Shapes of molecules exam questions + intermolecular forces

Question 1

Explain, in terms of electronegativity, why the boiling point of H2S2 is lower than
H2O2

Answer 1

• lower electronegativity difference between H and S
  -so no hydrogen bonding, only van der wals and dipole-dipole

Question 2

Explain why CF4 has a bond angle of 109.5°

Answer 2

Around carbon there are 4 bonding pairs of electrons (and no lone pairs)
1
Therefore, these repel equally and spread as far apart as possible

Question 3

Explain how the electron pair repulsion theory can be used to deduce the shape of,
and the bond angle in, PF3

Answer 3

• P has 5 electrons in the outside shell
• 3 of those form covalent bonds to fluorine atom so there is one lone pair remaining
• Electron pairs repel as far as possible
• Lone pair repels more than bonding pairs
• Therefore, tetrahedral shape
• With angle of 109(.5)° decreased to 107°

Question 4

Explain, in terms of electronegativity, why the boiling point of H2S2 is lower than
H2O2.
.

Answer 4

-electronegativity difference between H
and S is lower
-No hydrogen bonding between H2S2 molecules
-Or only van der Waals / only dipole-dipole forces between H2

( 2 marks)

Question 5

Give the meaning of the term electronegativity ( 2 marks)

Answer 5

The power of an atom or nucleus to attract a pair of electrons (towards itself)
in a covalent bond

Question 6

4.Ethanol can be oxidised by acidified potassium dichromate(VI) to ethanoic acid in a two-step
process.

(a) In order to ensure that the oxidation to ethanoic acid is complete, the reaction is
carried out under reflux.
Describe what happens when a reaction mixture is refluxed and why it is necessary,
in this case, for complete oxidation to ethanoic acid.

Answer 6

A mixture of liquids is heated to boiling point for a prolonged time

Vapour is formed which escapes from the liquid mixture, is changed back into
liquid and returned to the liquid mixture

Any ethanal and ethanol that initially evaporates can continue to be oxidised

Question 7

boiling point of ethanal - 21 degrees
ethanol - 78 degrees
ethanoic acid - 118 degrees

Use these data to describe how you would obtain a sample of ethanal from a mixture of these three compounds. Include in your answer a description of the apparatus you would use and how you would minimise the loss of ethanal.

Answer 7

Mixture heated in a suitable flask / container ie round bottomed flask

With still head containing a thermometer

Water cooled condenser connected to the still head and suitable cooled
collecting vessel

Collect sample at the boiling point of ethanal

Cooled collection vessel necessary to reduce evaporation of ethanal

Question 8

Use your knowledge of structure and bonding to explain why it is possible to
separate ethanal in this way

( 2 marks)

Answer 8

Hydrogen bonding in ethanol and ethanoic acid or no hydrogen bonding in
ethanal

Intermolecular forces / dipole-dipole are weaker than hydrogen bonding

Question 9

Describe how the student could use chemical tests to confirm that the liquid
contained ethanal and did not contain ethanoic acid.

Answer 9

-Add Fehling’s solution
- Warm the sample
- red/ orange precipitate is formed if aldehyde is present
- Add sodium hydrogencarbonate or sodium carbonate
- Result; no effervescence observed; hence no acid present

Question 10

Explain why methanol is easy to liquefy.

Answer 10

Hydrogen bonding is a strong enough force (to hold
methanol molecules together in a liquid)

Question 11

Explain why the AsCl4
+
ion has a bond angle of 109.5°.

Answer 11

There are 4 bonds or 4 pairs of electrons (around As)
electron pairs repel equally

Question 12

State the type of bond formed between the N atom and the B atom in F2HNBF3.
Explain how this bond is formed.

Answer 12

Coordinate / dative covalent / dative
Lone pair on N(HF2) donated (to BF3)

Question 13

Although phosphine molecules contain hydrogen atoms, there is no hydrogen
bonding between phosphine molecules.
Suggest an explanation for this.

Answer 13

electronegativity difference is too small between p and h

Question 14

Explain why boron trichloride has a trigonal planar shape with equal bond angles.

Answer 14

• 3 bonds surrounding central atom
• electron pairs repel equally

Question 15

Explain, in terms of bonding, why KBrF4 has a high melting point.

Answer 15

Ionic or (forces of) attraction between ions / bonds between ions

Strong (electrostatic) attraction / strong bonds / lots of energy needed to break
bonds

the forces of attraction are between oppositely charged ions

Question 16

Explain why iodine has a higher melting point than fluorine.

Answer 16

• iodine has more electron shells
• so has greater Van der Waals forces between the molecules

Question 17

Why does ice have a lower desnity than water

Answer 17

the water molecules in ice are further apart compared to liquid water

Question 18

Suggest one economic problem for the car user caused by incomplete
combustion of ethanol in the car engine.

Answer 18

waste of fuel

Question 19

What is the definition of a covalent bond

Answer 19

shared pair of electrons between 2 non metal atoms

Question 19

Explain why the boiling points increase from H2S to H2Te

Answer 19

Atoms/molecules get larger
therefore increased Van der Waals/IMF forces