WAEC SSCE - Chemistry - 2006 (Essay)

(a)(i) Draw and label a diagram to illustrate the preparation and collection of dry chlorine gas in the laboratory.

(ii) List two uses of chlorine.

(b)(i) Explain why river water flowing through an industrial town may be unsafe for drinking.

(ii) State the use of each of the following substances in water treatment: I. Sand, II. Chlorine, III. Calcium oxide, IV. Alum

(c)Consider the reaction represented by the following equation:

2Na\(_2\)CI\(_{(s)}\) + H\(_2\)SO\(_{4(aq)}\) \(\to\) Na\(_2\)SO\(_{4(aq)}\) + 2HCI\(_{(g)}\)

Calculate the volume of HCI gas that can be obtained at s.t.p. from 5.85 g of sodium chloride. [H = 1, Na = 23, CI = 35.5, Molar volume a 22.4 dm\(^3\) at s.t.p]

(d) Give one example in each case of a (i) metal that is a liquid at room temperature. (ii) non-metal that is a iiquid at room temperature, (iii) gas at room temperature that is monatomic.

(e) State two differences between metals and nom metals with respect to their: (i) physical properties; (ii) chemical properties.

(a)(i) Laboratory preparation and collection of dry chlorine gas

Chlorine is prepared by warming manganese(IV) oxide with concentrated hydrochloric acid:

\[\mathrm{MnO_2(s)+4HCl(aq)\rightarrow MnCl_2(aq)+Cl_2(g)+2H_2O(l)}\]

The gas is passed through saturated sodium chloride solution to remove hydrogen chloride, then through concentrated sulphuric acid to remove water vapour. Since chlorine is denser than air, it is collected by downward delivery, that is, upward displacement of air. It is not collected over water because it dissolves and reacts in water.

(a)(ii) Uses of chlorine

1. It is used to sterilise drinking water and swimming pools.
2. It is used as a bleaching agent for paper pulp and textiles.

(b)(i) River water flowing through an industrial town may contain poisonous industrial effluents, such as acids, oils and heavy-metal salts, as well as sewage and disease-causing microorganisms. It is therefore unsafe for drinking without treatment.

(b)(ii) Uses of substances in water treatment

Substance	Use
Sand	Filtration: removal of suspended solid particles.
Chlorine	Disinfection: kills germs and bacteria.
Calcium oxide	Neutralises acidic water and raises its pH.
Alum	Coagulates fine colloidal particles so that they settle or can be filtered off.

(c) Volume of hydrogen chloride gas

\[\mathrm{2NaCl(s)+H_2SO_4(aq)\rightarrow Na_2SO_4(aq)+2HCl(g)}\]

Molar mass of \(\mathrm{NaCl}=23+35.5=58.5\ \mathrm{g\ mol^{-1}}\).

\[\text{Moles of NaCl}=\frac{5.85}{58.5}=0.100\ \mathrm{mol}\]

From the equation, \(2\) mol of \(\mathrm{NaCl}\) produce \(2\) mol of \(\mathrm{HCl}\). Therefore, moles of \(\mathrm{HCl}=0.100\ \mathrm{mol}\).

\[\text{Volume of HCl at s.t.p.}=0.100\times22.4=\boxed{2.24\ \mathrm{dm^3}}\]

(d)

1. Mercury is a metal that is liquid at room temperature.
2. Bromine is a non-metal that is liquid at room temperature.
3. Argon is a monatomic gas at room temperature.

(e) Differences between metals and non-metals

Property	Metals	Non-metals
Physical	Good conductors of heat and electricity; malleable and ductile.	Usually poor conductors of heat and electricity; brittle when solid.
Chemical	Lose electrons to form positive ions and generally form basic oxides.	Gain or share electrons and generally form acidic oxides.

Read lesson note on Non-metals And Their Compounds (JAMB)

(a)(i) State two differences betwecii the properties of solids and gases

(ii) What process does each of X, Y and Z represent in the changes shown below?

(b)(i) State Charles' Law (ii) Draw a sketch to graphically illustrate Charles' Law.

(c) 60 cm of hydrogen diffused through a porous membrane in 10 minutes. The same volume of a gas G diffused through the same membrane in 37.4 minutes. Determine the relative molecular mass of G. [ H = I ]

(d)(i) State two assumptions X of the kinetic theory.

(ii) Consider the reaction represented by the Solid   of Liquid following equation:

H\(_{2(g)}\) + Cl\(_{2(g)}\) \(\to\) 2HCI\(_{(g)}\)

Use the kinetic theory to explain how the rate of formation of HCI\(_{(g)}\) would be affected by I. increase in temperature; II. decrease in pressure.

(e) Given different examples, mention one metal in each case vihich produces hydrogen on reacting with (i) dilute mineral acid; (ii) cold water; (iii) steam; (iv) hot, concentrated alkali.

(a)(i) Differences between solids and gases

Solid	Gas
It has a definite shape and a definite volume.	It has no definite shape or volume and fills its container.
Its particles are closely packed and it is not easily compressed.	Its particles are far apart and it is highly compressible.

(a)(ii) X is freezing (solidification); Y is sublimation; Z is condensation (liquefaction).

(b)(i) Charles' Law

At constant pressure, the volume of a fixed mass of gas is directly proportional to its absolute temperature.

Thus, \(V \propto T\), and \(\dfrac{V}{T}=\text{constant}\).

(b)(ii) Graphical illustration of Charles' Law

The straight line passes through the origin. For example, its gradient is \(\dfrac{160-40}{400-100}=0.40\ \text{cm}^3\text{K}^{-1}\), showing that \(V/T\) is constant.

(c) Relative molecular mass of G

By Graham's law, rate of diffusion is inversely proportional to the square root of relative molecular mass:

\[\frac{r_{H_2}}{r_G}=\sqrt{\frac{M_G}{M_{H_2}}}\]

Since equal volumes diffuse,

\[\frac{r_{H_2}}{r_G}=\frac{t_G}{t_{H_2}}=\frac{37.4}{10}=3.74\]

\[3.74=\sqrt{\frac{M_G}{2}}\]

\[M_G=2(3.74)^2=27.98\]

Therefore, the relative molecular mass of G is \(\boxed{28}\).

(d)(i) Assumptions of the kinetic theory of gases

1. A gas consists of tiny particles which are in continuous rapid random motion.
2. The particles are very far apart compared with their sizes, and forces of attraction between them are negligible.

(d)(ii) Effect on the rate of formation of HCl

\[H_{2(g)}+Cl_{2(g)}\rightarrow 2HCl_{(g)}\]

1. Increase in temperature: The molecules gain kinetic energy and move faster. They collide more frequently and with greater energy, so the number of effective collisions increases. Therefore, the rate of formation of HCl increases.
2. Decrease in pressure: The gas molecules become farther apart. Collision frequency and hence the number of effective collisions decrease. Therefore, the rate of formation of HCl decreases.

(e) Metals which produce hydrogen

1. With dilute mineral acid: zinc, for example \(Zn+H_2SO_4\rightarrow ZnSO_4+H_2\).
2. With cold water: sodium, for example \(2Na+2H_2O\rightarrow2NaOH+H_2\).
3. With steam: iron, for example \(3Fe+4H_2O\rightarrow Fe_3O_4+4H_2\).
4. With hot concentrated alkali: aluminium, for example \(2Al+2NaOH+2H_2O\rightarrow2NaAlO_2+3H_2\).

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

Read lesson note on Separation Of Mixtures And Purification Of Chemical Substances (JAMB)

(a) A solution of CuSO\(_4\) was electrolyzed between pure copper electrodes and the following results were obtained:

Mass of copper anode before experiment = 7.20 g

Mass of copper anode after experiment = 4.00 g

Mass of copper cathode before experiment = 5.75 g

From the information provided,

(i) calculate the mass of the cathode, after the experiment.

(ii) write an equation for the reaction at the I. anode, II. cathode.

(iii) state whether the colour of the solution would change during the electrolysis. Give a reason for your answer.

(iv) if the electrolysis was carried out for 1 hour 20 minutes with a current of 2.0 amperes, determine the value of the Faraday.

(b) Consider the reaction represented by the following equation:

MnO\(^-_4\) + I\(^-\) + H\(^+\)  \(\to\) I\(_2\) + H\(_2\)O + Mn\(^{2+}\) 

Write balanced half equation for the (i) oxidation reaction, (ii) reduction reaction.

(c)(i) Describe briefly how tin can be extracted from its ore.

(ii) State one use of tin.

(iii) Mention one property that makes tin suitable for the use stated in (c)(ii)

(d)(i) What is meant by the term pollution?

(ii) Explain why it is dangerous to run a generator in a closed room.

a) Define each of the following terms and indicate one use of each:

(i) Nuclear fission; (ii) Nuclear fusion.

(b) Alpha particle emission by \(^{293}_{25}U\) proceduces an element A. Beta particle emission by the particle A produces another element B. Element B also undergoes alpha particle emission to produce \(^{227}_{89}AC\). Write balanced equations to represent the above statement.

(c) The models below represent the filling of orbitals in an atom.

State which rule(s) is/are violated or obeyed by each model.

(d) Explain why the boiling point of H\(_2\)S with relative molecular mass of 34 is lower than that of H\(_2\)O with relative molecular mass of 18.

(e) HCI is passed into each of the following solvents:

(i) water;

(ii) methylbenzene. I. State the effect of each solution on blue litmus paper II. Compare the electrical conductivities of the two solutions.

(f) Zinc dust is added to copper (II) tetraoxosulphate (VI) solution. State;

(i) what is observed; (ii) the type of reaction that occurs.

(a) State the following laws of chemical combination: (i) Law of constant composition (ii) Law of multiple' proportion.

(b) Copper reacts with oxygen to form two oxides X and Y. On analysis, 1.535 g of X yielded 1.365 g copper and 1.450 g of Y yielded 1.160 g of cooper.

i) Determine the chemical formula of X and Y.

(ii) Calculate the mass of copper which can react with 0.500 g of oxygen to yield I. X  II. Y.

(iii) Which of the laws of chemical combination is illustrated by the result in (b)(i) above. [ = 16, Cu = 63.51]

(c) Write the structure of the product responsible for the observation in each of the following reactions:

(i) A mixture of butanoic acid and ethanol warmed in the presence of concentrated H\(_2\)SO\(_4\) gives off a fragrant odour.

(ii) Sodium dissolves in propan-2-ol with effervescence to give a solution which on evaporation to dryness leaves a white precipitate.

(d) Consider the compound CH\(_3\)CH\(_2\)COOCH\(_2\)CH\(_3\).

(i) Name the compound (ii) Write the structural formula of the compound (iii) State the reagents and conditions for the formation of the compound.

(a)(i) Draw the energy profile diagram for the reaction

\(\mathrm{H_{2(g)} + I_{2(g)} \longrightarrow 2HI_{(g)}}\) \(\Delta =\) —13 kJmol\(^3\)

(ii) If the concentration of HI increases from 0 to 0.001 mol dm\(^3}\) in 50 seconds, what is the rate of the reaction?

(b) State the type of salt represented by each of the following compounds:

(i) \(\mathrm{K_4Fe(CN)_6}\) (ii) \(\mathrm{(NH_4)_2Fe(SO_4)_2 6H_2O}\) (iii) \(\mathrm{Mg(OH)NO_3}\) (iv) \(\mathrm{NaH_2PO_4}\).

(c) Explain, giving equations, the following observation: When carbon (IV) oxide is passed into lime water, it turns milky initially but turns clear with excess carbon (IV) oxide.

(d)(i) Give one use for each of the following compounds: \(\mathrm{CaCO_3}\), \(\mathrm{CaSO_4}\), \(\mathrm{NaHCO_3}\).

(ii) State a drying agent for each of the following gases: i. \(\mathrm{NH_3}\), II. HCI Ill. \(\mathrm{SO_4}\).

(iii) Write an equation to illustrate the reaction of ammonia as a reducing agent.

(e) An industrial raw material has the following composition by mass:

Calculate the formula for the material. [ H = 1, 0 = 16, Cl = 35.5, Fe = 56 ].

(a)(i) Energy profile diagram

The reaction is exothermic, since \(\Delta H=-13\ \text{kJ mol}^{-1}\). Therefore, the products are at a lower energy level than the reactants.

(ii) Rate of reaction

\[\text{Rate}=\frac{\text{change in concentration of HI}}{\text{time taken}}=\frac{0.001-0}{50}=2.0\times10^{-5}\ \text{mol dm}^{-3}\text{s}^{-1}.\]

(b) Types of salts

1. \(\mathrm{K_4Fe(CN)_6}\): complex salt.
2. \(\mathrm{(NH_4)_2Fe(SO_4)_2\cdot6H_2O}\): double salt.
3. \(\mathrm{Mg(OH)NO_3}\): basic salt.
4. \(\mathrm{NaH_2PO_4}\): acid salt.

(c) Action of carbon(IV) oxide on lime water

Initially, carbon(IV) oxide reacts with lime water to form insoluble calcium trioxocarbonate(IV), \(\mathrm{CaCO_3}\), which is a white precipitate. This makes the lime water milky.

\[\mathrm{Ca(OH)_2(aq)+CO_2(g)\rightarrow CaCO_3(s)+H_2O(l)}\]

On passing excess carbon(IV) oxide, the calcium trioxocarbonate(IV) dissolves to form soluble calcium hydrogentrioxocarbonate(IV). The solution therefore becomes clear.

\[\mathrm{CaCO_3(s)+CO_2(g)+H_2O(l)\rightarrow Ca(HCO_3)_2(aq)}\]

(d)(i) Uses

1. \(\mathrm{CaCO_3}\): manufacture of cement.
2. \(\mathrm{CaSO_4}\): manufacture of plaster of Paris for casts.
3. \(\mathrm{NaHCO_3}\): used in baking powder.

(d)(ii) Suitable drying agents

1. \(\mathrm{NH_3}\): calcium oxide, \(\mathrm{CaO}\).
2. \(\mathrm{HCl}\): concentrated tetraoxosulphate(VI) acid, \(\mathrm{H_2SO_4}\).
3. \(\mathrm{SO_2}\): concentrated tetraoxosulphate(VI) acid, \(\mathrm{H_2SO_4}\).

(d)(iii) Ammonia as a reducing agent

Ammonia reduces copper(II) oxide to copper:

\[\mathrm{2NH_3(g)+3CuO(s)\rightarrow 3Cu(s)+N_2(g)+3H_2O(l)}\]

(e) Formula of the industrial raw material

Constituent	Mass in 100 g	Moles	Mole ratio
Fe	28.1 g	\(28.1/56=0.502\)	\(0.502/0.502=1\)
Cl	35.7 g	\(35.7/35.5=1.006\)	\(1.006/0.502=2\)
\(\mathrm{H_2O}\)	36.2 g	\(36.2/18=2.011\)	\(2.011/0.502=4\)

Hence, the mole ratio is \(\mathrm{Fe:Cl:H_2O}=1:2:4\).

\[\boxed{\mathrm{FeCl_2\cdot4H_2O}}\]

Read lesson note on Chemical Combination (JAMB)