Frequently Asked Ouestions Chemistry F4

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Chemistry SPM – Quick Quick Review F4


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Frequently Asked Questions in SPM

1) What is melting point? 2) State the meaning of isotopes.

3) Explain why the temperature remains unchanged during the melting process. 4) Explain why the temperature remains unchanged during the freezing process. 5) State the main subatomic particles of an atom. Compare and contrast the relative atomic mass and the relative charge of the subatomic particles of the atom.

Standard Answers • … is the temperature at which the substance at its solid state changes to liquid state at certain pressure. • … are atoms with the same number of protons but different numbers of neutrons (or with the same proton number but different nucleon numbers). • Heat energy absorbed by the particles is used to overcome the forces of attraction between the particles so that the solid can turn into liquid. • The heat energy released by the particles is exactly balanced by the heat loss to the surroundings as the particles attracted one another to form solid. • An atom contains three types of subatomic particles; proton, neutron arid electron. Subatomic particle

Symbol

Relative mass

Relative charge

Proton

P

1

+1

Electron

E

1/1840

-1

Neutron

n

1

Neutral


Standard Answers

6) State the meaning of empirical formula. 7) State the meaning of molecular formula. 8) Why does the method to determine the empirical formula of copper(Il) oxide cannot be used to determine the empirical formula for magnesium oxide? 9) Why does the method to determine the empirical formula of magnesium oxide cannot be used to determine the empirical formula of copper(Il) oxide?

• … i s the formula that shows the simplest whole number ratio of atoms of each element in the compound. • … is the formula that shows the actual number of atoms of each element that are present in a molecule of the compound. • Magnesium is more reactive than hydrogen. Hydrogen gas cannot reduce magnesium oxide to magnesium.


10) State the physical properties of Group 1.

11) State the physical properties of Group I7.


• Because copper is not a reactive metal . It reacts slowly with oxygen. Therefore, it is difficult to convert copper to copper(II) oxide completely.

Standard Answers • Alkali metals are good conductors of heat and electricity. • They are soft and can be cut easily with a knife. • All alkali metals are grey in colour with silvery and shiny surfaces when freshly cut. • They have relatively low melting and boiling points compare to other metals such as iron (MP : 1540°C). • There have low densities and float on surface of water. • Halogens exits as diatomic molecules. • Halogens have low melting and boiling points. • They are good insulator of electricity & poor conductor of heat.


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12) State the changes of the physical properties of Group 17 when going down the group. 13) State three special properties of transition elements.

14) State the position of element , 31 15 in Periodic table of Elements. Explain your answer 15) When across Period 3 from sodium to argon, the atomic size decreases. Explain why.

16) When across Period 3 from sodium to argon, the electronegativity increases. Explain why.

17) The reactivity of Group 1 increases when going down the group. Explain why. (Why potassium is more reactive than sodium and sodium is more reactive than lithium?)

18) The reactivity of Group 17 decreases when going down the group. Explain why.

19) Why helium gas is not reactive chemically? 20) Sodium burnt in oxygen and the product is dissolved in water. What is the property of the solution formed? Explain why. [email protected] : Frequently Asked Questions

• The physical state of halogens at room temperature changes from gas to liquid, then to solid. • The colour of halogen becomes darker. • Transition elements form coloured solutions or compounds. • Transition elements show various oxidation numbers in their compounds. • Transition elements act as catalyst in certain chemical reactions. • Transition elements can form complex ions. • Electron arrangement of X atoms is 2.8.5 • X atom has 5 valence electrons. Therefore, it is in Group 15. • X atom has 4 shells occupied with electrons. Therefore, it is in Period 4. When across Period 3 from left to right; • The number of protons in the nucleus increases. This increases the positive charge of the nucleus. • As a result, the nucleus attraction force towards the (valence) electrons becomes stronger . • (Valence) Electrons are pulled closer to the nucleus, causing the atomic size to decrease.

When across Period 3 from left to right. • The atomic size decreases • The proton numbers of elements increases. This increase the positive charge of the nucleus • As a result the attraction forces between the nucleus and the (valence) electrons in the outermost shell becomes stronger. • The tendency to attract electrons becomes lower. • Therefore, electronegativity increases. • When going down Group 1, the number of shell occupied by electrons increases. • The atomic size increases. • The single valence electron becomes further away from the nucleus. • Hence, the force of nucleus attraction towards valence electron becomes weaker. • As a result, it is easier for the atom to lose the single valence electron to achieve the octet or duplet electron e lectron arrangement. When going down Group 17, • The atomic size increases. • The outermost occupied shell of each halogen atom becomes further away from the nucleus. • Thus, the force of attraction between the nucleus and electrons ele ctrons become weaker. • As a result it is more difficult to attract one electron el ectron into the outermost occupied shell. • Helium has 2 valence electrons. This is stable duplet electron los e or share electrons arrangement. It does not need to gain, lose with other atom of elements. • The solution is an alkaline solution. • Sodium burns in oxygen to produce sodium oxide. Sodium oxide dissolves in water to produce sodium hydroxide solution. 

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21) Chlorine gas is dissolved in water. What can you observe if a piece of blue litmus paper is immersed into the solution formed? Explain why.


22) State some physical properties of ionic compounds. 23) State the physical properties of covalent compounds.

24) Explain why sodium chloride can conduct electricity in aqueous state but cannot conduct electricity in solid state.

25) Magnesium chloride and hydrogen chloride are two compounds of chlorine. At room condition, magnesium chloride exists as a solid but hydrogen chloride exists in gaseous state. Explain why.

26) Explain the formation of ionic bond between magnesium atom, 24 12 and oxygen atom, 168

27) Explain the formation of covalent bond between carbon atom, 126 and chlorine atom, 35 17 .



• Blue litmus paper turns red and then bleached. • When chlorine gas dissolves in cold water, it forms hydrochloric acid, HCI and hypochlorous acid, HOCl. • Hydrochloric acid is a strong acid which turns blue litmus paper red while hypochlorous acid is a strong bleaching agent which decolourise the litmus paper.

Standard Answers • High melting point and boiling point • Conduct electricity in liquid - aqueous solution molten state. • Generally soluble in water but do not dissolve in organic solvent. • Low melting point and boiling point. • Insulators of electricity in all state. • Usually dissolves in organic solvents such as benzene but do not dissolve in water. • Sodium chloride is an ionic compound. • In solid state, sodium ions and chloride ions are held by strong electrostatic forces of attraction in the lattice structure. • Ions are in fixed positions and do not move freely. Hence, ionic compounds in the solid state do not conduct electricity. • In aqueous or molten state, ions are freely moved. Therefore the compound can conduct electricity. • Magnesium chloride is an ionic compound whereas hydrogen chloride is a covalent compound. • Magnesium ion and chloride ions are held together by strong electrostatic forces of attraction. Therefore, it has very high melting point and boiling point and exists as solid at room condition. • Hydrogen chloride consists of molecules. • The intermolecular forces (van der Waals forces) between the molecules is week. Therefore, it has low melting point and boiling point and exist as gas at room condition. • Magnesium atom has electron arrangement of 2.8.2 • It donates 2 valence electrons to achieve the stable octet electron arrangement, 2.8. • A posit ive ion, Mg2+ is formed. • Oxygen atom has electron arrangement of 2.6 • It receives 2 electrons from magnesium atom to achieve stable octet electron arrangement, 2.8. • A negative ion, O2- is formed • Positive ion, Mg2+, and negative ion, O 2-, are attracted together by strong electrostatic forces to form an ionic compound with the formula MgO. • Carbon atom has electron arrangement of 2.4 • Chlorine atom has electron arrangement of 2.8.7 • 1 carbon atom contributes 4 valence electrons to 4 Cl atoms. • 4 chlorine atoms, contribute 1 valence electron each to C atom. • 1 C atom shares 4 pairs of electrons with 4 Cl atoms to form 4 single covalent bonds.


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• All of C and Cl atoms achieve stable octet electron arrangement. • A covalent compound with formula CCI4 is formed.


28) State the meaning of electrolyte. 29) Explain why solution of hydrogen chloride in water can conduct electricity but solution of hydrogen chloride in methylbenzene cannot conduct electricity? 30) Explain how copper electrodes affect the selective discharge of ions at the anode during electrolysis of copper(ll) sulphate solution. 31) Explain how concentration of ions in electrolyte affects the selective discharge of ions at the anode during electrolysis of hydrochloric acid. 32) Explain why the blue copper(ll) sulphate solution remains unchanged when copper is used as electrodes.

33) Explain the process of electrolysis of sodium nitrate solution using carbon electrode.

34) Explain how Daniell Cell can generate electricity.


Standard Answers • Electrolyte is the substance that can conduct electricity in molten or aqueous (liquid) state and undergo chemical change. • Hydrogen chloride is a covalent compound. • In methylbenzene, methylbenzene, hydrogen chloride exists as molecules. Therefore, it cannot conduct electricity. • In water, hydrogen chloride ioni ses to produce hydrogen ions and chloride ions which are free to move . ele ctricity. • Therefore, it can conduct electricity. • In an electrolysis of copper(ll) sulphate solution using copper electrodes, no ion is discharged at anode. • Instead, the copper anode itself dissolves to form copper(ll) ion. Cu  Cu2+ + 2e (Cu atoms atoms ionise) • When using dilute hydrochloric acid (0.0001 mol dm -3), hydroxide ion is discharged at anode to release oxygen gas. 4 OH-  2 H2O + O2 + 4e • When using concentrated hydrochloric acid (1.0 mol dm -3), chloride ion is discharged at anode to produce chlorine gas. 2 Cl-  Cl2 + 2e • The rate of the discharged of copper(ll) ions at the cathode is the same as the rate of ionisation of copper anode. The concentration of copper(ll) ions remains unchanged. OR

• The copper(ll) ion s discharged at the cathode is replaced with the copper(Il) ion formed from the anode. The concentration of copper(ll) ions remains unchanged. • Sodium nitrate solution consists of Na +, NO3-, H+, and OH- ions. • During electrolysis, positive ions - Na+ and H+ ions move to cathode and negative ions - NO3- and OH- ions move to anode. • At cathode, H + ion is selectively discharged because its position is lower than Na+ ion in electrochemical series. • Hydrogen gas is given out at cathode. • At anode, OH - ion is selectively discharged because it is lower than NO3- ion in electrochemical series. • Oxygen gas is given out at anode. • In Daniell Cell, zinc is more electropositive than copper. • Zinc acts as the negative terminal and copper acts as the positive terminal. • Zinc donates electrons to form zinc ions, Zn 2+. • Electrons f low low from zinc electrode to copper electrode through the external circuit. • At copper electrode, electrons are received by the copper(Il) ions, Cu2+ in the copper(II) sulphate solution and discharge to form copper atoms. • The continuous flow of electrons and movement of ions produce electric current in the Daniell Cell.

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35) What is the meaning of strong acid and weak alkali.

36) What is neutralisation? 37) The pH value of glacial ethanoic acid is 7 but the pH value of dilute ethanoic acid is 5. Explain why the pH values of the two substances are different

38) The pH value of solution ammonia in water is 9 but the pH value of solution of ammonia in trichloromethane is 7. Explain why does the pH values of the two substances are different.

39) The pH values of 1.0 mol dm-3 of ethanoic acid and 1.0 mol dm -3 hydrochloric acid is 4 and 1 respectively. Explain the differences.

40) Given dilute nitric acid and dilute sulphuric acid have the same concentration of 0.5 mol dm -3. In a neutralisation experiment, 20 cm 3 of nitric acid is required to neutralise 20 cm3 of sodium hydroxide solution but only 10 cm3 of sulphuric acid is required to neutralise 20 cm 3 of sodium hydroxide solution. Explain why.


41) What is salt? 42) Describe a chemical test to identify the presence of carbonate ions, CO32- in a solution.



Standard Answers • A strong acid is an acid t hat ionizes / dissociates completely in water to form high concentration of hydrogen ions, H+ • A wea k alkali is an alkali that ionises / dissociates partially in water to form low concentration of hydroxide ions, OH• Neutralisation is a reaction between an acid and a base to produce salt and water only • Glacial ethanoic acid, CH 3COOH is a dry acid (without water) which is consists of neutral molecules only. There is no hydrogen ions, H+. Therefore, it does not show acidic property. • In dilute ethanoic acid, the molecules of ethanoic acid ionise to produce hydrogen ions, H+. • The presence of hydrogen ions, H+ will cause the aqueous solution of ethanoic acid, show its acidic property. • Dry ammonia gas (or ammonia gas) dissolved in trichloromethane consists of neutral molecules, NH3 only. • There is no hydroxide ions, OH -. Therefore, it does not show alkaline property. • In water, the NH 3 molecules will ionise to produce hydroxide ions, OH-. • The presence of hydroxide ions, OH- will cause the aqueous ammonia solution to show alkaline properties. • Ethanoic aci d is a weak acid. • It ionizes partially in water, to produce hydrogen ions with low concentration. • Hydrochloric acid is a strong acid. • It ionizes completely in water to produce hydrogen ions with high concentration. • Nitric acid, HNO3 is a monoprotic acid whereas sulphuric acid, H2SO4 is a diprotic acid. • The concentration of hydrogen ions in sulphuric acid is double than that in nitric acid. • As a result, the volume of sulphuric acid required (to neutralise NaOH(aq)) is half compared to nitric acid.

Standard Answers • A salt is a compound formed when the hydrogen ion, H+ from an acid is replaced by a metal ion or an ammonium ion, NH4+. • 2 cm3 of the solution is poured into a test tube. • 2 cm3 of dilute hydrochloric acid is added into the test tube • The gas liberated is immediately bubbled / flown through lime water. • The l ime water turns milky / cloudy / chalky.

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43) Describe a chemical test to identify the presence of chloride ions, CI - in a solution. 44) Describe a chemical test to identify the presence of sulphate ions, SO 42in a solution. 45) Describe a chemical test to identify the presence of nitrate ions, NO 3-, in a solution.

46) Describe a chemical test to identity the following ions a) Copper(ll) ion, Cu 2+ b) Magnesium ion, Mg 2+ c) Zinc ion, Zn2+

47) Describe a chemical test to identify the following ions; a) iron(ll) ion, Fe 2+ b) iron(Ill) ion, Fe3+

48) Describe a chemical test to differentiate the lead(Il) ion, Pb 2+ and aluminium ion, Al 3+. 49) Describe a chemical test to identify the presence of ammonium ions, [email protected] : Frequently Asked Questions


• 2 cm3 of the solution is poured into a test tube. • 2 cm3 of dilute nitric acid is added into the test tube follow by about 2 cm3 of silver nitrate solution. • A white precipitate is formed . • 2 cm3 of the solution is poured into a test tube. • 2 cm3 of dilute hydrochloric acid is added into the test tube followed by about 2 cm3 of barium chloride solution. • A white precipitate is formed. • 2 cm3 of the solution is poured into a test tube. • 2 cm3 of dilute sulphuric acid is added into the test tube follow ed by 2 cm3 of iron(II) sulphate solution. The mixture is shaken well. • The test tube is slanted slightly. A few drops of concentrated sulphuric acid are dropped carefully and slowly along the side of slanting test tube into the mixture. • A brown ring is formed. (a) Copper(Il) ion • Sodium hydroxide solution is added drop by drop until in excess. A blue precipitate is formed and insoluble in excess sodium hydroxide solution. • Ammonia solution is added drop by drop until in excess. A blue precipitate is formed and dissolves in excess ammonia solution to produce a dark blue solution. (b) Magnesium ion • Sodium hydroxide solution is added drop by drop until in excess. A white precipitate is formed and insoluble in excess sodium hydroxide solution. • Ammonia solution is added drop by drop until in excess. A white precipitate is formed and insoluble in excess ammonia solution. (c) Zinc ion • Sodium hydroxide solution is added drop by drop until in excess. A white precipitate is formed and dissolves in excess sodium hydroxide solution to form a colourless solution. • Ammonia solution is added drop by drop until in excess. A white precipitate is formed and dissolves in excess ammonia solution to form a colourless solution. (a) Iron(II) ion • 2 cm3 of X solution is poured into a test tube. • Potassium hoxacyanoferrate(III) solution is added into the test tube. • A dark blue precipitate is formed. (b) Iron (Ill) ion • 2 cm3 of Y solution is poured into a test tube. • Potassium thiocynate solution is added into the test tube. • A blood red colouration is formed • 5 cm3 of each solution is poured into two separated test tubes. • Add 2 cm3 potassium iodide solution, KI into each of test tube. • The solution in the test tube produce yellow precipitate consists of lead(Il) ion – lead(II) iodide, PbI 2. • 2 cm3 ammonia solution is poured into a test tube. • Sodium hydroxide solution is added into the test tube


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NH4+ in a solution

• The mixture is heated and a damp red litmus paper is placed near the mouth of test tube. • The damp red litmus paper turns blue.


50) What is the meaning of alloy? 51) State the aims of making alloys

52) Bronze is an alloy consists of copper and tin. Explain why bronze is harder than copper.

53) Explain the industrial process involved in the manufacture of sulphuric acid Write all the chemical equations involved.

54) State the optimum conditions for the Contact process 55) State the optimum conditions for the Haber process 56) What is the meaning of polymer? Name the monomer of polythene and polyvinyl chloride. 57) What is the difference in terms of major component between glass and ceramic? 58) What does it means by composite materials?

Standard Answers • Alloy is a mixture of two or more elements with a certain fixed composition in which the major component is a metal. • Improve the appearance of the pure metal • Improve the strength and hardness of the pure metal • Increase the resistance to corrosion of the pure metal • Copper atoms in copper block are of the same sizes and arranged in an orderly arrangement. • The layers of copper atoms slide easily if force is applied. • The copper atoms and the tin atoms are in different sizes. • The presence of tin atoms disrupts the orderly arrangement of copper atoms to bronze block. • This reduces the layers of copper atoms from sliding over one another easily. • Molten sulphur is burnt in excess air to produce sulphur dioxide gas. S + O2  SO2 • Sulphur dioxide and excess oxygen gas are passed over vanadium(V) oxide catalyst at 450°C and pressure of 1 atm. 2 SO2 + O2  2 SO3 • Sulphur trioxide is dissolved in concentrated sulphuric acid to form oleum SO3 + H2SO4  H2S2O7 • The oleum is then diluted in water to produce dilute sulphuric acid H2S2O7 + H2O  2 H2SO4 • Catalyst - Vanadium(V) oxide, V2O5 • Temperature - 450°C • Pressure - 1 atm. • Catalyst - Iron filling • Temperature - 450°C - 550°C • Pressure - 200 atm • Polymer is a long chain molecules made up of many identical & repeating units called as monomers. • Ethene and chloroethene respectively. Major component; glass – silica / silicone dioxide ceramic – aluminosilicate .. is a new and advance structural materials with a superior properties that formed by combining two or more different materials or components.

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Frequently Asked Ouestions Chemistry F4

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