STPM Chemistry 2011 – Extra Tips and Predictions for Paper 962/2: Answering Techniques (Part 3)

by BerryBerryTeacher

in Berry Super Tips *SPM/STPM

Continuing from the Part 2 of the Form 6 STPM Chemistry extra answering techniques from Berry Berry Easy for paper 2 (Paper 962/2) , this Part 3 of the series focuses on the “teknik menjawab” for thermochemistry, organic chemistry and inorganic chemistry.

For thermochemistry, you’ll learn answering tips on definitions, chemical equations, enthalpy of atomisation, energy level diagram. As for organic chemistry, tips are dispensed for free radicals, Le Chatelier’s principle and multi-steps organic synthesis. Similarly, general answering tips are available for inorganic chemistry. For Berry Readers who want to score in your exams with flying colours in Form 6 STPM Chemistry, do read the following post.

STPM Chemistry 2011 – Extra Tips and Predictions for Paper 962/2: Answering Techniques (Part 3)

Answering techniques for Thermochemistry questions

Definition must be precise

  • Ionisation energy – keyword: heat required (not heat change); 1 mol gaseous atom, 1 mol electron, 1 mol gaseous ion.
  • Standard enthalpy of combustion – keyword: heat released, 1 mol, standard condition (25˚C, 101 325 Pa).

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Chemical equation in Thermochemistry

  • MUST mention the condition (physical) state: solid (s), liquid (l), gas/vapour (g) and aqueous (aq).
  • Value and sign/symbol (+ or -)

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Writing the correct equation for enthalpy of atomisation

For elements (for the formation of 1 mol gaseous atom)

  • ½ Cl2(g) –> Cl(g)

For compounds (for compounds 1 mol compounds)

  • H2O(l) –> 2H(g) + O(g)

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Energy level diagram

Construct an energy level diagram must take notes on:

  • Electron affinity – the arrow always going down

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Answering techniques for organic chemistry questions

Free Radical

MUST write:

  • Initiation step. Exampke: Cl2 –uv–> 2Cl•
  • Propagation step.
  • Termination step.

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Equation questions (Le Chatelier’s Principle)

Example: STPM 2009 Q6 (b) (i)

Water gas, a mixture of hydrogen and carbon monoxide gases, is an important industrial fuel formed according to the following chemical equilibrium: C(s) + H2O(g) <—-> CO(g) + H2(g) At 800˚C, the value of Kc for the equilibrium is 0.160 mol dm-3 and the value of ΔH is +131.3 kJ. (b) Explain the effect of an increase in temperature on (i) the concentration of each component in the equilibrium (4 marks)

Answer:

  • Forward reaction is endothermic
  • Forward reaction is favoured at higher temperature
  • Concentration of CO gas and H2 gas increase
  • Concentration of H2O vapour / gas decrease (NOT water, NOT solid C)

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Multi-steps organic synthesis

Example: STPM 2005 Q10 (c)

Two isomers M and N have structural formulae as follows. (c) By giving reagents and reaction conditions, show the synthesis of M from 2-nitrotoluene. Hence, show how M can be converted to N. (8 marks)

Answer:

  • Step1: Sn/HCl or H2/Ni or NaBH4
  • Step 2: KMnO4/H+, Δ
  • Step 3: HNO2 (0 – 5˚C)
  • Step 4: H2O, Δ
  • Step 5: NH3, Δ
  • Students can interchange step 1 and step 2

Wrong response: Step 2: Use LiAlH4 / Na & ethanol as reducing agent. (It is not accepted for aromatic reaction)

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Answering techniques for inorganic chemistry questions

Read the question carefully and understand the question

Example: STPM 2007 Q1 (b) (i)

The complex ion dichlorobis(ethylenediamine)cobalt(III) exist as cis and trans isomer. (i) Name the types of ligands in the complex ion.

  • Wrong response: -chloro or ethylenediamine
  • Answer: monodentate and bidentate

Example: STPM 2005 Q3 (a) (ii)

An aqueous solution containing X2+ ions forms a white precipitate when an aqueous solution of sodium sulphate is added to it… (ii) Name the precipitate formed.

  • Wrong response A: BaSO4
  • Wrong response B: Barium(II) sulphate
  • Answer: Barium sulphate (name only)

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