M.33/34

Pre-Leaving Certificate Examination, 2009

Chemistry

Marking Scheme

Ordinary Pg. 2

Higher Pg. 18

ªM.34MS5¬

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Dublin Examining Board

Pre-Leaving Certificate Examination, 2009

Chemistry

Ordinary Level Marking Scheme (400 marks)

Answer two questions from Section A and six questions from Section B. All questions carry equal marks (50). Notes: (i) In calculations, 3 marks are deducted for a mathematical error but no further penalty

is incurred if the problem, otherwise correct, is completed. (ii) Alternative valid answers are, of course, acceptable.

SECTION A (100 marks) Answer at least two questions from this section. 1. A group of students prepared a sample of ethanal (CH3CHO) was prepared in the laboratory by adding

an aqueous solution of ethanol (C2H5OH) and sodium dichromate (Na2Cr2O7.2H2O) to an almost boiling aqueous solution of sulfuric acid. The apparatus drawn below was used.

The equation for the reaction is:

3C2H5OH + Cr2O −27 + 8H+ → 3CH3CHO + 2Cr3+ + 7H2O

(a) Outline two safety precautions that should be observed when diluting the sulfuric acid in the flask. (8)

Any 2: (2 × 4m) – add slowly / – swirling / – cool in water (b) State the change in colour that was observed inside

the flask as the ethanol solution was added. Explain this change. (2 × 4m) (12)

Change – from orange – to green

Explanation (4m) – Cr2O −2

7 to Cr3+

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(c) Describe another change that could be seen inside the flask. Why did this change occur? (6)

Another change (3m) – it boiled

Why (3m) – the reaction is exothermic (d) What was added to the flask along with the sulfuric acid? Why was this necessary? (6)

Added (3m) – boiling chips / anti-bumping chips

Why (3m) – to heat evenly (e) Why was the receiving vessel cooled in ice-water? (6)

Any 1: (6m) – ethanal is volatile / – ethanal has a low boiling point (f) Why was it important not to have an excess of sodium dichromate in the mixture? (2 × 6m) (12)

– to prevent further oxidation (of ethanal) – to ethanoic acid

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2. A standard solution of sodium hydroxide (NaOH) was used to find the concentration of a hydrochloric acid (HCl) solution by titration. The pieces of equipment A, B and C shown in the diagram were used in the experiment.

(a) Name the pieces of equipment A, B and C. (3 × 3m) (9)

A – pipette B – conical flask C – burette (b) What is a standard solution? (6)

Any 1: (6m) – solution of exact known concentration / – concentration is known (c) Outline how to use A, which is clean and dry, to accurately deliver 25 cm3 of sodium hydroxide

solution into B. (2 × 3m) (6)

– fill above the mark and allow solution to run out until the bottom of the meniscus is on the mark at eye level – to empty touch tip against inside of flask (d) Describe how C, which is clean and dry, should be prepared before using it to measure the

volume of hydrochloric acid in the titration. (9)

Any 3: (3 × 3m) – clamp vertically / – fill space below tap / – read bottom of meniscus / – read at eye level / – remove funnel (e) Name a suitable indicator for this titration. What colour change was observed at the end-point? (11)

Indicator (3m) Colour change (2 × 4m) Any 1: − methyl orange – yellow → orange / – methyl red – orange → red / etc. ** Accept any other suitable indicator and corresponding colour change. (f) The balanced equation for the reaction is: (9)

HCl + NaOH → NaCl + H2O

When the hydrochloric acid (HCl) solution was titrated a number of times against 25 cm3 portions of the 0.1 M sodium hydroxide (NaOH) solution, the average titre was 22.4 cm3.

Calculate the concentration of the hydrochloric acid solution in moles per litre.

– 1

1.025 × (3m) = 1

4.22 M× (3m)

– M = 0.11 M (3m)

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3. Hydrogen peroxide decomposes rapidly in the presence of manganese dioxide into water and oxygen gas, according to the equation.

H2O2 → H2O + ½O2

In an experiment using this reaction, the oxygen gas was collected and its volume measured every minute until the reaction was complete. The data obtained is presented in the table.

Time / minutes 0 1 2 3 4 5 6 7 8 Volume of O2 / cm3 0 34 58 71 78 81 83 83 83

(a) What is the role of the manganese dioxide in this reaction? (3m) (3)

– catalyst (b) Draw a labelled diagram of the apparatus which could be used to carry out the reaction and to

measure the volume of gas produced. (6m + 3m + 3m) (12)

** Each item(s) must be shown and labelled for marks to be awarded. – reaction vessel – delivery tube – measuring device / graduated cylinder (upside down) (c) Outline how to begin the experiment. (2 × 3m) (6)

– catalyst in small test tube, add MnO2 and stopper – start clock (d) Plot a graph of the volume of oxygen collected against time (x-axis). (18)

Axes scaled and labelled (2 × 3m) Points plotted (6m) Curve drawn (6m) (e) What was the average rate of oxygen production over the first 4 minutes? (2 × 4m) (8)

– 19.5 – cm3 / min (f) Why did the rate of oxygen production decrease as time passed? (3m) (3)

– the concentration fell

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SECTION B (300 marks) 4. Answer eight of the following items (a), (b), (c), etc. Award 1 bonus mark to the first two fully correct answers. (a) Name the scientist who discovered radioactivity in 1896. (6m) (6)

– Becquerel (b) Calculate the percentage by mass of calcium in Ca(NO3)2. (6)

– 164

10040 × (3m)

– 17% (3m) (c) Which halogen is the strongest oxidising agent? (6m) (6)

– fluorine (d) Name the catalysts that are produced by living cells. (6m) (6)

– enzymes (e) Name the instrument that is used to measure the calorific value of foods. (6m) (6)

– bomb calorimeter (f) State Avogadro’s law. (2 × 3m) (6)

– equal volumes of gases have equal number of particles (molecules) – under the same temperature and pressure (g) Define reduction in terms of electron transfer. (6m) (6)

– gain of electrons (h) Define relative atomic mass (Ar). (2 × 3m) (6)

– mass of average atom – relative to one-twelfth of C12 (i) What does the term HPLC stand for? (6m) (6)

– high performance liquid chromatography

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(j) Name a hydrocarbon that is produced by the reaction of water and calcium carbide. (6m) (6)

– ethyne (k) Answer part A or part B. (6)

A Name the gas, present in air, that is used to flush out dangerous vapours from oil tankers. (6m)

– nitrogen or B Name the metal that is used to galvanise another metal. (6m)

– zinc

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5. (a) Answer the following questions:

(i) What are isotopes? (6m) (12)

– atoms of the same element with different mass numbers

How do the nuclei of isotopes differ from each other? (6m)

– the number of neutrons (ii) What term describes the time it takes for half of the radioactive nuclei in a sample

to decay? (6m) (6)

– half life (iii) Radioisotopes can emit three kinds of radiation. Which kind

of radiation is not affected by electric or magnetic fields? (6m) (6)

– gamma (iv) Name the instrument that uses a magnetic field to separate atoms (ions) of different

mass and is used to determine relative atomic mass. (6m) (6)

– mass spectrometer (b) The following scientists have made an important contribution to our knowledge of atomic structure,

the elements or radioactivity.

Curie Dalton Rutherford Thomson

Identify the scientist from the above list who:

(i) described atoms as small invisible particles. (5m) (5)

– Dalton (ii) discovered the electron as a negatively charged particle. (5m) (5)

– Thomson (iii) discovered the nucleus of the atom. (5m) (5)

– Rutherford (iv) was one of the discoverers of polonium and radium (5m) (5)

– Curie

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6. Crude oil is a mixture of hydrocarbons, which are separated into various fractions, and are used as fuels and for other purposes. (8)

(a) Name the process that is used in an oil refinery to separate the fractions in crude oil. (4m)

– fractionation / distillation

What property of the molecules causes them to be separated in this way? (4m)

– molecular mass (size) / boiling point (b) What is LPG? (6m) (12)

– liquid petroleum gas

Name one of its components.

Any 1: (6m) – propane / – butane (c) What is added to gas fuels to give them a strong odour by which leaks can be detected? (6m) (6)

– mercaptans (d) Name the fraction that is used as a fuel in jet engines and for home heating. (6m) (6)

– kerosene (e) Petrol requires substances that have a high octane rating. What is meant by octane rating? (18)

Any 1: (6m) – measure of tendency to cause or prevent knocking / – to resist auto-ignition

Apart from the addition of lead, name two processes that are used to improve the octane rating of petrol.

Any 2: (2 × 6m) – catalytic cracking / – isomerisation / – dehydrocyclisation

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7. (a) What is meant by an iconic bond? (6m) (6)

– the attraction of oppositely charged ions (b) Explain why a sodium atom has a neutral charge but a sodium ion has a

positive charge. (3 × 3m) (9)

– protons have a positive charge and electrons are negative – atom has equal number of protons and electrons – sodium ion has an extra proton (lost one electron) (c) Apart from its solubility in water, outline two properties of sodium chloride that indicate it is an

ionic compound. (2 × 6m) (12)

– high melting point – conducts electricity when molten or dissolved (d) Use a dot and cross diagram to show the bond formation in water. (3 × 3m) (9)

– pair of electrons shared with a hydrogen atom – oxygen with 6 valence electrons (4 non-bonding electrons) – 2 hydrogen atoms in diagram with oxygen (e) What is the shape of a water molecule? (6)

Any 1: (6m) – bent / – v-shaped (f) Describe what you would expect to see if a charged rod was brought close to the jet of

water flowing from the burette. What does this indicate about the nature of water as a solvent? (8)

What happens (4m) – deflected / attracted to rod Nature of water (4m) – polar

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8. Examine the reaction scheme and answer the questions that follow: CH3CHO ← C2H5OH → CH2CH2

A X B Y C (a) Which one of the compounds A, B or C does not have a planar carbon? (5m) (5)

– B (b) The apparatus on the right can be used in the

laboratory for reaction Y. Name substance Z. (6)

Any 1: (6m) – aluminium oxide / – Al2O3 (c) Classify (i) reaction X, (ii) reaction Y as an addition, elimination, redox or substitution

reaction. (2 × 6m) (12)

(i) – X – redox (ii) – Y - elimination (d) Name A and draw its structural formula. (12)

Named (6m) – ethanal

Structure (6m) –

(e) Name the polymer that is formed from substance C. (6m) (9)

– poly(ethene) / polythene

What is the main industrial source of C? (3m)

– (crude) oil (f) B is produced in the brewing industry. Name B and name the process by which it is

produced in a brewery. (6)

Named (3m) – ethanol

Process (3m)

– fermentation

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9. (a) Hardness in water is caused by dissolved calcium salts. Name a calcium compound that causes temporary hardness and show by equation how it changes when the water is boiled. (2 × 6m) (12)

– calcium hydrogencarbonate – Ca(HCO3)2 → CaCO3 + H2O + CO2 (b) A 500 cm3 sample of water was taken from a treatment plant for analysis in the laboratory. The

sample was filtered using a weighed filter paper. After filtering the filter paper was dried and reweighed. It was found that the mass of the filter paper had increased by 0.18 g. Calculate the total suspended solids in ppm (mg/l). (9)

– mass of suspended solids = 0.12 g / 500 cm3 = 0.18 × 2 g/l (3m) = 0.36 g/l = 0.36 × 1000 mg/l (3m) = 360 mg/l / 360 ppm (3m) (c) What is the purpose of fluoridation? (5)

Any 1: (5m) – to strengthen teeth / – to prevent tooth decay (d) Explain the difference between primary and secondary treatment of sewage. (12)

Primary (6m) – physical removal of solid matter (screening and settling)

Secondary (6m) – bacterial (biological) breakdown (e) What is meant by eutrophication? (6m) (12)

– enrichment of lakes or rivers with nutrients (plants, algae)

Name a substance that causes eutrophication.

Any 1: (6m) – nitrate / – phosphate

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10. Answer any two of the parts (a), (b), and (c). (2 × 25) (a) (i) What is an anion? (7m) (7)

– a negative ion (ii) Describe a test to show the presence of sulfate ions in solution. (2 × 6m) (12)

– add barium chloride / barium solution – white precipitate (iii) What colour is emitted when a sample of copper salt is heated in the flame of a

Bunsen burner? (6m) (6)

– green / blue green (b) A 50 gram sample of calcium carbonate was heated until it had all decomposed. The equation for

the reaction is CaCO3 (s) → CaO(s) + CO2 (g)

(i) What is a mole? (7)

Any 1: (7m) – (the amount that) contains Avogadro’s number of particles / – it contains as many particles as in 12 g of C12 (ii) How many moles are in 50 g of calcium carbonate? (2 × 3m) (6)

– mr of CaCO3 = 100 – moles = 50/100 = 0.5 (iii) What mass of calcium oxide was produced? (2 × 3m) (6)

– mr of CaO = 56 – mass = 56 × 0.5 = 28g (iv) What volume (at s.t.p.) of carbon dioxide was produced? (2 × 3m) (6)

– molar volume = 22.4 litres – volume = 22.4 × 0.5 = 11.2 L

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(c) The diagram shows the apparatus used to measure the heat of reaction between hydrochloric acid and sodium hydroxide.

(i) What is the significance of a plus sign for a heat of reaction, e.g., ∆H = +57 kJ mol–1? (6m) (6)

– an endothermic reaction (ii) Why was a cup made of polystyrene used? (6)

Any 1: (6m) – good insulator / – does not absorb much heat / – to reduce heat loss (iii) What term is used to describe the reaction of an acid and a base? (6m) (6)

– neutralisation (iv) The solution had a heat capacity of 420 J K–1 and an increase in temperature rise of 6 °C

was recorded. Calculate the amount of heat that was released in the reaction. (7m) (7)

– heat = 420 × 6 = 2520 J

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11. Answer any two of the parts (a), (b) and (c). (2 × 25) (a) (i) Define a base according to the Arrhenius theory. (6m) (6)

– it provides OH– ions in solution (ii) Stomach indigestion medicines usually contain a base. Explain how the medicine works. (4m) (4)

– they neutralise (stomach) acid (iii) Calculate the pH of a 0.2 M solution of sodium hydroxide. (3 × 3m) (9)

– p(OH) = -log (0.2) – = 0.7 – pH = 14 – 0.7 = 13.3 (iv) How many moles are in 25 cm3 of a 0.2 M solution of sodium hydroxide? (2 × 3m) (6)

– 0.2 × 25/100 – 0.005 (b) (i) State Le Chatelier’s principle. (4m + 3m) (7) – reactions at equilibrium – oppose applied stress (ii) What is meant by dynamic equilibrium? (2 × 3m) (6)

– forward and reverse reactions – at the same time (iii) Write the equilibrium expression (Kc) for the reaction: (6) 2SO2 (s) + O2 ↔ 2SO3 (6m)

– ]O[ ]SO[

]SO[

22

2

23

×

(iv) In the Haber process, ammonia gas is made according to the reaction: (6)

N2 (g) + 3H2 (g) ↔ 2NH3 (g)

What is the effect on the amount of ammonia in the equilibrium mixture if the pressure is increased? (6m)

– increased

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(c) Answer part A or part B. A

Acid rain is a problem that has arisen from air pollution in industrial areas due to the formation of compounds of nitrogen and sulfur.

(i) Define acid rain in terms of the pH scale. (7m) (7)

– pH is less than 5.5 (5.3 - 5.7) (ii) Give the formulas of two gases that cause acid rain. (6)

Any 2: (2 × 3m) – NO / – NO2 / – SO2 / – SO3 (iii) State two harmful effects of acid rain on the environment. (6)

Any 2: (2 × 3m) – kills plants / – kills fish / – corrodes limestone / – releases plant toxins / etc. (iv) Name a substance that is mixed with coal in a power station to reduce the release of

gases that cause acid rain. (6m) (6)

– limestone (lime)

or

B (i) What is an alloy? (3m) (3)

– a mixture of metals (ii) Name an element (other than iron) that is present in all kinds of steel. (4m) (7)

– carbon

Name an element that is added to make stainless steel. (3m)

– chromium

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(iii) The diagram on the right shows a blast furnace for the production of iron. Identify the three materials that are added at the top of the blast furnace. (3 × 3m) (9)

– coke (charcoal) – iron ore – limestone (iv) Steel is manufactured by the electric arc process.

Name (a) the gas, (b) the solid that are added to the molten iron to remove impurities. (2 × 3m) (6)

(a) – oxygen (b) – lime (CaO)

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Dublin Examining Board

Pre-Leaving Certificate Examination, 2009

Chemistry

Higher Level Marking Scheme (400 marks)

Answer two questions from Section A and six questions from Section B. All questions carry equal marks (50). Notes: (i) In calculations, 3 marks are deducted for a mathematical error but no further penalty

is incurred if the problem, otherwise correct, is completed. (ii) Alternative valid answers are, of course, acceptable.

SECTION A (100 marks) Answer at least two questions from this section. 1. A student carried out an experiment to determine the concentration of sodium hypochlorite (NaClO) in

household bleach. A 25.0 cm3 sample of the bleach was diluted in deionised water and made up to 250 cm3. A 25.0 cm3 portion of the diluted bleach was placed in a conical flask and an excess of acidified potassium iodide solution was added. This solution was then titrated against a 0.2 M sodium thiosulfate solution. The average titration volume was 20.2 cm3.

The equations for the reactions are:

ClO– + 2I– + 2H+ → Cl– + I2 + H2O 2S2O

−23 + I2 → S4O

−26 + 2I–

(a) Why is hydrochloric acid unsuitable for acidifying the potassium iodide solution? (5)

Any 1: (5m) – it reacts / – it can be oxidised / – it is a reducing agent (b) Outline the correct procedure for diluting the bleach to 250 cm3. (4 × 3m) (12)

– with pipette / burette – place in volumetric flask – add deionised water to the meniscus to the graduation mark – stopper and invert many times

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(c) Why was an excess of potassium iodide used? (3)

Any 1: (3m) – to react with all of the hypochlorite / – so bleach / hypochlorite is the limiting reactant (d) What was the colour of the solution when the potassium iodide was added? (6m) (6)

– reddish brown (e) Name the indicator for this experiment and describe the colour change at the

end point. (3 × 3m) (9)

– starch solution – blue-black – colourless (f) Calculate the concentration of sodium hypochlorite (NaClO) in (i) the diluted bleach, (ii) the

original bleach, in moles per litre. Express the concentration in the original bleach in terms of % (w/v). (15)

(i) – 1

25 M× = 2

2.02.20 ×

– M = 225

2.02.20×× (3m)

– M = 0.08 M (3m) (ii) – undiluted bleach = 0.08 × 10 = 0.8 M (3m) – % w/v: 0.8 × 74.5 g/l = 59.6 g/l (3m) = 5.96 – 6.02% (3m)

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2. A group of students prepared ethanoic acid (CH3COOH) in the laboratory by the reaction of ethanol (C2H5OH), sodium dichromate (Na2Cr2O7) and sulfuric acid. The reaction is described by the following equation:

3C2H5OH + 2Cr2O −27

- + 16H+ → 3CH3COOH + 4Cr3+ + 11H2O In this part of the experiment the students prepared the mixture of reactants (Diagram 1).

(a) What is the purpose of the condenser? (3m) (3)

– to cool the ethanol (b) Explain why a bath of ice-water was used. (3m) (3)

– the reaction is exothermic (c) There is an outlet between the tap funnel and the condenser.

What is the purpose of this outlet? (3)

Any 1: (3m) – for escape of gas / – to prevent apparatus coming apart / – to release pressure In the next part of the experiment the flask was heated for 20 minutes (Diagram 2).

(d) What is the purpose of this refluxing? (3m) (3)

– for completion of the reaction (e) What is the function of the condenser in this arrangement? (3m) (3)

– to return ethanol / ethanal to the flask for further reaction (f) Describe and explain the colour change in the flask. (3 × 3m) (9)

– from orange – to green – Cr2O −2

7 is reduced to Cr3+

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(g) Describe how the apparatus drawn in Diagram 2 must be changed to remove a pure sample of ethanoic acid. (3 × 3m) (9)

– position condenser – for distillation – insert thermometer (h) Calculate the maximum amount (in grams) of ethanoic acid that could be expected if 20 cm3

of ethanol (density 0.8 g cm–3) was used with an excess of sodium dichromate and sulfuric acid. (4 × 3m) (12)

– 20 cm3 = 20 × 0.8g = 16g – moles of ethanol = 16/46 = 0.35 moles – moles of ethanoic acid = 0.35 – mass of ethanoic acid = 0.35 × 60 = 21g (i) Name the industrial product that is formed by the reaction of ethanoic acid and wood. (5m) (5)

– cellulose acetate

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3. To investigate the effect of temperature on the rate of a chemical reaction, a group of students measured 100 cm3 of a sodium thiosulfate solution into a conical flask and placed the flask on top of a cross drawn on a sheet of white paper. They placed a thermometer in the solution. They added 10 cm3 of a hydrochloric acid solution, swirled the mixture and started a stop clock. The experiment was repeated a number of times varying the temperature. At each temperature they calculated the reaction rate and obtained the results given in the table below.

Temperature (°C) Reaction rate (sec–1) 20 0.005 30 0.008 40 0.020 50 0.035 60 0.080

(a) How could the students change the temperature of the reaction? (5m) (5)

– heating the solutions on a hot plate / over a Bunsen burner (b) Describe how the students measured the rate of reaction. (4 × 3m) (12)

– measured the time – for the mark to disappear – under the sulphur – calculated the inverse of the time (c) Name one factor that had to be kept constant each time they measured the reaction rate.

How could they make sure this factor was kept constant? (2 × 3m) (6)

– the concentration of the reactants – same volumes of same solutions (d) Plot a graph to show the relationship between the temperature and the reaction rate. (18)

Graph axes correctly labelled and scaled (2 × 3m) Points correctly positioned (3m) Graph drawn (3m)

State two reasons why temperature has this relationship to the rate of a reaction. (2 × 3m)

– (rate increases) due to increased rate of collisions – more effective collisions (more molecules with activation energy) (e) Use your graph to (9) (i) measure the reaction rate at 45 °C, (3m)

– find rate on graph [0.027; allow 0.026 – 0.028]

(ii) calculate how long the reaction took at that temperature. (2 × 3m)

– time = 1/rate – time = 37s (allow 35s – 39s)

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SECTION B (300 marks) 4. Answer eight of the following items (a), (b), (c), etc. Award 1 bonus mark to the first two fully correct answers. (a) Name the scientist who put forward the theory that elements with similar properties occurred in

groups of three and give an example. (6)

– Dobereiner (3m)

Any 1: (3m) – lithium / sodium / potassium / – chlorine / bromine / iodine / – sulfur / selenium / tellurium (b) What are isotopes? (2 × 3m) (6)

– atoms of the same element – with different mass number (different numbers of neutrons) (c) Which metal and what type of radiation was used by Rutherford when he discovered the

nucleus? (2 × 3m) (6)

– gold – alpha particles (d) Name a change of state that is exothermic. (6)

Any 1: (6m) – freezing / – condensing

(e) What is the oxidation number of Mn in the manganate ion (MnO−4 )? (6m) (6)

– +7 (f) How many atoms are present in 1 kg of carbon dioxide? (6m) (6)

– 4.09 × 1025 (g) Calculate the percentage phosphorus, by mass, in ammonium phosphate (NH4)3PO4. (6m) (6)

– 20.8% (h) A 100 cm3 sample of water was found to contain 0.05g of suspended solids. Calculate the

concentration of suspended solids in the sample in parts per million. (6m) (6)

– 500 ppm

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(i) Name an oxygenate suitable for adding to petrol. (6)

Any 1: (6m) – methanol / – ethanol / – MTBE (j) Name and draw the structural formula of the compound C8H10. (2 × 3m) (6)

– ethylbenzene

– (k) Answer part A or part B. (6)

A Name (i) a gas that is produced in power stations which causes acid rain and (ii) a substance that is used to scrub it from the waste gases. (2 × 3m)

(i) – NO2 / SO2 (ii) – limestone or B Name the materials used as (i) anode and (ii) cathode in a Downs cell.

(i) Anode (3m) – graphite

(ii) Cathode (3m) – steel

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5. (a) (i) Define atomic orbital. (6m) (6)

– a region where there is a high probability of finding an electron (ii) How many orbitals are in a d energy sublevel? (3m) (3)

– 5 (iii) Write the electron configuration (s, p, etc.) of the copper atom. (6m) (6)

– 1s2 2s2 2p6 3s2 3p6 4s1 3d10 (iv) What is the chemical formula for copper I oxide? (3m) (3)

– Cu2O (b) The first ionisation energies of the elements lithium to carbon are given in the table below.

Element Li Be B C First ionisation energy / kJ/mol 519 900 799 1090

(i) Explain why Be has a higher first ionisation energy than lithium. (2 × 3m) (6)

– Be has a greater nuclear charge / smaller radius – greater attraction for electron (ii) Explain why the first ionisation energy of boron is less than that of beryllium. (2 × 3m) (6)

– outermost electron of boron is in an unstable 2p orbital – outermost electron of beryllium is in a stable 2s orbital (iii) Write an equation to represent the first ionisation energy of carbon. (6m) (6)

Any 1: (6m) – C(g) → C+

(g) + e– /

– C(g) – e– → C+(g)

(iv) Carbon has three main isotopes, carbon-12, carbon-13 and carbon-14. Suggest why there

is very little difference between the first ionisation energies of these isotopes. (6m) (6)

– all have the same number of protons so nuclear attraction is the same (c) Name the reagents that are used to identify phosphate in an aqueous solution. (2 × 4m) (8)

– ammonium molybdate – concentrated nitric acid

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6. (a) C3H6O2 is the molecular formula for two esters. (i) Name one of these esters and draw its structural formula. (12)

Named Any 1: (6m) – ethylmethanoate / – methylethanoate

Structural formula Any 1: (6m) – – (ii) Identify one planar carbon atom in the molecule you have drawn. (3)

** Correct identification of a planar carbon. (3m) (iii) Esterification involves the reaction of an alcohol and an acid. What type of reaction

is this? (3)

Any 1: (3m) – condensation / – substitution (iv) State one common use for esters. (3)

Any 1: (3m) – solvents for paints / – perfumes / – drugs / – flavourings / – soap making / – clothing / – energy storage / etc.

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(b) (i) Name the compound represented by the molecular formula CH3CH2COOH and draw its structural formula. (12)

Named (6m) – propanoic acid

Structural formula (6m) – (ii) State one use for this acid and its salts. (3m) (3)

– preserving food

(iii) Explain why this molecule behaves as an acid when it is dissolved in water. (2 × 3m) (6)

– negative ion (anion) can become stabilised – by delocalising the charge over both O atoms (c) Draw the structural formula for chloroethane. (4m) (8) – What kind of reaction is involved in its formation from ethane? (4m)

– substitution

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7. (a) Define reduction in terms of electron transfer. (3m) (15)

– reduction is a gain of electrons

A redox reaction occurs when scrap iron is added to copper sulfate solution. Describe, using equations, what happens in this reaction. (2 × 6m)

– Cu2+ + 2e - → Cu – Fe → Fe2+ + 2e -

(b) What is the oxidation number of (i) chromium in CrO −24 and (ii) oxygen in H2O2? (2 × 3m) (18)

(i) – Cr: + 6 (ii) – O: –1

Using oxidation numbers balance the following equation: (3 × 4m)

Fe2+ + MnO−4 + H+ → Fe3+ + Mn2+ + H2O

– (2) (7) (3) (2) – 5Fe2+ + MnO−

4 + H+ → 5Fe3+ + Mn2+ + H2O – 5Fe2+ + MnO−

4 + 8H+ → Fe3+ + Mn2+ + 4H2O (c) A student conducted an experiment to investigate the electrolysis of a potassium iodide

solution using platinum electrodes. A few drops of phenolphthalein were added to the solution. Phenolphthalein is an indicator that is pink in alkaline solution and colourless in acidic solution.

(i) Describe what you would observe at the anode and at the cathode. (9)

– cathode: pink colour / gas is produced (2 × 3m) – anode: brown colour (3m)

(ii) Explain what happened at each electrode with an equation. (2 × 4m) (8)

– cathode 2H2O + 2e– → H2 + 2OH– – anode 2I– – 2e– → I2

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8. (a) Define heat of combustion of a substance. (2 × 4m) (8)

– heat change when 1 mole of a substance – completely burned (in excess of oxygen) (b) The combustion of ethyne may be described by the following equation: (12)

C2H2 + 2½ O2 → 2CO2 + H2O

Given that the heats of formation of water, carbon dioxide and ethyne are –286, –394 and 227 kJ mol–1, respectively, calculate the heat of combustion of ethyne. (4 × 3m)

– heat of combustion = heat of formation of products – heat of formation of the reactants – 2(–394) + (–286) –227 – –788 – 286 –227 – –1301 kJ mol–1 (c) What is the common name for the ethyne-containing mixture that is burned for high

temperature welding and cutting? (6m) (6)

– oxyacetylene (d) One of the fractions produced from crude oil is refinery gas. Name the components that are

liquefied as LPG. (2 × 3m) (9)

– propane – butane

What use is made of the other components? (3m)

– to heat (boil) the crude oil (e) What property of petrol is measured by its octane rating? (3m) (15)

– measure of its tendency to cause knocking (or to resist autoignition)

Name a hydrocarbon with an octane rating of 0. (3m)

– heptane

Describe three properties that give molecules a high octane rating. (3 × 3m)

– branched – short chain – aromatic or cyclic

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9. (a) Describe what happens in a reversible reaction at equilibrium. (2 × 3m) (12)

– forward and reverse reactions proceed – at the same rate

What is the effect of adding a catalyst to a reversible reaction? (2 × 3m)

– catalyst causes equilibrium to be achieved faster – but no change on the equilibrium mixture (b) State Le Chatelier’s principle. (2 × 3m) (6)

– when a system at equilibrium is subjected to a stress – the system alters to oppose the stress (c) Potassium dichromate (K2Cr2O7.2H2O) was dissolved in water and allowed to

reach equilibrium. (15) (i) Write an equation for the reaction. (3m)

– Cr2O −27 + H2O ↔ 2CrO −2

4 2H+

(ii) Suggest how to adjust this equilibrium mixture to change the orange colour to yellow. (2 × 3m)

– add NaOH (or other alkali) – the OH- ions react with the H+ ions to form water

(iii) Explain clearly why the colour changed. (2 × 3m)

– system alters to produce more H+ – system moves to the right (or more CrO −2

4 is produced) (d) When 0.5 mole of ethanoic acid (CH3COOH) and 0.5 mole of ethanol (C2H5OH) were mixed

together in a vessel and allowed to reach equilibrium it was found that the mixture contained 0.16 mole of ethanoic acid. Calculate the value of the equilibrium constant (Kc) (17)

CH3COOH + C2H5OH → CH3COOC2H5 + H2O Initially 0.5 0.5 0 0 (3m) At equilibrium 0.16 0.16 0.34 0.34 (4m) (0.5-0.16) (0.5-0.16)

Vol. at equilibrium V16.0

V16.0 →

V34.0

V34.0

– Kc = ]OHHC[]COOHCH[

]OH[]HCOOCCH[

523

2523 (3m)

– = 16.016.0

)34.034.0(××

(4m)

– = 4.5 (3m)

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10. Answer any two of the parts (a), (b) and (c). (2 × 25) (a) (i) What is the purpose of chromatography? (3m) (16)

– to separate components of mixtures

Outline the principle by which chromatography works. (3 × 3m)

– there is a stationary phase – mobile phase moves forward – separation based on relative absorbance / affinities

Why is GC unsuitable for certain vitamins or other substances? (4m)

– GC not suitable if substance not volatile / or unstable when heated (ii) What is the main use for IR spectrometry? State two properties that are measured when

an IR spectrometer is used by an analytical chemist. (9)

Main use Any 1: (3m) – used to identify a substance / – to identify nature of bonds / – to determine the concentration of substance in a sample

Properties (2 × 3m) – absorbance (amount absorbed) – for spectrum (spread) of wavelength / frequency (b) (i) State Charles’s law. (2 × 3m) (10)

– volume of a fixed mass of gas – directly proportional to Kelvin temperature

Draw a simple graph to illustrate Charles’s law.

– Correct axes (2 × 1m) – Straight line through origin (2m) (ii) Name two assumptions in the kinetic theory of gases that explain the behaviour of

an ideal gas. (6)

Any 2: (2 × 3m) – volume of molecules insignificant to volume they occupy / – insignificant intermolecular forces / – collisions perfectly elastic

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(iii) Use the equation of state for an ideal gas to calculate the number of moles in 500 cm3 of a gas that is measured at 20 °C and at an atmospheric pressure of 99,000 Pa. (9)

– n = RTPV (3m)

– = 000,000,12938.3

50099000××

× (3m)

– = 0.02 moles (3m) (c) (i) What is a strong base? (4m) (10)

– a good proton acceptor

Identify one base and its conjugate acid in the following system: H2O + NH3 NH+

4 + OH–

Any 1 (2 × 3m) – base: NH3 – conjugate acid: NH+

4 /

– base: OH– – conjugate acid: H2O (ii) The dissociation constant for methanoic acid is 1.8 × 10–4. Calculate the pH of a 0.02 M

solution of methanoic acid. (3 × 3m) (9)

– pH = log [square root Ka × concentration of acid] – = -log [square root of 1.8 × 10–4 × 0.02] – = 3.57 (iii) Sketch the titration curve you would expect when a strong acid is titrated against a

strong base. (6m) (6)

–

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11. Answer any two of the parts (a), (b) and (c). (2 × 25) (a) Ethene is a reactive hydrocarbon. (i) Name the type of mechanism by which ethane reacts with bromine. (3m) (3)

– ionic addition (ii) Name the product of this reaction. (6m) (6)

– 1,2-dibromoethane (iii) What visual change provides evidence for the reaction? (4m) (4)

– decolourisation / orange to colourless (iv) As part of the mechanism an intermediate species with a positive carbon atom is formed.

Draw this ion. (6m) (6)

– (v) Evidence for the formation of this ion is found if the reaction takes place in a solution

of sodium chloride which results in the formation of two new products. Name these products. (2 × 3m) (6)

– 2-bromoethanol – 1-bromo-2chloroethane (b) Water pollution can occur through the uncontrolled release of industrial, domestic and

agricultural wastes to rivers and lakes. (i) Explain how an excess of nitrate can lead to water pollution. (2 × 3m) (6)

– eutrophication (excess plant growth) – O2 depletion (ii) Describe how you would test for the presence of nitrates in a water sample. (3 × 3m) (9)

– fresh iron II sulfate – concentrated sulfuric acid – brown ring (iii) Name the process by which the tertiary treatment of sewage removes nitrates. (3m) (3)

– precipitation

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(iv) Disposal of batteries in a landfill can produce waste that contains heavy metals. Identify one heavy metal. (7)

Any 1: (3m) – lead / – cadmium / – mercury / etc.

Name an instrument that can measure the concentrations of heavy metals in wastes. (4m)

– AAS (atomic absorption spectrometer) (c) Answer part A or part B. A The ozone layer is an important part of the stratosphere. (i) Show, by equation, the formation of ozone in the stratosphere and state one benefit

of the ozone layer. (6)

Equation (3m) – O2 + O* → O3

Benefit (3m) – protection from UV radiation (ii) (15) What are CFCs? (3m)

– chlorofluorocarbons

Show by equations how CFCs damage the ozone layer. (3 × 3m)

– Cl* produced – Cl* + O3 → ClO* + O2 – chain reaction (ClO* + O* → Cl* + O2)

What other environmental damage have CFCs caused to the atmosphere? (3m)

– greenhouse effect (iii) Name a group of substances that have been used to replace CFCs. (4m) (4)

– HFCs / HCFCs or

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B Polymers are important synthetic materials that have many uses in modern life. (i) Define addition polymer. (4m + 3m) (7)

– monomers joined – to form a large molecule (ii) Show by equation how poly(ethene) is formed. (2 × 3m) (6)

– nCH2CH2 – → [CH2CH2]n– (iii) (12) There are two forms of poly(ethene). Name these forms and outline the structural

difference between the forms. (3 × 3m)

– high density (HDPE) – low density (LDPE) – high density molecules have fewer branches

Name the chemist who discovered the catalyst for the process. (3m)

– Ziegler

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