hkep supplementary notes

8/18/2019 HKEP Supplementary Notes

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New 21st Century Chemistry

Supplementary Notes 1 © Jing Kung Educational Press. All rights reserved.

Supplementary Notes

Topic 2 Microscopic World I

Unit 6 The periodic table

6.7 Group 0 elements — noble gases

Density of gas

Density is the mass of an object divided by its volume.

density of gas =

The following diagram shows two containers of equal volumes containing helium and carbon

dioxide gases at the same temperature and pressure. As equal volumes of gases contain equal

numbers of molecules (refer to Avogadro’s Law), the mass of gas in the second container is higher

because the relative molecular mass of carbon dioxide is higher. Thus, the density of carbon dioxide

is higher than that of helium.

helium (He) carbon dioxide (CO2)

Relative molecular mass 4.0 44.0

Density (g cm –1

) 0.00018 0.001977

Suppose a helium gas sample contains 1% of carbon dioxide gas, the mass and density of this

impure helium sample will be higher than those of pure helium.

mass

volume

Supplement to Section 6.7 (P 46)

Refer to HKDSE Practice

Paper 2012 Paper 1A Q7


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John Rayleigh and William Ramsay — the noble gases discovered

By the 1770s, chemists thought that the main components of the

atmosphere had been well identified. But in 1892, John Rayleigh

found that nitrogen isolated from the air had a density slightly

higher than that of nitrogen prepared from nitrogen compounds.One dm

3 of pure nitrogen gas generated from a chemical reaction

weighed 1.2505 g. On the other hand, one dm3 of nitrogen gas

generated from air by removing oxygen, carbon dioxide and

water vapour weighed 1.2572 g (at the same temperature and

pressure). A possible explanation was the presence of an

unknown gas denser than nitrogen in the atmosphere.

Both William Ramsay and John Rayleigh tried to isolate

this unknown gas using different methods. Each found evidence

of the presence of an unknown gas in the atmosphere. This gas

was a new element. Rayleigh and Ramsay announced their discovery in 1894, claiming that they

had found a new element which did not fit into any group of the periodic table, and named it argon

(from the Greek word argos, meaning ‘inactive’). Ramsey went on to discover helium, neon,

krypton and xenon.

John Strutt Lord Rayleigh

(1842 – 1919)


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Topic 2 Microscopic World I

Unit 6 The periodic table

6.7 Group 0 elements — noble gases

Using argon to preserve wine in an opened bottle

Argon can be used to replace oxygen which causes food to oxidize, the chemical reaction that turns

crisps stale and apples brown. Oxygen in the air can oxidize ethanol in wine to form ethanol or

ethanoic acid, giving the wine a sour taste. Argon is denser than air. Pumping argon into an opened

bottle of wine displaces air from the bottle, thus preventing the wine from contact with air.

Traditionally nitrogen has been used in a similar manner for food preservation. However,argon is denser and fills spaces around the food more completely, making it a more efficient

preservative agent.

The first noble gas helium is less dense than air and cannot displace air from the bottle. Thus,

it is not used for preserving wine in an opened bottle.

The vacuum pump method is another wine preservation method. This is to pump air out of the

opened bottle of wine and stop the bottle. A vacuum is created inside the bottle. However, pumping

air out may also remove volatile organic compounds that give the wine a pleasant odour.


Refer to HKDSE Practice Paper

2012 Paper 1B Q2(a) & (c)


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When three identical steel balls are dropped through the three alcohols, the ball in

propane-1,2,3-triol takes the longest time to reach the bottom because this alcohol is the most

viscous.


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Topic 9 Chemical Reactions and Energy

Unit 34 Energy changes in chemical reactions

34.7 Enthalpy change of an exothermic reaction

Self-heating food / beverage containers

Today, many people wish to pursue outdoor activities in environments where modern conveniences

such as stoves and microwave ovens are not readily available. The food and beverage industry has

developed self-heating food / beverage containers so that people can enjoy a hot meal or beverage

while engaging in such pursuits.

A self-heating container generally includes a reaction chamber and a food / beverage chamber.The reaction chamber contains reactants separated by a breakable membrane. Breaking of the

membrane allows contact between the reactants which react exothermically. The reaction generates

a sufficient amount of heat to warm the food / beverage.

The diagram below shows the design of a can of self-heating coffee.

When the bottom of the can is pushed, the rod causes the membrane to break, allowing water

to mix with the calcium oxide. The following reaction occurs.

CaO(s) + H2O(l) Ca(OH)2(s)

The reaction between calcium oxide and water is used because it generates a substantial

amount of heat. Calcium oxide is cheap and readily available. An alternative is to dissolve

anhydrous calcium chloride in water, which has the advantage of producing no reaction products,

but generates a less amount of heat.


Refer to HKDSE Practice

Paper 2012 Paper 1B Q7(b)


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The reaction chamber of the can is made of polypropene. Polypropene is a poor conductor of

heat. Using it to hold the calcium oxide can prevent the fingers from being burnt. It can also

withstand the high temperature caused by the reaction between calcium oxide and water.

The beverage chamber of the can is made of aluminium. Aluminium is used because it is

non-toxic and will not poison the coffee. It is a good conductor of heat. The heat generated from the

reaction between calcium oxide and water can be transferred to the coffee readily. Aluminium is

also covered by a layer of aluminium oxide, which prevents the metal from corrosion.


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Topic 10 Rate of Reaction

Unit 37 Factors affecting the rate of a reaction

37.7 Applications of catalysts

Autocatalysis

A chemical reaction is said to have undergone autocatalysis if one of the products acts as a catalyst

for the reaction itself. From industrial production point of view, autocatalytic reactions play an

important role. The rate of reaction can be maximized by making sure that the optimum

concentrations of reactants and products are always present.

Consider the reaction between permanganate ions and oxalate ions under acidic conditions:

2MnO4–(aq) + 5C2O4

2–(aq)

+ 16H

+(aq) 2Mn

2+(aq) + 10CO2(g) + 8H2O(l)

The following graph shows how the concentration of permanganate ions in the reaction

mixture varies with time in an experiment conducted to study the kinetics of the reaction.

The concentration of MnO4–(aq) ions changes slowly at the beginning of the reaction,

indicating the reaction is quite slow at this stage. Both MnO4–

(aq) and C2O42–

(aq) ions arenegatively charged, so they are unlikely to make fruitful collision with each other.

However, the concentration of MnO4–(aq) ions decreases rapidly after the initial stage, i.e. the

reaction proceeds rapidly. It is likely to be due to the building up of the concentration of a product

which catalyzes the reaction. The reaction is slow at the beginning because of the low concentration

of the product.

When MnO4–(aq) ions are almost used up, the reaction slows down.



2012 Paper 1B Q10(b)(ii)


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We can use the following experiment to show that Mn2+

(aq) ion is a catalyst for the reaction.

1. Add MnO4–(aq) ions to beakers

containing C2O42–

(aq) ions.

2. Add a little manganese(II)

sulphate to beaker 2.

3. After 2 minutes, the contents of

beaker 2 become colourless

while that of beaker 1 remains

purple. Manganese(II) sulpahte

catalyzes the reduction of

MnO4–(aq) ions to colourless

Mn2+

(aq) ions.

4. The contents of beaker 1

become colourless also after 6

minutes. The rate of the reaction

in beaker 1 increases as the

concentration of Mn2+

(aq) ions

builds up.


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Topic 11 Chemical Equilibrium

Unit 40 Factors affecting chemical equilibrium systems

40.4 The effect of concentration changes on chemical equilibrium system

Adding aqueous ammonia to copper(II) sulphate solution until excess

When aqueous ammonia is added to copper(II) sulphate solution, a blue precipitate (copper(II)

hydroxide) is produced initially. With the addition of excess aqueous ammonia, a deep blue solution

results due to formation of complex ions. The following equilibrium is established.

Cu2+

(aq) + 4NH3(aq) Cu(NH3)42+

(aq)

Suppose the above equilibrium mixture contains 2 x 10–16 mol dm–3 of Cu2+(aq) ions,

3.20 mol dm–3

of NH3(aq) and 0.200 mol dm–3

of Cu(NH3)42+

(aq) ions. We can calculate the value

of K c for this reaction in the following way:

K c =

=

= 9.54 x 1012 dm12 mol–4

Effect of adding dilute sulphuric acid to the system

Addition of dilute sulphuric acid to the system introduces H+(aq) ions, which react with NH3(aq) to

form NH4+(aq) ions. Decreasing the concentration of NH3(aq) causes the equilibrium position of the

above system to shift to the left, producing Cu2+

(aq) ions in the process.

Ammonia also reacts with water to form the NH4+

(aq) and OH

–

(aq) ions.

NH3(aq) + H2O(l) NH4+(aq) + OH

–(aq)

The Cu2+

(aq) ions react with the OH–(aq) ions to form a pale blue copper(II) hydroxide

precipitate.

Cu2+

(aq) + 2OH–(aq) Cu(OH)2(s)

0.200 mol dm–3

(2 x 10–16 mol dm–3)(3.20 mol dm–3) 4

[Cu(NH3)42+(aq)]

[Cu2+(aq)][NH3(aq)]4

Supplement to Section 40.4 (P 49)Refer to HKDSE Practice

Paper 2012 Paper 1B Q13(c)


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When excess dilute sulphuric acid is added, the acid reacts with the copper(II) hydroxide to

give a deep blue solution.

Cu(OH)2(s) + H2SO4(aq) CuSO4(aq) + 2H2O(l)


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Topic 15 Materials Chemistry

Unit 51 Metals, alloys and other synthetic materials in modern life

51.4 Unit cells

Calculating the density of metal

Nickel has a cubic close-packed structure. The unit cell of a nickel structure is shown below:

We can calculate the number of atoms per unit cell as follows:

• one atom at the centre of each of the six faces, giving 6 x atoms;

• one atom at each of the eight corners, giving 8 x atoms.

∴ number of atoms per cell = [ (6 x ) + (8 x ) ]

= 4

Given that the edge length of a unit cell of nickel is 3.52 x 10–8 cm, we can calculate the

density of solid nickel as follows.

Volume of one unit cell = (3.52 x 10–8)3 cm3 = 4.36 x 10–23 cm3

Mass of one nickel atom = =

= 9.75 x 10–23 g

Mass of nickel atoms in one unit cell = 4 x 9.75 x 10–23 g

Density of solid nickel =

=

= 8.94 g cm–3

58.7 g mol–1

6.02 x 1023 mol–1

molar mass

Avogadro number

mass of one unit cell

volume of one unit cell

4 x 9.75 x 10–23 g

4.36 x 10–23 cm3

1

8

1

2

1

8

1

2


Refer to HKDSE Practice Paper2012 Paper 2 Q2(a)(i)(III)


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Topic 15 Materials Chemistry

Unit 49 Natural and synthetic polymers

49.14 Production of polypropene, polyvinyl chloride and polystyrene

Copolymer formed from styrene and acrylonitrile

When two different types of monomers are joined in the same polymer chain, the polymer is called

a copolymer.

Styrene acrylonitrile resin (SAN) is thermoplastic copolymer of styrene ( ) and

acrylonitrile ( ). SAN consists of styrene units and acrylonitrile units in a ratio of

approximately 70 to 30.


Remark:

SAN is not made from styrene and acrylonitrile in 1:1 mole ratio. The two types of monomers

distribute randomly along the polymer molecule. Thus, the structure of SAN CANNOT be

represented as shown below.


2012 Paper 2 Q2(b)(ii)


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SAN combines the clarity and rigidity of polystyrene with the hardness, strength, and heat and

solvent resistance of polyacrylonitrile. It was introduced in the 1950s and is employed in

automotive parts, battery cases, kitchenware, computer products, packaging material and furniture.

Weak instantaneous dipole-induced dipole attractions exist between polymer chains of

polystyrene. On the other hand, acrylonitrile has a polar –C N group. Stronger permanent

dipole-permanent dipole attractions exist between polymer chains of SAN. Thus, SAN can

withstand higher temperatures than polystyrene and it is widely used in place of polystyrene.

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