Sodium is manufactured by electrolysis.
(a) Explain why sodium is manufactured by electrolysis and not by reduction with carbon.
(b) The diagram shows the equipment for the production of sodium.
(i) The anode is inert.
Suggest a suitable substance that can be used for the anode.
(ii) Label the anode on the diagram.
(iii) Describe, by reference to the diagram, how you know that sodium is less dense than molten sodium chloride.
(c) When concentrated aqueous sodium chloride is electrolysed, gases are produced at each electrode.
State the names of the products and give the observations at each electrode.
product at the negative electrode
observations at the negative electrode
product at the positive electrode
observations at the positive electrode
(d) Give two ways in which the physical properties of sodium are different from the physical properties of transition elements.
(e) The symbol equation for the production of sodium hydride is shown.
\(2Na + H_2 → 2NaH\)
(i) Write a word equation for this reaction.
(ii) Suggest why the hydrogen must be dry.
(iii) Sodium hydride reduces iron(III) oxide to iron.
\(Fe2O_3 + 3NaH → 2Fe + 3NaOH\)
Explain how this equation shows that iron(III) oxide is reduced.
(f) State the colour observed in the flame test for sodium.
▶️ Answer/Explanation
(a) Sodium is too reactive (higher in the electrochemical series) to be reduced by carbon. The temperature required would be impractically high, making electrolysis the preferred method.
(b) (i) Graphite or platinum are suitable inert anode materials. (ii) The anode should be labeled in the center of the diagram. (iii) Sodium collects at the top, indicating it’s less dense than molten NaCl.
(c) Negative electrode: Hydrogen gas (bubbles observed). Positive electrode: Chlorine gas (greenish-yellow bubbles observed).
(d) 1. Sodium is softer/more malleable than transition metals. 2. Sodium has a lower melting point than transition metals.
(e) (i) Sodium + hydrogen → sodium hydride. (ii) Dry hydrogen prevents reaction with water. (iii) Iron oxide loses oxygen (reduction).
(f) Yellow flame color in flame test.
This question is about metals and compounds of metals.
(a) Identify two correct statements about transition elements. Tick two boxes.
(b) The table compares the ease of reduction of some metal oxides with carbon.
Put the four metals in order of their reactivity. Put the least reactive metal first.
(c) Crystals of copper(II) sulfate, $\mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$, can be prepared by heating excess copper(II) oxide powder with dilute sulfuric acid.
(i) Describe how to prepare a sample of pure dry copper(II) sulfate crystals after the reaction is complete. In your answer describe how to:
- remove the excess copper(II) oxide from the reaction mixture
- crystallise the copper(II) sulfate
- dry the crystals. [4]
(ii) Identify the word that best describes copper(II) sulfate. Draw a circle around the correct answer.
acid $\bigcirc$
halogen $\bigcirc$
polymer $\bigcirc$
salt $\bigcirc$ [1]
(d) A few drops of water are added to a sample of solid anhydrous copper(II) sulfate, $\mathrm{CuSO}_4$.
(i) The reaction is reversible. $$\mathrm{CuSO}_4+5 \mathrm{H}_2 \mathrm{O} \quad\square\quad \mathrm{CuSO}_4 \cdot 5 \mathrm{H}_2 \mathrm{O}$$ Draw in the box, the sign for a reversible reaction. [1]
(ii) State the colour change observed when water is added to anhydrous copper(II) sulfate. from —————– to ——————————– [2]
▶️ Answer/Explanation
(a) Ans: 2nd box (have high densities) and 4th box (high melting points) are correct.
Transition elements are known for their high densities and high melting points due to strong metallic bonding.
(b) Ans: copper < cobalt < manganese < magnesium
The ease of reduction decreases as reactivity increases. Magnesium oxide is hardest to reduce (most reactive), while copper oxide is easiest (least reactive).
(c)(i) Ans:
1. Filter to remove excess copper(II) oxide.
2. Heat the filtrate to evaporate water until crystals form.
3. Filter to collect the crystals.
4. Wash with cold water and dry between filter papers.
(c)(ii) Ans: salt $\bigcirc$
Copper(II) sulfate is a salt formed from the reaction of an acid (sulfuric acid) and a base (copper oxide).
(d)(i) Ans: ⇌
The reversible reaction sign shows the reaction can go both directions.
(d)(ii) Ans: from white to blue
Anhydrous copper(II) sulfate is white, but turns blue when hydrated as water molecules are incorporated into its structure.