Marking Guidelines Chemistry 2015

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University of Technology Sydney *

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Nov 24, 2024

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STHS 2015 Chemistry Preliminary Examination . Marking Guidelines and model Answers. Part A Multiple Choice 1 2 3 4 5 6 7 8 9 1 0 11 1 2 1 3 1 4 1 5 1 6 1 7 1 8 1 9 2 0 C B C C C A C B A A B B D D C A A C C A Part B FAULDER 21.a. Marking Criteria Marks  Both equations correct 2  One equation correct 1 Suggested Answer: 2Na (s) + 2H 2 O (l)  2NaOH (aq) + H 2(g) 2Na (s) + 2HCl (aq)  2NaCl (aq) + H 2(g) 21.b. Marking Criteria Marks  Equation correct 1 Suggested Answer: Na  Na + + e - 22 Marking Criteria Marks  Compares and contrasts the molecular shape, bonding and BP of H 2 O and H 2 S, and includes a correct electron dot diagram. Response contains both similarities and differences. 3  Describes the structure, bonding and BP of H 2 O and H 2 S  OR  Describes the structure and bonding of H 2 O and H 2 S, and includes a correct electron dot diagram. 2  Describes the structure and/or bonding and/or properties of H 2 O or H 2 S or correct diagram 1 Suggested Answer Molecular Shape: Both are bent due to the two pairs of nonbonding electrons in oxygen and sulfur. Bonding: Similarity - The hydrogen atoms are bonded to the O or S atom through covalent single bonds, where each bond contains one electron from each of the atoms in the bond. 1

Both molecules have dispersion forces between them, and both are polar. Differences. Oxygen is more electronegative than sulfur so water is capable of hydrogen bonding, which is stronger than dipole-dipole interactions of hydrogen sulfide. Boiling Point Boiling point of water is higher than that of hydrogen sulfide due to the hydrogen bonding. 23.a. Marking Criteria Marks  Correct Element chosen and corresponding reason 2  Correct element but incorrect reason OR  Incorrect element but correct reason 1 Suggested Answer: Either B or C – because they are malleable and conduct electricity 23.b. Marking Criteria Marks  Correct element and reason 2  Correct element but incorrect reason OR  Incorrect element but correct reason 1 Suggested Answer: D as is melting point is -101  C. All other elements are solid at room temperature 23.c. Marking Criteria Marks  Correct balanced equation and correctly refers to it to give volume of hydrogen gas required 2  Correct balanced equation OR  Correct volume of hydrogen stated 1 Suggested Answer: N 2(g) + 3H 2(g)  2NH 3(g) From stoichiometry of equation 300 mL of hydrogen is required 24.a. Marking Criteria Marks  Correctly calculates mass of Mg and O  Correctly calculates moles  Determines empirical formula 3  Correctly calculates two of above 2  Correctly calculates one of above 1 Suggested Answer: Mass of Mg = 32.63 - 32.14 = 0.49 g Mass of MgO = 32.95 - 32.14 = 0.81 2

Mass of O = 0.81 - 0.49 = 0.32 Mg : O Mass 0.49 : 0.32 Moles 0.49/24.31 : 0.32/16 0.020156 : 0.02 Simplest 1 : 1 Empirical formula is MgO 24b. Marking Criteria Marks  Recognises oxygen in air is O 2  Calculates mass correctly 2  Uses mass of O calculated above 1 Suggested Answer Mass of O atoms = 0.32 therefore the mass of O 2 = 0.32/2 = 0.16g 25.a. Marking Criteria Marks  Correct description of trend with reference to Na and Ar. 2  Correct description of trend Or compares Na and Ar only 1 Suggested Answer: Ionisation energy is the energy required to remove an electron from a gaseous atom of an element. The first ionisation energy generally increases across a period from left to right. Thus Na has the lowest value and Ar has the highest value. 25.b Marking Criteria Marks  Explains the trend down group VII with reference to elements 3  Describes the trend down group VII with reference to elements 2  Identifies trend Or gives examples 1 Suggested Answer: Electronegativity is a measure of the ability of an atom to attract bonding electrons towards itself and form compounds. Electronegativity decreases down a group because each element going down has another electron shell. This greater distance between the protons and the outer electrons decreases the ability to attract electrons. Therefore F is more electronegative than iodine. 26. 3

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Marking Criteria Marks  Defines a decomposition reaction and describes two examples of this type of reaction the student has observed, including the energy needed to decompose the compound.  Includes a relevant chemical equation. 3  Defines a decomposition reaction and describes one example of this type of reaction the student has observed, including the energy needed to decompose the compound. 2  Defines a decomposition reaction OR  Describes an example of this type of reaction the student has observed, including the energy needed to decompose the compound  OR includes a relevant chemical equation of a decomposition reaction. 1 Decomposition reactions involve breaking a more complex chemical into simpler components. Two ways that this can be done are by strongly heating a compound or by electrical energy: (or exposing to uv light) Thermal decomposition Heat 1g copper(II) carbonate in a test tube with a Bunsen burner. It decomposed into CuO and CO 2 : CuCO 3 (s)  CuO (s) + CO 2 (g) Electrolytic Decomposition We also used electrolysis to decompose water into hydrogen and oxygen in a voltameter. This process uses electricity to break the chemical bonds in the water. 2H 2 O (l)  2H 2 (g) + O 2 (g) Photolytic decomposition Silver chloride was prepared by adding 5mL 0.1 mol/L silver nitrate to 5mL 0.1 mol/L potassium chloride. Silver chloride precipitated was exposed to UV light and it turned dark grey as silver was deposited. 2AgCl(s)  2Ag(s) + Cl 2 (g) (NOTE: Outline a method usually means numbered steps with substance, quantities and equipment. As this was only worth 3 marks, the marking was more lenient) 27. Marking Criteria Marks  Gives reasons why Al should be recycled AND  Gives a judgement 2  Gives reasons why Al should be recycled OR  Gives judgement 1 Suggested Answer: Recycling helps conserve valuable metal resources and avoids the environmental damage resulting from mining and the discharge of pollutants to the atmosphere. Aluminium requires 5 times more energy to mine than is does to obtain the same mass of metal by recycling. Judgement: Therefore Al and other metals should be recycled. CARRINGTON 28 a. 4

Marking Criteria Marks  Identifies process 1 Suggested Answer: Fractional distillation 28b. Marking Criteria Marks  Correctly describes filtration and distillation with the addition of water 2  Correctly describes filtration or distillation with the addition of water 1 Suggested Answer: 1. Enough water was added to cover the mixture containing X(solid), Y (liquid) and Z (solid) 2. The mixture was poured through a folded filter paper that was placed in a filter funnel that was supported in a filter ring attached to a retort stand. 3. The residue was collected and dried 4. The filtrate containing X(solution) and Y (liquid) was then separated using the process of distillation. 29. Marking Criteria Marks  Explains the difference between two properties of diamond and graphite in terms of structure and bonding. 3  Explains the difference between one property of diamond and graphite in terms of structure and bonding, and outlines one other difference. 2  Outlines the difference between two properties of diamond and graphite in terms of structure and bonding. 1 Suggested Answer: Diamond and graphite have different electrical conductivity and hardness because of their structure and bonding. In diamond each C atom is covalently bonded to 4 others in a tetrahedral arrangement, forming 6 membered rings in a 3D giant covalent lattice. All of the valence electrons are involved in forming bonds. Because of the strength of the covalent bonding throughout the lattice, the substance is extremely strong, and finds application in industrial drills and saws. The lack of electrical conductivity is because there are no delocalised, mobile electrons in the structure to carry an electric charge. Graphite on the other hand is composed of 2D layers of carbon atoms arranged in 6 membered rings, and there is one electron per carbon atom not involved in bonding. These are delocalised between the layers, and mobile, and this is what results in the electrical conductivity of graphite. Graphite is softer than diamond because the 2D layers can slide over each other, and because not all of the atoms in the structure are connected to the entire lattice. Only the atoms in the 2D layers are connected by covalent bonds. 5

30a. Marking Criteria Marks  Identifies observations and correctly identifies the gas produced 2  Identifies the gas or one observation 1 Suggested Answer: Hydrogen gas was produced and small bubbles of gas were produced and there was twice as much volume of gas produced as compared to the other electrode. The test used was the hydrogen pop test, where a flame was brought close to the gas being tested and a distinctive “pop” is heard. 30b. Marking Criteria Marks  Identifies observations and correctly identifies the gas produced 2  Identifies the gas or one observation 1 Suggested Answer: Oxygen gas was produced and large bubbles of gas were produced and there was half as much volume of gas produced as compared to the other electrode. The test used was the glowing splint test, where a piece of wood that is quickly extinguished was brought close to the gas being tested and it is readily ignited 31a. Marking Criteria Marks  Identifies X,Y and Z 2  Identifies X,Y or Z 1 Suggested Answer: X: Pb(NO) 3 Y: CaCl 2 .......................................... Z: HCl 31b. Marking Criteria Marks  Correctly writes the NET ionic equation 1 Suggested Answer: Pb 2+ + 2Cl -  PbCl 2(s) 6

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NOYES 32.a. Marking Criteria Marks  Accurately plots data for both compounds  Draws 2 line graphs, including axes titles, suitable scales and a key. 4  Draws a satisfactory graph of the data, with one or two errors in the presentation of the graph. 2-3  Makes a limited attempt at graphing the data, including at least ONE correct feature of the graph. 1 Suggested Answer: (NOTE: the line of best fit should be a smooth line) 32.b. Marking Criteria Marks  Explains why the acid took longer to react. 2  Identifies the acid was less concentrated and took longer to react. 1 Suggested Answer The less concentrated acid took longer to react and produce the 30 mL of CO 2 (g). This is because there are less acid particles available to react and therefore less chance of collisions and product (gas) being formed. 33.a . Marking Criteria Marks  Correct answer 1 but-1-ene 7

33.b. Marking Criteria Marks  Discusses the benefits and limitations of the model (two of each),  identifies how it can be used to derive the correct name of the compound  Writes a judgement 3  Discusses at least one benefit and one limitation of the model, and uses it to derive the correct name of the compound.  OR  Discusses the benefits or limitations of the model (two).  OR  Writes a Judgement 2  Identifies one benefit or limitation of the model.  OR  Writes a judgement 1 Judgement: The model shown in Figure 2 is very useful because it allows the molecule to be visualised despite its size. One of the main benefits of the 3D model is that it can be flipped over, so that the student can see that the molecule is really 1-butene, and not 3-butene. It shows the difference between a double bond and a single bond, and shows how the single bonded carbon has a tetrahedral structure, whereas the double bonded carbon has a planar structure. It also shows that the double C=C bond does not rotate, but the single C-C bond does. The model has some limitations though, such as the relative sizes of H and C not being accurate, and the electrons in the bonds are not visualised. 34.a. Marking Criteria Marks  Correct balanced equation with states 1  Correct balanced equation with no states 0.5 Suggested Answer: States are always marked in combustion, precipitation and redox! C 2 H 6 (g) + 7/2 O 2 (g)  2CO 2 (g) + 3H 2 O(l) 34.b. Marking Criteria Marks  Identifies a problem 1 Suggested Answer: Toxic CO(g) is produced (Harmful is a motherhood statement & must be backed up with toxic or pollutant) 35.a. Marking Criteria Marks  Identifies endothermic reaction 1 8

Suggested Answer: Endothermic 35.b. Marking Criteria Marks  Correct value of E A read off graph 1 Suggested Answer: 30 kJ 23.c. Marking Criteria Marks  Line drawn to show lower E A 1 Suggested Answer: . 9

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