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 (e.g., can be cut with a knife).
(c) Astatine is predicted to be a solid at room temperature (trend: F₂, Cl₂ are gases, Br₂ is liquid, I₂ is solid).
(d) (i) Colour change: colourless (KBr) → orange/brown (Br₂).
(d) (ii) Species oxidised: Br⁻. Explanation: It loses electrons to form Br₂ (oxidation).
(e) Energy calculations:
- Energy to break bonds (Br₂ + Cl₂): 193 + 242 = 435 kJ.
- Energy released forming 2BrCl: 2 × 218 = 436 kJ.
- Overall energy change: 435 − 436 = −1 kJ/mol (exothermic).
This question is about transition elements.
(a) Transition elements are harder and stronger than Group I elements.
Describe two other differences in physical properties between transition elements and Group I elements.
(b) State one physical property of transition elements that is similar to Group I elements.
(c) State two chemical properties of transition elements.
(d) Cobalt is a transition element. Anhydrous cobalt(II) chloride is used to test for water.
State the colour change that occurs when water is added to anhydrous cobalt(II) chloride.
(e) Iron is a transition element.
(i) Which two substances react with iron to form rust?
(ii) Which metal is used to galvanize iron?
(f) The hull of a ship is made from steel (mainly iron). Metal blocks are placed on the ship’s hull to prevent rusting.
Use your knowledge of the reactivity series to explain why:
- magnesium is suitable to use as the metal blocks
- copper is not suitable to use as the metal blocks.
(g) Rust contains iron(III) oxide.
Phosphoric acid, H3PO4, can be used to remove rust from an iron object and prevent further rusting.
(i) Write a chemical equation for the reaction between iron(III) oxide and phosphoric acid to form iron(III) phosphate and water.
(ii) Iron(III) phosphate is an insoluble salt.
Suggest how the formation of iron(III) phosphate prevents further rusting.
▶️ Answer/Explanation
(a) Two physical differences between transition elements and Group I elements:
1. Transition elements have higher melting/boiling points.
2. Transition elements are more dense.
(b) Both transition and Group I elements are good conductors of heat/electricity.
(c) Two chemical properties of transition elements:
1. They act as catalysts (e.g., iron in Haber process).
2. They form colored compounds (e.g., blue CuSO4).
(d) Anhydrous cobalt(II) chloride changes from blue to pink when water is added.
(e) (i) Rust forms when iron reacts with oxygen and water.
(ii) Zinc (Zn) is used to galvanize iron.
(f)
1. Magnesium is more reactive than iron, so it corrodes first (sacrificial protection).
2. Copper is less reactive than iron, so iron would rust preferentially.
(g) (i) The equation is: Fe2O3 + 2H3PO4 → 2FePO4 + 3H2O.
(ii) Iron(III) phosphate forms a protective layer, blocking oxygen/water from reaching the iron.