2012 olevel pure chemistry paper 2 questions and answers

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METHODOLOGY OF SCIENCEMETHODOLOGY OF STUDIESBlock 513 Bishan St 13 #01-514 (2nd Floor) Singapore 570513 | ROC No. : 53176143J Office Tel: 63540340REALISE YOUR MOSTCHEMISTRYO-LEVEL 2012QUESTIONS AND ANSWERS SECTION A (50 marks)1. The diagran below shows reactions involving dilute sulfuric acid. Six unknown substances A to F are shown. dilute sulfuric acid A Fsalt +B + waterCDEa blue solution forms a red solution formssalt + hydrogensalt + water(a)Using the letters A to

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METHODOLOGY OF SCIENCE

METHODOLOGY OF STUDIESBlock 513 Bishan St 13 #01-514 (2nd Floor) Singapore 570513 | ROC No. : 53176143J Office Tel: 63540340

REALISE YOUR MOST

CHEMISTRYO-LEVEL 2012QUESTIONS AND ANSWERS SECTION A (50 marks)1. The diagran below shows reactions involving dilute sulfuric acid. Six unknown substances A to F are shown. dilute sulfuric acid A F

salt +B + water

C

D

E

a blue solution forms a red solution forms

salt + hydrogen

salt + water

(a)

Using the letters A to F, (i) which substance is Universal Indicator? E (ii) which substance is carbon dioxide? B (iii) which substance is a metal? C (iv) [ which substance could be sodium hydrogencarbonate? A . ] [1] [1] [1] [1] [1]

(b)

F is an oxide of metal. Suggest a possible name of this substance. Copper(II) oxide [ ( ) ]

(c)

D can be used in titration with dilute sulfuric acid to produce the substances listed in the diagram above. What could be a possible name of D? [ Potassium hydroxide . ]

[1]

METHODOLOGY OF SCIENCE

METHODOLOGY OF STUDIESBlock 513 Bishan St 13 #01-514 (2nd Floor) Singapore 570513 | ROC No. : 53176143J Office Tel: 63540340

REALISE YOUR MOST2. The names and symbols of isotopes of some elements are given in the table below. name symbol Use the isotopes from the table to answer the questions below. (a) State the names of two isotopes that have the same number of protons in their atoms. CarboncarbonCarbon-12 and carbon-14 (b) State the names of two isotopes that have the same number of neutrons in their atoms. FluorineneonFluorine-19 and neon-20 (c) State the names of two isotopes that have 7 electrons in the valence shell of their atoms. [ (d) FluorineiodineFluorine-19 and iodine-131 ] [1] [1] [1] [1] fluorine19 carbon-12 iodine131 strontium90 neon-20 carbon-14 magnesium24

Which two isotopes form stable compounds that have ions of charge 2+? [ StrontiummagnesiumStrontium-90 and magnesium-24 ]

3.

Oxides of nitrogen in the stratosphere cause damage to the ozone layer and engines of aircraft are a source of oxides of nitrogen. (a) (i) Explain how oxides of nitrogen can be formed in the engine of aircraft. From internal combustion of nitrogen in air inside the engine under high temperature. temperature. (ii) Suggest a natural source of oxides of nitrogen in air. [ (b) During lightning activites ] [1] [2]

Nitrogen monoxide also damages the ozone layer through a two-step reaction. Step 1: Step 2: (i) + + + +2 [2]

State which element is oxidised in step 1 using its oxidation states. element : change in oxidation state : nitrogen / N from +2 to +4 +

METHODOLOGY OF SCIENCE

METHODOLOGY OF STUDIESBlock 513 Bishan St 13 #01-514 (2nd Floor) Singapore 570513 | ROC No. : 53176143J Office Tel: 63540340

REALISE YOUR MOST(b) (ii) Each nitrogen monoxide molecule can destroy thousands of ozone molecules. Using equations from both steps, explain this phenomenon. Each NO molecule in step 1 reacts to form one molecule of which in turn [2]

reacts to form back one molecule of NO. As such, each molecule of NO reacts with 2 molecules of more . and gets formed back into NO to further react continuously with

(c)

Oxides of nitrogen are removed in car exhaust by the usage of catalytic converters by reacting nitrogen monoxide with carbon monoxide. (i) Write a balanced equation for the reaction. + (ii) + [1] [1]

Gases that are harmful to the environment are still emitted from cars fitted with catalytic converters. Name one such gas and describe the problem it causes. Carbon dioxide. It is a greenhouse gas which causes global warming and the [ sea-levels, melting of polar caps which results to the rise in sea-levels, causing floods. / ]

4.

The diagram below shows the structures of ethanoic acid and ethanoyl chloride.

ethanoic acid (a)

ethanoyl chloride

Both compounds react with methanol to make the same ester. (i) Draw the structure of the ester. Ans: [1]

slanted] [Allow: -COO to be slanted] (ii) State the formula of the other produt formed when: 1. 2. methanol reacts with ethanoic acid: methanol reacts with ethanoyl chloride: [1]

[Does not accept compound names]

METHODOLOGY OF SCIENCE

METHODOLOGY OF STUDIESBlock 513 Bishan St 13 #01-514 (2nd Floor) Singapore 570513 | ROC No. : 53176143J Office Tel: 63540340

REALISE YOUR MOST(a) (iii) Give two uses of esters. [Avoid short unclear answers such as perfume and flavourings] (b) A homologous series contains a member known as ethanoyl chloride. (i) Give the name and structural formula of the next member of this homologous series. name : propanoyl chloride structural formula : [2] As food flavourings and in the manufacturing of perfumes. [1]

[Does not accept (ii)

] [1]

What is the general formula of the members of this homologous series?

(iii)

As the size of the molecules in the homologous series increases, the physical properties changes. Describe the trends in two properties as the sizes increase. As the molecular size increases, the melting and boiling points of the substances Increases. increases. Increases. As the molecular size increases, the density of the substances increases. Accept: viscosity - viscosity increases - less flammable [Do not accept any chemical properties] State your two properties clearly and avoid joining two points into one sentence.

[2]

5.

The diagram below shows a cooling pack that contains a small bag of solid ammonium nitrate surrounded by a bag of water. water

solid ammonium nitrate

When the pack is squeezed, the small bag will break open and result in a reaction between ammonium nitrate and water. This reaction is endothermic and the bag becomes very cold. (a) Given that the become cold. is positive, suggest why this endothermic change cause the water to [1]

During reaction, ammonium chloride takes in heat from the surrounding which results water, in the decrease of temperature of water, hence the bag becomes cold..

METHODOLOGY OF SCIENCE

METHODOLOGY OF STUDIESBlock 513 Bishan St 13 #01-514 (2nd Floor) Singapore 570513 | ROC No. : 53176143J Office Tel: 63540340

REALISE YOUR MOST(b) The reaction produces ammonium nitrate solution as shown by the equation below. ( )+( ) ( ) +2 / [3]

Complete the energy profile diagram using the axes provided, clearly indicating the reaction energy. enthalpy change and the activation energy enthalpy activation energy ( )+( ) reaction enthalpy change, + ( )

progress of reaction [1M1M[1M-correct shape with product written 1M-activation energy shown with one arrowhead in 1Mbold] the right direction 1M-enthalpy change including the word reaction as it appears in bold] (c) (i) Ammonium nitrate, which is made from ammonia, is also used as a fertiliser. Complete the table below. gas nitrogen hydrogen source liquefied air / air crude oil method of production fractional distillation cracking [2]

[Note spelling of liquefied does not accept fractional distillation for crude oil] (ii) Crops need the nitrogen content found in fertilisers such as ammonium nitrate for healthy growth. 50kg of ammonium nitrate was found in some fertilisers. Calculate the mass of nitrogen in this fertiliser. (1kg 1000g) Ans: Mr of 80 , Mass of nitrogen in ,

[3]

17,500g

[Accept answer as 17.5kg marks deducted if working did not indicate conversion of kg to g indicate clearly the formula] (c) (iii) Fertilisers that contain ammonium nitrate are slightly acidic, hence calcium carbonate are usually added to soil to reduce the acidity. However, calcium hydroxide should not be used. Explain why. base, Calcium hydroxide, which is a base, will react with ammonium nitrate and result in the reduction of the composition by mass of nitrogen in the form of ammonia [1Mthe 1M[1M-suggest reaction in the form of ammonium compound + base 1M-nitrogen content decrease due to ammonia produced and escaped] surrounding. released into the surrounding.

[2]

METHODOLOGY OF SCIENCE

METHODOLOGY OF STUDIESBlock 513 Bishan St 13 #01-514 (2nd Floor) Singapore 570513 | ROC No. : 53176143J Office Tel: 63540340

REALISE YOUR MOST6. The table below shows the trends of the melting points and boiling points of Group I and Group VII elements. element lithium Group I sodium potassium chlorine bromine iodine melting point/ 181 98 64 1 1 7 114 boiling point/ 1347 883 774 34 59 184

Group VII

(a)

(i)

The elements in Group I and Group VII have different trends in melting points and boiling points. Describe the trends down each group. I, The melting point and boiling point decreases down Group I, but the melting point and boiling point increases down Group VII. VII.

[2]

(ii)

The melting point and boiling point of sodium is higher than that of chlorine. Explain this phenomenon using concept from bonding. Sodium is a metal with metallic structure where positive metal ions and sea of delocalised' electrons are held by strong electrostatic forces and a huge amount of energy is required to overcome to change state. However, chlorine has simple molecular structure where molecules are held by weak intermolecular forces and only a small amount of energy is required to overcome to change state. Hence, sodium has a higher melting and boiling point than chlorine.

[3]

[To include structure/bonding(forces of attraction)/energy required; deduction of at least 1M for any missing component.] (b) The densities of chlorine and bromine at room temperature and pressure is provided. element chlorine bromine density (g/cm3) . 3 3.12

A student states that: The diffe