Paper 3

Question 1

Give a chemical explanation for why water has a concentration of approx 56moldm-3

Answer 1

1dm3 water has a mass 1000g.
n(H2O) in 1dm3 = 1000/18 = 56mol

Question 2

Give a chemical explanation for why the reaction of ethane with chlorine under UV radiation is a poor method for preparing a high yield of chloroethane

Answer 2

Further substitution occurs

Question 3

The value of Kw at 37degrees is 2.4x10^-14.
Calculate the pH of water.

Answer 3

Kw = [H+][OH-].
[H+] = root(2.4x10^-14) = 1.55 × 10–7 
pH = –log (1.55 × 10–7) = 6.81

Question 4

Why would use of excess HCOOH reactant ensure that the order with respect to HCOOH is effectively zero?

Answer 4

Concentration of HCOOH would be constant

Question 5

The equilibrium constant can be calculated using the amounts, in mol, of the components in the equilibrium mixture.
Why can this be used to calculate Kc?

Answer 5

Volume cancels
OR
Same number of moles on each side of equation

Question 6

Give the test for NH4+ ions.

Answer 6

Test: Add NaOH(aq) and WARM
Observation: gas turns red litmus indicator blue
Equation: NH4+ + OH– → NH3 + H2O

Question 7

Suggest the interactions which could occur between Cu2+ ions and protein molecules (polyamides)

Answer 7

Coordinate/dative covalent bonds between protein and Cu2+ 
N atoms OR O atoms in protein donate electron pairs 

Question 8

Predict and explain the effect, if any, of a larger volume of water, but same mass of H2SO4, when calculating the enthalpy change of solution of H2SO4, on the following:
- The temp change
- The calculated value of ∆solH

Answer 8

• ∆T is less AND ∆solH is the same 

Reason for ∆T less
- (same) energy/heat spread over larger volume of water 
- State new value for ∆T 

Reason for ∆solH same
- Same energy released per mole of H2SO4 

Question 9

Suggest a modification to experiments for calculating enthalpy change which would reduce the % errors in the measurements.

Answer 9

• Greater mass of Na2O 
• larger ∆T so reduces % uncertainty in ∆T 

OR

• One mass measurement OR measure mass directly 
• % uncertainty reduced by half 

Question 10

What type of reaction is a G2 metal + acid? What is formed?

Answer 10

Redox reaction.
–> Salt + H2

Question 11

Describe how to prepare 100cm3 of a standard solution with a salt.

Answer 11

• Dissolve known mass of solid in distilled water (less than 100cm3) in beaker
• Transfer solution to 100cm3 volumetric flask AND wash/rinse (from beaker to flask)
• Make up to mark / up to 100cm3 with (distilled water AND invert flask several times to ensure mixing

Question 12

Explain why a small proportion of molecules in water have a relative molecular mass of 20

Answer 12

ISOTOPES OF H AND O

Molecules contain
* 2H OR deuterium/D
* 3H OR tritium/T
OR O/H atoms have more neutrons (than 1H)
OR (different) O/H isotopes are present
OR (Molecules are) D2O 

Question 13

Explain why oxidation mixture of an alcohol (in alkaline solution) must be acidified so that crystals of benzoic acid appear

Answer 13

• Acid reacts with OR protonates carboxylate ion to form the carboxylic acid

Question 14

Describe a test that would confirm the presence of a phenol group

Answer 14

Indicator/pH paper turns red / orange OR pH < 7
OR pH meter < 7

AND

No reaction with sodium carbonate 