CE CHEM SII P1 Microscopic World Answ

1993 - 2(b) Marking Scheme
(i)

Diamond is a giant structure with covalent bonds/ giant covalent structure/ consist of a large number of covalent bond ∴ melting requires supply of a large amount of energy. Melting of tetrachloromethane involves the breaking of weak van der Waals forces between molecules/ intermolecular forces.
1+1

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(ii)
There are mobile electrons in solid sodium but the ions in solid sodium chloride are not free to move/ solid sodium chloride has no mobile ion.
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(iii)
Tetrachloromethane which is a covalent compound, does not possess any mobile electrons (ions)/ exists as (discrete) molecules in liquid state, therefore is not an electrical conductor
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1993 - 4(a) Marking Scheme
(i)
Group II/ 2
Because W has 2 electrons in its outer/ outermost shell/ W is magnesium (Mg)

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(ii)
(1)
X is the cation/ an ion of W/ W and X are atom and ion of the same element

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(2)
Oxidation of W, e.g., reacting W with dil. acid/ reacting W steam/ burning of W in air/ reacting W with chlorine

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(iii)
(1)

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(2)
Yes, because Y and Z have the same electron structure/ are the same element/ have the same atomic number/ are isotopes.
(Do not accept Y and Z have same no. of e－ in outermost shell)

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1994 - 7(b) Marking Scheme
(i)
Bromine exists as simple molecules with weak intermolecular attraction/ weak van der Waal’s force.
∴ it has low b.p.
(Do not accept bromine has weak covalent bond.)
In Lead(II) bromide, the attraction between ions is strong/ ionic bond/PbBr2 has strong electrostatic attraction. ∴ it has higher a m.p.
(Do NOT accept lead(II) bromide is an ionic compound/ ionic structure)

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(ii)
Lead is a metal/ The bonding between lead atoms in solid is metallic bond. There exists mobile/ delocalized (freely-moving) electrons in lead ∴ it conducts electricity.
In solid lead(II) bromide, the ions are not mobile ∴ it does not conduct electricity

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(iii)
Yes, when molten/ in liquid state, the Pb2+ and Br－ ions can move freely/ are mobile ∴ it conducts electricity.
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1995 - 1 Marking Scheme
(a)
Rb is more reactive than K because Rb can give away/donate/release its (outermost) electron more easily/readily.

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or, Rb can form ion/cation/positive ion more easily/readily.

(1)

or, The strength of metallic bond between Rb atoms is weaker than that between K atoms/Rb can atomize more readily than K.

(1)

(DO NOT accept Rb has one more electron shell than K/Rb atom is larger than K atom/Rb is below K in the Periodic Table.)

(d)
Wear gloves/do not touch directly/use a pair of forceps/wear safety spectacles (goggles)/use a safety screen (DO NOT accept fume cupboard)

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1995 - 4 Marking Scheme
Chemical knowledge (5 marks)

When atoms of non-metals combine, they tend to share their (valence. outermost) electrons to form molecules.

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Electronic structure of a molecule e.g., HCl/ etc.
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(Any appropriate example)

/

When metal and non-metal combine, atoms of the metal donate electrons (to atoms of non-metal) to form cations/positive ions/metallic ions, while atoms of the non-metal accept electrons to form anions/negative ions/non-metallic ions.

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1
(or, award 1 mark for electron transfer, 1 mark for the formation of cation and anion)

Electronic structure of an ionic compound e.g. NaCl etc.
1
(Any appropriate example)

or

Effective communication (3 marks)

1996 7(a) Marking Scheme
(i)
isotope
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(ii)
One/1
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1997 - 6(a) Marking Scheme
(i)

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(ii)
The sodium sulphate solution provides ions for the conduction of electricity/acts as an electrolyte/complete the circuit

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(iii)
(1) The orange colour becomes paler/colourless/fades

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(2) Green/brown/purple colour was observed

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Explanation: Under the influence of the electric field, cations in the deep blue solution are attracted to the negative pole (move to the left) and negative/Cr2O72- ions are attracted to the positive pole (move to the right).

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or, Under the influence of the electric field, the cations and anions are respectively attracted towards the negative and positive poles

(1)

The orange negative ions and the blue positive ions mix/meet at B to give the green colour.

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(iv)
reverse the polarity of the d.c. supply

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or, connect the left hand electrode to the positive pole and the right hand electrode to the negative pole

(1)

Observation: orange colour will appear at the left of A and blue colour will appear at the right of C

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1998 1 Marking Scheme
(a)
Atoms with the same atomic number but different mass number/atoms with the same no. of protons but different no. of neutrons

1

(Accept any correct definition of isotope.)

(b)
Relative atomic mass = 6×0.074 + 7×0.926
= 6.93
(Also accept 6.9 and 6.926)
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1999 4 Marking Scheme
Chemical knowledge (6 marks)

For MgCl2, each magnesium atom donates / loses (two) electrons and each chlorine atom accepts (one) electron to form an ionic compound.

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In , the carbon atom shares (a pair of) electrons with (each of the four) chlorine atoms to form a covalent compound.

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1 has a higher melting point than because the attraction / van der Waals’ forces between molecules of is weak and the attraction between ions in is strong.

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2000 - 2 Marking Scheme
(a)
The average isotopic mass of the natural isotopes/ average mass of an atom of the element on the 12C (=12.000) scale
OR, relative atomic mass =

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1
(1,1)
(b)
(i)
Y/ potassium (K)
Y is a reactive metal and reacts readily with oxygen/ water in air
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(ii)
X/ argon (Ar)
X is chemically inert/ is a noble gas/ will not react with the hot tungsten filament.
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2001 - 8(a) (i) and (iv) Marking Scheme
(i)
Atomic size (atomic radius)/ electronegativity/ metallic character
1
(iv)
The balloon falls to the ground
Because Xe is much denser than air
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2002 - 6(b) Marking Scheme
(i)
Isotopes are atoms with the same atomic number but different mass number/ the same number of protons but different number of neutrons.

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(ii)
Relative atomic mass = 24 × 0.786 + 25 × 0.101 + 26 × 0.113 = 24.3

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(iii)
No, because isotopes of an element have the same chemical properties
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2002 - 8(b) Marking Scheme
(i)

1
(ii)
Carbon dioxide is denser than air
It can exclude air from the fuel/ can blanket the fire from air

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(iii)
Silicon dioxide has a covalent network structure.
Attraction between CO2 molecules is van der Waals’ forces.
The attraction between CO2 molecules is weak while the attraction between atoms in SiO2 is strong covalent bond
(1 mark for the structure of SiO2; 1 mark for the attraction between CO2 molecules; 1 mark for comparing the strength of inter-particle attractions.)

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(iv)
(1)
SiO2 + 2C  Si + 2CO
OR SiO2 + C  Si + CO2

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(1)

(2)
Making computer chips/ electronic parts/ alloys/ semi-conductors/ silicone
(Accept other uses of silicon.)
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2003 – 1 marking scheme
1.
(a)
(i)
carbon dioxide/ CO2
1

(ii)
Sodium chloride/ NaCl
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(iii)
Neon/ Ne
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2003 3(a) marking scheme
3.
(a)
(i)

1

(ii)

(All electrons must be shown in the electronic diagrams.)
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2004 – 5 marking scheme
5. Na2O and MgO are ionic compounds. The cations and anions pack together to form a giant ionic structure.
The attraction between cations and anions in Na2O and MgO is strong ionic bond.∴ Na2O and MgO have high melting points.
SiO2 has a covalent network structure.
Melting of SiO2 required the breaking of strong covalent bonds. ∴ SiO2 has a high melting point.
SO2 has a simple molecular structure.
Intermolecular attraction is weak van der Waals’ forces. ∴ SO2 has a low melting point.

2004 – 9(a) marking scheme
9.
(a)
(i)
Silicon

(ii)
Atoms of Group 0 elements have stable electronic structure (a duplet or octet structure in the outermost shell).

(iii)
Metals can be considered as making up of positive ions and a ‘sea’ of delocalized electrons. The attraction between the positive ions and the delocalized electrons holds the particles together.
Metallic bond is non-directional. Layers of atoms can easily slide over each another.
∴ metals have high ductility

(iv) potassium and fluorine

(v)
(1)
y = 7 x = 18

(2)
Br2＋2OH－ BrO－＋Br－＋H2O

2005 1(a), (b), (c) marking scheme
1.
(a)
(i)

1

(ii) flame test
Calcium compounds give a brick-red flame,
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(b)
(i)
Isotopes are atoms of the same element with same proton number but different neutron numbers.
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(ii)

Number of protons
Number of neutrons
90Sr
38
52

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(c)
(i)
They have the same number of electrons in their outermost shells.
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(ii)
Sr has similar chemical properties as Ca does, thus can replace some of the Ca required.
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2006 – 5(a),(b) marking scheme
5.
(a) covalent crystal
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2007 – 1 (a),(b), (c) marking scheme

1.
(a)

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(b)

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(c)
Ions of A are linked by strong ionic bonds/electrostatic force forming giant crystal lattice.
Molecules of B are attracted by weak van der Waals’s forces/intermolecular forces.

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