WAEC SSCE - Chemistry - 1999 (Essay)

(a)(i) List two elements which react with steam at red heat to produce hydrogen.

(ii) Explain why an aqueous solution of potassium bromide turned reddish brown on bubbling chlorine through it.

(iii) Write an equation for the reaction in (a)(ii).

(b)(i) Name two types of chemical industry that use limestone as raw material.

(ii) Give one example of hygroscopic substances.

(iii) Copy and complete the table below.

(c) In the contact process for the manufacture of tetraoxosulphate (VI) acid;

(i) State how sulphur (IV) oxide is obtained;

(ii) Write an equation for the reaction that takes place in the catalyst chamber;

(iii) Give the steps required to convert sulphur (VI) oxide to acid.

(d)(i) List two disadvantages of hard water.

(ii) Mention three methods which can be used to remove both permanent and temporary hardness in water at the same time.

(iii) State the role of alum and chlorine respectively in the purification of water for town supply.

(a)(i) Define allotropy.

(ii) Name the allotrope of carbon used in gas masks.

(iii) Mention two other elements which exhibit allotropy apart from carbon.

(b) List the products of each of the following reactions:

(i) Heating coal in the absence of air.

(ii) Burning of candle wax in plentiful supply of air

(c)(i) State the two properties of carbon (IV) oxide which make it useful in extinguishing fire.

(ii) Write an equation for the reaction of carbon (IV) oxide with lime water

(iii) Calculate the volume of oxygen that was in excess if 150cm\(^3\) of carbon (II) oxide was burnt in 80cm\(^3\) of oxygen according to the following equation: 2CO\(_{(g)}\) + O\(_{2(g)}\) \(\to\) 2CO\(_{2(g)}\).

(d)(i) State how nitrogen can be obtained from ammonia gas.

(ii) Name the gaseous fuels obtained when steam and air are passed over red hot coke.

(iii) Which of the fuels in (d)(ii) has the lower heating ability? Give reason for your answer.

(a) The letters R,S,T represent an alkene, an alkene and a terminal alkyne respectively. Which of R, S and T typically undergo(es) the following reactions?

(i) Addition reaction;

(ii) Combustion;

(iii) Substitution reaction.

(b)(i) Name the process by which an alkanol can be converted to an alkene.

(ii) Write the name and structural formula of the third member cf the alkanol series.

(iii) State what would be obtained if a primary alkanol reacted with excess acidified KMnO\(_4\) solution.

(c)(i) Give one chemical test for alkanoic acids.

(ii) Write an equation to show how methanoic acid reacts with ethanol in the presence of mineral acids.

(iii) What is the role of mineral acid in the reaction in (c) ii?

(d) In an experiment, cassava was pressure-cooked to release starch granules, followed by treatment with malt for about 2 hours at 55°C. Yeast was then added and the mixture was left for 2 days at 27°C. An organic product J and a gas H were obtained.

(i) Identify H

(ii) State the class of carbohydrates to which starch belongs and explain what happened to the starch during treatment with malt.

(iii) Draw a labelled diagram of a suitable set-up for obtaining a sample of J from the reaction mixture.

(a)

Taking R as an alkane, S as an alkene and T as a terminal alkyne:

1. Addition reaction: S and T.
2. Combustion: R, S and T.
3. Substitution reaction: R and T.

(b)

1. The process is dehydration of the alkanol.
2. The third member is propan-1-ol.
   Structural formula: CH₃CH₂CH₂OH.
3. A primary alkanol is oxidised to the corresponding alkanoic acid by excess acidified potassium manganate(VII) solution.
   For example:
   \[\mathrm{CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O}\]

(c)

1. Add aqueous sodium hydrogencarbonate or sodium carbonate to the acid. Brisk effervescence of carbon dioxide is produced.
   \[\mathrm{RCOOH + NaHCO_3 \rightarrow RCOONa + CO_2 + H_2O}\]
2. \[\mathrm{HCOOH + CH_3CH_2OH \xrightleftharpoons[\ ]{conc.\ H_2SO_4} HCOOCH_2CH_3 + H_2O}\]

   The ester formed is ethyl methanoate.

3. The mineral acid acts as a catalyst in the esterification reaction.

(d)

1. H is carbon dioxide, CO₂.
2. Starch is a polysaccharide. The diastase (amylase) enzyme in malt hydrolyses starch to maltose, a reducing sugar.
   \[\mathrm{(C_6H_{10}O_5)_n + \tfrac{n}{2}H_2O \xrightarrow[55^{\circ}C]{diastase} \tfrac{n}{2}C_{12}H_{22}O_{11}}}\]

   Yeast subsequently ferments the sugar to ethanol, J, and carbon dioxide:

   \[\mathrm{C_6H_{12}O_6 \xrightarrow{yeast} 2C_2H_5OH + 2CO_2}\]
3. J is ethanol. A sample is obtained from the fermented mixture by simple distillation.

Distillation set-up for obtaining ethanol (J)

The fermented mixture is heated gently in a water bath. Ethanol distils over, condenses in the Liebig condenser and is collected as the distillate.

Read lesson note on Chemistry Of Carbon Compounds (Nigeria Only) (WAEC)

(a)(i) Give two differences between a conductor and an electrolyte.

(ii) State three applications of electrolysis.

(iii) Write equation for the reaction at each electrode when a dilute solution of sodium chloride is electrolysed using carbon electrodes.

(b)(i) What is an electrochemical cell?

(ii) Give two examples of primary cells.

(iii) Split the following equation into two balanced hall cell equations. Mte + Fe\(^{2+} \to Mg^{2+} + Fe\).

(c)(i) A current of 0.72 amperes was passed through dilute tetraoxosulphate (VI) acid for 3 hours 20 minutes. Calculate the quantity of electricity that was passed

(ii) If 1 dm\(^3\) of gas evolved at the cathode during the electrolysis of acidified water, what was the volume of gas evolved at the anode?

(d)(i) 20g of copper(II) oxide was warmed with 0.05 mole of tetraoxosulphate (VI) acid. Calculate the mass of copper (II) oxide that was in excess. The equation for the reaction: CuO\(_{(s)}\) + H\(_2\)SO\(_{4(aq)}\) ---> CuSO\(_{4(aq)}\) + H\(_2\)O\(_l\) [0 = 16 ; Cu = 64]

(ii) What type of reaction was involved in (d)(i)?

(a)(i) State three postulates of the kinetic theory of gases.

(ii) Draw a sketch to illustrate Boyle's law.

(iii) Explain what is meant by "absolute zero of temperature".

(b) Equal volumes of a gas X and oxygen are at the same pressure but temperature of 200 K and 800K respectively.

(i) Giving reason for your answer, state which of the gas samples contains the greater number of molecules.

(ii) If the mass of one molecule of the gas X is \(2.19 x 10^{-22}\) g, determine the molar mass of X. [Avogadro constant = \(6.02 x 10^{-23}\)]

(iii) Hence, state with reason which of X and oxygen will diffuse faster under the same conditions. [O = 16].

(c)(i) Mention two reaction conditions that can increase the yield of ammonia in the reaction represented by the following equation: \(N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)}\ \Delta H = -46\text{kJmol}^{-1}\). Draw a labelled energy profile diagram for an endothermic reaction.

(iii) A solid W decomposes on heating according to the following equation: \(W_{(s)} \to Y_{(s)} + Z_{(g)}\) List two factors apart from temperature, which can affect the rate of the reaction.

(a)(i) Postulates of the kinetic theory of gases

1. A gas consists of a very large number of tiny particles which are in continuous rapid random motion.
2. The actual volume of the gas particles is negligible compared with the volume of the container, and intermolecular forces are negligible.
3. Collisions between gas particles and with the walls of the container are perfectly elastic. The average kinetic energy of the particles is proportional to the absolute temperature.

(a)(ii) Boyle's law

At constant temperature, the pressure of a fixed mass of gas is inversely proportional to its volume: \(P \propto \frac{1}{V}\), or \(PV=\text{constant}\).

(a)(iii) Absolute zero

Absolute zero is the lowest theoretically attainable temperature, \(0\text{ K}\) or approximately \(-273^\circ\text{C}\), at which the volume of an ideal gas would theoretically become zero and molecular motion is at its minimum.

(b)(i)

Gas X contains the greater number of molecules. For equal volumes at the same pressure,

\[n=\frac{PV}{RT}\]

Hence, \(n\propto \frac{1}{T}\). Since X is at \(200\text{ K}\), whereas oxygen is at \(800\text{ K}\), X has more molecules.

(b)(ii)

Mass of one molecule of X \(=2.19\times10^{-22}\text{ g}\).

\[M=2.19\times10^{-22}\times6.02\times10^{23}\]

\[M=(2.19\times6.02)\times10=131.8\text{ g mol}^{-1}\]

Therefore, the molar mass of X is approximately \(132\text{ g mol}^{-1}\).

(b)(iii)

\[M(\mathrm{O_2})=2(16)=32\text{ g mol}^{-1}\]

By Graham's law, rate of diffusion is inversely proportional to the square root of molar mass. Since \(32<132\), oxygen diffuses faster than X.

(c)(i) Conditions for increasing the equilibrium yield of ammonia

1. Use a high pressure, because the forward reaction changes four moles of gaseous reactants into two moles of gaseous product.
2. Use a low temperature, because the forward reaction is exothermic, \(\Delta H=-46\text{ kJ mol}^{-1}\).

Continuous removal of ammonia as it is formed also increases its yield.

(c)(ii) Labelled energy profile diagram for an endothermic reaction

(c)(iii)

Two factors, apart from temperature, which affect the rate of decomposition of W are:

1. Surface area (particle size) of W. A finely divided solid decomposes faster.
2. Presence of a catalyst.

Read lesson note on Kinetic Theory Of Matter And Gas Laws (JAMB)

a)(i) State three differences between covalent compounds and electrovalent compounds.

(ii) Two elements represented as K and L have atomic numbers of 12 and 8 respectively. Write their electronic structures and state the group to which each belongs in the Periodic Table.

(iii) If an alkali metal M exists naturally as the oxide, state with reason whether or not M can be extracted by reduction of the oxide with coke.

(b)(i) What is meant by the half-life of a radioactive element?

(ii) Mention the radioactive isotope used in dating archaeological specimens.

(iii) Balance the following equation and identify Q,

\(^{28}_{13}Al\) —> \(^{24}_{13}Si + \(^{0_1Q\)

(c)(i). Give three chemical properties of metals.

(ii) State two reasons why duralumin preferred to steel in aircraft manufacture.

(iii) What term is used to describe the ability of metals to be hammered into thin sheets?

(iv) Calculate the number of mole of electrons involved in the oxidation of 2.8g of iron filings to iron (II) ions. [Fe = 56].