(a) Magnesium is manufactured by the electrolysis of molten magnesium chloride.
(i) The negative electrode is made of iron.
Suggest a non-metal which could be used for the positive electrode.
Give a reason for your answer.
(ii) Predict the products of the electrolysis of molten magnesium chloride at:
the positive electrode
the negative electrode.
(b) The following statements are about the procedure for making crystals of hydrated magnesium chloride from magnesium and dilute hydrochloric acid.
- Leave the mixture until no more bubbles are seen.
- Leave the mixture at room temperature to form more crystals.
- Add an excess of magnesium to dilute hydrochloric acid.
- Warm the filtrate to the point of crystallization.
- Filter off the crystals and dry between filter papers.
- Filter off the excess magnesium.
Put the statements A, B, C, D, E and F in the correct order.
The first one has been done for you.
(c) Magnesium is a metal in Group II of the Periodic Table.
Copper is a transition element.
Copper has a higher melting point and a higher boiling point than magnesium.
Describe two other properties of copper which are different from those of magnesium.
(d) Chromatography can be used to separate a mixture of ions from different transition element compounds.
Four samples, R, S, T and U, each containing transition element ions, were placed on a piece of chromatography paper.
Two solutions, Y and Z, each containing only one type of transition element ion were also placed on the same piece of chromatography paper.
The results of the chromatography are shown.
(i) Which sample, R, S, T or U, contains the same ions as both solution Y and solution Z?
(ii) Which sample, R, S, T or U, does not contain the same ions as either solution Y or solution Z?
(iii) In which sample, R, S, T or U, has the greatest number of transition element ions been separated?
▶️ Answer/Explanation
(a)(i) The non-metal used for the positive electrode is graphite because it conducts electricity and is inert (does not react with chlorine).
(a)(ii) The products are:
– Chlorine gas (\(Cl_2\)) at the positive electrode (anode).
– Magnesium metal (Mg) at the negative electrode (cathode).
(b) The correct order is: C, A, F, D, B, E.
Explanation: First, add excess magnesium to acid (C), wait until no more bubbles (A), filter excess magnesium (F), warm filtrate to crystallize (D), leave to form more crystals (B), and finally filter and dry (E).
(c) Two properties of copper different from magnesium:
1. Forms coloured compounds (e.g., blue \(CuSO_4\)), unlike magnesium which forms white compounds.
2. Has variable oxidation states (e.g., \(Cu^+\) and \(Cu^{2+}\)), while magnesium only forms \(Mg^{2+}\).
(d)(i) Sample R contains the same ions as both Y and Z (matches both spots).
(d)(ii) Sample T does not match Y or Z (no overlapping spots).
(d)(iii) Sample R shows the greatest separation of ions (multiple distinct spots).
Alcohols are a ‘family’ of organic molecules which have the same general formula.
(a) What is the name given to any ‘family’ of organic molecules which have the same general formula and similar chemical properties?
(b) Give the general formula of alcohols.
(c) Propan-1-ol can be made from propene.
(i) Name the reagent and give the conditions needed to convert propene into propan-1-ol.
(ii) Write a chemical equation for the complete combustion of propan-1-ol.
(d) A simple sugar can be represented as shown.
Simple sugars can be polymerised to make more complex carbohydrates.
(i) Complete the diagram to show part of a carbohydrate polymer made from the simple sugar shown.
(ii) Name the chemical process which occurs when a carbohydrate polymer is broken down into simple sugars.
(iii) What conditions are needed for this process to occur?
(e) Chromatography can be used to identify simple sugars in a mixture.
(i) What is the name given to the type of substance used to identify the positions of the simple sugars on the chromatogram?
(ii) The student calculated the Rf value of a spot on the chromatogram.
Complete the expression for the Rf value of the spot.
(iii) How could a student identify a simple sugar from its Rf value?
(iv) Sometimes not all the substances in a mixture can be identified from the chromatogram produced.
Explain why this may happen.
▶️ Answer/Explanation
(a) The name is homologous series. Members have the same general formula and exhibit similar chemical properties.
(b) The general formula of alcohols is CnH2n+1OH or CnH2n+2O.
(c) (i) Reagent: Steam (H2O)
Conditions: Phosphoric acid (H3PO4) catalyst at 300°C and high pressure.
(ii) The combustion equation is:
\[ 2C_3H_7OH + 9O_2 \rightarrow 6CO_2 + 8H_2O \]
(d) (i) The polymer structure should show glycosidic (–O–) bonds linking sugar units.
(ii) The process is called hydrolysis (breaking bonds with water).
(iii) Conditions: Enzymes (e.g., amylase) or heat with acid (e.g., HCl).
(e) (i) The substance is a locating agent (e.g., ninhydrin for amino acids).
(ii) Rf = distance travelled by substance / distance travelled by solvent.
(iii) Compare the Rf value to known values in a database or reference chromatogram.
(iv) Some substances may have identical or very similar Rf values, making them indistinguishable.