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Thermochemistry & Structure · Part 1 of 3
15 exam-style questions with model answers, plus 20 quick multi-choice questions — every question on this part of the standard, grouped by the 5 pages of notes they come from.
Write a full answer before you reveal the model one. That comparison is where the learning happens.
Write the full electron configuration of a sulfur atom (Z = 16) and of a calcium atom (Z = 20).
Write the electron configurations of a nickel atom (Z = 28) and the Ni2+ ion, and explain which electrons are removed when the ion forms.
A student writes that 'the 4s electrons must be the hardest to remove from a titanium atom, because 4s was the last sublevel to be filled before 3d, and electrons are removed in the reverse order to the order in which they were added.' Evaluate this claim, and support your evaluation by writing the configurations of Ti (Z = 22), Ti2+ and Ti4+.
Define first ionisation energy, and state how it changes across a period from left to right.
Explain why the atomic radius of potassium is larger than the atomic radius of sodium, but the atomic radius of magnesium is smaller than that of sodium.
The ions O2−, F−, Na+ and Mg2+ all have the electron configuration 1s2 2s2 2p6. Their radii, in no particular order, are 0.065 nm, 0.140 nm, 0.095 nm and 0.133 nm. Assign each radius to its ion and fully justify your assignment. Compare and contrast your reasoning with the reasoning you would use to order the radii of the neutral atoms O, F, Na and Mg.
State the number of bonding pairs and lone pairs around the central atom in PF5 and in NH3, and name the shape of each.
The bond angle in CH4 is 109.5°, in NH3 it is 107°, and in H2O it is 104.5°. Explain why the angle decreases across these three molecules.
Both XeF4 and CF4 contain four bonds to a central atom, yet XeF4 is square planar and CF4 is tetrahedral. Justify the difference in shape, and explain why a student who assumed 'four bonds always means tetrahedral' would also get the shape of ClF3 wrong.
State whether each of CO2, H2O and CCl4 is a polar or a non-polar molecule.
Both CO2 and SO2 contain two polar double bonds to oxygen, yet CO2 is non-polar and SO2 is polar. Explain this difference.
Consider XeF2, XeF4 and SF4. Each has lone pairs on the central atom, yet only one of the three is polar. Identify the polar molecule and justify your answer fully, explaining why the presence of lone pairs does not by itself make a molecule polar.
Identify all the types of attractive force between the molecules in liquid methane, CH4, and in liquid hydrogen fluoride, HF.
Explain why ethanol, CH3CH2OH, boils at 78 °C while ethanal, CH3CHO, of very similar molar mass, boils at only 21 °C.
Propanone, CH3COCH3, boils at 56 °C and is completely miscible with water, yet propanone molecules cannot hydrogen bond to one another. Explain this apparent contradiction, and compare propanone with propan-1-ol (boiling point 97 °C), which has a similar molar mass.