Chemistry Exam Questions Flashcards
(17 cards)
Define the mass number of an atom.
The number of protons and neutrons.
Define the term relative atomic mass.
The average mass of an atom of an element.
Compared to 1/12th the mass of an atom of carbon-12.
State, in terms of the numbers of fundamental particles, one similarity and one difference between atoms of 50Cr and 53Cr.
Similarity:
- Same electron number.
Difference:
- Different neutron number.
Give two reasons why it is necessary to ionise the isotopes of chromium before they can be analysed in a TOF mass spectrometer.
So the ions can be accelerated to a negative plate.
Ions create a current in the detector.
Explain the pattern in the first ionisation energies of the elements from lithium to neon.
1) Ionisation energy increases.
2) Increased nuclear charge
3) Electrons are in the same shell so shielding is the same.
4) Stronger attraction between nucleus and outer electron
5) There is a deviation at Be and B. B is lower than Be. Outer electron in 2p is higher in energy than 2s.
6) There is a deviation at N and O.
O is lower than N. 2 electrons in the 2p need to pair. Pairing causes repulsion.
Write an equation, including state symbols, to represent the process that occurs when the third ionisation energy of manganese is measured.
Mn^2+ -> Mn^3+ + e-
State which of the elements magnesium and aluminium has the lower first ionisation energy.
Explain your answer.
Aluminium. Outer electron is in 3p orbital. Further from the nucleus so easier to remove.
Explain why it is necessary to ionise molecules when measuring their mass in a TOF mass spectrometer.
Ions will interact with and be accelerated by an electric field.
Only ions will create a current when hitting the detector.
Deduce which of Na+ and Mg2+ is the smaller ion.
Mg2+ because it has more protons and has the same shielding.
Explain how ions are accelerated, detected and have their abundance determined in a time of flight (TOF) mass spectrometer.
Ions are accelerated by attraction to the negatively charged plate.
Ions are detected by gaining electrons.
Abundance determined by the size of current flowing in the detector.
Explain why pentan-2-ol has a higher boiling point than pent-1-ene.
Pentan-2-ol has stronger intermolecular forces than pent-1-ene.
Pent-1-ene only has van der waals forces.
Pentan-2-ol has hydrogen bonds.
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.
1) XeF4 has 4 bp and 2lp so it is square planar.
2) PF3 has 3bp and 1lp so it is pyramidal.
3) Lone pairs repel more than bonding pairs.
4) XeF4 has VDW forces and PF3 has dipole-dipole forces and VDW.
5) More intermolecular forces in XeF4 due to the more electrons.
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.
1) Fluoride ion contains a lower nuclear charge.
2) Weaker attraction between nucleus and outer electron.
Explain, in terms of structure and bonding, why the melting point of sodium fluoride is high.
1) Electrostatic forces of attraction between oppositely charged ions.
2) Lots of energy needed to overcome forces.
Explain how the lone pairs of electrons on the oxygen atom influence the bond angle in oxygen difluoride.
1) Lone pairs repel more than bonding pairs.
2) Bond angle will be lower.
Explain how permanent dipole-dipole forces arise between hydrogen chloride molecules.
1) Difference in electronegativity leads to bond polarity.
2) Dipoles don’t cancel out and there is an attraction between the S+ on one molecule and S- on another.
State why Si(CH3)4, CCl4 and CDCl3 are used in 1H NMR spectroscopy. Explain how their properties make them suitable for use in 1H NMR spectroscopy.
1) TMS is used as a reference as it only gives one signal away from other typical H signals. It has a low boiling point so it is easy to remove.
2) CDCL3 and CCl4 are both solvents.
3) Both CDCL3 and CCl4 don’t have hydrogen.
4) CCl4 is non polar so it is a good solvent for non polar organic molecules.
5) CDCL3 is polar so it is good for polar organic compounds.
