Not just about what you know

6 downloads 231 Views 1MB Size Report
... 3 & 4. • Biology • Chemistry • Physics • Psychology • English ... For enquiries & enrolments phone Neap on. 8341 8341 or ... VCAA Units 3-4 November exams.
Monday, September 7, 2009 THE AGE

7

EDUCATION VCE EXAM GUIDE

Not just about what you know Changes to the testing process mean you need to apply the appropriate data, writes Philip Ponder. CHEMISTRY ITH the change of Study Design in 2008, a decision was made to include a far more comprehensive Data Book with the Chemistry examination than in previous years. As a result, students can now expect more examination questions to involve applications of the data provided and fewer questions that merely rely on blind regurgitation of memorised material. Essential preparation for the exam should therefore include a thorough familiarisation with the contents of the Data Book and an understanding of how to use the information. The Data Book will be the same as the one you used for the Unit 3 examination. The purpose of this article is to guide you through the relevant information, and to point out some of the common mistakes that can be made under the pressure of exam conditions.

W

1. Periodic Table Its main use will be as a source of relative atomic masses for calculations but students should never forget that the Group Numbers give clues to the charges on the cations and anions formed by many common elements. A frequent mistake on the 2008 November exam was to write the formula of magnesium chloride as MgCl. A quick check of the Periodic Table should reveal that magnesium, a Group 2 element, would form Mg2+, and chlorine, in Group 17, would exist as Cl– in an ionic compound, thus combining to make the formula MgCl2. 2. The Electrochemical Series Many students find it useful to write “oxidant” at the top left of the table (above F2 (g)) and “reductant” at the top right (above 2F–(aq)). This will remind them that F2, the strongest oxidant, is a substance

VCE Headstart

that causes the oxidation of something else and is itself reduced. BEWARE: A common mistake is naming as “the oxidant” the substance being oxidised. The half equations for a hydrogen-oxygen fuel cell are readily found in the Electrochemical Series. The overall equation is the same as for the combustion of hydrogen: 2H2 (g) + O2 (g) ➝ 2H2O(l) Being an exothermic redox reaction, electrical energy can be obtained directly from chemical energy if the two half reactions can be separated, with oxidation of hydrogen occurring at an anode and reduction of oxygen at a cathode. The fuel cell can operate with either an acid or an alkaline electrolyte. For the acid electrolyte, you want H+ in the half equations (you should be able to find them at Eo = +1.23V and 0.00V) and for the alkaline electrolyte, you want OH– in the half equations (at Eo = +0.40V and –0.83V). Note that in both electrolytes, the predicted cell voltage under standard conditions is 1.23V, as you would expect, given that both cells would have the same overall reaction. 3. Physical Constants Here we find some data that was not relevant for Unit 3 but is now highly likely to be required for Unit 4 calculations. The unit for the Faraday constant (C mol–1) allows you to deduce the relevant formula F = Q (in coulomb) / n(e–) You will also need to remember Q = It (which looks like advice to smokers from the anti-cancer council!) and E = VIt , from which can be derived a useful relationship for the energy produced by a cell, E = VQ = V x F x n(e–). Many students who sat the November 2008 exam seemed unaware of this relationship.

VCAA Units 3-4 November exams

Small classes, expert staff, a wide range of subjects.

25 Sept. – 2 Oct. 2009 Caulfield Grammar School Caulfield Campus, 217 Glen Eira Road, East St Kilda Bookings essential phone 9524 6222 [email protected] www.caulfieldgs.vic.edu.au

however increase at T > 25˚C, so [H+] > 10–7 M and pH of a neutral solution will be less than 7. Likewise, at T < 25˚C, the pH of a neutral solution will be more than 7. Not a lot more, mind you, but enough for the examiners to set some interesting traps. 4. SI Prefixes Make sure you can recognise all the prefix symbols and do not confuse mega (M) with mol L–1 (M) or even molar mass (usually given the symbol M, although it probably ought to be Mm) 5. – 10. Mostly of relevance for Unit 3 11. & 12. Acidity Constants A very poorly handled question on last year’s Unit 4 examination involved the change in pH when a weak acid is diluted. Very few students recognised that equilibrium and Le Chatelier’s Principle would be relevant. An interesting exercise is to compare the dilution of 1.0 M HCl to 0.10 M, with the dilution of 1.0 M CH3COOH to 0.10 M. The calculation is simple for

the strong acid. Because HCl can be regarded as essentially fully ionised, [H+] goes from 100 M to 10–1 M, so the pH changes from 0 to 1. For the weak ethanoic acid, however, you will need to look up the Ka value (1.7 x 10–5 M). This applies to the equation: CH3COOH(aq) + H2O(l) ⇌ H3O+(aq) + CH3COO–(aq) Making the usual 2 assumptions ([CH3COOH]at eq’m ≈ [CH3COOH]initially, and that [H+] from self-ionisation of water is negligible) leads to: For 1.0 M CH3COOH [H+]2 / 1.0 = 1.7 x 10–5 ∴ [H+] = 0.0041 M and pH = 2.4 For 0.10 M CH3COOH [H+]2 / 0.10 = 1.7 x 10–5 ∴ [H+] = 0.0013 M and pH = 2.9 Note that the pH change is smaller for dilution of the weak acid. Dilution has increased the % hydrolysis of the ethanoic acid, as predicted by Le Chatelier’s Principle. Addition of one of the reactants in the equilibrium has resulted in a net forward reaction, leading to an increase in the n(H+)

Continued PAGE 8

J1930\1

Revision Classes The Headstart program has 17 years experience preparing students for exam success.

Any calculation that refers to the heating of water may require use of its Specific heat capacity (c). You might prefer to memorise the formula: E = 4.18 x m x ΔT but BEWARE: the m in this formula is the mass of water. It is a common mistake for students to use (as m) the mass of the fuel used to heat the water. In some ways it is preferable to look at the unit (J g–1 K–1) and deduce that 4.18J of energy will be required to heat 1g (1mL) of water by 1K (1˚C). The problem can then be solved using ratios. Many students have an unshakeable conviction that a neutral solution must have a pH of 7. It is therefore essential to recognise the significance of the Ionic product for water (Kw = [H+]x[OH–]). Because this is an equilibrium constant, it will vary with temperature, and the self ionisation is an endothermic reaction: H2O(l) ⇌ H+(aq) + OH–(aq) ; ΔH = +ve BEWARE: Kw will only equal 1.00 x 10–14 at 25˚C. At this temperature, [H+] in a neutral solution = [OH–] = 10–7 M, and hence pH = 7. Kw will,

IMPROVE YOUR ENTER SCORE

VCE

EXAM REVISION YEARS 11 & 12

SEPTEMBER HOLIDAY PROGRAM 24 September – 1 October 2009 VENUE: University of Melbourne

Year 11 Units 1 & 2

• Chemistry • Physics • Maths Methods

Year 12 Units 3 & 4

• Biology • Chemistry • Physics • Psychology • English s • Accounting • Maths Methods • Specialist Maths s • Legal Studies • Economics This comprehensive and flexible revision program offers the opportunity for in-depth revision of whole subjects or individual Areas of study. By attending, students will benefit from experienced teachers who will facilitate focused, high quality exam preparation and revision. Comprehensive and concise lecture notes will be provided.

For enquiries & enrolments phone Neap on 8341 8341 or visit www.neap.com.au

Study counselling, coaching, management, human resources, case management or community services and make a difference to people’s lives. We offer: • Vocational courses • Bachelor in Applied Social Science counselling, coaching or management

Study: • On-campus, online, by distance or a mix of all three • In small classes

GOVERNMENT FEE-HELP AVAILABLE

Find out how you can make a difference for life by: Calling: 1800 061 199 Emailing: [email protected] Visiting: www.acap.edu.au Australian College of Applied Psychology

National Educational Advancement Programs (Neap) Pty Ltd NATAGE F007

THE AGE Monday, September 7, 2009

8

EDUCATION VCE EXAM GUIDE

How to get pumped for grand final A clear revision timetable will help provide a winning strategy, writes Anita Forsyth. ECONOMICS O DO yourself justice, your economics exam requires careful preparation – just like getting ready for a grand final. You’ve been practising your knowledge and skills all year; now, in these final weeks, it is time to hone them through the development of a revision strategy. But what is a good approach? Set up a revision timetable so you have a detailed and effective strategy for exam preparation. This strategy might include the following activities: ■ Prepare a study summary to cover all aspects of the key knowledge and skills learnt in Units 3 and 4. The summary notes can be organised under specific headings. For example, in Unit 3’s Area of Study 2, organise material under each economic objective. Use your class notes, textbook(s), study guides, internet sources and newspaper articles to inform your summary. Cross-reference with the study design to make sure all the key skills and knowledge are covered. ■ Review this summary and condense the revision notes into the main ‘‘big ideas’’ of the course. Make links and connections across these big ideas by drawing up mind/concept maps. This will allow you to show your understanding of the relationships between key concepts and ideas. Perhaps get together with other students and divide up the key knowledge topics, with one classmate taking responsibility for preparing a ‘‘big ideas’’ summary and recording it as a podcast. This can then be put on your iPod and listened to regularly. ■ Write down the definitions of the main economics concepts and terms and compare your definitions with those in your notes or in the text. Ask yourself these questions: How accurate were my definitions? Which definitions need improvement? ■ The study of economics involves a special way of thinking, that is the use of cost/benefit analysis. For example, economists use this analysis to evaluate the desirability of a proposed policy change. This is a strategy that can be applied to your revision of economic

T

Students are encouraged to dig deep for the exams by developing a thorough preparation strategy that includes plenty of practice with sample questions. principles, theories and examples. When analysing any changes in economic performance or policies, practise listing the costs and benefits of the situation to help deepen your economic understanding and make you aware of the possible positive and negative aspects associated with the change. ■ Review the past three years’ VCE Economics Examination Assessment Reports at vcaa.vic.edu.au/vce/studies/ economics/exams.html. Outline

the strengths and weaknesses of students’ responses to past exams. This will allow you to see how to maximise your performance by avoiding past mistakes and by following recommendations that may enhance your approach. For example, the assessment reports regularly say students who perform well are those who are aware of, and understand, current economic issues and conditions and are able to apply this knowledge to their answers. So think about how current economic

issues and events can be used to illustrate the economic theory you have been studying this year. Draw up a list of topics and match current examples. Think, for example, about how the global financial crisis may have affected the performance of Australia’s economic objectives this year and how economic policy was used to manage a response to the challenges related to the crisis. There are various sources that can help to keep you up to date, such as the Reserve Bank of

Australia’s monetary policy media releases of February 3, March 3, April 7, May 5, June 2, July 7, August 4, etc at rba.gov.au and the 2009-10 Commonwealth Budget Paper No. 1 at www.budget.gov.au. ■ Immerse yourself in economic activity. Talk economics at every opportunity: in the classroom, over lunch with classmates, at home around the dinner table, when reading the newspaper. Teach parts of the economics course to friends and family. ■ Review all completed assessments from throughout the year, including school-assessed course work. Assessment is part of the learning process and helps you diagnose your strengths and weaknesses. Observe the sorts of questions asked, note the teacher’s feedback, rewrite parts of the assessments that you initially struggled with and check your new response against the textbook, or ask your teacher. These approaches can enhance your understanding and help you improve in the areas you found more challenging. Once you feel you have a thorough understanding of the study requirements and have completed a range of revision tasks, such as those recommended: ■ Practise sample multiple choice and written response questions as much as possible under timed conditions. Examples of questions can be found in a range of study guides, in your textbook and at various websites, such as economicstutor.com.au and mrwood.com.au. ■ Practise making up your own questions on a range of topics and writing solutions. Alternatively, swap questions with classmates. Make sure you set an appropriate mark allocation for the written response questions because this will act as a guide to how much detail should be put in an answer. ■ Work through the past three years’ exam papers (available at vcaa.vic.edu.au/vce/studies/ economics/exams.html) under timed conditions. Remember, 30 marks are allocated to Section A (multiple choice) and 60 marks to Section B (written responses). Spend about 30 minutes on Section A and 90 minutes on Section B. ■ Mark and assess each other’s responses to sample questions from other sources, to the questions you might have devised and to those for the past three years’ examinations. Anita Forsyth is a senior lecturer in education at Monash University and an experienced economics assessor.

It’s not just what you know, it’s how you use it CHEMISTRY continued From PAGE 7 in the solution. Of course, the greater volume of solution means that the overall [H+] will decrease but not by as much as it did for the strong acid, where there was effectively no equilibrium consideration. Another way of explaining this is to consider the relationship between reaction rates and equilibrium. When a system is at equilibrium, NATAGE F008

the rate of the forward reaction equals the rate of the back reaction. Addition of water to the ethanoic acid equilibrium will probably slightly increase the rate of the forward reaction (more frequent collisions between the reactant particles) but will definitely decrease the rate of the back reaction (lower concentrations of the products will lead to less frequent collisions between product particles). With the two rates no longer equal, the system is out of equilibrium, and the forward reaction will occur to a greater extent until the [CH3COOH] has

decreased enough and the [H3O+] and [CH3COO–] increased enough for the two rates to be equal again. 13. Molar Enthalpy of Combustion BEWARE: It is important to recognise the difference between the Heat of Combustion of a fuel, and the ΔH for the reaction in which the fuel reacts with oxygen. Note that ΔH normally applies to a specific equation as written, with the unit kJ mol–1 signalling that the co-efficients in the equation should be read as amounts in mole. Using butane as an example, its Heat of Combustion is 2874 kJ

mol–1, meaning that when 1 mole of gaseous butane undergoes complete reaction with oxygen (which usually implies excess oxygen), 2874 kJ of energy will be released (at 25˚C and 1 atm). A quoted value of ΔH requires an accompanying balanced equation. The value given in the Data Book is telling you to write the equation for combustion of butane as: C4H10 (g) + 13/2 O2 (g) ➝ 4 CO2 (g) + 5 H2O(l) ; ΔH = –2874 kJ mol–1 This tells you that if 1 mole of gaseous butane reacts with 6.5 mole of gaseous oxygen to produce

gaseous carbon dioxide and liquid water, then 2874 kJ of energy will be released. However, most students will write the equation as: 2 C4H10 (g) + 13 O2 (g) ➝ 8 CO2 (g) + 10 H2O(l) ; ΔH = –5748 kJ mol–1 This equation refers to 2 mole of gaseous butane reacting with 13 mole of gaseous oxygen. In this case, 5748 kJ of energy will be released but the unit of ΔH is still kJ mol–1. Phil Ponder is a Chemistry Assessor, and teaches Chemistry at Penleigh and Essendon Grammar School.