Exam Qs Paper 3

Question 1

Describe the bonding occurring in a delocalised system and explain why this can lead to
organic compounds being coloured

Answer 1

alternating double and single bonds OR
conjugated system OR
delocalisation of electrons 

split into different electronic energy levels OR electron(s) promoted OR excited from lower to
higher level OR electron(s) excited to a higher energy level

can absorb VISIBLE/COLOURED light (causing electrons to be promoted to upper level)

reflected/transmitted light complimentary colour 

Question 2

Two students decide to study the reaction of magnesium with dilute hydrochloric acid to find the
order of this reaction with respect to the acid
The students run five experiments each using a different concentration of hydrochloric acid.
They use the same length of magnesium ribbon each time.
(i) Why was the length of the magnesium ribbon kept the same?

Answer 2

Only changes in HCL conc affect rate
Constant SA/mass of Mg

Question 3

The students decided to investigate carbonated drinks such as diet and regular cola.
These drinks contain phosphoric acid (to increase the flavour) and aspartame.
Regular cola has a lower concentration of acid and keeps for longer than diet cola.
Suggest why regular cola keeps for longer.

Answer 3

Phosphoric acid is catalyst for hydrolysis of aspartame
Lower conc of acid=slower hydrolysis

Question 4

Amino acids such as phenylalanine exist as zwitterions.
Use the structure of phenylalanine to help explain how amino acids form zwitterions.

Answer 4

(Draw zwitterion of phenylalanine)
Internal acid/base reaction
OR
NH2 gains proton
OH loses proton

Question 5

Suggest the purpose of adding the anti-bumping granules.

Answer 5

Controls boiling/ Prevents boiling over/allows to boil more gently

Question 6

50 cm3 acid is used each time.
The concentration ranged from 0.1 to 0.5 mol dm–3.

Calculate the maximum mass of magnesium ribbon that would completely react with the
acid in all five experiments

Answer 6

Moles acid at lowest concentration = 0.05x0.1=0.005 
moles Mg wiil be half above= 0.0025 
mass 0.0025 x 24.3 = 0.06(075) (g) 

Question 7

Ocimenol and cinnamyl alcohol are both unsaturated molecules.
Explain how you could use a solution of bromine water to show which of the molecules has
the greater degree of unsaturation.

Answer 7

Use equal volume of cinamoyl alcohol and ocimenol
Add bromine water dropwise/from burette
Bromine water decolourises
Greater titre/ greater volume of bromine water needed=greater unsaturation
(can also use calorimetry and absorbance)

Question 8

Name the piece of apparatus which is most suitable for removing the 25.0 cm3 of the
Cu2+ ion solution.

Answer 8

VOLUMETRIC pipette