The Alkali Metals
5 marks · EasyThis question is about alkali metals and their reactions with air and predictions for francium.
A student weighs out 1.50 g of sodium and exposes it to air. The sodium rapidly forms a white compound. Give the formula for this compound and calculate the maximum mass that could be formed.
Francium is even more reactive than potassium. Predict what you would observe if a piece of francium metal was freshly cut and left in the air, and explain your answer.
Use the information about group trends to estimate whether francium would have a higher or lower melting point than potassium. Justify your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) Na2O, 2.03 g · must show Correct formula: Na2O, Correct calculation of mass
(b) Francium would tarnish almost instantly, forming a white oxide. This happens even faster than with potassium because francium is more reactive. · must show Explains instant/rapid tarnishing, Relates to increased reactivity
(c) Lower melting point than potassium · must show States melting point is lower, Reason linked to group trend
Common mistakes: Incorrect formula for oxide (e.g., NaO); Forgetting mass conservation in calculation; Incorrect trend for melting point
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · Correctly writes formula for sodium oxide as Na2O.
A1 (a) · 1 mark · Calculates the theoretical mass of Na2O formed from 1.50 g Na (accept correct working using molar masses and stoichiometry, allow error carried from wrong formula). (follow-through)
B1 (b) · 1 mark · Predicts francium tarnishes instantly or 'almost instantly' in air.
B2 (b) · 1 mark · Explains tarnishing happens faster than potassium due to greater reactivity.
C1 (c) · 1 mark · States francium has a lower melting point than potassium AND links this to the group trend.
Worked solution
a) Sodium reacts with air to form sodium oxide. The formula is Na2O.
Relative atomic mass: Na = 23, O = 16
Molar mass Na2O = (2 × 23) + 16 = 62
Moles Na = 1.50 g / 23 = 0.0652 mol
From the equation, 4 mol Na → 2 mol Na2O, or 2 mol Na → 1 mol Na2O, so:
moles Na2O formed = 0.0652 mol Na / 2 = 0.0326 mol Na2O
Mass Na2O = 0.0326 mol × 62 g/mol = 2.02 g (rounded to 2.03 g for 3 significant figures)
b) Francium would tarnish almost instantly when cut and exposed to air, much faster than potassium, because it is even more reactive.
This reactivity trend is down the group: Li < Na < K < Rb < Cs < Fr.
c) Francium would have a lower melting point than potassium, because melting point decreases as you go down Group 1.