Similar to Part 3 of the Form 6 STPM Chemistry Extra Answering Techniques from Berry Berry Easy for Paper 2 (Paper 962/2) , this Part 4 of the series focuses on the “teknik menjawab” for electrochemistry questions. For the electrochemistry questions, Berry Readers will learn about contradicting answers, writing answers in the correct question number, important formula to memorise, important concepts and tips on organic structures. 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 4 – Final)
Answering techniques for electrochemistry questions
Always refer to the Data Booklet provided
Example: STPM 2007 Q7 (b)
An aqueous solution of copper(II) nitrate and an aqueous solution of aluminium nitrate are separately electrolysed using inert electrodes. Based on the standard reduction potentials of the species involved, explain these electrolyses. Write the equations for all the reactions involved. (9 marks)
- Need to refer to the Data Booklet provided.
- Specific answer with reason – copper is deposited & reduction potential of Cu2+ more positive.
- Answer: (1) Aqueous solution of Cu(NO3)2 contains Cu2+, NO3- and H2O while aqueous solution of Al(NO3)3 contains Al3+, NO3- and H2O. (2) Electrolysis of Cu(NO3)2 gives copper metal at the cathode and O2 gas at the anode. (3) Copper deposited at the cathode because its SBP is more positive (+0.34V) than water (-0.83V). (4) Cathode: Cu2+ + 2e –> Cu. (5) Electrolysis of Al(NO3)3 gives H2 at the cathode and O2 gas at the anode. (6) It is because SRP of water is more positive (-0.83V) than Al (-1.66V). (7) Cathode: 2H2O + 2e –> H2 + 2OH- (8) At anode, O2 gas is formed because its SRP of water is less positive than NO3- ion. (9) Anode: H2O –> ½ O2 + 2H+ + 2e.
Do not write contradicting answer
Example: Be have 2 valence electrons which can act as noble ion.
Answer in the correct question number
Students must write (in Paper 2) question number which stated in the question / according to the question number.
Formula expected to remember
Students need to remember these equation because it is not stated in the Data Booklet.
- Henderson-Hasselbalch equation: pH = pKa + lg ([A-] / [HA])
- Nernst equation: E = Eθ – (0.0591 / n) lg ([Product] / [Reactant])
Bonding vs intermolecular forces
- Physical changes (boiling point)
- Van der Waals forces – intermolecular and weak hydrogen bond is stronger (but not strong)
- Covalent bond is not weak
Organic structure must complete
Students need to write all H, show multiple bonds and correct geometry cis / trans.
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.