WAEC SSCE - Chemistry - 2013 (Essay)
Question 1 Report
(a)(i) List two characteristics of homologous series.
(ii) Consider the compound represented by the following formula: CH\(_3\)(CH\(_2\))\(_2\)CH\(_3\). I. Which homologous series does the compound belong? II. Write the structures of three possible isomers of the coumpound. III Name the three possible isomers in (a)(ii)II.
(b) Write the structure of the major product formed in each of the following reactions: (i) ethanol with excess acidified potassium tetraoxomanganate (VII);
(ii) excess ethane with chlorine in the presence of sunlight;
(iii) ethanol with propanoic acid in the presence of few drops of concentrated tetraoxosulphate(VI) acid.
(c) Name the major product in each reaction in (b).
(d) Consider the following organic compounds:
(i) Give the IUPAC name of each compound.
(ii) State a chemical test for the functional group in each compound.
(e) An organic compound with relative molecular mass 136 contains 70.57% carbon, 5.90% hydrogen and 23.53% oxygen. Determine its: (i) empopirical formula; (ii) molecular formula. [H= 1.00, C= 12.0, H = 16.0]
(a)(i) Two characteristics of a homologous series
- All members are represented by the same general molecular formula.
- Successive members differ from one another by a
CH2unit (relative mass 14).
(They also share the same functional group, show a steady gradation in physical properties, and have similar chemical properties.)
(a)(ii) The compound CH3(CH2)2CH3 is butane, molecular formula C4H10.
I. Homologous series: the alkanes (saturated aliphatic hydrocarbons).
II. Possible isomers. The formula C4H10 has only two possible structural (chain) isomers; a third isomer of this formula cannot be drawn. The two are:
CH3-CH2-CH2-CH3CH3-CH(CH3)-CH3i.e. a central carbon bearing three methyl groups and one hydrogen.
III. Names: (1) butane (n-butane); (2) 2-methylpropane (isobutane). No third isomer exists for C4H10.
(b) & (c) Major products and their names
| Reaction | Structure of major product | Name of product |
|---|---|---|
| (i) ethanol + excess acidified KMnO4 (full oxidation) | CH3COOH | ethanoic acid |
| (ii) excess ethane + chlorine in sunlight (monosubstitution favoured) | CH3CH2Cl | chloroethane |
| (iii) ethanol + propanoic acid + few drops conc. H2SO4 (esterification) | CH3CH2COOCH2CH3 | ethyl propanoate |
(d) The two compounds shown
X is (CH3)3COH and Y is a benzene ring bearing -COOH, i.e. C6H5COOH.
(i) IUPAC names: X = 2-methylpropan-2-ol (2-methyl-2-propanol); Y = benzoic acid (benzenecarboxylic acid).
(ii) Chemical test for the functional group:
- X (hydroxyl, -OH): add solid phosphorus(V) chloride, PCl5; misty white fumes of hydrogen chloride are evolved, confirming the -OH group. (Alternatively a piece of sodium gives effervescence of hydrogen.)
- Y (carboxyl, -COOH): add aqueous sodium hydrogentrioxocarbonate(IV), NaHCO3; brisk effervescence of carbon dioxide occurs, and the gas turns lime water milky, confirming the -COOH group.
(e) Empirical and molecular formula (using C = 12.0, H = 1.00, O = 16.0; the third figure in the data is O, not H).
Divide each percentage by the atomic mass:
\[C:\ \frac{70.57}{12.0}=5.88,\qquad H:\ \frac{5.90}{1.00}=5.90,\qquad O:\ \frac{23.53}{16.0}=1.47\]
Divide through by the smallest ratio (1.47):
\[C:\ \frac{5.88}{1.47}=4,\qquad H:\ \frac{5.90}{1.47}=4,\qquad O:\ \frac{1.47}{1.47}=1\]
(i) Empirical formula = C4H4O.
Empirical formula mass \(=4(12.0)+4(1.00)+16.0 = 68\).
\[n=\frac{\text{molar mass}}{\text{empirical mass}}=\frac{136}{68}=2\]
(ii) Molecular formula = (C4H4O)2 = C8H8O2.
Answer Details
(a)(i) Two characteristics of a homologous series
- All members are represented by the same general molecular formula.
- Successive members differ from one another by a
CH2unit (relative mass 14).
(They also share the same functional group, show a steady gradation in physical properties, and have similar chemical properties.)
(a)(ii) The compound CH3(CH2)2CH3 is butane, molecular formula C4H10.
I. Homologous series: the alkanes (saturated aliphatic hydrocarbons).
II. Possible isomers. The formula C4H10 has only two possible structural (chain) isomers; a third isomer of this formula cannot be drawn. The two are:
CH3-CH2-CH2-CH3CH3-CH(CH3)-CH3i.e. a central carbon bearing three methyl groups and one hydrogen.
III. Names: (1) butane (n-butane); (2) 2-methylpropane (isobutane). No third isomer exists for C4H10.
(b) & (c) Major products and their names
| Reaction | Structure of major product | Name of product |
|---|---|---|
| (i) ethanol + excess acidified KMnO4 (full oxidation) | CH3COOH | ethanoic acid |
| (ii) excess ethane + chlorine in sunlight (monosubstitution favoured) | CH3CH2Cl | chloroethane |
| (iii) ethanol + propanoic acid + few drops conc. H2SO4 (esterification) | CH3CH2COOCH2CH3 | ethyl propanoate |
(d) The two compounds shown
X is (CH3)3COH and Y is a benzene ring bearing -COOH, i.e. C6H5COOH.
(i) IUPAC names: X = 2-methylpropan-2-ol (2-methyl-2-propanol); Y = benzoic acid (benzenecarboxylic acid).
(ii) Chemical test for the functional group:
- X (hydroxyl, -OH): add solid phosphorus(V) chloride, PCl5; misty white fumes of hydrogen chloride are evolved, confirming the -OH group. (Alternatively a piece of sodium gives effervescence of hydrogen.)
- Y (carboxyl, -COOH): add aqueous sodium hydrogentrioxocarbonate(IV), NaHCO3; brisk effervescence of carbon dioxide occurs, and the gas turns lime water milky, confirming the -COOH group.
(e) Empirical and molecular formula (using C = 12.0, H = 1.00, O = 16.0; the third figure in the data is O, not H).
Divide each percentage by the atomic mass:
\[C:\ \frac{70.57}{12.0}=5.88,\qquad H:\ \frac{5.90}{1.00}=5.90,\qquad O:\ \frac{23.53}{16.0}=1.47\]
Divide through by the smallest ratio (1.47):
\[C:\ \frac{5.88}{1.47}=4,\qquad H:\ \frac{5.90}{1.47}=4,\qquad O:\ \frac{1.47}{1.47}=1\]
(i) Empirical formula = C4H4O.
Empirical formula mass \(=4(12.0)+4(1.00)+16.0 = 68\).
\[n=\frac{\text{molar mass}}{\text{empirical mass}}=\frac{136}{68}=2\]
(ii) Molecular formula = (C4H4O)2 = C8H8O2.
Question 2 Report
(a)(i) Define the term standard electrode potential.
(ii) State three factors that affect the discharge of ions during electrolysis.
(iii) State two functions of a salt bridge in an electrochemical cell.
(b) Describe briefly what happens when a solution of copper (II) tetraoxosulphate (VI) is electrolyzed using copper electrodes.
c) Calculate the mass of copper deposited at the cathode when a current of 0.2A is passed through a solution of copper (II) tetraoxosulphate (VI) for 35 minutes using copper electrodes. [H = 1.00, O = 16.0, S = 32.0, Cu = 64.0, IF = 96,500C]
(d)(i) State three characteristics of a catalyst.
ii) Name one manufacturing process in which each of the following metals is used as catalyst: I. iron; II. nickel; Ill. platinum.
c-NA
(a)(i) The standard electrode potential of an element is the potential difference developed when the element is in contact with a solution of its ions of concentration \(1\ \text{mol dm}^{-3}\) at 298 K and 1 atmosphere, measured relative to the standard hydrogen electrode (which is taken as zero).
(a)(ii) Three factors affecting the discharge of ions during electrolysis: the position of the ion in the electrochemical series (relative ease of discharge), the concentration of the ion in solution, and the nature of the electrode used.
(a)(iii) Two functions of a salt bridge: it completes the electrical circuit by allowing ions to flow between the two half-cells, and it maintains electrical neutrality in the half-cells (preventing charge build-up).
(b) Electrolysis of \(CuSO_4\) with copper electrodes
Copper is deposited at the cathode: \(Cu^{2+} + 2e^- \to Cu\). At the anode the copper electrode itself dissolves: \(Cu \to Cu^{2+} + 2e^-\). Copper is therefore transferred from anode to cathode, the anode loses mass while the cathode gains mass, and the blue colour of the solution stays constant because the \(Cu^{2+}\) concentration is unchanged (this is the basis of electro-refining).
(c) Mass of copper deposited
\[Q = It = 0.2 \times (35 \times 60) = 420\ \text{C}\]From \(Cu^{2+} + 2e^- \to Cu\), \(2 \times 96500\ \text{C}\) deposits 64 g of copper.
\[\text{mass} = \frac{420}{2 \times 96500} \times 64 = 0.139\ \text{g}\]About 0.14 g of copper is deposited.
(d)(i) Three characteristics of a catalyst: it is chemically unchanged in mass and composition at the end of the reaction; it is needed only in a small amount; and it is specific in its action (a given catalyst works for a particular reaction).
(d)(ii) Manufacturing processes using the metals as catalysts:
- I. Iron: the Haber process (manufacture of ammonia).
- II. Nickel: the hydrogenation of vegetable oils (manufacture of margarine).
- III. Platinum: the Ostwald process (catalytic oxidation of ammonia in the manufacture of nitric acid); it is also used in the Contact process.
Answer Details
(a)(i) The standard electrode potential of an element is the potential difference developed when the element is in contact with a solution of its ions of concentration \(1\ \text{mol dm}^{-3}\) at 298 K and 1 atmosphere, measured relative to the standard hydrogen electrode (which is taken as zero).
(a)(ii) Three factors affecting the discharge of ions during electrolysis: the position of the ion in the electrochemical series (relative ease of discharge), the concentration of the ion in solution, and the nature of the electrode used.
(a)(iii) Two functions of a salt bridge: it completes the electrical circuit by allowing ions to flow between the two half-cells, and it maintains electrical neutrality in the half-cells (preventing charge build-up).
(b) Electrolysis of \(CuSO_4\) with copper electrodes
Copper is deposited at the cathode: \(Cu^{2+} + 2e^- \to Cu\). At the anode the copper electrode itself dissolves: \(Cu \to Cu^{2+} + 2e^-\). Copper is therefore transferred from anode to cathode, the anode loses mass while the cathode gains mass, and the blue colour of the solution stays constant because the \(Cu^{2+}\) concentration is unchanged (this is the basis of electro-refining).
(c) Mass of copper deposited
\[Q = It = 0.2 \times (35 \times 60) = 420\ \text{C}\]From \(Cu^{2+} + 2e^- \to Cu\), \(2 \times 96500\ \text{C}\) deposits 64 g of copper.
\[\text{mass} = \frac{420}{2 \times 96500} \times 64 = 0.139\ \text{g}\]About 0.14 g of copper is deposited.
(d)(i) Three characteristics of a catalyst: it is chemically unchanged in mass and composition at the end of the reaction; it is needed only in a small amount; and it is specific in its action (a given catalyst works for a particular reaction).
(d)(ii) Manufacturing processes using the metals as catalysts:
- I. Iron: the Haber process (manufacture of ammonia).
- II. Nickel: the hydrogenation of vegetable oils (manufacture of margarine).
- III. Platinum: the Ostwald process (catalytic oxidation of ammonia in the manufacture of nitric acid); it is also used in the Contact process.
Question 3 Report
(a)(i) Explain briefly the term chemical industry.
(ii) State three factors that should be considered in siting a chemical industry.
(b)(i) Describe briefly twin is extracted from its ore.
(ii) Give two uses of tin.
(c)(i) Name the constituents of cement.
(ii) How does mortar set?
(d)(i) Explain briefly the term pollution.
(ii) Give two examples of air pollutants.
(e) Consider the following reversible reaction which occurred at the temperature of 298K:
N\(_{2(g)}\) + 3H\(_{2(g)}\) \(\rightleftharpoons\) 2NH\(_{3(g)}\); \(\bigtriangleup\)H = —92.37kJ
(a)(i) A chemical industry is an industry that uses chemical reactions and processes to convert raw materials into useful finished products on a large (commercial) scale.
(a)(ii) Three factors to consider in siting a chemical industry: availability of raw materials, a cheap and adequate source of energy/power and water, and good transport and nearness to the market (availability of labour and safe waste disposal are also acceptable).
(b)(i) Extraction of tin
Tin is obtained from its ore cassiterite, \(SnO_2\). The ore is first concentrated by washing (froth flotation/gravity), then reduced with carbon (coke) in a furnace: \[SnO_2 + 2C \to Sn + 2CO\] The molten tin is run off and refined.
(b)(ii) Two uses of tin: tin-plating (coating) of steel to make food cans, and making alloys such as solder and bronze.
(c)(i) Constituents of cement: limestone (\(CaCO_3\)), clay/shale (aluminosilicates), and a little gypsum (with silica and iron oxide).
(c)(ii) Mortar (slaked lime, sand and water) sets by absorbing carbon(IV) oxide from the air, forming hard calcium trioxocarbonate(IV): \[Ca(OH)_2 + CO_2 \to CaCO_3 + H_2O\]
(d)(i) Pollution is the introduction of harmful (unwanted) substances into the environment, making it unclean and injurious to living things.
(d)(ii) Two air pollutants: carbon(II) oxide (\(CO\)) and sulphur(IV) oxide (\(SO_2\)) (oxides of nitrogen and unburnt hydrocarbons are also acceptable).
(e) \(N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)};\ \Delta H = -92.37\ \text{kJ}\). This is the exothermic Haber synthesis of ammonia; a high yield is favoured by low temperature and high pressure, while an iron catalyst is used to reach equilibrium quickly. (No specific sub-question is stated for part (e).)
Answer Details
(a)(i) A chemical industry is an industry that uses chemical reactions and processes to convert raw materials into useful finished products on a large (commercial) scale.
(a)(ii) Three factors to consider in siting a chemical industry: availability of raw materials, a cheap and adequate source of energy/power and water, and good transport and nearness to the market (availability of labour and safe waste disposal are also acceptable).
(b)(i) Extraction of tin
Tin is obtained from its ore cassiterite, \(SnO_2\). The ore is first concentrated by washing (froth flotation/gravity), then reduced with carbon (coke) in a furnace: \[SnO_2 + 2C \to Sn + 2CO\] The molten tin is run off and refined.
(b)(ii) Two uses of tin: tin-plating (coating) of steel to make food cans, and making alloys such as solder and bronze.
(c)(i) Constituents of cement: limestone (\(CaCO_3\)), clay/shale (aluminosilicates), and a little gypsum (with silica and iron oxide).
(c)(ii) Mortar (slaked lime, sand and water) sets by absorbing carbon(IV) oxide from the air, forming hard calcium trioxocarbonate(IV): \[Ca(OH)_2 + CO_2 \to CaCO_3 + H_2O\]
(d)(i) Pollution is the introduction of harmful (unwanted) substances into the environment, making it unclean and injurious to living things.
(d)(ii) Two air pollutants: carbon(II) oxide (\(CO\)) and sulphur(IV) oxide (\(SO_2\)) (oxides of nitrogen and unburnt hydrocarbons are also acceptable).
(e) \(N_{2(g)} + 3H_{2(g)} \rightleftharpoons 2NH_{3(g)};\ \Delta H = -92.37\ \text{kJ}\). This is the exothermic Haber synthesis of ammonia; a high yield is favoured by low temperature and high pressure, while an iron catalyst is used to reach equilibrium quickly. (No specific sub-question is stated for part (e).)
Question 4 Report
(a)(i) What is the common name given to the group VII elements?
(ii) Name the hydrides of the first two elements in group VII.
(iii) State three chemical properties of group VII elements.
(b) Copy and complete the following table:
| Particles | Number of Neutrons |
Number of electrons |
Number of prontons |
Mass Number |
| \(W^{2+}\) | 12 | 24 | ||
| \(X^{2+}\) | 8 | 16 | ||
| Y | 13 | 27 | ||
| Z | 12 | 11 |
(c)(i) Define each of the followinc processes: I. nuclear fission; II. nuclear fusion.
(ii) Give one use of each process in (c)(i).
(d)(i) List three types of radiation that are produced during radioactivity.
(ii) Arrange the radiations listed in
(d)(i) in order of increasing: I. penetrating power; II. ionizing power.
(a)(i) The Group VII elements are commonly called the halogens.
(a)(ii) The hydrides of the first two elements (fluorine and chlorine) are hydrogen fluoride, \( HF \) and hydrogen chloride, \( HCl \).
(a)(iii) Three chemical properties of Group VII elements
- They are strong oxidizing agents (their oxidizing power decreases down the group).
- They react with metals to form salts (metal halides), for example \( 2Na + Cl_2 \rightarrow 2NaCl \).
- They react with hydrogen to form hydrogen halides, and a higher halogen displaces a lower one from its salt solution.
(b) Completed table
Using mass number \( = \) protons \( + \) neutrons, and for an ion the electrons \( = \) protons \( - \) (positive charge):
- \( W^{2+} \): neutrons 12, mass 24, so protons \( = 24 - 12 = 12 \); electrons \( = 12 - 2 = 10 \).
- \( X^{2+} \): protons 8, mass 16, so neutrons \( = 16 - 8 = 8 \); electrons \( = 8 - 2 = 6 \).
- \( Y \): electrons 13 (neutral, so protons 13), mass 27, so neutrons \( = 27 - 13 = 14 \).
- \( Z \): electrons 11 (neutral, so protons 11), neutrons 12, so mass number \( = 11 + 12 = 23 \).
| Particle | Number of neutrons | Number of electrons | Number of protons | Mass number |
|---|---|---|---|---|
| \( W^{2+} \) | 12 | 10 | 12 | 24 |
| \( X^{2+} \) | 8 | 6 | 8 | 16 |
| \( Y \) | 14 | 13 | 13 | 27 |
| \( Z \) | 12 | 11 | 11 | 23 |
(c)(i) I. Nuclear fission: the splitting of a heavy nucleus into two lighter nuclei of comparable mass, with the release of a large amount of energy. II. Nuclear fusion: the combination of two light nuclei to form a heavier nucleus, with the release of a large amount of energy.
(c)(ii) Fission is used in nuclear power stations to generate electricity (and in the atomic bomb). Fusion is the source of the energy of the sun and stars (and of the hydrogen bomb).
(d)(i) The three radiations are alpha, beta and gamma radiation.
(d)(ii) I. Increasing penetrating power: alpha \( < \) beta \( < \) gamma. II. Increasing ionizing power: gamma \( < \) beta \( < \) alpha.
Answer Details
(a)(i) The Group VII elements are commonly called the halogens.
(a)(ii) The hydrides of the first two elements (fluorine and chlorine) are hydrogen fluoride, \( HF \) and hydrogen chloride, \( HCl \).
(a)(iii) Three chemical properties of Group VII elements
- They are strong oxidizing agents (their oxidizing power decreases down the group).
- They react with metals to form salts (metal halides), for example \( 2Na + Cl_2 \rightarrow 2NaCl \).
- They react with hydrogen to form hydrogen halides, and a higher halogen displaces a lower one from its salt solution.
(b) Completed table
Using mass number \( = \) protons \( + \) neutrons, and for an ion the electrons \( = \) protons \( - \) (positive charge):
- \( W^{2+} \): neutrons 12, mass 24, so protons \( = 24 - 12 = 12 \); electrons \( = 12 - 2 = 10 \).
- \( X^{2+} \): protons 8, mass 16, so neutrons \( = 16 - 8 = 8 \); electrons \( = 8 - 2 = 6 \).
- \( Y \): electrons 13 (neutral, so protons 13), mass 27, so neutrons \( = 27 - 13 = 14 \).
- \( Z \): electrons 11 (neutral, so protons 11), neutrons 12, so mass number \( = 11 + 12 = 23 \).
| Particle | Number of neutrons | Number of electrons | Number of protons | Mass number |
|---|---|---|---|---|
| \( W^{2+} \) | 12 | 10 | 12 | 24 |
| \( X^{2+} \) | 8 | 6 | 8 | 16 |
| \( Y \) | 14 | 13 | 13 | 27 |
| \( Z \) | 12 | 11 | 11 | 23 |
(c)(i) I. Nuclear fission: the splitting of a heavy nucleus into two lighter nuclei of comparable mass, with the release of a large amount of energy. II. Nuclear fusion: the combination of two light nuclei to form a heavier nucleus, with the release of a large amount of energy.
(c)(ii) Fission is used in nuclear power stations to generate electricity (and in the atomic bomb). Fusion is the source of the energy of the sun and stars (and of the hydrogen bomb).
(d)(i) The three radiations are alpha, beta and gamma radiation.
(d)(ii) I. Increasing penetrating power: alpha \( < \) beta \( < \) gamma. II. Increasing ionizing power: gamma \( < \) beta \( < \) alpha.
Question 5 Report
(a)(i) Define the term hygroscopic.
(ii) Give two difference: between a physical change and a chemical change.
(iii) Using the kinetic theory of gases, explain briefly the Charles' law.
(b)(i) Arrange the following compounds in order of increasing boiling points: CS\(_2\); CO\(_2\); NaH. Give reasons for your answer.
(ii) Write a balanced chemical equation to illustrate the reaction of chlorine gas with cold dilute sodium hydroxide.
(c) In a certain reaction, 15.0 g of impure magnesium sample reacted with excess hydrochloric acid liberating 8.6 dm\(^2\) of hydrogen gas at s.t.p.
(i) Write a balanced equation for the reaction.
(ii) Calculate the: I. mass of pure magnesium in the sample; I. percentage purity of the magnesium sample; III. number of ions produced in the reaction. [Mg = 24.0; volume at s.t.p. 22.4 dm\(^{-3}\), Avagadro's constant = 6.02 x 10\(^{23}\)mol\(^{-1}\)]
(a)(i) Hygroscopic
A hygroscopic substance is one which absorbs moisture (water vapour) from the atmosphere without dissolving in it or forming a solution.
(a)(ii) Two differences between physical and chemical changes
| Physical change | Chemical change |
|---|---|
| No new substance is formed. | One or more new substances are formed. |
| It is usually easily reversible. | It is usually not easily reversible. |
(a)(iii) Charles' law using the kinetic theory of gases
At constant pressure, when the temperature of a fixed mass of gas is increased, the gas molecules gain kinetic energy and move faster. The molecules move farther apart, causing the gas to expand. Hence, the volume of the gas increases directly with its absolute temperature.
(b)(i) Order of increasing boiling points
\[ CO_2 < CS_2 < NaH \]
Both \(CO_2\) and \(CS_2\) are simple molecular substances held together by van der Waals' forces. The forces in \(CS_2\) are stronger because its molecules are larger and more polarizable than those of \(CO_2\). Sodium hydride, \(NaH\), is an ionic compound with strong electrostatic forces of attraction between its ions; therefore, it has the highest boiling point.
(b)(ii) Reaction of chlorine with cold dilute sodium hydroxide
\[ Cl_{2(g)} + 2NaOH_{(aq)} \rightarrow NaCl_{(aq)} + NaOCl_{(aq)} + H_2O_{(l)} \]
(c)(i) Balanced equation for the reaction
\[ Mg_{(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2(g)} \]
(c)(ii)
I. Mass of pure magnesium in the sample
Volume of hydrogen gas evolved \(= 8.6\text{ dm}^3\).
\[ n(H_2)=\frac{8.6}{22.4}=0.384\text{ mol} \]
From the equation, \(1\) mole of \(Mg\) produces \(1\) mole of \(H_2\).
\[ n(Mg)=0.384\text{ mol} \]
\[ \text{Mass of pure }Mg = 0.384 \times 24.0 = 9.21\text{ g} \]
II. Percentage purity of the magnesium sample
\[ \%\,\text{purity}=\frac{\text{mass of pure magnesium}}{\text{mass of impure sample}}\times100 \]
\[ =\frac{9.21}{15.0}\times100=61.4\% \]
III. Number of ions produced
The \(0.384\) mol of magnesium forms \(0.384\) mol of \(MgCl_2\).
\[ MgCl_2 \rightarrow Mg^{2+} + 2Cl^- \]
Therefore, \(1\) mole of \(MgCl_2\) produces \(3\) moles of ions.
\[ \text{Moles of total ions}=3 \times 0.384=1.152\text{ mol} \]
\[ \text{Number of ions}=1.152 \times 6.02\times10^{23} \]
\[ =6.94\times10^{23}\text{ ions} \]
Answer Details
(a)(i) Hygroscopic
A hygroscopic substance is one which absorbs moisture (water vapour) from the atmosphere without dissolving in it or forming a solution.
(a)(ii) Two differences between physical and chemical changes
| Physical change | Chemical change |
|---|---|
| No new substance is formed. | One or more new substances are formed. |
| It is usually easily reversible. | It is usually not easily reversible. |
(a)(iii) Charles' law using the kinetic theory of gases
At constant pressure, when the temperature of a fixed mass of gas is increased, the gas molecules gain kinetic energy and move faster. The molecules move farther apart, causing the gas to expand. Hence, the volume of the gas increases directly with its absolute temperature.
(b)(i) Order of increasing boiling points
\[ CO_2 < CS_2 < NaH \]
Both \(CO_2\) and \(CS_2\) are simple molecular substances held together by van der Waals' forces. The forces in \(CS_2\) are stronger because its molecules are larger and more polarizable than those of \(CO_2\). Sodium hydride, \(NaH\), is an ionic compound with strong electrostatic forces of attraction between its ions; therefore, it has the highest boiling point.
(b)(ii) Reaction of chlorine with cold dilute sodium hydroxide
\[ Cl_{2(g)} + 2NaOH_{(aq)} \rightarrow NaCl_{(aq)} + NaOCl_{(aq)} + H_2O_{(l)} \]
(c)(i) Balanced equation for the reaction
\[ Mg_{(s)} + 2HCl_{(aq)} \rightarrow MgCl_{2(aq)} + H_{2(g)} \]
(c)(ii)
I. Mass of pure magnesium in the sample
Volume of hydrogen gas evolved \(= 8.6\text{ dm}^3\).
\[ n(H_2)=\frac{8.6}{22.4}=0.384\text{ mol} \]
From the equation, \(1\) mole of \(Mg\) produces \(1\) mole of \(H_2\).
\[ n(Mg)=0.384\text{ mol} \]
\[ \text{Mass of pure }Mg = 0.384 \times 24.0 = 9.21\text{ g} \]
II. Percentage purity of the magnesium sample
\[ \%\,\text{purity}=\frac{\text{mass of pure magnesium}}{\text{mass of impure sample}}\times100 \]
\[ =\frac{9.21}{15.0}\times100=61.4\% \]
III. Number of ions produced
The \(0.384\) mol of magnesium forms \(0.384\) mol of \(MgCl_2\).
\[ MgCl_2 \rightarrow Mg^{2+} + 2Cl^- \]
Therefore, \(1\) mole of \(MgCl_2\) produces \(3\) moles of ions.
\[ \text{Moles of total ions}=3 \times 0.384=1.152\text{ mol} \]
\[ \text{Number of ions}=1.152 \times 6.02\times10^{23} \]
\[ =6.94\times10^{23}\text{ ions} \]
Question 6 Report
(a)(i) Define each of the following terms: I. normal salt. II. acid salt.
(ii) Tetraoxosulphate (VI) acid and sodium hydroxide react to produce salt and water. Write a balanced chemical equation fir the formation of: I. a normal salt; II. an acid salt.
(b)(i) Explain briefly the term acid-base indicator.
(ii) Copy and complete the following table.
| Indicator | , Colour in acidic medium | , Colour in basic medium |
| Methyl orange | ||
| Phenolphthalein |
(iii) For each of the following titrations, state the most suitable indicator: I. strong acid against strong base; II. strong acid against weak base; iii. weak acid against strong base.
(c) Baking soda and hydrochloric acid react according to the following equation:
\(\mathrm{NaHCO}_{3(aq)} + \mathrm{HCI}_{(aq)} \longrightarrow \mathrm{NaCl}_{(aq)} \mathrm{CO}_{2(g)} + \mathrm{H}_2\mathrm{O}_{(l)}\). Calculate the mass of baking soda that would produce 10g of ccrbon (IV) oxide. [H = 1.00, C = 12.0, 0 = 16.0, Na = 23.0]
(d) Give a reason why a given mass of sodium hydroxide pellets cannot be used to prepare a standard solution.
\(\mathrm{NaHCO}_3 + \mathrm{HCI} \longrightarrow \mathrm{NaCI} + \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O}\)
\(84\mathrm{g}\ \mathrm{NaHCO}_3 \longrightarrow 44\mathrm{g}\ \mathrm{CO}_2\)
\(X\mathrm{g} \longrightarrow 10\mathrm{g}\ \mathrm{CO}_2\)
\(X\mathrm{g} = \frac{84 \times 10}{44}\)
\(= 19.09\mathrm{g}\)
\(= 19.1\mathrm{g}.\)
(d) give a reason why a given mass of sodium hydroxide pellets cannot be used to prepare a standard solution: Sodium hydroxide absorbs water/deliquescent and absorbs carbon IV oxide from air/and this would make mass taken unreliable/add to its mass.
(a)(i) I. Normal salt: a salt formed when all the replaceable hydrogen ions of an acid have been completely replaced by a metal ion (or the ammonium ion), for example \( Na_2SO_4 \). II. Acid salt: a salt formed when only part of the replaceable hydrogen ions of an acid has been replaced by a metal ion, so it still contains replaceable hydrogen, for example \( NaHSO_4 \).
(a)(ii)
I. Normal salt: \[ H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O \]
II. Acid salt: \[ H_2SO_4 + NaOH \rightarrow NaHSO_4 + H_2O \]
(b)(i) An acid-base indicator is a substance (usually a weak organic acid or base) that shows different colours in acidic and in basic media, and so is used to detect the end point of a titration.
(b)(ii) Completed table
| Indicator | Colour in acidic medium | Colour in basic medium |
|---|---|---|
| Methyl orange | Red (pink) | Yellow |
| Phenolphthalein | Colourless | Pink |
(b)(iii) I. Strong acid against strong base: methyl orange or phenolphthalein. II. Strong acid against weak base: methyl orange. III. Weak acid against strong base: phenolphthalein.
(c) Mass of baking soda producing 10 g of \( CO_2 \)
\[ NaHCO_3 + HCl \rightarrow NaCl + CO_2 + H_2O \]Molar mass of \( NaHCO_3 = 23 + 1 + 12 + (3 \times 16) = 84\ \text{g mol}^{-1} \); molar mass of \( CO_2 = 12 + 32 = 44\ \text{g mol}^{-1} \).
From the equation, \( 84\ g \) of \( NaHCO_3 \) gives \( 44\ g \) of \( CO_2 \). Let \( x \) be the mass needed for \( 10\ g \) of \( CO_2 \):
\[ x = \frac{84 \times 10}{44} = 19.09\ g \approx 19.1\ g \]Mass of baking soda = 19.1 g.
(d) A given mass of sodium hydroxide pellets cannot be used to prepare a standard solution because sodium hydroxide is deliquescent and also absorbs carbon(IV) oxide from the air; its mass therefore changes and is unreliable, so it is not a primary standard.
Answer Details
(a)(i) I. Normal salt: a salt formed when all the replaceable hydrogen ions of an acid have been completely replaced by a metal ion (or the ammonium ion), for example \( Na_2SO_4 \). II. Acid salt: a salt formed when only part of the replaceable hydrogen ions of an acid has been replaced by a metal ion, so it still contains replaceable hydrogen, for example \( NaHSO_4 \).
(a)(ii)
I. Normal salt: \[ H_2SO_4 + 2NaOH \rightarrow Na_2SO_4 + 2H_2O \]
II. Acid salt: \[ H_2SO_4 + NaOH \rightarrow NaHSO_4 + H_2O \]
(b)(i) An acid-base indicator is a substance (usually a weak organic acid or base) that shows different colours in acidic and in basic media, and so is used to detect the end point of a titration.
(b)(ii) Completed table
| Indicator | Colour in acidic medium | Colour in basic medium |
|---|---|---|
| Methyl orange | Red (pink) | Yellow |
| Phenolphthalein | Colourless | Pink |
(b)(iii) I. Strong acid against strong base: methyl orange or phenolphthalein. II. Strong acid against weak base: methyl orange. III. Weak acid against strong base: phenolphthalein.
(c) Mass of baking soda producing 10 g of \( CO_2 \)
\[ NaHCO_3 + HCl \rightarrow NaCl + CO_2 + H_2O \]Molar mass of \( NaHCO_3 = 23 + 1 + 12 + (3 \times 16) = 84\ \text{g mol}^{-1} \); molar mass of \( CO_2 = 12 + 32 = 44\ \text{g mol}^{-1} \).
From the equation, \( 84\ g \) of \( NaHCO_3 \) gives \( 44\ g \) of \( CO_2 \). Let \( x \) be the mass needed for \( 10\ g \) of \( CO_2 \):
\[ x = \frac{84 \times 10}{44} = 19.09\ g \approx 19.1\ g \]Mass of baking soda = 19.1 g.
(d) A given mass of sodium hydroxide pellets cannot be used to prepare a standard solution because sodium hydroxide is deliquescent and also absorbs carbon(IV) oxide from the air; its mass therefore changes and is unreliable, so it is not a primary standard.
