chem bonding section a ans

Nanyang Junior College H2/H1 Chemistry JC1 2014

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Chemical Bonding Tutorial

Section A: Review Questions Questions in Section A are slight variations of what was covered in the lectures. Try not to refer to your lecture

notes while attempting these questions, but please ensure you know which section of the notes to refer to if you

get stuck. Also look out for problem solving skill set (PS3) boxes along the way, they will guide you to solve

trickier questions.

Spend no more than 1.5 min on each MCQ.

1 Which of the following statements explains why copper conducts electricity when a potential difference is applied?

A Copper(II) ions move to the cathode B The crystal lattice breaks down C Electrons combine with copper(II) ions D Bonding electrons in the crystal lattice move

2 Which bonding type does not correspond to its description of physical properties?

Bonding type Physical properties A Giant covalent High melting point, conducts electricity when in solution but not

when solid B Simple covalent Low melting point, does not conduct electricity in any state C Metallic Variety of melting points, conducts electricity when solid and

when molten D Ionic High melting point, conducts electricity when molten but not

when solid

3 The C2H2 molecule is linear. What can be deduced from this about the numbers of and pi bonds present in the molecule?

pi

A 2 2 B 2 3 C 3 1 D 3 2

PS3: What is the structure of

C2H2? Will C or H be the terminal

atom?


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4 Why is the molecule of BCl3 planar, whereas the molecule of PH3 is pyramidal?

A The boron atom has no d-orbitals available for bonding. B The boron atom in BCl3 has 6 electrons in its valence shell, while the phosphorous

atom in PH3 has 8. C The repulsion between chlorine atoms is greater than that between hydrogen atoms. D The covalent radius of phosphorus is greater than that of boron.

5 MBTE is a constituent of petrol.

What are the values of angle P and angle Q in a molecule of MTBE?

Angle P Angle Q A 90 105 B 90 180 C 109 105 D 109 180

6 AlCl3 reacts with LiAlH4 and (CH3)3N to give (CH3)3NAlH3. Which statement about (CH3)3NAlH3 is correct?

A It contains hydrogen bonding B It is dimeric C The Al atom is electron deficient D The shape about the Al atom is tetrahedral

7 Why is the boiling point of methane greater than that of neon? [Ar: H, 1; C, 12; Ne, 20]

A A molecule of methane has a greater mass than a molecule of neon. B A molecule of methane has more electrons than a molecule of neon. C Molecules of methane have stronger intermolecular forces than those of neon. D Molecules of methane form hydrogen bonds, but those of neon do not.

PS3: Why do these 2

substances react? What type

of bond is formed?

PS3: Do not be fooled by the

diagram! Is MTBE 2D? Which

theory should you use to

determine the angles?


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8 Which of the following solids consists of atoms or molecules held together only by van der Waals forces?

A CO2 B Cu C H2O D MgO

9 Which of the following molecules will not form a hydrogen bond with another of its own molecules?

A CH3CHO B CH3NH2 C CH3OH D NH3

10 When heated, solid iodine readily forms iodine vapour. What does this information suggest about the nature of the particles in these 2 physical states of iodine?

Solid Vapour A ionic atomic B ionic molecular C molecular atomic D molecular molecular

11 Which set of properties could apply to a non-ionic compound which has a giant lattice?

Physical state at room temperature

Electrical conductivity in molten state

Melting point / C

A liquid Does not conduct -114 B liquid Does not conduct Melts over a range C solid Conducts well 808 D solid Does not conduct 1610

PS3: What are the structures

of these molecules? What is

the criteria for H-bond

formation?


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The responses A to D should be selected on the basis of

A B C D 1, 2 and 3

are correct

1 and 2 only are correct

2 and 3 only are correct

1 only is

correct

No other combination of statements is used as a correct response.

12 Which of the following are features of the structure in metallic copper?

1 Ionic bonds 2 Delocalised electrons 3 Lattice of ions

13 In microwave ovens, the wave energy produced is absorbed by certain polar molecules. Which of the following would absorb microwave energy?

1 C2H5OH 2 NaCl 3 SiO2

14 The boiling points of pentane and 2,2-dimethylpropane are 36 C and 9 C respectively. Which of the following suggested factors help to account for this difference in boiling points?

Pentane 2,2-dimethylpropane

1 The molecule of 2,2-dimethylpropane is more compact than that of pentane. 2 The covalent bonds in pentane are stronger than those in 2,2-dimethylpropane. 3 There is more effective hydrogen bonding in pentane than in 2,2-dimethylpropane.

15 Many ceramic materials based on silicon(IV) oxide have recently been developed. Which properties apply to these materials?

1 They are heated during manufacture and form solids. 2 They are heat-resistant solids. 3 They are good conductors of electricity due to delocalised electrons.


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16 a) Explain what is meant by sigma () and pi (pi) bonds. Illustrate your answers with suitable diagrams.

Refer to lecture notes Page 13 and 14.

b) How many sigma () and pi (pi) bonds are present in the propene (C3H6) molecule?

8, 1pi

17 For the following particles, draw their dot-and-cross diagrams count the total number of lone pairs and bond pairs state their shapes according to VSEPR theory draw their shapes give the bond angles about the central atom state whether they are polar or non polar (only applicable for molecules)

Particle Dot-and-cross diagram / Structure Bond pairs

/ Lone pairs

Shape, Bond Angle, Polarity

NF3

3 bp 1 lp

Trigonal pyramidal 107

Polar

SiF4

4 bp 0 lp

tetrahedral 109

Non Polar

PS3: What is the structure of

propene? Recall from Organic

Chem in Sec Sch. Draw its

structure out first.


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/ Lone pairs


CH2Cl2

4 bp 0 lp

tetrahedral 109 Polar

ICl3

3 bp 2lp

T shape 90

Polar

ClO

NH2

2 bp 2 lp

Bent 105

H3O+

3 bp 1 lp

Trigonal pyramidal

107

NO3

3 bp 0 lp

Trigonal planar 120

PS3: For the polyatomic ions,

how do you decide which atom

to give the extra electrons to or

take electrons away from?


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/ Lone pairs


AlH4

4 bp 0 lp

tetrahedral 109

BrCl2

2 bp 3 lp

Linear 180

I3

2 bp 3 lp

Linear 180

SO42

4 bp 0 lp

tetrahedral 109

18 a) Describe the bonding present in

(i) solid krypton, 36Kr,

Solid krypton exists as a lattice of atoms with weak van der Waals forces between Kr atoms.

(ii) solid rubidium, 37Rb.

Rubidium has a giant metallic lattice with strong metallic bonds between the cations and the sea of delocalised electrons.

PS3: Structure? Bonding?

Particles?


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b) Use your descriptions of their bonding to explain why the boiling point of krypton is 152C whereas that of rubidium is 686C, despite their having nearly the same relative atomic mass, Ar.

More energy is required to overcome the stronger metallic bonds in Rb than the weak van der Waals forces in Kr. Hence Rb has a higher b.p. than Kr.

19 The table below gives the boiling points of four organic compounds:

formula relative molecular mass boiling point /oC A CH3CH2CH2CH2CH3 72 36 B (CH3)4C 72 10 C CH3CH2CH2CH2OH 74 117 D CH3CH2OCH2CH3 74 35

By comparing i) A with B, ii) A with C and iii) C with D, suggest explanations for the differences in the boiling points within each of the three pairs.

i) A with B

Both A and B have simple molecular structure with weak van der Waals forces between molecules.

A has a straight chain structure while B has a more spherical shape due to its highly branched structure.

The vdW forces between A molecules are stronger than that of B due to the large surface area of contact. More energy is required to overcome the stronger vdW forces, higher bp.

ii) A with C

Both A and C have simple molecular structure. There are weak van der Waals forces between A molecules, and H-bonds between C molecules. More energy is required to overcome the stronger H-bonds than vdW forces, so C has a higher bp.

iii) C with D

Both C and D have simple molecular structure. There are H-bonds between C molecules and pd-pd interactions between D molecules. More energy is required to overcome the stronger H-bonds than pd-pd interactions, so C has a higher bp.

PS3: Compare the strength of

the forces between particles.

Use your answers in a) to help.

What should we always link bp

to?

PS3: Structure, bonding,

particles? Reasons for differences

in strength of forces? Dont

forget to always link mp/bp to

energy required!


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20 Explain the following observations in terms of the structure and bonding present.

a) 4-hydroxybenzoic acid is more soluble in water than 2-hydroxybenzoic acid.

OH

O OH

4-hydroxybenzoic acid

O OH

OH

2-hydroxybenzoic acid

In 2-hydroxybenzoic acid, COOH and OH groups are close to each other.

They can form intramolecular H-bonds This reduces the chance of forming H-bonds

with water molecules. Hence it is less soluble in water

b) In benzene solution, the measured relative molecular mass of methanoic acid is approximately 92.0. Explain the observation and include a relevant diagram in your answer.

In benzene solution, methanoic acid dimerises through hydrogen bonds.

PS3: There is a very similar example on

Page 37 of the lecture notes. However,

that example focuses on bp, not

solubility. How can we tweak our

explanation accordingly? What are the

factors affecting solubility in water?

PS3: What is the expected Mr of

methanoic acid? What should

we be looking out for when

drawing the diagram?

chem bonding section a ans

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