PJC_H1_CHEM_P2 ANS

1 Section A Answer all the questions in this section in the spaces provided. 1 This question is about aluminium, an elem ent in the third peri od of the Periodic Table. (a) (i) On the grid below, sketch the trend of the first ionisation energies of elements of Period 3 from sodium to argon. [1] (ii) Write an equation for the first ionisation energy of aluminium. Al(g) → Al+(g) + e- (iii) Explain why the first ionisation ener gy of aluminium is lower than magnesium. first electron of A l is removed from a higher sub-energy level as compared to that of Mg [3] (b) A beam of particles of 27Al+ and 9Be+ is passed through an electric field. If the angle of deflection of the 27Al+ particle is 3o, state the angle of deflection for the 9Be+ particle. 9 o [1] For Examiner’s Use first ionisation energy / kJ mol -1 Na Mg Al Si P S Cl Ar element

2 A lCl3 is a covalent compound that exists as a white solid at room temperature and pressure. When A lCl3 is heated to a sufficiently high temperature, it sublimes to gives the Al2Cl6 dimer. (c) Draw a dot-and-cross diagram for the A l2Cl6 dimer. Name the type of bond responsible for the formation of the dimer. Type of bond: _dative bond _ [2] (d) 2.0 mol of gaseous A lCl3 is heated at 800 K in a 750 cm 3 vessel. When dynamic equilibrium is achieved, 0.971 mol of gaseous Al 2Cl6 is formed in the vessel. (i) Define the term dynamic equilibrium. rate of forward reaction equals rate of reverse reaction (ii) Write a balanced equati on for the equilibrium. 2AlCl 3 ⇌ Al2Cl6 (iii) Calculate the value and state the units of the equilibrium constant, K c, for this reaction at 800 K. equation 2AlCl3 ⇌ Al2Cl6 initial amount / mol 2 0 change in amount / mol -2(0.971) +0.971 equilibrium amount / mol 0.058 0.971 equilibrium [ ] / mol dm-3 0.058 / 0.75 = 0.07733 0.971 / 0.75 = 1.2947 [1 (amount) +1 (concentration)] Kc = 2 3 6 2 ] [ ] [ AlCl Cl Al = 2) 07733 . 0 ( ) 2947 . 1 ( = 216 mol-1 dm3 [1] [5] For Examiner’s Use

3 (e) AlCl3 dissolves in water to give an acidic solution, as shown by the following equation. [Al(H2O)6]3+(aq) + H2O(l) ⇌ [Al(H2O)5(OH)]2+(aq) + H3O+(aq) (i) Define the term Bronsted base . Hence, identify the two Bronsted bases and state one conjugate acid-base pair in the equation above. Definition of Bronsted base: _proton acceptor _. The two Bronsted bases are _H2O_ and _[Al(H2O)5(OH)]2+ One conjugate acid-base pair: [Al(H 2O)6]3+ and [Al(H2O)5(OH)]2+_. OR H3O+ and H2O (ii) Write an expression for the acid dissociation constant, Ka, of [Al(H2O)6]3+. Ka = ] ) ( [ ] ][ ) ( ) ( [ 3 6 2 3 2 5