The Periodic Table can be used to classify elements.
(a) Group I elements react with cold water to form alkaline solutions.
(i) Place the Group I elements caesium, lithium, potassium, rubidium and sodium in their order of reactivity with water.
Put the most reactive element first.
(ii) Name the alkaline solution formed when caesium reacts with cold water.
(b) Group I elements have lower melting points than transition elements.
Describe one other difference in the physical properties of Group I elements and transition elements.
(c) Group VII elements are known as the halogens.
Astatine is below iodine in Group VII.
Predict the physical state of astatine at room temperature and pressure.
(d) Some Group VII elements react with aqueous solutions containing halide ions.
When aqueous chlorine is added to aqueous potassium bromide a reaction occurs.
The ionic half-equations for the reaction are shown.
Cl2(aq) + 2e– → 2Cl– (aq)
2Br– (aq) → Br2(aq) + 2e–
(i) Describe the colour change of the solution.
(ii) Identify the species that is oxidised.
Explain your decision.
(e) Bromine monochloride, BrCl, is made by the reaction between bromine and chlorine. The chemical equation is shown.
Br2(g) + Cl2(g) → 2BrCl(g)
Calculate the overall energy change for the reaction using bond energies.
Use the following steps.
-
- Calculate the total amount of energy required to break the bonds in 1 mole of Br2(g) and 1 mole of Cl2(g).
- Calculate the total amount of energy released when the bonds in 2moles of BrCl(g) are formed.
- Calculate the overall energy change for the reaction.
▶️ Answer/Explanation
(a)(i) The order of reactivity (most reactive first): caesium → rubidium → potassium → sodium → lithium.
(a)(ii) The alkaline solution formed is caesium hydroxide (CsOH).
(b) Group I elements are softer than transition elements (or less dense, or weaker in strength).
(c) Astatine is predicted to be a solid at room temperature (like iodine).
(d)(i) The colour changes from colourless (KBr solution) to orange/brown/yellow (Br2 formed).
(d)(ii) The species oxidised is Br– because it loses electrons to form Br2.
(e)
- Energy to break bonds: Br2 (193 kJ) + Cl2 (242 kJ) = 435 kJ.
- Energy released forming 2BrCl: 2 × 218 kJ = 436 kJ.
- Overall energy change: 435 kJ – 436 kJ = −1 kJ/mol (exothermic).
This question is about iron.
(a) Iron is extracted from its main ore in a blast furnace.
(i) Name the main ore of iron used in the blast furnace.
(ii) Name the substance that enters the blast furnace at A.
(iii) Name the substance that leaves the blast furnace at B.
(iv) Give two reasons for using coke in the blast furnace.
(b) Another ore of iron is iron pyrites, \(FeS_2\). Iron pyrites contains the positive ion, \(Fe^{2+}\).
Deduce the formula of the negative ion in \(FeS_2\).
(c) Iron is a transition element.
A list of properties of iron is shown.
- Iron is a good conductor of electricity.
- Iron forms soluble salts.
- Iron forms coloured compounds.
- Iron has variable oxidation states.
- Iron acts as a catalyst.
- Iron forms a basic oxide.
(i) Give two properties from the list in which iron differs from Group I elements.
(ii) Give two properties from the list in which iron is similar to Group I elements.
(d) Steel consists mainly of iron.
Iron forms rust when it reacts with water and oxygen.
Magnesium blocks can be attached to the bottom of steel boats. The magnesium does not completely cover the steel.
(i) Explain how the magnesium blocks prevent iron from rusting.
(ii) Explain why replacing the magnesium blocks with copper blocks will not prevent the bottom of the boat from rusting.
▶️ Answer/Explanation
(a) (i) The main ore of iron is hematite (Fe₂O₃).
(ii) Substance entering at A: Air (O₂) for combustion.
(iii) Substance leaving at B: Slag (calcium silicate), a byproduct.
(iv) Coke is used because:
1. Acts as a fuel (combustion releases heat).
2. Reduces iron oxide to iron (forms CO, a reducing agent).
(b) The negative ion in \(FeS_2\) is \(S_2^{2-}\) (disulfide ion).
(c) (i) Iron differs from Group I elements in:
1. Forms coloured compounds (Group I compounds are usually white).
2. Variable oxidation states (e.g., Fe²⁺/Fe³⁺; Group I has only +1).
(ii) Similarities:
1. Good conductor of electricity (metallic property).
2. Forms a basic oxide (e.g., FeO reacts with acids).
(d) (i) Magnesium blocks prevent rusting via sacrificial protection: Mg oxidizes instead of iron (Mg is more reactive).
(ii) Copper blocks accelerate rusting because Cu is less reactive than iron, causing iron to corrode preferentially.