The combustion of elements in oxygen
5 marks · HardSodium and magnesium both burn in oxygen, forming new compounds. The reactions are more vigorous in pure oxygen than in air.
Write a balanced chemical equation for the reaction of sodium with oxygen.
Describe what you would observe when magnesium burns in pure oxygen.
Explain, using ideas about the percentage of oxygen in air, why sodium and magnesium burn more vigorously in pure oxygen than in air.
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) \ce{4Na + O2 -> 2Na2O} · must show correct balancing, state symbols optional but accepted
(b) A bright white light is produced and a white solid forms. · must show reference to bright white light, mention of white (solid) product
(c) In pure oxygen, the higher concentration (100%) of oxygen means more oxygen is available to react with the metals. In air, only about 21% of the gas is oxygen. The increased oxygen concentration makes the reaction with sodium and magnesium more vigorous. · must show reference to oxygen concentration difference, correct link to combustion rate
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correct balanced chemical equation for sodium and oxygen: \ce{4Na + O2 -> 2Na2O}
B1 (b) · 1 mark · States bright white light or flame when magnesium burns
B2 (b) · 1 mark · Mentions formation of white solid (magnesium oxide)
C1 (c) · 1 mark · States that oxygen is a higher percentage in pure oxygen than in air (100% vs 21%)
C2 (c) · 1 mark · Links higher oxygen concentration to more vigorous/faster/brighter burning
Worked solution
a) The balanced chemical equation is:
b) Magnesium burns with a bright white flame and forms a white powder (magnesium oxide).
c) In air, only about 21% is oxygen, whereas pure oxygen is 100% oxygen. The greater oxygen concentration causes sodium and magnesium to burn faster/more brightly in pure oxygen.