ALS9A - Section 9/ The s-Block Elements / Suggested...

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Section 9/ The s-Block Elements / Suggested solutions / SKH Bishop Baker Secondary School AL Chemistry Section 9 The s-Block Elements 81 IIB Q.7 (a) As the ions get larger from Li + to Cs + [1], the electrostatic interaction between the polar [1] H 2 O molecules and the ions become less [1] or charge density falls from Li + to Cs + [1], hence coulombic energy falls. OR Distance between effective charge (supposedly in centre) (1) and polar (1) H 2 O molecule increases (1) so that electrostatic interactions decreases . (1) (b) The attractive force between the Be 2+ and the O of the H 2 O is so great due to the small size of the Be 2+ (charge density is very high [1]) that the O - H bond is heavily polarized [1], hence H 2 O attacks to give H 3 O+. [1] [Be(OH 2 ) 4 ] 2+ (aq) + H 2 O(l) → [Be(OH)(OH 2 ) 3 ] + (aq) + H 3 O + (aq) [1] 83 IA Q.4 (b) Yellow/orange flame and white solid [1] (0 if only one mentioned) Product : sodium peroxide (or sodium oxide and peroxide) [1] 83 IIB Q.8 (a) (i) With strontium compounds an intense (crimson) red colouration is observed. [1] (ii) Cleaned platinum(nichrome) wire dipped into solution (solid) under test. [1] Wire dipped into concentrated hydrochloric acid. [1] Wire put to Bunsen flame moving from cooler to hottest part of the flame. [1] (b) Abundance is expected to be low . [1] This is predicted from trends in the Group. The trend : decrease in abundance descending the Group . [1] Strontium below calcium but above barium in Group II. [1] Probable compound of the ore is strontium carbonate/chloride/sulphate formula SrCO 3 /SrCl 2 /SrSO 4 (name and formula needed). [1] (c) (i) Abundance in seawater is low. [1] This is because of low solubility. [1] (ii) EITHER Electrolysis of molten chloride (all 3 terms needed). [1] Carbon electrodes or carbon anode/steel cathode. [1] OR Heat strongly to form oxide (can mention via sulphide). (1) Reduce oxide at high temperature using carbon/Al/Mg. (1) (d) Lattice energy depends on size of cation and anion. [1] Large anion (SO 4 2 - ) dominates. [1] Lattice energy nearly constant for all sulphates. [1] Hydration energy dependent on individual ions. [1] This decreases considerably from MgSO 4 to BaSO 4 . [1] Hydration energy is smaller than lattice energy low solubility. [1] Strontium ion confirmed by addition of (NH 4 ) 2 SO 4 . [1] White ppt. is formed. [1] 85 IA Q.2 (c) (i) K 2 O, K 2 O 2 , KO 2 (1 correct [ ½ ]; 2 correct [1]; 3 correct [2]) (ii) K 2 O + H 2 O → 2KOH [1 or 0] K 2 O 2 + 2H 2 O → 2KOH + H 2 O 2 [1 or 0] (or 2K 2 O 2 + 2H 2 O → 4KOH + O 2 ) 2KO 2 + 2H 2 O → 2KOH + H 2 O 2 + O 2 [1 or 0] 1
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Section 9/ The s-Block Elements / Suggested solutions / (or 4KO 2 + 2H 2 O → 4KOH + 3O 2 ) 86 IA Q.2 (b) O 2 O 2 2 - O 2 - relative bond energy : O 2 2 - < O 2 - < O 2 [1] 86 IA Q.2 (g) The piece of sodium burns more vigorously [ ½ ] with a yellow flame [ ½ ] to give pale yellow solid (or a white solid) [ ½ ]. A brown colour (a violet solid or brown ppt.) [ ½
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This note was uploaded on 08/17/2011 for the course CHEM 100 taught by Professor Fleet during the Fall '08 term at Oxford University.

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ALS9A - Section 9/ The s-Block Elements / Suggested...

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