Advanced Level Chemistry Examination Paper Review 2009 Question 09 (Essay question)

Essay Question (2009 AL Chem) 9. Write an essay to compare the chemistry of the s-block elements with that of the d-block elements. A: Introduction/Definition(Max:1) B: Physical properties (Max:2) C: Chemical reactions (Max:3) D: Oxidation states (Max:2) E: Complex formation(Max:3) F: Catalytic activity(Max:3)

Marking of Essay question Chemical knowledge : 10% - Maximum mark for C.K. is 10 even when the sum of 6 areas > 10 Organization + Presentation : 10% - O+P≤C.K.; O+P = C.K. – 0, -1, -2 but rarely -3 - Deduct marks for no sub-headings, no paragraphing, poor English & organization - Deduct marks for irrelevant or incorrect materials

A 1 B 3  2 C 3 D 1 E 2 F 3 C.K. = 12  10 O + P = 09 (deduct 1 mark for P) Total= 19 Example of marking

Observation-Overall 1. Insufficient analysis and planning before writing Imbalance of the coverage Some put nearly equal weightings on similarities and differences (weighting on differences should be higher) Some only list facts but unable to compare s- and d- 2. Student only remember facts, but do not demonstrate understanding and apply it in different contexts A lot of irrelevant or excess materials Incorrect definition for d-block / confused with transition elements Misuse of ‘low lying vacant orbitals’ to explain all characteristics of d-block elements

Observation-Area A Most can answer the definition of s block elements Many give definition of transition elements instead of d-block elements Unclear concept of ‘outermost electrons’ and ‘valence electrons’ Mistaken d electrons as outermost shell electrons

Observation-Area B Spend too much time in this section Give a lot of excess materials Melting points, boiling points and hardness better answered. List group trend of s- block & period trend of d-block, particularly atomic radius and IE. Correct explanations but no comparison  ZERO marks List similar physical properties, e.g. they are metals, can conduct electricity, have higher density than non-metals  ZERO marks

Observation-Area B Misconception: all s-block metals have open structures all d-block metals are close packed colour of flame test and colour of complexes have same origins ‘nuclear charge’ & ‘effective nuclear charge’ are the same do not understand ‘d-d transition’- interaction between d orbitals and lone pairs of ligands to cause splitting of d orbitals into 2 energy levels.

Observation-Area C poorly answered, seldom answered incomplete products names for reactions no comparison with d-block elements incorrect formulae and incorrect equations little attempt to explain the reactivity Misconception: s-block metals react to give alkalis, d-block metals react to give complexes

Observation-Area D can point out fixed O.S. in s-block and variable O.S. in d-block, but forget to give examples explanation is not satisfactory. Many mention that d-block elements use low lying vacant d orbitals / extend the octet (since they do not have the concepts of hybridization)- confusion with NH 3 VS PH 3

Observation-Area E Best performance among all areas. Many score 2 marks or more Mistakes include ‘incorrect electric charges of the examples’, ‘misunderstanding between polarizing power and charge density’

Observation-Area F Usually score 1 – 2 marks Some mismatch between explanation and the type of catalyst Common misspelling: ‘Habour process’, ‘Harbour’, ‘absorb’, ‘absorbtion’, adsorbtion’ Incorrect equation: 2 Fe I -  I Fe 3+

Distribution of Marks for Q9 Paper 1C AL Chem 2009 Sample size: 328 papers MarksFrequency