succeeding in the exams, 2017

Transcription

1 Chemistry

2 author(s) succeeding in the exams, 2017 These materials represent the collective effort of many teachers across the state. The principal authors of this booklet are: Mr Jon-Paul Grant, B.Sc. (Hons) Env. Sci, PGCE (Sci) (Senior Chemistry Teacher Loreto Normanhurst) Ms Jane Wolter, B.Sc., Dip. Ed., Grad. Cert. (Physics) (Senior Chemistry Teacher The Scots College) lecturer(s) To ensure that students are afforded every possible advantage in their examinations, our lectures are prepared and delivered by qualified, currently practising HSC teachers and official HSC exam markers who possess the knowledge and experience to demonstrate the means by which students can achieve the higher ATAR scores. Further details regarding our teachers (including qualifications and experience) may be obtained at important notes Our policy at TSFX is to provide students with the most detailed and comprehensive set of notes that will maximise student performance and reduce study time. These materials, therefore, include a wide range of questions and applications, all of which cannot be addressed within the available lecture time i.e. Due to time constraints; it is possible that some of the materials included in this booklet will not be addressed during the course of these lectures. Where applicable, fully worked solutions to the questions in this booklet will be handed to students on the last day of each subject lecture. Although great care is taken to ensure that these materials are mistake free, an error may appear from time to time. If you believe that there is an error in these notes, please let us know asap (admin@tsfx.com.au). Errors, as well as clarifications and important updates, will be posted at The views and opinions expressed in this booklet and corresponding lecture are those of the authors/lecturers and do not necessarily reflect the official policy or position of TSFX. TSFX - voted number one for excellence and quality in HSC programs. copyright notice These materials are the copyright property of The School For Excellence and have been produced for the exclusive use of students attending this program. Reproduction of the whole or part of this document constitutes an infringement in copyright and can result in legal action. No part of this publication can be reproduced, copied, scanned, stored in a retrieval system, communicated, transmitted or disseminated, in any form or by any means, without the prior written consent of The School For Excellence (TSFX). The use of recording devices is STRICTLY PROHIBITED. Recording devices interfere with the microphones and send loud, high-pitched sounds throughout the theatre. Furthermore, recording without the lecturer s permission is ILLEGAL. Students caught recording will be asked to leave the theatre, and will have all lecture materials confiscated. it is illegal to use any kind of recording device during this lecture

3 THE ACIDIC ENVIRONMENT SECTION 1 Indicators were identified with the observation that the colour of some flowers depends on soil composition. REVISION: COMMON REACTIONS OF ACIDS acid + metal carbonate metal salt + carbon dioxide + water acid + metal hydrogencarbonate metal salt + carbon dioxide + water acid + metal hydroxide metal salt + water acid + active metal metal salt + hydrogen CLASSIFY COMMON SUBSTANCES AS ACIDIC, BASIC OR NEUTRAL Identify that indicators such as litmus, phenolphthalein, methyl orange and bromothymol blue can be used to determine the acidic or basic nature of a material over a range, and that the range is identified by change in indicator colour. Indicator Colour Change at different ph levels Methyl orange Red below 3.1 Yellow above 4.4 Bromothymol Blue Yellow below 6.2 Green around 7 Blue above 7.6 Phenolphthalein Colourless below 8.3 Red above 10.0 Litmus Red below 5.5 Purple / Blue above 8.0 Note: These ph values are approximate and can be slightly different in different texts. It is not necessary to know the exact ph ranges of each indicator, but you will need to know the colours. For example you will need to know that bromothymol blue is yellow in the acidic ph range SOLVE PROBLEMS BY APPLYING INFORMATION ABOUT THE COLOUR CHANGES OF INDICATORS TO CLASSIFY SOME HOUSEHOLD SUBSTANCES AS ACIDIC, NEUTRAL OR BASIC Using a variety of indicators the ph levels of vinegar, baking soda, crème of tartar, detergent, water, lemon juice, bleach and/or other household substances can be tested. This could be done using small amounts of each sample in a test tube or on a spotting tray and adding a drop or two of each indicator (using a fresh sample and clean glassware for each test). The results of these tests will enable you to determine the ph of each household substance to a fairly narrow range. The School For Excellence 2017 Succeeding in the Exams Chemistry Page 1

4 EXAMPLE RESULTS Household substance ph from ph probe Colour in universal indicator Lemon juice 5 Red Vinegar 4 Red Detergent 8 Blue Oven cleaner 10 Purple IDENTIFY DATA AND CHOOSE RESOURCES TO GATHER INFORMATION ABOUT THE COLOUR CHANGES OF A RANGE OF INDICATORS Chemical substances can be classified as acidic, basic or neutral. This classification is based on the concentration of hydrogen ions produced when the substance is dissolved in water. The concentration of hydrogen ions can be determined by change in colour of indicators. Examples of indicators include litmus, phenolphthalein, methyl orange and bromothymol blue. To see if the colours of the indicators match the colours in the chart above, some buffer solutions at particular ph values could be prepared and tested with each indicator. A well calibrated ph probe could also be used to match the colour change to ph. PERFORM A FIRST-HAND INVESTIGATION TO PREPARE AND TEST A NATURAL INDICATOR Natural indicators such as the above can be created by taking a red cabbage, crushing it and adding water to extract the dye. This dye solution is the natural indicator. It can then be used to test solutions known to be acidic, basic and neutral (e.g. HCl, NaOH and pure water). The colour of the natural indicator can be seen to change colours in these different ph solutions. Method to prepare a natural indicator: 1. Crush 50 g red cabbage with a mortar and pestle. 2. Place the crushed red cabbage in a 250 ml beaker with 100 ml deionised water. 3. Heat over a Bunsen for 15 min (do not let solution boil over) 4. Allow to cool. 5. Decant the aqueous, coloured solution into a clean beaker (this is the indicator). 6. Use 1 ml of the indicator for each solution to be tested. The School For Excellence 2017 Succeeding in the Exams Chemistry Page 2

5 IDENTIFY AND DESCRIBE SOME EVERYDAY USES OF INDICATORS INCLUDING THE TESTING OF SOIL ACIDITY/BASICITY Indicators provide a cheap and convenient way of determining the acidity or alkalinity of substances. Some everyday uses of indicators are: Testing the acidity or alkalinity of soils for plant health. Testing home swimming pools for safety. Testing home and commercial aquariums for fish and plant health. Monitoring wastes from photographic processing (which are often highly alkaline) to minimise the environmental impact. QUESTION 1 (HSC 2004:23) A natural indicator was made by heating yellow flowers with water. The resulting yellow solution was tested with a range of substances. The results are recorded below: Substance Colour 0.1 M hydrochloric acid Red 0.1 M ammonia Yellow Lemon juice Orange Oven cleaner Yellow Pure water Yellow Assess the usefulness of the yellow solution as an acid-base indicator. (3 marks) Solution The School For Excellence 2017 Succeeding in the Exams Chemistry Page 3

6 QUESTION 2 (HSC 2001:6-7) (a) The graph shows the colour ranges of the acid-base indicators methyl orange, bromothymol blue and phenolphthalein. A solution is yellow in methyl orange, blue in bromothymol blue and colourless in phenolphthalein. What is the ph range of the solution? A 4.5 to 6.0 B 6.0 to 7.5 C 7.5 to 8.5 D 8.5 to 10.0 (b) A group of students produced a red solution by boiling red cabbage leaves in water. When dilute sodium hydroxide was added to the solution, it turned purple. When dilute hydrochloric acid was added to the red solution, no colour change occurred. Which of these substances, when added, is most likely to cause the red solution to change colour? A B C D Cleaning solution containing ammonia Concentrated hydrochloric acid Orange juice Vinegar The School For Excellence 2017 Succeeding in the Exams Chemistry Page 4

7 QUESTION 3 (HSC 2006:23) Correct swimming pool maintenance requires regular monitoring of the ph level of the water. Select the best indicator from the graph to check that the ph of swimming pool water lies within the correct range of Justify your choice. (3 marks) Solution QUESTION 4 An unknown chemical was extracted from a soil sample and sent to your laboratory. One of the first tests you carried out was to determine the ph of the chemical by using indicators. The following are the results: Indicator Phenolphthalein Methyl orange Bromothymol blue Colour Colourless Yellow Blue How should your laboratory classify the soil sample? A B C D Strongly acidic Slightly acidic Neutral Slightly alkaline The School For Excellence 2017 Succeeding in the Exams Chemistry Page 5

8 SECTION 2 While we usually think of the air around us as neutral, the atmosphere naturally contains acidic oxides of carbon, nitrogen and sulphur. The concentrations of these acidic oxides have been increasing since the Industrial Revolution. IDENTIFY OXIDES OF NON-METALS WHICH ACT AS ACIDS AND DESCRIBE THE CONDITIONS UNDER WHICH THEY ACT AS ACIDS Oxides are a class of compound that often displays acidic or basic properties. ACIDIC OXIDES (NON-METAL OXIDES) Reacts with bases to form salts OR Reacts with water to form an acid (or does both) Are oxides of non-metals Common acidic oxides are carbon dioxide ( CO2 ) and diphosphorus pentoxide ( PO 2 5) because: CO + H O H CO 2( g) 2 ( l) 2 3( aq) (carbonic acid) OR CO + 2NaOH H O + 2Na + CO + 2 2( g ) ( aq) 2 ( l ) ( aq) 3( aq) Write an equation to represent the acidic nature of the following acidic oxides: SO 2 NO 2 The School For Excellence 2017 Succeeding in the Exams Chemistry Page 6

9 BASIC OXIDES (METAL OXIDES) Reacts with acids to form salts but Does not react with alkali solutions (such as NaOH or KOH ) Some react with water to form OH - Common basic oxides are magnesium oxide, copper oxide and iron (III) oxide ( Fe2O 3) because: MgO (s) + H 2 O (l) Mg(OH) 2(aq) CuO + H SO CuSO + H O ( s) 2 4( aq) 4( aq) 2 ( l) Fe O + 6HNO 2 Fe( NO ) + 3H O 2 3( s) 3( aq) 3 3( aq) 2 ( l) ANALYSE THE POSITION OF THESE NON-METALS IN THE PERIODIC TABLE AND OUTLINE THE RELATIONSHIP BETWEEN POSITION OF ELEMENTS IN THE PERIODIC TABLE AND ACIDITY/BASICITY OF OXIDES Metal oxides are basic. Metals are generally found on the left of the periodic table. Generally, the further left the metal the more basic the oxide. Some metals that are found close to non-metals on the periodic table can act as acidic or basic oxides (these are called amphoteric oxides). Non-metal oxides are acidic. Non-metals are found on the right side of the periodic table. The noble gases (group VIII) do not form oxides. The School For Excellence 2017 Succeeding in the Exams Chemistry Page 7

10 Amphoteric oxides are oxides of Zn, Al, Pb or Sn that can therefore react with acids or bases. They tend to be in the semi-metal region of the periodic table. For example: Aluminium oxide acting as a BASE: Al 2 O 3 + 6H + 2Al H 2 O Aluminium oxide acting as an ACID: Al 2 O 3 + 2OH - 2AlO H 2 O You will need to know an example equation of an amphoteric oxide acting as a base and an example equation of the amphoteric oxide acting as an acid for your exam. QUESTION 5 Identify the set that contains an acidic oxide, an amphoteric oxide and a basic oxide. A B C Al 2 O 3, CaO, CuO Na 2 O, ZnO, FeO Cl 2 O 7, BaO, ZnO D CO 2, SO 2, Al 2 O 3 Using Period 3 as an example we can see the change from basic to acidic oxide as you move from left to right: Strong Base Weak Base Na 2 O MgO Amphoteric Al 2 O 3 Very Weak Acid SiO 2 Moderate Acid P 4 O 10 Strong Acid SO 3 Strongest Acid Cl 2 O 7 QUESTION 6 (HSC 2001:5) The ph of unpolluted rainwater is about 6.0. Which substance contributes most to this? A CO 2 B N 2 C NO 2 D O 3 The School For Excellence 2017 Succeeding in the Exams Chemistry Page 8

11 QUESTION 7 Describe the trend in the acid base properties of the following oxides down Group (IV): Solution CO SiO 2 GeO 2 SnO PbO IDENTIFY NATURAL AND INDUSTRIAL SOURCES OF SULFUR DIOXIDE AND OXIDES OF NITROGEN Sources Sulfur dioxide (SO 2 ) Oxides of nitrogen NO x : Nitrous oxide (N 2 O), nitric oxide (NO), nitrogen dioxide (NO 2 ) Natural Geothermal hot springs, volcanoes Lightning (O 2(g) + N 2(g) 2NO (g) ) N 2 O formed by bacterial action on nitrogenous materials Industrial Processing or burning fossil fuels S compounds + O 2(g) SO 2(g) Extracting metals from sulfide ores (e.g. 2ZnS (g) + 3O 2(g) 3ZnO (s) + 2SO 2(g) ) Same in high temps of combustion chambers (cars, power stations) Further reaction of 2NO (g) + O 2(g) 2NO 2(g) The School For Excellence 2017 Succeeding in the Exams Chemistry Page 9

12 ANALYSE INFORMATION FROM SECONDARY SOURCES TO SUMMARISE THE INDUSTRIAL ORIGINS OF THE ABOVE GASES AND EVALUATE REASONS FOR CONCERN ABOUT THEIR RELEASE INTO THE ENVIRONMENT Since the Industrial Revolution there has been an increase in air pollution. This has led to a significant increase in the levels of acidic oxides (includes SO 2 and NO x ) released into the atmosphere. However, because SO 2 and NO 2 are washed out of the atmosphere by rain, there appears not to have been any significant build-up of their concentrations over the last century or so in the atmosphere (unlike CO 2 which is contributing to Global Climate Change). However, there are still reasons for concern about the release of these oxides. Direct health effects. Nitrogen monoxide, nitrogen dioxide and sulfur dioxide irritate the respiratory tract and cause breathing discomfort, exacerbating asthma and emphysema. Acid rain. Acid rain can lower the ph of soils and waterways, as well as destroy buildings and statues that have been made with materials containing calcium carbonate. This is primarily an issue with sulfur dioxide. Photochemical smog. The main problem with nitrogen dioxide is that it combines with other gases to produce toxic photochemical smog. Photochemical smog is a complex mixture of gases that can cause breathing difficulties and also can produce more toxic gases, such as ozone (O 3 ). Ozone is toxic to life, so is a significant pollutant in the lower atmosphere. DESCRIBE, USING EQUATIONS, EXAMPLES OF CHEMICAL REACTIONS WHICH RELEASE SULFUR DIOXIDE AND CHEMICAL REACTIONS WHICH RELEASE OXIDES OF NITROGEN CHEMICAL ORIGINS OF THESE ACIDIC OXIDES: The sulfur dioxide in the air comes from: 1. Burning of coal and oil to generate electricity and to fuel internal combustion engines. Small amounts of S are found in ALL fossil fuels. 2. The smelting of metals such as zinc, copper (this is an aspect often examined on, do not underestimate the significance of smelting as a source of air pollution, particularly as Australia is a big exporter of metals). The following equations will satisfy this dot point: Metal refining processes produce sulfur dioxide directly: 2ZnS (s) + 3O 2 (g) 2ZnO (s) + 2SO 2 (g) Sulfur in fossil fuels is converted into sulfur dioxide during combustion by the following equation: S (s) + O 2 (g) SO 2 (g) The School For Excellence 2017 Succeeding in the Exams Chemistry Page 10

13 The sulfur dioxide is then available to react with water to form acids: SO 2(g) + H 2 O (l) H 2 SO 3(aq) (H 2 SO 3 is sulfurous acid, do not confuse it with sulfuric acid) For the NO X gases it is the high temperatures of an internal combustion engine or lightning that cause N 2 and O 2 (both in high concentrations in the atmosphere) can react to form NO (g). There is then a secondary reaction of nitrogen oxide with oxygen: N 2 (g) + O 2 (g) 2NO (g) then 2NO (g) + O 2 (g) 2NO 2 (g) The nitrogen dioxide is then available to react with water form acids: 2NO 2(g) + H 2 O (l) HNO 3(aq) + HNO 2(aq) (nitric acid) (nitrous acid) EXPLAIN THE FORMATION AND EFFECTS OF ACID RAIN Acid rain occurs when SO 2 and NO x dissolve in rain making the rain acidic. Acid rain is rain with a higher H + concentration than normal, [H + ] > 10-5 moll -1. Note that regular rain unaffected by these pollutants is still slightly acidic. Regular rain contains some acidic carbonic acid (from CO 2 see equation below), but is not considered acid rain unless the [H + ] > 10-5 moll -1 CO 2(g) + H 2 O (l) H 2 CO 3(aq) Effects of acid rain include: 1. Defoliation of leafy plants, destroying crops and forests. 2. Lowering of soil ph, limiting plant growth and solubilising toxic ions (such as Al 3+ ) which can then move into the food chain. 3. Lowering the ph of water ways, preventing aquatic animals from reproducing and killing aquatic plants 4. Destruction of the human built environment, such as buildings with sandstone, limestone, marble and metals. Sandstone, limestone and marble contain calcium carbonate that reacts with acids in acid rain, e.g. CaCO 3(s) + 2H + (aq) Ca 2+ (aq) + CO 2(g) + H 2 O (l) Metals such as zinc can react with the acids in acid rain e.g. Zn (s) + 2H + (aq) Zn 2+ (aq) H 2(g) The School For Excellence 2017 Succeeding in the Exams Chemistry Page 11

14 ASSESS THE EVIDENCE THAT INDICATES INCREASES IN ATMOSPHERIC CONCENTRATION OF OXIDES OF SULFUR AND NITROGEN. The evidence which indicates increases in atmospheric concentrations of these gases have been gathered from measurements on bubbles of ancient air trapped in ice sheets of Antarctica and Greenland. This is then compared to more modern levels. This evidence successfully shows the increase in the atmospheric oxides of sulfur and nitrogen. The burning of coal and petroleum products as well as the smelting of minerals has led to a significant increase in the levels of acidic oxides in the atmosphere. But we must also remember that they are continually washed from the atmosphere by rain and thus their concentrations do not continue to build. It is thus the increased ph of soils and waterways that could be monitored and compared in such a way as described above. QUESTION 8 (HSC 2009:18) There has been an increase in the concentration of the oxides of nitrogen in the atmosphere as a result of combustion. Assess both the evidence to support this statement and the need to monitor these oxides. (5 marks) Solution The School For Excellence 2017 Succeeding in the Exams Chemistry Page 12

15 QUESTION 9 (HSC 2006:22) The atmosphere contains acidic oxides of sulfur which have been increasing in concentration since the Industrial Revolution. Discuss the evidence for this statement, and include relevant balanced chemical equations. (4 marks) Solution The School For Excellence 2017 Succeeding in the Exams Chemistry Page 13

16 QUESTION 10 (HSC 2002:21) Evaluate the impact of industrial sources of sulfur dioxide and nitrogen oxides on the environment, making use of appropriate chemical equations. (7 marks) Solution The School For Excellence 2017 Succeeding in the Exams Chemistry Page 14

17 CHEMICAL EQUILIBRIUM Not all reactions proceed to completion. In many reactions, the final reaction mixture consists of both products and reactants. These reactions are in chemical equilibrium, which means that the rate of the forward reaction equals the rate of the reverse reaction and therefore, the concentrations of reactants and products are constant. For equilibrium to be established the system must be closed. A closed system is one in which no energy or reactants/products can escape. When a system is at equilibrium there will be no macroscopic changes to the system i.e. no changes to pressure, temperature, concentration, colour. Note that equilibrium does not necessarily mean that the amount of products and reactants are equal. The system could contain mostly products and only a small amount of reactant, mostly reactant and only a small amount of product, or anywhere in between. This is often referred to as the equilibrium position. DEFINE LE CHATELIER S PRINCIPLE When a system at equilibrium is disturbed the equilibrium shifts to minimise the disturbance. This means that either the forward or reverse reaction proceeds at a faster rate until equilibrium is re-established. You will need to quote Le Chatelier s principle every time you explain a change in equilibrium, so memorise this phrase. IDENTIFY FACTORS WHICH CAN AFFECT THE EQUILIBRIUM IN A REVERSIBLE REACTION Some of these factors include: Changes in the concentration of a product or reactant. Changes in temperature. Changes in pressure (for reactions involving gases with different moles of gas on each side of the equation). Note: Addition of a catalyst does not constitute a disturbance to an equilibrium system. Catalysts speed up reactions by lowering the activation energy (remember Preliminary Chemistry!), but it will lower the activation energy of the forward AND reverse reaction. Therefore a catalyst will speed up each reaction, but will not shift the position of the equilibrium. Example: 2NO 2 (g) N 2 O 4 (g) ΔH < 0 Brown colourless The School For Excellence 2017 Succeeding in the Exams Chemistry Page 15

18 (a) How will the reaction respond when heated? The forward reaction is exothermic, so we can think of heat as a PRODUCT of this reaction. According to Le Chatelier s principle When a system at equilibrium is disturbed the equilibrium shifts to minimise the disturbance, so the system will want to reverse the disturbance. Adding more heat will favour the reverse reaction as the reverse reaction is endothermic, removing the excess heat. (b) How will the reaction respond when cooled? The forward reaction is exothermic, so we can think of heat as a PRODUCT of this reaction. According to Le Chatelier s principle When a system at equilibrium is disturbed the equilibrium shifts to minimise the disturbance, so the system will want to reverse the disturbance. Removing heat will favour the forward reaction as the forward reaction is exothermic, replacing the lost heat. (c) How will the reaction respond when pressure is increased? There are more gas moles on the reactants side of the equation. According to Le Chatelier s principle When a system at equilibrium is disturbed the equilibrium shifts to minimise the disturbance, so the system will want to reverse the disturbance. When we increase the pressure, the system will respond by favouring the forward reaction as it has less gas moles, therefore reducing the pressure. (d) How will the reaction respond when pressure is decreased? There are more gas moles on the reactants side of the equation. According to Le Chatelier s principle When a system at equilibrium is disturbed the equilibrium shifts to minimise the disturbance, so the system will want to reverse the disturbance. When we decrease the pressure, the system will respond by favouring the reverse reaction as it has more gas moles, therefore increasing the pressure. The School For Excellence 2017 Succeeding in the Exams Chemistry Page 16

19 QUESTION 11 (HSC 2008:7) Which of the following changes will always shift this equilibrium reaction to the right? 2HI(g) H 2 (g) + I 2 (g) ΔH = 52 kj A B C D Adding a catalyst Increasing the pressure Increasing the temperature Adding more of the reactant QUESTION 12 (HSC 2004:10) Phosgene is prepared from the reaction of carbon monoxide and chlorine in the presence of a catalyst: CO + Cl ( g) 2( g) COCl2( g ) Δ H = 9.93 kj mol 1 Which of the following sets of conditions would produce the highest yield of phosgene? A B C D High temperature, high pressure Low temperature, low pressure Low temperature, high pressure High temperature, low pressure QUESTION 13 (HSC 2001:10) The following equations describe some reactions in the formation of acid rain: SO 2 (g) + H 2 O(l) H + (aq) +HSO 3- (aq) 2H + (aq) + 2 HSO 3- (aq) + O 2 (g) 4H + (aq) + 2SO 4 2- (aq) What would occur if some solid sodium sulfate (Na 2 SO 4 ) were added to a sample of acid rain? A B C D The amount of SO 2 (g) would increase and the acidity of the solution would decrease. The amount of SO 2 (g) would increase and the acidity of the solution would increase. The amount of SO 2 (g) would be unchanged and the acidity of the solution would be unchanged. The amount of SO 2 (g) would be unchanged and the acidity of the solution would decrease. The School For Excellence 2017 Succeeding in the Exams Chemistry Page 17

20 QUESTION 14 (HSC 2009:23) The graph shows the variation in concentration of reactant and products as a function of time for the following system. COCl 2 (g) Cl 2 (g) + CO(g) ΔH = +108 kj Identify and explain each of the changes in conditions that have shaped the curves during the time the system was observed. (6 marks) Solution The School For Excellence 2017 Succeeding in the Exams Chemistry Page 18

21 DESCRIBE THE SOLUBILITY OF CARBON DIOXIDE IN WATER UNDER VARIOUS CONDITIONS AS AN EQUILIBRIUM PROCESS AND EXPLAIN IN TERMS OF LE CHATELIER S PRINCIPLE CO 2 dissolves in water in a series of steps, producing carbonic acid, which ionises as shown in the equation: CO 2 (g) + H 2 O (l) H 2 CO 3 (aq) H < 0 H 2 CO 3 + H 2 O (l) H 3 O + (aq) + HCO 3 - (aq) H < 0 HCO 3 - (aq) + H 2 O (l) H 3 O + (aq) + CO 3 2- (aq) H < 0 Disturbance Change in pressure Change in concentration of CO 2 in system Change in temperature Changing ph Effect on Equilibrium (Right Means Increased Solubility) Increased pressure favours reaction with fewer moles of gas in product. Increasing pressure, therefore shifts right, CO 2 dissolves more. Reduced pressure shifts equilibrium left to produce more CO 2. Increased concentration causes system to try and reduce concentration of CO 2 and thus it reacts, i.e., shifts equilibrium right, a decrease in concentration cause a shift to the left to try to balance out concentrations. Increased temp. favours endothermic reaction, i.e. back reaction, i.e. shifts left, and CO 2 is less soluble, therefore more CO 2 released. As the temperature decreases, the exothermic reaction is favoured; therefore more carbon dioxide is dissolved. Adding NaOH to equilibrium neutralises carbonic acid and the system moves to the right to compensate for this change. Adding H + causes the reaction to shift to the left. QUESTION 15 (HSC 2005:7) A series of observations was made about a first-hand investigation. A cup of carbonated soft drink was poured from a newly-opened, chilled bottle. A lot of bubbles were observed in the cup. The cup was then left on the kitchen bench for several hours. After this time, it was observed that there were no bubbles visible in the cup. Why were there no bubbles visible in the cup after several hours? A B C D The bubbles of carbon dioxide had dissolved into the soft drink. The carbon dioxide had been neutralised by the water in the soft drink. Carbon dioxide is less soluble at higher temperatures and lower pressures. Carbon dioxide is more soluble at higher temperatures and lower pressures. The School For Excellence 2017 Succeeding in the Exams Chemistry Page 19

22 QUESTION 16 The following equation describes the carbon dioxide equilibria in soda water. CO 2 (aq) + H 2 O(l) e H 2 CO 3 (aq) Describe how the solubility of carbon dioxide in water is affected by each of the following changes. (a) (b) (c) Addition of drops of sodium hydroxide solution. Increasing the partial pressure of carbon dioxide above the soda water. Adding nitrogen gas to raise the total gas pressure above the soda water. Solution The School For Excellence 2017 Succeeding in the Exams Chemistry Page 20

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24 QUESTION 1 SOLUTIONS The indicator is not useful in distinguishing between neutral and basic substances as it appears yellow in pure water (neutral) as well as ammonia and oven cleaner, which are both basic. QUESTION 2 (a) Answer is C the colours described only overlap between 7.5 and 8.5. (b) Answer is A changes red to purple in basic solution (NaOH). Ammonia is the only basic solution in the options given, all others are acidic. QUESTION 3 Identifies bromothymol blue as the best indicator. Provides a reason why bromothymol blue is the best indicator. Provides a reason why each of the other indicators are not suitable. QUESTION 4 QUESTION 5 Answer is D Answer is C QUESTION 6 Answer is A CO 2 is quite common in the atmosphere and reacts with water in the atmosphere to form carbonic acid. QUESTION 7 The oxides change from neutral to acidic to amphoteric for these oxides. CO is neutral and SiO 2 is acidic. The remaining oxides (GeO 2, SnO, PbO) are amphoteric. QUESTION 8 The evidence of increasing concentrations of NO and NO 2 in the atmosphere comes from direct measurement, knowledge of sources and measurement of effects. Thus there is clear factual evidence to support this statement. Monitoring is necessary due to detrimental environmental and health effects. Combustion of fossil fuels in motor vehicles and in power stations results in the formation of NO. N 2 (g) + O 2 (g) 2NO(g) In the lower atmosphere, in the presence of sunlight NO is oxidised to NO 2 2NO(g) + O 2 (g) 2NO 2 (g) The School For Excellence 2017 Succeeding in the Exams Chemistry Page 22

25 Direct measurements are made by statutory bodies, such as NSW EPA, and also by scientists researching the atmosphere. Nitrogen oxides were found to be part of pollution generated over large cities from the onset of the industrial revolution and the use of the internal combustion engine. This was essentially unrestricted till the 1950s, when concerns grew from loss of life due to smog. Some reduction of NO was achieved by the introduction of catalytic converters in motor vehicles where 2CO(g) + 2NO(g) N 2 (g) + 2CO 2 (g) This removes NO from the exhaust gas. However the increased number and use of motor vehicles has offset further major reductions. Measurement of effects of NO x highlights both issues of quantities produced and detrimental effects which necessitates their monitoring. One effect is production of photochemical smog. NO x is part of a complex set of chemical species which have several reactions in the presence of sunlight resulting in photochemical smog. Another effect is the direct toxicity of NO x to people, affecting especially the respiratory tract and eyes. There is a need to monitor NO x in cities in order to warn people most susceptible to acute effects. NO x are also contributors to acid rain: 2NO(g) + HO( ) HNO(aq) + HNO(aq) 2HNO 2 (g) + O 2 (g) 2HNO 3 (aq) The effects of acid rain are multiple: Lowering ph of natural waters affecting all biota in that water body. Acidic precipitation affecting vegetation by acid burning or defoliating plants Acidic runoff from soils leaching some nutrients, specifically Ca 2+, Mg 2+ Acidic leaching of toxic species, e.g. Al 3+ Acidic attack on manmade structures, e.g. marble or limestone buildings. QUESTION 9 Smelting and fuel combustion (e.g. coal) has increases since then to make more metals and energy available. This has caused more SO 2 going into the air making rain acidic. CuS + O 2 Cu + SO 2 SO 2 + H 2 O H 2 SO 3 We see more of an effect of acid rain now. Metal and stone buildings are deteriorating quicker and forests are dying where more factories are. The School For Excellence 2017 Succeeding in the Exams Chemistry Page 23

26 QUESTION 10 Demonstrates an extensive knowledge of the industrial production of nitrogen oxides and sulfur dioxide and their impact on the environment. Includes examples and chemical equations. Provides an evaluation of these impacts. QUESTION 11 Answer is D QUESTION 12 Answer is C With an increase in pressure the equilibrium will shift to the side with less moles of gas (right) to minimise the disturbance; increasing the yield of phosgene. The reaction is exothermic (heat is a product), so reducing the temp forces the equilibrium to the right to make more heat; increasing the yield of phosgene. QUESTION 13 Answer is C These reactions are not equilibrium reactions, so more sulfate does not change anything. Sodium sulfate is a neutral salt. QUESTION 14 Concentrations vary at 4 to 8 minutes, 10 to 12 minutes and minutes. At 4 to 8 minutes the system is moving to the right. There is no indication that Cl 2 or CO have been initially removed or that COCl 2 was initially added, hence the temperature has been changed. The reaction is endothermic to the right, so this means heat has been added (temperature has been raised). The sharp drop in [CO] at 10 minutes means that CO has been removed. The system then moves to the right, seen as an increase in [CO] and [Cl 2 ] and a decrease in [COCl 2 ]. At 14 min all concentrations abruptly decrease, all by a 2/3 factor hence the volume of the system has increased. As the right side is proportionally more affected than the left hand side, the system moves to the right to compensate, hence [CO], [Cl 2 ] increases and [COCl 2 ] decreases. QUESTION 15 Answer is C QUESTION 16 (a) The addition of hydroxide ions will neutralise the carbonic acid and hydrogen ions in the soda water. This will shift equilibrium to the right to counteract the change. As a result the solubility of carbon dioxide in water will increase. (b) (c) This will increase the carbon dioxide solubility as equilibrium will shift to the right to counteract the partial pressure increase. No change; the addition of nitrogen does not affect the partial pressure of carbon dioxide as these gases both fill the space above the liquid. Although the total gas pressure has increased, this has not happened due to volume reduction. The effective concentration of carbon dioxide above the water has not changed. The School For Excellence 2017 Succeeding in the Exams Chemistry Page 24