii) order of reaction

(iii) Molecularity

(b)The data in the table 1 were obtained for the reaction between a bromoalkane (RBr) and hot aqueous sodium hydroxide.

Table I

Time(min)	0	9	18	27	40	54	72	105
Conc of RBr(moldm-3)	0.106	0.096	0.086	0.077	0.065	0.054	0.043	0.030

(i) Plot a graph of concentration of the bromoalkane against time.

(ii) From the graph determine the order of reaction; Give a reason for your answer.

(iii) State whether the bromoalkane is primary, secondary or tertiary.

(iv) Suggest a mechanism for the reaction.

(v)Draw a labelled energy diagram for the reaction.

SECTION B

5. Complete each of the following equations in each case outline a mechanism for the reaction

6. (a) (i) State Hess's law.

(ii) Explain the terms standard state of a substance and standard enthalpy change for a reaction.

(b) Describe briefly how the enthalpy and neutralization of hydrochloric acid can be determined.

(c) The standard enthalpies of combustion of carbon, sulphur, and carbon disulphide are 394,-297 and -1075 kj mol ^-1 respectively.

Calculate the standard enthalpy of formation of carbon disulphide.

7. Explain each of the following observations:

(a) The pH of pure water is 7.0 at 25⁰C and 6.4 at 75⁰C.

(b) The Ka of methanoic acid is 1.7x10^-4 and that of ethanoic acid is 1.7x10^-5 at 25⁰C.

(d)Benzene and cyclohexene can be hydrogenated to cyclohexane. The enthalpies of hydrogenation of benzene and cyclohexene are 200 kj mol^-1 and 121 kj mol^-1 respectively.

(e) A solution containing ammonium chloride and ammonia has a buffer action.

8. (a) (i) Outline a chemical process involved in the manufacture of nitric acid. Illustrate your answer with suitable equations.

(ii) State two large scale uses of nitric acid.

(b) Concentrated nitric acid 70%(w/w) and has a density of 1.42g cm^-3

Calculate the molarity of the concentrated nitric acid.

(c) 12.68cm³ of the acid in (b) was dissolved in water and the solution made up to 250cm³ with distilled water. Calculate the volume of the solution that would react completely with 25.0cm³ of a 0.2M sodium carbonate solution.

(d) Briefly discuss the reaction between copper and nitric acid

Illustrate your answers with equations

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Wednesday, 14 September 2016

1992 uace chemistry

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