exam questions Flashcards
The sample of chromium is analysed in a time of flight (TOF) mass spectrometer.
Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.
- (Ions will interact with and) be accelerated (by an electric field)
- Ions create a current when hitting the detector
The first ionisation energies of the elements in Period 2 change as the atomic
number increases.
Explain the pattern in the first ionisation energies of the elements from lithium to
neon.
Stage 1: General Trend (Li → Ne)
1a. 1st IE increases
1b. More protons/increased nuclear charge
1c. Electrons in same energy level / shell
1d. No extra/similar shielding
1e. Stronger attraction between nucleus and outer e OR outer e closer to
nucleus
Stage 2: Deviation Be → B
2a. B lower than Be
2b. Outer electron in (2)p
2c. higher in energy than (2)s
Stage 3: Deviation N → O
3a. O lower than N
3b. 2 electrons in (2)p need to pair
3c. pairing causes repulsion
State which of the elements magnesium and aluminium has the lower first
ionisation energy.
Explain your answer.
Al
(Outer) electron in (3)p sublevel / orbital
Higher in energy / further from the nucleus
so easier to remove
State how, if at all, the chemical properties of these isotopes differ.
Give a reason for your answer.
Chemical properties
No difference in chemical properties
reason
Because all have the same electronic structure (configuration)
A TOF mass spectrometer can be used to determine the relative molecular
mass of molecular substances.
Explain why it is necessary to ionise molecules when measuring their mass
in a TOF mass spectrometer.
Ions, not molecules, will interact with and be accelerated by an electric field
Only ions will create a current when hitting the detector
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
The positive ions with m / z of 104 have the same kinetic energy as
those with m / z of 118 and move faster
Therefore, ions with m / z of 104 arrive at the detector first
The melting point of XeF4 is higher than the melting point of PF3
Explain why the melting points of these two compounds are different.
In your answer you should give the shape of each molecule, explain why each
molecule has that shape and how the shape influences the forces that affect the
melting point.
Stage 1 electron pairs
-XeF4 4BP and 2LP around Xe
-PF3 3BP and 1LP around P
Stage 2 explanation of shapes
- XeF4 is square planar
-PF3 is pyramidal (allow tetrahedral)
-Electron pairs repel as far as possible or Lone pair repels more than bonding
pairs
Stage 3 IMF
-XeF4 has vdw forces and PF3 has dipole-dipole forces (and vdw)
-Stronger/more intermolecular forces in XeF4
Due to larger Mr or more electrons or larger molecules or packs more closely
together
Sodium fluoride contains sodium ions (Na+) and fluoride ions (F–).
Na+ and F– have the same electron configuration.
Explain why a fluoride ion is larger than a sodium ion.
(Electrostatic) forces of attraction between
oppositely charged ions/Na+ and F–
Lots of energy needed to overcome/break forces
Explain, in terms of structure and bonding, why the melting point of sodium
fluoride is high
Fluoride ion has (two) fewer protons/lower nuclear charge
Weaker attraction between nucleus and (outer) electrons
Methoxymethane (CH3OCH3) is an isomer of ethanol.
The table shows the boiling points of ethanol and methoxymethane.
Compound Boiling point / °C
ethanol 78
methoxymethane −24
In terms of the intermolecular forces involved, explain the difference in boiling points.
Hydrogen bonds (between ethanol molecules)
(permanent) dipole-dipole OR van der Waals force (between methoxymethane molecules)
Hydrogen bonds are stronger/est intermolecular force
