NATIONAL SENIOR CERTIFICATE EXAMINATION

Transcription

1 NATIONAL SENIOR CERTIFICATE EXAMINATION PHYSICAL SCIENCES: CHEMISTRY (P2) JUNE 2015 QUESTION PAPER GRADE 11 MARKS: 150 TIME: 3 HOURS This paper consists of 10 pages, 2 data sheets and 1 graph paper.

2 INSTRUCTIONS AND INFORMATION 1. Write your name in the appropriate space on the FOLIO PAPER. 2. This question paper consists of SEVEN questions. Answer ALL the questions in the FOLIO PAPER. 3. Start EACH question on a NEW page in the FOLIO PAPER. 4. Number the answers correctly according to the numbering system used in this question paper. 5. Leave ONE line between two sub questions, for example between QUESTION 2.1 and QUESTION You may use a non-programmable calculator. 7. You may use appropriate mathematical instruments. 8. You are advised to use the attached DATA SHEETS. 9. Show ALL formulae and substitutions in ALL calculations. 10. Round off your final numerical answers to a MINIMUM of TWO decimal places. 11. Give brief motivations, discussions, et cetera where required. 12. Write neatly and legibly. 2

3 QUESTION 1 (MULTIPLE-CHOICE QUESTIONS) Four possible options are provided as answers to the following questions. Each question has only ONE correct answer. Choose the best answer and write down A, B, C or D next to the question number ( ) in your FOLIO PAPER. 1.1 A certain mass of gas occupies a volume (V) in a closed container. If the Pressure is doubled and the temperature is halved, the volume will be A ¼ V B ½ V C V D 4 V 1.2 The forces between the molecules of ice crystals are A London forces B Polar covalent bonding C Covalent bonding D Hydrogen bonding. 1.3 Which ONE of the following gases occupies the smallest volume at STP? A 4 g helium B 14 g nitrogen C 32 g oxygen. D 34 g ammonia 1.4 An ideal gas is enclosed in a gas syringe. The pressure on the gas is now increased but the temperature of the gas is kept constant. The average kinetic energy of the gas molecules will... A increase B remain constant C decrease. D double 3

4 1.5 Consider the following balanced equation: N 2 (g) + 3H 2 (g) 2NH 3 (g) The amount of nitrogen gas required to prepare 17 mol of ammonia is A 17 mol B 25 mol C 18,5 mol D 8,5 mol g of a sample of each of the following substance is measured off. Which of the following will contain the largest number of molecules or formula units. A ammonia B carbon dioxide C nitric acid D potassium hydroxide. 1.7 In which ONE of the following molecules is the bond the most polar in nature? A HF B NO C HCl D OF Which ONE of the following species contains a dative covalent bond? A HCl + B NH 4 C CH 4 D NaCl 1.9 The behaviour of a fixed mass of gas will deviate from ideal gas behaviour when A the kinetic energy of the particle increases B the temperature of the gas is very high C the pressure on the gas is very low D observable forces exist between the gas molecules 4

5 1.10 As the bond order between two atoms increases, A bond energy increases and the bond length increases B bond energy decreases and the bond length decreases C bond energy increases and the bond length decreases D bond energy increases but the bond length either increases or decrease [20] QUESTION 2. ( START ON A NEW PAGE) Consider the substances below and answer the questions which follow: CO 2, CH 4, N 2, H 2 O, HCl, NaCl, NH Explain the difference between a covalent bond and an ionic bond. 2.2 Name the type of chemical bond formed between the particles in NaCl (1) HCl (1) 2.3 Draw the Lewis structure for CH H 2 O 2.4 Write down the shapes of CO 2 and NH 3 according to the VSEPR theory. 2.5 Is the NH 3 molecule POLAR OR NON POLAR, Explain your answer? (3) 2.6 Consider the following molecules NO 2, AlF 3, HF, KCl, SO Determine whether the bonds that are formed are POLAR COVALENT, NONPOLAR COVALENT or IONIC using electronegativity. (5) Define electronegativity? Explain why a BF 3 molecule is non-polar even if it contains polar bonds? (3) [23] 5

6 QUESTION 3 (START ON A NEW PAGE) Air is a mixture of gases that contains various pollutants. The gases can be separated by fractional distillation of liquid air. The table below gives information about some of the gases found in air. Name Formula Boiling point ( 0 C) Helium He -269,0 Oxygen O Carbon dioxide CO 2-78,5 Nitrogen N Sulphur dioxide SO 2-72,0 Argon Ar -185,9 3.1 Define the term boiling point. 3.2 Which gas will not be a liquid at -200 (1) 3.3 Helium, oxygen, nitrogen and argon all have the same type of force between their particles. Name this type of force between their particles. 3.4 Explain the trend in the boiling point of the substance that have this type of force between their particles Both water and carbon tetrachloride are popular solvents in which other substances are dissolved. Complete the table below Solvent molecular shape polar/non-polar Type of intermolecular force between molecules H 2 O CCl 4 (6) (3) 3.6 Consider the following substances and show in each case if it will most likely dissolve in water or carbon tetrachloride Na 2 S (1) HCl (1) CS 2 (1) Br 2 (1) [18] 6

7 QUESTION 4 The graph below shows the change in energy that takes place when a hydrogen (H) atom approaches a bromine (Br) atom. 4.1 Define the term bond energy. 4.2 From the graph, write down the Bond length in pm of the H-Br bond Energy in KJ.mol -1 needed to break the H-Br bond Name of the energy represented by E in the graph (1) 4.3 What is the bond order of this bond (1) 4.4 How will the bond length of an H-Cl bond compare to that of H-Br bond? Write down EQUAL TO, SHORTER THAN OR LONGER THAN. Give a reason for the answer. [10] 7

8 QUESTION 5 In an experiment to determine the relationship between the pressure and temperature of a fixed mass of a gas, a group of learners obtained the following results: Pressure (kpa) ,5 138 Temperature ( Total gas volume (cm 3 ) On the graph paper provided draw a straight line graph of pressure (on the dependent axis) versus temperature (on the independent-axis). (4) 5.2. Extrapolate (extend) the graph and determine the temperature (in ) at which the graph will pass through the temperature axis. (1) 5.3. Write down in words the relationship between pressure and temperature in Kelvin From your graph, determine the pressure (in kpa) at 173 K. Indicate on your graph how you obtained this value How would the gradient of the graph be affected if a larger mass of the gas is used? Write down only INCREASES, DECREASES or REMAINS THE SAME. (1) 5.6. At a temperature of 27, 5,60 g of a diatomic gas occupies a volume of m 3 at a pressure of 166 kpa Calculate the molar mass of the gas. Assume that the gas behaves like an ideal gas. (6) Write down the NAME or FORMULA of the gas in (1) Calculate the volume at STP of a given mass of gas which occupies 200dm 3 at a pressure of 260kPa and a temperature of -13 (5) [22] 8

9 QUESTION Water forms the hydrosphere. This molecule plays an important role in moderating our climate and in the continued survival, of living organisms on earth Name the inter-atomic bonds that exist in water molecule. (1) Name the intermolecular forces that exist between water molecules (1) Explain why the water molecule is polar. (3) 6.2 Water transports essential nutrients from the roots of the plant right up to the highest leaf on the plant through narrow tubes called xylem found in the stem of a plant An example of a nutrient a plant needs is KNO 3.Explain why water is able to dissolve this substance. (3) Name the property responsible for the transportation of nutrients in plants. (1) Name the type of intermolecular force that exists between the dissolved KNO 3 and the water. (1) 6.3 Water is a solid at 0 at atmospheric pressure.i t has an unusual change in density with temperature that makes it possible for aquatic life to live in very cold climate Compare the density of ice to liquid water.explain why the difference in density occurs Explain which property of water is the reason for the low fluctuations in temperature between winter and summer QUESTION 7 (4) [16] 7.1 Define the term molar mass of a substance. 7.2 Calculate the number of moles in 32 g of bromine. (3) 7.3 A sample of 10 g of a liquid is analysed and it is established that the sample consist of 9,23 g of carbon and 0,77 g of hydrogen. The molar mass of the substance is 78 g.mol Define the term empirical formula Determine the empirical formula of the sample. (6) Determine the molecular formula of the substance. [15] 9

10 QUESTION 8 Diiodine pentoxide is useful in devices such as respirators because it reacts with the dangerous gas carbon monoxide to produce relatively harmless CO 2 according to the equation In testing a respirator, 2 g of CO gas is passed through 5,5 g of I 2 O Calculate the mass of iodine that will be produced (7) 8.2 Upon analysis, it is found that 3,17g of iodine was produced. Calculate the percentage yield of the reaction. (3) [10] QUESTION 9 Ozone (O 3 ) reacts with nitrogen monoxide (NO) to produce NO 2 gas.the NO gas forms largely as a result of emissions from the exhausts of motor vehicles and from certain jets. The NO 2 gas also causes the brown smog (smoke and fog), which is seen over most urban areas. This gas is also harmful to humans, as it causes breathing problems. The following equation indicates the reaction between ozone and nitrogen monoxide In one such reaction 0,74g O 3 reacts with 0,67g NO. 9.1 Define the term limiting reagent. 9.2 Calculate the number of moles of O 3 and NO present at the start of the reaction. (5) 9.3 Identify the limiting reagent in the reaction. Justify your answer. (5) 9.4 Calculate the mass of NO 2 produced from the reaction. (4) [16] TOTAL MARKS :

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14 NATIONAL SENIOR CERTIFICATE EXAMINATION PHYSICAL SCIENCES: CHEMISTRY (P2) JUNE 2015 MEMORANDUM GRADE 11 MARKS: 150 This memorandum consists of 8 pages

15 2 QUESTION A 1.2 D 1.3 B 1.4 B 1.5 D 1.6 A 1.7 A 1.8 B 1.9 D 1.10 C [20] QUESTION A covalent bond is a bond formed by the sharing of electrons between atoms, whilst an ionic bond is formed when there is a transfer of electrons from one atom to another atom Ionic bond (1) Covalent bond (1)

16 CO 2 ; Linear NH 3 : Trigonal planar 2.5 Polar. Because electronegativity of nitrogen is higher than that of hydrogen thus the bonding electrons are shifted towards nitrogen than hydrogen. This results in a slight negative charge on nitrogen and a slight positive charge on hydrogen, which makes ammonia a polar molecule (3) NO 2, 3.5-3,.0= 0.5 Polar covalent bond AlF 3, =2.5 Ionic bond HF, Polar covalent KCl Ionic bond SO 2 Polar covalent bond (5) Electronegativity of an atom is the ability of an atom in a molecule to attract the bonding electrons to form a polar covalent bond The three bonds in are polar. They are arranged symmetrically around the central boron atom. Because of the trigonal planar, no side of the molecule is negative or positive than another side which makes the molecule non polar (3) [23]

17 4 QUESTION The temperature at which the vapour pressure of the liquid equals the atmospheric pressure 3.2 Helium (1) 3.3 London/dispersion/dipole-induced dipole forces. 3.4 They all have weak London forces, as the molar mass increases, the strength of Intermolecular force also increases, so more energy is required to overcome the forces. The more the energy needed the higher the temperature to reach the boiling point. (3) 3.5 solvent Molecular shape Polar/non polar Intermolecular force water Angular polar Hydrogen bonding Carbon tetrachloride Tetrahedral Non-polar London/dispersion forces (6) Na 2 S Soluble in water (1) HCl soluble in water (1) CS 2 Soluble in carbon tetrachloride (1) Br 2 - soluble in carbon tetra chloride. (1) QUESTION 4 [18] 4.1 Is the amount of energy required to break a chemical bond between two atoms (pm) ±330 KJ.mol Bond Energy (1) (1) 4.4 Shorter than Cl atoms are smaller than Br atoms. [10]

18 5 QUESTION labelling of axes correct scale shape correct value of the temperature (4) 5.2 Approximately -273 (1) 5.3 The pressure for a fixed mass of a gas is directly proportional to temperature in kelvin provided volume remains constant k Pa (±5) 5.5 Increases (1) PV =nrt Nitrogen/ N 2 (1) (6) (5) [22]

19 6 QUESTION Covalent bond (1) Hydrogen Bond (1) The water molecule has a polar bond and due to its angular shape it has a permanent dipole moment. This results in the oxygen side of the molecule being slightly negative and hydrogen side of the molecule being slightly positive. This makes the molecule polar. (3) As per the rule like dissolves like. Water has strong hydrogen bonds and is polar and KNO 3 has strong ELECTROSTATIC FORCES OF ATTRACTION. THESE FORCES AND POLAR AND THE NATURE OF THE SUBSTANCES ARE COMPARABLE and so they dissolve. (3) Capillarity (1) Ion-dipole (1) Ice is less dense than liquid water. Because of the strong hydrogen bonding leads to an open structure with a large space in it and thus ice is less dense than water High specific heat capacity. Because of the strong hydrogen bonding, water takes more heat to raise the temperature of 1kg of water by one degree than any other liquid. The heat energy has to partially overcome the strong hydrogen bonds as well as increase the kinetic energy of the water molecules. As a result a lot of energy transfer brings about small temperature change. This keeps the temperature near the ocean relatively constant. (4) QUESTION The mass of one mole of atoms, molecules or formula units [16] 7.3 = =0,20 mol. (3) Chemical formula that shows the simplest ratio between the atoms in a compound.

20 Element, C : H 9,23 : 0,77 = = 0,769 mol = = 0,77 mol molar ratio= 0,769 : 0,77 = 1 : 1 Empirical formula = (6) Molecular formula =n(empirical formula) n= = =6 molecular formula of the compound is [15] Question n(co) = = =0,071mol n(i 2 O 5 ) = = = 0,061 mol = = = n(i 2 ) = 0,0142mol = 1 n(i 2 ) = 0,061mol CO is the limiting reagent. n (I 2 ) =0,0142 mol m (I 2 ) =n = =3,6 g (7)

21 8 8.2 percentage yield (%) = = = 88,1 % (3) [10] QUESTION The limiting reagent in a reaction is the reactant that is consumed first completely or is used up first 9.2 n(o 3 ) = = mol n(no) = = mo l (5) 9.3 From the equation O 3 : NO = : mol NO available Required ratio = actual ratio = = 0.69< 1 Ozone is the limiting reagent. (5) 9.4 From the equation O 3 : NO = : m(no 2 ) =nm =( ) = 0.71 g (4) [16] TOTAL = 150