Continuing from the Part 1 STPM chemistry extra answering techniques from Berry Berry Easy for paper 2 (Paper 962/2) , this Part 2 of the series focuses on the correct utilisation of information from questions, reaction equation writing methods, chemical tests suggestions, observation statements and IUPAC nomenclatures. For Berry Readers who wish to score A in Form 6 STPM Chemistry, please read the following post.
STPM Chemistry 2011 – Extra Tips and Predictions for Paper 962/2: Answering Techniques (Part 2)
Must use information from the question
Example: STPM 2009 Q3 (a)
Ammonia is a colourless gas with an unpleasant pungent smell.
Write a balanced for the formation of ammonia from calcium hydroxide. (1 mark)
- Student A: 2NH2+ + Ca(OH)2 –> 2NH3 + Ca2+ + 2H2O
- Student B: 2NH2Cl + Ca(OH)2 –> 2NH3 + CaCl2 + 2H2O
- Student C: 2NH2Cl + 2OH- –> 2NH3 + 2Cl- + 2H2O
Answer: Student A and B are correct. It is because they use the information from question. “…Write a balanced … from calcium hydroxide.”
Writing reaction equation correctly
Example: STPM 2009 Q4 (c) (ii)
Compound Y is an optically active isomer of X.
(ii) Compound Y reacts with a hot acidified solution of potassium manganate(VII). Write the equation for the reaction. (1 mark)
- Student A: CH3CH(OH)CH2CH3 –KMnO4 / H+, Δ –> CH3COCH2CH3
- Student B: CH3CH(OH)CH2CH3 — [O] –> CH3COCH2CH3
- Student C: CH3CH(OH)CH2CH3 — Potassium manganate(VII) –> CH3COCH2CH3
Answer: Only student A is correct. It is because “Write the equation for the reaction.” Student B answer is not accepted because the condition of the reaction is very important. Student C is naming the chemical reagent not answering the question.
Suggest a chemical test
- Choose simple reagent and state the conditions (acidic / heat / solvent)
- Results are observable (colour change, precipitate, bubbles)
- If to distinguish two compounds, must state observations for both reactions, cannot write “No reaction”, write “No observable change”.
Example: STPM 2009 Q10 (a) (ii)
Describe chemical tests which could be performed on dopamine to confirm the presence of phenolic and amino groups in it. Write equations for the reactions involved (6 marks)
Answer: FeCl3 or Br2 (aq) for phenolic group & NaNO2 / HCl or HNO2 (Nitrous acid in cold) for amino group
- Br2 (without stating aqueous or bromine water) for phenolic group
- HNO3 or NaCl for amino group
- HNO2, Nitrous acid, Δ
Stating the observation
- Student A: Nitrogen gas is evolved.
- Student B: Bubbles (or effervescence) observed
- Student C: No reaction
- Student D: No deposition of white solid
Answer: Students A, C and D are wrong. Students need to state their observation correctly. Results are observable (colour change, precipitate, bubbles). Correct answer: B
IUPAC Nomenclature – follow the rules
- Answer: nitrogen monoxide
- Wrong answer: nitrogen oxide
- Answer: dinitrogen monoxide
- Answer: sodium chlorate(V)
- Wrong answer: natrium chlorate(V) , sodium chlorate, natrium chlorate
Berry Berry Easy wishes all STPM Chemistry candidates good luck. Please share this tips with your friends and return credits (with links) to Berry Berry Easy if you wish to share them in your blogs.