pmt questions Flashcards
Explain, in terms of electronegativity, why the boiling point of H2S2 is lower than
H2O2. 3
Electronegativity of S lower than O or electronegativity difference between H
and S is lower
Mark independently
1
Hydrogen bond(ing) / H bonding / H bonds
Not just hydrogen
No hydrogen bonding between H2S2 molecules
Or only van der Waals / only dipole-dipole forces between H2S2 molecules
Ammonia gas readily condenses to form a liquid when cooled.
(i) Name the strongest attractive force between two ammonia molecules.
hydrogen bonding
Ethanol can be oxidised by acidified potassium dichromate(VI) to ethanoic acid in a two-step
process.
ethanol ethanal ethanoic acid
(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. 3
A mixture of liquids is heated to boiling point for a prolonged time
1
Vapour is formed which escapes from the liquid mixture, is changed back into
liquid and returned to the liquid mixture
1
Any ethanal and ethanol that initially evaporates can then be oxidised
Write a half-equation for the overall oxidation of ethanol into ethanoic acid.
CH3CH2OH + H2O —> CH3COOH + 4H+4e–
The boiling points of the organic compounds in a reaction mixture are shown in the
following table.
ethanol 78
ethanal 21
ethanoic acid 118
Boiling point / °C
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. 5
b) Use your knowledge of structure and bonding to explain why it is possible to
separate ethanal in this way.
c) A student obtained a sample of a liquid using the apparatus in part (c).
Describe how the student could use chemical tests to confirm that the liquid
contained ethanal and did not contain ethanoic acid.
Mixture heated in a suitable flask / container
A labelled sketch illustrating these points scores the marks
1
With still head containing a thermometer
1
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
b)Hydrogen bonding in ethanol and ethanoic acid or no hydrogen bonding in
ethanal
Intermolecular forces / dipole-dipole are weaker than hydrogen bonding
c)Add Tollens’ reagent / ammoniacal silver nitrate / aqueous silver nitrate
followed by 1 drop of aqueous sodium hydroxide, then enough aqueous
ammonia to dissolve the precipitate formed
OR
Add Fehling’s solution
Warm
M2 and M3 can only be awarded if M1 is given correctly
Result with Tollen’s reagent:
Silver mirror / black precipitate
6.The table below contains some entropy data relevant to the reaction used to synthesise
methanol from carbon dioxide and hydrogen. The reaction is carried out at a temperature
of 250 °C.
Substance
CO2 (g) 214
H2 (g) 131
CH3OH(g) 238
H2O(g) 189
Entropy (SƟ) / J K−1 mol−
CO2(g) + 3H2 (g) =–>CH3OH(g) + H2 O(g) ∆H = −49 kJ mol−1
(a) Use this enthalpy change and data from the table to calculate a value for the
free-energy change of the reaction at 250 °C.
Give units with your answer
(b) Calculate a value for the temperature when the reaction becomes feasible. in kelivin
ΔS = 238 + 189 – 214 – 3 × 131 = –180 J K–1 mol–1
ΔG = ΔH – TΔS
= –49 (523+(-180)/1000 = +45.1 kJ mol–1
Units essential
(b)
When ΔG = 0, ΔH = TΔS therefore T = ΔH / ΔS
= –49 × 1000 / –180 = 272 (K)
Mark consequentially to ΔS in part (a)
Suggest what might cause the relative atomic mass of this sample to be
different from the relative atomic mass given in the Periodic Table.
Other isotopes present / some isotopes absent / different abundances of
isotopes
A sample of ethanedioic acid was treated with an excess of an unknown alcohol in the
presence of a strong acid catalyst. The products of the reaction were separated and
analysed in a time of flight (TOF) mass spectrometer. Two peaks were observed at m / z =
104 and 118.
(Outline how the TOF mass spectrometer is able to separate these two species to
give two peaks.
Positive ions are accelerated by an electric field
To a constant kinetic energy
1
The positive ions with m / z of 104 have the same kinetic energy as those with
m / z of 118 and move faster
1
Therefore, ions with m / z of 104 arrive at the detector first
Explain why the second ionisation energy of boron is higher than the first ionisation
energy of boron.
Electron being removed from a positive ion (therefore needs more energy) /
electron being removed is closer to the nucleus
what can mass spect give accurate info about
Relative mass (m/z ratio)
Relative abundance
Explain why second ionisation energy of calcium is lower than secound ionisation energy of potassium 2)
WRITE ELECTRON CONFIGURATION OUT
IN K e- removed from a lower energy level that’s closer to the nucleus so required more energy
