Inorganic Chemistry
8 marks · NormalThis question concerns the reactions of alkali metals and halogens.
Describe what you would observe when a small piece of sodium is added to water. Give the word equation for the reaction.
Compare the observations when potassium is added to water with those for sodium.
State what you would observe when chlorine reacts with heated iron filings. Write the balanced chemical equation for the reaction.
A solution contains sodium chloride and potassium iodide. Describe how you could use chlorine water to show a displacement reaction occurs, stating your observations.
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(a) Sodium fizzes, moves on surface, dissolves, forms a colourless solution and hydrogen gas is released. Word equation: sodium + water → sodium hydroxide + hydrogen. · must show Fizzing, Movement, Hydrogen gas released, Word equation
(b) Potassium reacts more rapidly, with more vigorous movement, and a lilac flame is seen. · must show Faster/more vigorous, Lilac flame
(c) Iron burns with orange sparks, brown solid (iron(III) chloride) forms. Equation: 2Fe(s) + 3Cl2(g) → 2FeCl3(s) · must show Burning/sparks, Brown solid, Balanced equation
(d) On adding chlorine water, the solution turns brown as iodine is displaced from potassium iodide; no change from sodium chloride. Observation: solution becomes brown. · must show Brown colour, Iodine displaced, No change for chloride
Common mistakes: Missing flame for potassium; Missing colour change in displacement
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Marking points
M1 (a) · 1 mark · Appropriate observations for sodium in water: fizz/move/dissolve/hydrogen
A1 (a) · 1 mark · Correct word equation
B1 (b) · 1 mark · States that potassium reacts more vigorously/rapidly
B2 (b) · 1 mark · Mentions lilac flame for potassium
M2 (c) · 1 mark · Correct observations for iron and chlorine
A2 (c) · 1 mark · Correct balanced equation
M3 (d) · 1 mark · Brown colour forms showing iodine displaced
B3 (d) · 1 mark · No change with chloride; only iodide is displaced
Worked solution
a. When sodium is added to water, it fizzes, moves on the surface, may melt into a ball, and hydrogen gas is released. The solution becomes colourless. Word equation: sodium + water → sodium hydroxide + hydrogen.
b. When potassium is added to water, the reaction is more vigorous; the potassium moves faster and a lilac flame appears as the hydrogen ignites.
c. When chlorine reacts with iron, orange sparks are seen and a brown solid forms. 2Fe(s) + 3Cl2(g) → 2FeCl3(s)
d. Adding chlorine water to a solution containing sodium chloride and potassium iodide will turn the solution brown as iodine is displaced from the potassium iodide. Sodium chloride does not react (no change for chloride).