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CHEMISTRY NCEA LEVEL 2

STEP UP 2010. CHEMISTRY NCEA LEVEL 2. AS 2.4. FACILITATOR: MR KANE RAUKURA. QUESTION ONE: Properties of Substances. 1 . The table below shows the properties of some substances. yes. no. 1.(a) 2 Correct (Ach) Only. no. Complete the table above.

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CHEMISTRY NCEA LEVEL 2

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  1. STEP UP 2010 CHEMISTRY NCEA LEVEL 2 AS 2.4 FACILITATOR: MR KANE RAUKURA

  2. QUESTION ONE: Properties of Substances 1. The table below shows the properties of some substances yes no 1.(a) 2 Correct (Ach) Only no • Complete the table above.

  3. Using information from the table above: • Give the state of tetrachloromethane at room temperature (250C) • Explain your answer: • Choose a substance which contains non-polar molecules. • (iii) Choose a substance which contains polar molecules. 1.(b) i State Only = (Ach) Complete Answer = (Merit) LIQUID The melting point is below 250C and the boiling point is above 250C 1.(b)ii 1 Correct = (Ach) Methane or tetrachloromethane 1.(b)iii 1 Correct = (Ach) Ammonia

  4. (c) Explain why ammonia is very soluble in water, while methane is not soluble in water. • Ammonia is a polar substance and will dissolve in polar water / The strength of the van der Waals forces between solute molecules are approx. the same as the strength of the van der Waals forces between solvent molecules. • 2. Methane is a non-polar substance and will not dissolve in polar water/ The strength of the van der Waals forces between solute molecules are very different to the strength of the van der Waals forces between the solvent molecules. 1.(c) 1 Idea = (Ach) 2 Ideas with clear links between polarity in the solvent and the solute = (Merit)

  5. (d) Explain why copper can conduct electricity as a solid and as a liquid. 1.(d) Electrons carry the current = (Ach) Plus Idea of electrons moving = (Merit) QUESTION TWO: Polarity and Shape of Molecules The electronegativity values for elements are given on the data page at the front of this booklet. (a) Explain the term ‘electronegativity’ Electronegativity is the force of attraction an atom has for bonding electrons within a molecule. 2.(a) Correct answer = (Ach)

  6. + - - + + - Cl P Cl Cl O O C • Use + and - to indicate the polarity of the bonds in each of the following: • (i) H – S (ii) F – Cl (iii) C – O 1.(b) 2 Correct (Ach) Only (c) Draw Lewis diagrams of carbon dioxide and phosphorus trichloride and give the shape of the carbon dioxide molecule. 2.(c)i 16 val. Elec. = (Ach) Plus all correct = (Merit) (c)ii Correct answer = (Ach) (c)iii As (c)i but 26 val.elect. Linear

  7. (d) Explain why phosphorus trichloride (PCl3) is polar. 2.(d) 1 idea = (Ach) Both ideas with clear links = (Merit) • Phosphorus has a different electronegativity to chlorine, so the bonds are polar. • 2. Since shape is trigonal pyramid, the bond dipoles do not cancel out, therefore PCl3 is polar (e) Explain why phosphorus trichloride (PCl3) has a trigonal pyramid shape. 2.(e) 1 idea = (Ach) 2 ideas = (Merit) 3 ideas clearly expressed = (Exc) • There are four areas of electron density around the central atom. • Due to electron-electron repulsion, they are pushed to the corners of a tetrahedron. • 3. Since one of the areas of electron density is a lone pair, shape is trigonal pyramid

  8. QUESTION THREE: Chemistry of some Elements Use the information from the table below to answer the questions that follow. Use the letters A to D in all your answers. 3.(a-d) Each Correct answer = (Ach) (a) Which element would be the most un-reactive? C (b) Which element would be a metal? D

  9. (c) Which element would react with B to form a covalent molecule? A 3.(e) Ratio of A:D is 2:1 Correct answer = (Merit) (d) Which element is likely to exist in its compounds as a -1 ion? A (e) What is the formula of the compound formed between A and D? A2D 3.(f) One Idea = (Ach) Two ideas = (Merit) All 3 plus links = (Exc) (f) Explain the very high boiling point of element B (b.p.= 48300C) by discussing the nature of the bonding within the element. • B has a giant macromolecular covalent structure. • 2. Strong covalent bonds between the atoms, holding the lattice together. • 3. A very large amount of energy is required to break the covalent bonds, therefore B has very high boiling point.

  10. QUESTION FOUR: Allotropes Carbon can exist in three allotropes – diamond, graphite and buckminsterfullerene. These are shown in the diagrams below. diamond graphite buckminsterfullerene (a) Explain the term ‘allotropes’: These are different forms of the same element, (in the same phase), due to different bonding arrangement. 4.(a) Correct Answer = (Ach)

  11. (b) Complete the following table to explain the properties given: 4.(b)i One Idea = (Ach) Two Ideas = (Merit) 1. Graphite has weak van der Waals forces between the layers. 2. The layers can easily slide over each other, therefore graphite is greasy 4.(b)i One Idea = (Ach) Two Ideas = (Merit) • Buckminsterfullerene is a discrete • molecule, and has weaker van der Waals • forces between the molecules. • 2. Diamond is a covalent network, which has • strong covalent bonds holding the • network together. • 3. More energy is needed to overcome the • covalent bonds between the atoms in • diamond than the van der Waals forces • between the ‘bucky ball’ molecules.

  12. QUESTION FIVE: Chemistry of Seawater The sea covers two-thirds of the Earth’s surface and is a vast resource of various chemicals. The sea contains many dissolved ions such as chloride ions, sodium ions, calcium ions, carbonate ions, and iodide ions. (a) (i) Which is the better conductor of electricity – sea water or distilled water? 5.(a)i Correct answer = (Ach) 5.(a)ii One Idea = (Ach) Two Ideas = (Merit) Sea water is a better conductor of electricity than distilled water (ii) Explain your answer in detail: 1. Seawater contains lots of ions that can move and carry current. 2. Distilled water consists of neutral molecules that cannot carry current. (only slight ionisation within distilled water, few ions)

  13. Sea salt is commercially obtained from sea water by the process of evaporation and crystallization. • The main component of sea salt is sodium chloride. • What type of attractive force or bond holds the sodium ions and chloride ions together in a crystal of sodium chloride? Ionic bonds/electrostatic forces of attraction 5.(b) Correct Answer = (Ach) (c) Iodide ions are found in seawater and become fairly concentrated in a seaweed called Kelp. Iodine can be extracted from the ash left behind when Kelp is burnt, using a redox reaction to convert the iodide ions present in the ash, into elemental iodine. When solid iodine is warmed it quickly sublimes forming a purple vapour. Write a paragraph to explain the following statement: ‘Warming solid iodine is an illustration of the fact that the bonds within molecules are a different strength to the forces between molecules.’

  14. The paragraph needs to include all 3 of the following points to gain excellence. • With a comparison between the strength of attraction between atoms and the strength of the attraction between molecules AND • an understanding that intermolecular attractions are broken, not the intramolecular attractions • Iodine (I2) is made out of molecules with a single covalent bond between two atoms. • 2. When iodine is warmed, the weak van der Waals forces between the molecules are broken and the iodine sublimes (to form a purple vapour). • 3. The (purple) vapour still contains molecules of iodine, so the strong covalent bond is not broken. 5.(c) One Idea = (Ach) Two ideas = (Merit) All 3 plus links = (Exc)

  15. QUESTION SIX: Periodic trends Boiling point (oC) 1600 1400 1200 1000 800 600 400 200 0 NaCl MgCl2 AlCl3 SiCl4 PCl3 SCl2 Cl2 -200 Period three chlorides QUESTION SIX: Periodic trends Each of the elements in Period Three (sodium to chlorine) will react with chlorine to form a chloride compound given suitable conditions. The graph below shows the trend in boiling points of the Period Three chlorides.

  16. Using your knowledge of bonding, discuss the trend shown by the graph • NaCl and MgCl2 are ionic substances so will need lots of energy to break the strong ionic bonds between the cations and anions, therefore high boiling points. • 2. AlCl3, SiCl4, PCl3, SCl2 and Cl2 are molecules with weaker van der Waals forces between them, therefore less energy needed to overcome them, so relatively lower boiling points. • Note: AlCl3 needs to be grouped with molecular substances based on relative b.pt. 6. General statement = (Ach) One idea with links to substance & b.p. = (Merit) Two ideas with links between type of substance & b.p. = (Exc)

  17. ACHIEVED MERIT EXCELLENCE SUFFICIENCY STATEMENT: Achievement: 14 of the 27 Achievement opportunities Merit: 7 of the 13 Merit opportunities Excellence: 3 of the 5 Excellence opportunities WHAT WAS YOUR TOTAL? FINAL GRADE AFTER CASCADING (IF NECESSARY)

  18. STEP UP 2010 THANK-YOU FOR ATTENDING GOOD LUCK IN THE EXTERNALS!

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