IGCSE Chemistry 0620 Paper 4: Chemical Bonding, Structure and Properties

Level: Year 10-11 (IGCSE) | Subject: Chemistry 0620 | Time: 60 minutes | Total: 100 marks | Parts: 33

Paper at a glance

Paper	Time	Marks	Calculator
Paper 4 (Extended, structured)	1 h 15 min	75	Not allowed

Questions

Question 1 (16 marks) – Ionic bonding and ionic structures

(a) Describe how a sodium atom and a chlorine atom form sodium chloride, and state the charge on each ion. (3 marks)

(b) Write the chemical formulae of magnesium oxide and calcium chloride. (2 marks)

(c) Explain why solid sodium chloride does not conduct electricity but molten sodium chloride does. (3 marks)

(d) Explain why the radius of a sodium ion is smaller than the radius of a sodium atom. (2 marks)

(e) Explain why ionic compounds generally have high melting points and are hard. (3 marks)

(f) How many electrons are there in total in one particle of magnesium chloride, MgCl2? Show your working. (3 marks)

Question 2 (17 marks) – Covalent bonding, simple molecules and giant covalent structures

(a) Explain how two hydrogen atoms form a covalent bond in a hydrogen molecule and state the number of shared pairs. (3 marks)

(b) State the number of bonds formed by the carbon atom in a methane molecule and in a carbon dioxide molecule. (2 marks)

(c) Explain why carbon dioxide is a gas at room temperature while diamond has a very high melting point. (3 marks)

(d) Explain why graphite conducts electricity but diamond does not. (3 marks)

(e) A diamond sample contains 3.0 g of carbon. Calculate the number of carbon atoms in the sample (Avogadro constant = 6.02 x 10^23 /mol). (3 marks)

(f) Explain why a simple molecular substance such as tetrachloromethane does not conduct electricity when pure. (3 marks)

Question 3 (16 marks) – Metallic bonding, properties of metals and alloys

(a) Describe the model of metallic bonding and state how the metal ions and electrons are arranged. (3 marks)

(b) Explain why metals are malleable and can be hammered into thin sheets. (3 marks)

(c) Explain why an alloy such as bronze is harder than pure copper. (3 marks)

(d) Bronze contains 88% copper by mass, the rest being tin. Calculate the mass of tin in 500 g of bronze. (3 marks)

(e) Explain why metals conduct electricity and heat, and why their electrical conductivity falls as temperature rises. (4 marks)

Question 4 (17 marks) – Linking structure to properties

(a) For each of ionic, simple molecular, giant covalent and metallic structures, give one feature of its melting point and one feature of its electrical conductivity. (4 marks)

(b) Explain why solid potassium nitrate does not conduct electricity but a solution of potassium nitrate does. (3 marks)

(c) Explain why diamond is extremely hard while graphite is soft and can be used as a lubricant. (3 marks)

(d) Sodium chloride melts at 801 °C while magnesium oxide melts at 2852 °C. Use ionic charge and ionic radius to explain the difference. (4 marks)

(e) Methane boils at -162 °C and butane boils at -0.5 °C. Explain the difference in boiling point. (3 marks)

Question 5 (17 marks) – Calculations linked to bonding and structure

(a) Calculate the relative formula mass of sodium chloride and the percentage by mass of sodium in it. (3 marks)

(b) How many moles are there in 117 g of sodium chloride, and how many ions in total (Avogadro constant = 6.02 x 10^23 /mol)? (3 marks)

(c) Calculate the mass of 0.250 mol of magnesium chloride. (3 marks)

(d) How many moles are there in 112 g of calcium oxide, and what is the total number of ions? (3 marks)

(e) A sample of copper(II) oxide weighs 8.0 g. Given that the relative atomic mass of Cu is 63.5 and of O is 16.0, calculate the mass of copper in the sample. (3 marks)

(f) Compare the amount of ions, in moles, contained in 1 mol of sodium chloride and in 1 mol of magnesium chloride. (2 marks)

Question 6 (17 marks) – Synoptic: deducing properties from structure

(a) Explain why magnesium oxide has a higher melting point than sodium chloride, naming two structural factors involved. (4 marks)

(b) 100 g of water is heated from 20 °C to 80 °C; the specific heat capacity of water is 4.18 J/(g K). Calculate the energy needed, in kJ. (3 marks)

(c) A plant melts sodium chloride using a 2.0 kW supply for 1.5 hours. Calculate the electrical energy used in kWh and explain why melting, rather than dissolving, sodium chloride needs so much energy. (3 marks)

(d) A student claims that the higher the melting point, the stronger the bonds in the substance. Using one simple molecular substance and one giant covalent structure, explain why this claim is not fully correct. (4 marks)

(e) Explain why copper is used for electrical wiring and graphite is used for electrodes in electrolysis cells. (3 marks)

Answers and marking notes

Question 1 – Ionic bonding and ionic structures

(a) A sodium atom loses one outer electron to form Na+ (one mark); the chlorine atom gains that electron to form Cl- (one mark); the oppositely charged ions are held together by electrostatic attraction, which is the ionic bond (one mark). [3 marks]

(b) Magnesium oxide is MgO (one mark); calcium chloride is CaCl2 (one mark). [2 marks]

(c) In the solid state the ions are fixed in the lattice and cannot move (one mark); on melting the ions gain enough energy to leave their lattice positions (one mark); the ions are then free to move and carry charge, so the liquid conducts (one mark). [3 marks]

(d) The sodium atom loses the single electron of its outer shell, so that shell disappears (one mark); the remaining electrons feel a larger effective nuclear charge and the electron cloud is pulled closer to the nucleus (one mark). [2 marks]

(e) Each ion in the lattice is surrounded by several ions of opposite charge, so the electrostatic attraction is strong (one mark); melting or deforming the crystal requires many of these ionic bonds to be broken (one mark); a high temperature is therefore needed and the crystal resists scratching (one mark). [3 marks]

(f) Magnesium has atomic number 12, so the magnesium ion has 10 electrons (one mark); each chloride ion has 18 electrons, so two have 36 (one mark); the total is 10 + 36 = 46 electrons (one mark). [3 marks]

Question 2 – Covalent bonding, simple molecules and giant covalent structures

(a) Each hydrogen atom contributes one electron (one mark); the two electrons are attracted by both nuclei and form one shared pair (one mark); a hydrogen molecule therefore has only one shared pair, a single bond (one mark). [3 marks]

(b) In methane the carbon atom forms four single bonds, one to each hydrogen atom (one mark); in carbon dioxide the carbon atom forms a double bond with each oxygen atom, that is two double bonds (one mark). [2 marks]

(c) Carbon dioxide consists of simple molecules held together only by weak intermolecular forces, which are easily overcome (one mark); diamond is a giant covalent structure in which each carbon atom is joined to four others by strong covalent bonds in a three-dimensional network (one mark); melting diamond means breaking many covalent bonds, so very high energy is required (one mark). [3 marks]

(d) In graphite each carbon atom bonds to only three others, leaving one electron delocalised within the layer (one mark); these delocalised electrons can move along the layers and carry charge (one mark); in diamond all four outer electrons are used in bonding, so there are no free electrons (one mark). [3 marks]

(e) Moles of carbon = 3.0 / 12.0 = 0.25 mol (one mark); number of atoms = 0.25 x 6.02 x 10^23 (one mark); = 1.505 x 10^23 atoms (one mark). [3 marks]

(f) Molecules as a whole carry no net charge, and although the bonds within them may be polar the molecule itself is neutral (one mark); in the liquid or gas state there are no free ions or electrons (one mark); with no charged particles to carry current, the substance does not conduct (one mark). [3 marks]

Question 3 – Metallic bonding, properties of metals and alloys

(a) Metal atoms lose their outer electrons to become positive ions (one mark); the positive ions are arranged in a regular lattice (one mark); the delocalised electrons move freely through the whole structure and hold the ions together (one mark). [3 marks]

(b) Layers of metal ions can slide over one another without breaking the metallic bonding (one mark); the delocalised electrons move with the layers and continue to hold the ions together (one mark); the metal therefore deforms instead of cracking and can be hammered into sheets (one mark). [3 marks]

(c) An alloy contains atoms of different sizes (one mark); these atoms disturb the regular arrangement of the layers (one mark); the layers can no longer slide easily, so the alloy is harder and stronger (one mark). [3 marks]

(d) Percentage of tin by mass = 100% – 88% = 12% (one mark); mass of tin = 500 x 12 / 100 (one mark); = 60 g (one mark). [3 marks]

(e) Delocalised electrons move freely through the metal and carry charge, so it conducts electricity (one mark); collisions of electrons transfer energy from hot to cooler regions, so it conducts heat (one mark); at higher temperature the metal ions vibrate more strongly (one mark); electrons collide with the ions more often, which impedes their flow and lowers conductivity (one mark). [4 marks]

Question 4 – Linking structure to properties

(a) Ionic: high melting point, does not conduct when solid but conducts when molten or dissolved (one mark); simple molecular: low melting point and generally does not conduct (one mark); giant covalent: very high melting point, does not conduct except graphite (one mark); metallic: melting point varies widely but is often high, and conducts in both solid and liquid states (one mark). [4 marks]

(b) In the solid the ions are fixed in the lattice and cannot move (one mark); when dissolved, water molecules break up the lattice and the ions become separated in the solution (one mark); the hydrated ions are free to move and carry charge, so the solution conducts (one mark). [3 marks]

(c) In diamond every carbon atom is joined to four others by strong covalent bonds in a three-dimensional network, so it cannot deform in any direction (one mark); graphite has a layered structure in which the bonds within a layer are strong but the forces between layers are weak (one mark); the layers slide over each other easily, so graphite is soft and works as a lubricant (one mark). [3 marks]

(d) Magnesium oxide contains Mg2+ and O2- ions, with charges of 2+ and 2- (one mark); the ions in sodium chloride carry only 1+ and 1- charges, which are smaller (one mark); the magnesium and oxide ions are also smaller than the sodium and chloride ions, so the ions are closer together (one mark); the electrostatic attraction is much stronger and far more energy is needed to melt the solid, hence the much higher melting point (one mark). [4 marks]

(e) Both are simple molecular substances whose boiling points depend on intermolecular forces (one mark); a butane molecule has more electrons and a larger surface, so the intermolecular forces are stronger (one mark); more energy is needed to separate the molecules, so butane has the higher boiling point (one mark). [3 marks]

Question 5 – Calculations linked to bonding and structure

(a) Relative formula mass = 23.0 + 35.5 = 58.5 (one mark); percentage by mass = 23.0 / 58.5 x 100% (one mark); = 39.3% (one mark). [3 marks]

(b) Moles = 117 / 58.5 = 2.0 mol (one mark); each formula unit gives two ions, so the amount of ions is 4.0 mol (one mark); number of ions = 4.0 x 6.02 x 10^23 = 2.408 x 10^24 (one mark). [3 marks]

(c) Relative formula mass = 24.3 + 2 x 35.5 = 95.3 (one mark); mass = 0.250 x 95.3 (one mark); = 23.8 g (one mark). [3 marks]

(d) Relative formula mass = 40.0 + 16.0 = 56.0 (one mark); moles = 112 / 56.0 = 2.0 mol, giving 4.0 mol of ions (one mark); total ions = 4.0 x 6.02 x 10^23 = 2.408 x 10^24 (one mark). [3 marks]

(e) Relative formula mass = 63.5 + 16.0 = 79.5 (one mark); mass fraction of copper = 63.5 / 79.5 (one mark); mass of copper = 8.0 x 63.5 / 79.5 = 6.39 g, quoted as 6.4 g (one mark). [3 marks]

(f) 1 mol of sodium chloride contains 2 mol of ions while 1 mol of magnesium chloride contains 3 mol of ions (one mark); the latter therefore contains 1.5 times as many ions (one mark). [2 marks]

Question 6 – Synoptic: deducing properties from structure

(a) The ions in magnesium oxide carry 2+ and 2- charges, larger than the 1+ and 1- charges in sodium chloride (one mark); the magnesium and oxide ions are smaller, which brings the ions closer together (one mark); both factors greatly increase the electrostatic attraction (one mark); more energy is therefore needed to break the ionic bonds on melting, so the melting point is higher (one mark). [4 marks]

(b) Temperature rise = 80 – 20 = 60 K (one mark); energy = 100 x 4.18 x 60 = 25080 J (one mark); = 25.1 kJ (one mark). [3 marks]

(c) Energy = 2.0 x 1.5 = 3.0 kWh (one mark); melting requires the whole ionic lattice to be broken apart against the electrostatic attraction, whereas dissolving merely lets water molecules separate the ions (one mark); melting therefore needs far more energy than dissolving (one mark). [3 marks]

(d) Melting a simple molecular substance only requires the intermolecular forces to be overcome, not the covalent bonds (one mark); in carbon dioxide, for example, the C=O bonds are strong yet the melting (sublimation) point is very low (one mark); a giant covalent structure such as diamond only melts when covalent bonds are broken (one mark); the melting point therefore reflects which forces must be overcome on melting and is not a direct measure of bond strength (one mark). [4 marks]

(e) Copper has delocalised electrons, conducts well and is malleable, so it can be drawn into thin wire (one mark); graphite conducts, has a very high melting point and is relatively unreactive at high temperature and in contact with the electrolyte (one mark); graphite is also easily shaped into rods or plates and is inexpensive (one mark). [3 marks]

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