shapes of ions and molecules as answers

7/30/2019 Shapes of Ions and Molecules as Answers

1/7

NT Exampro 1

1. The following ions may be formed as intermediates in chemical reactions. Their shapes can bepredicted in the usual way. Draw a clear diagram of each, indicating values for the bond angles.

(i) CH3Pyramidal diagram (1), angle < 109 (1)

(ii) CH3+

Trigonal planar (1), 120 (1)

(4)

2. (a) Draw a diagram to show the arrangement of ions in the sodium chloride crystal.

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Na+Na+

Na+

Na+

Na+

Na+

Na+

and

Diagram must be 3DMust show/state 6:6coordinationMinimum of 8 cubes forboth marks or as drawn

(1)

(1)

or diagram showing Na+: CI as 6:6diagram showing 6:6 over-rules conflicting words

(2)

(b) Identify the products of the action of heat on(i) Li2O and CO2 (1)

(ii) MgO + NO2 + O2 (1)

(iii) SrO + NO2 + O2 (1)

If (ii) and (iii) incorrect, oxide in both (1)but (1) mark max of(2) in this case

(3)

(c) A solution contains 50.0 g dm3 of the chloride of an alkaline earth metal; To 25.0 cm3of this solution was added an excess of aqueous silver nitrate; 3.77 g of silver chloride,AgCl, was precipitated. Calculate the value of the relative atomic mass of the metal andsuggest its identity.

(XCI2 + 2AgNO3 X(NO3)2 + 2AgCI)

number of moles AgCI = )1(5.143

77.3 = 0.0263 0.0131 mol XCI2(1)

Mr(XCI2) = 25/1000 50 (1) = 95.2 71 = 24.2 (1)

0.0131 (1)

Final 3 marks consequential

(5)


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NT Exampro 2

(d) Write ionic equations for the following reactions:

(i) the precipitation of silver chloride from the solution containing the metalchloride in (c);Ag+ + CI AgCI (1)Ignore absence of state symbols since credited in 1(d)(i)

(ii) the formation of a white precipitate when aqueous sodium hydroxide is added to thesolution containing the metal chloride in (c).M2+ + 2OH M(OH)2(1)consequential on metal in (c) but must be a metal with 2+ charge

(2)

(e) Give ONE property of each substance which makes it suitable for the particularapplication.

(i) The addition of calcium hydroxide to soil:basici ty / reduces acidity / increases pH / alkaline / notsolubility (1)

(ii) The potential use of sodium metal in electricity cables:(good electrical) conductor / low density / not l ight (1)

(2)(Total 14 marks)

3. Draw a diagram to show the arrangement of ions in the sodium chloride crystal.

Cl

Cl

Cl

Cl

Cl

Cl

Cl

Na+Na+

Na+

Na+

Na+

Na+

Na+

and

Diagram must be 3DMust show/state 6:6coordinationMinimum of 8 cubes forboth marks or as drawn

(1)

(1)

or diagram showing Na+: CI as 6:6diagram showing 6:6 over-rules conflicting words

(Total 2 marks)

4. (a) H C11.1 / 1 88.9 / 12 (1)

=11.1 =7.41.5 1 (1)

Empirical formula C2H3 (1) 3

(b) HI has more electrons (1)

has greater induceddipoleinduced dipole / vdW forces (1) 2


3/7

NT Exampro 3

(c) (i) pyramidalP

H

HH

..

Need to show evidence of three dimensional or state it is pyramidal with twodimensional diagram (1)

3 bond pairs and 1 lone pair to get as far apart as possible (1) 2

(ii) tetrahedral

H

HH

H

Al

Need to show evidence of three dimensional or state it is tetrahedral with twodimensional diagram (1)4 bond pairs around aluminium as far apart as possible (1) 2

(d) Amount of phosphine =8.0/24000 (1)

=3.33 104 mol

Number of molecules of phosphine =6.0 1023 3.33 104 (1) 2

=2.0 1020

[11]

5. (a) (i) C 1

(ii) A 1

(iii) D 1(iv) B 1

(b) allow multiples of equations and ignore state symbols

(i) 2K +2H2O 2KOH + H2 1

(ii) 2Ca + O2 2CaO 1

(iii) MgO + 2HNO3 Mg(NO3)2 + H2O 2

1 mark for species; 1 mark for balancing

[8]

6. (a) 4 bond pairs around 1 carbon(1)

Cl C H

H

H+ +

+

+

all electrons shown around chlorine(1) 2


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NT Exampro 4

(b) Correct tetrahedral diagram

C

Cl

H

H

H

Or poor diagram +tetrahedral (1)

4 pairs (in words or diagram) of electrons around C arranged to minimise repulsion or asfar apart as possible / four electron pairs repel each other equally (1) 2

(c) chloromethane has a (permanent) dipole / is polar (1)methane does not / only

Increase in number of electrons in molecule(1) causes increase in vdW forces ofattraction between molecules(1) Scores maximum of 2 marks 4

has temporary dipoles or van der Waals forces(1)attraction (forces) between dipoles (1)stronger than van der Waals in CH4(1)

(d) hydrogen bonding in methanol (1)

between molecules (1)even stronger than dipole-dipole / vdW / hydrogen strongest of all intermolecularforces(1) 3

[11]

7. (a) For diagrams there must be some attempt at 3D

Octahedral diagram

Angle =90

Repulsion between 6 bonding pairs / bonding pairs as far apart as possible 3

(b) Trigonal pyramidal diagram Angle =106o 108o

3 bp and 1 lp (or shown by dot and cross diagram) / lp repels more than bp 3

(c) Tetrahedral diagram

Angle =109o 110o

Repulsion between 4bp/ 4 bonding pairs as far apart as possible 3[9]

8. (a) (i) attraction between positively and negatively charged species or ions formed by electron transfer between atoms (2)

(ii) bond formed as a result of sharing a pair of electrons one electron from each atom (2)

(iii) sideways overlap of two p-orbitals

to give a two part orbit above and below a bond(marks could be awarded for a diagram) (2)

(iv) a covalent bond in which both electrons come from the same atom (2) 8


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NT Exampro 5

(b) pairs of electrons around a central atom repel each other arranging themselves as far apart as possible to minimise repulsion or reach lowest energy state 5

(c) (i) diagram (mark if it looks like diamond is)

covalent bonding each carbon with 4 pairs around tetrahedral structure (4)

(ii) diagram (mark if it looks like graphite is) sigma and pi bonding each carbon with three pairs around planar triangular structure (4)

(iii) diagram (mark if it looks like ice is) covalent in water molecules hydrogen bonds between molecules each water with four waters round tetrahedral (4) 12

[25]

9. (a) (i) Valid diagram 4 carbons overall C4H10 (2)

(ii) Valid diagram 4 carbons C =C (2) 4

(b) Valid chlorine substituted product for the structure given in (a) HCl (2) 2

(c) Any chlorine addition product for the structure given in (b) Correctly drawn (2) 2

(d) Bromine solution added to unknown

Red/orange colour of bromine Goes colourless (3) 3

(e) (i) Repeating unit of CH2

No double bonds shown (2)

(ii) Electrophilic addition (1)

(iii) Any valid use e.g. packaging, containers, electrical insulation, piping, guttering.Any other valid use (1) 4

[15]

10. (a)

Cl

Cl

B

Cl+

++

(1)

Must show all the outer electrons around the chlorineDo not have to be and + 1

(b) (i)

B

Cl

ClCl (1) 1


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NT Exampro 6

(ii) The (three) bonding (electron) pairs (1)repel as far apart as possible / position of minimum repulsion(1)not stand alonenot just equal repulsion 2

(c) (i) Power (of an atom) to attract (the pair of) electrons(1)in a covalent bond / bonding pair (1) 2

(ii) Bonds arranged symmetrically /molecule symmetrical /bondpolarities directional/ are vectors(1)Bond polarities cancel (1)Could be shown as a diagram

Note:The answer to (b) is consequential on the answer to (a) in thefollowing situationIf the candidate puts a lone pair of electrons on the boron

the shape mark can be given for a clear, 3-D diagramof amolecule with the same shape as ammonia

the explanation will need to refer to both bond and lonepairs of electrons 2

[8]

11. (a)

C N BeCl Cl

H H

H HH H

H

(i) (ii) (iii)

109 110 106 108 180Shape: 1 mark3Angle marked on diagram in correct place: 1 mark 3 6Must be some attempt to show 3-D.Poor diagram can be salvaged by correct name or correct bond angle.Ignore

(b) (i) Temporary and/ or induced dipole forces(1) allowinstantaneous in place of temporary 1Allow London/dispersion/van der Waals forces

lone pair on ammonia if shown.If angle just written alongside diagram, penalise once

(ii) Hydrogen bonding(1) 1

(c) HF (1)consequential on some attempt at explanation..hydrogen bonding stronger / requires more energyto overcome (than vdW forces) / HF has strongerintermolecular force(1) 2

[10]

12. (a) (i) Bond pairs 3(1)

lone pairs / (1) 2


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NT Exampro 7

(ii)P

HHH

(1)

Angle (actual figure is 93) any value 2between 108 and 93 is acceptable(1)

(b) (i) Hydrogen bonds(1) Induced dipole-dipole interactions / van der Waals /London / dispersion(1) 2

(ii) Phosphine does not have hydrogen bonds (1) Lack of hydrogen bond not compensated by / increased

induced dipole-dipole(1) 2

(c) (i) When the pair of electrons shared by two atoms (in covalentbond) (1)

both come from the same atom(1) 2

(ii) The lone pair on the nitrogen(1) 1

(iii) Tetrahedral (1) has four pairs of bonding electrons(1)repel as far away from each other as, possible / minimum

repulsion(1) 3[14]

13. (a) Bent / v- shaped(1) non-linear (0) unless clarified by diagram

Oxygen has two lone pairs and two bonding pairs (1)

Basic shape of electron pairs is tetrahedral / shape based on 2bonds or 3 atoms / electron pairs repel to positions of maximumseparation / minimum repulsion(1) 3

(b) Pyramidal (1)

O

H

H

H (1) 2

(c) (i) Hydrogen bond is force of attraction between the hydrogen of oneand the oxygen in a second molecule(1)It arises because of the electronegativity difference between theoxygen and the hydrogen in the molecule(1)which sets up a+and a charge on the atoms (1) 3

(ii) Water is more dense than solid ice(1)

The hydrogen bonds in solid ice which hold the molecules togetherare in fixed positions and lead to an open structure(1)In water the hydrogen bonds are (constantly) being broken andmade(1) 3

[11]

shapes of ions and molecules as answers

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