Observation

This was a compulsory question. Majority of the candidates that responded to it performed fairly.
In part (a), majority of the candidates did not describe acid-base indicator as weak organic acids or weak bases. Hence, their explanations were incomplete.
In part (b), majority of the candidates stated the property exhibited by nitrogen (IV) oxide in the reaction equation. In addition, quite a number of the candidates could not identify its action as a mixed anhydride.
In part (c), majority of the candidates defined enthalpy of combustion as heat change rather than heat evolved.
In part (d), some candidates mixed up the properties of primary cell and secondary cell. Meanwhile, examples of primary cell and secondary cell were correctly given.
In part (e), majority of the candidates could not correctly define the term “mole”.
In part (f), few candidates could calculate the amount of hydrochloric acid in 40.0cm3 of 0.40moldm-3 dilute HCl.
In part (g), candidates named two substances which can be used as electrodes during the electrolysis of acidified water.
In part (h), majority of the candidates listed forces that can exist between covalent molecules.
In part (i), majority of the candidates wrote carbon monoxide instead of carbon (II) oxide as one of the products formed when butane undergoes incomplete combustion.
In part (j), majority of the candidates could not write the correct electron configuration of 26Fe3+.

The expected answers include:

1. (i) It is either a weak (organic) acid or weak base which gives different colours in an acid / basic medium.
   OR
   It is a dye (organic) which changes colour according to to the pH / hydrogen ion concentration of the solution.
   (ii) - phenolphthalein
   - methyl red
   - methyl orange
   - litmus ( paper / solution )
   - methyl yellow
   - methyl violet
   - bromothymol blue
   - bromocresol green
   - alizarin yellow
2. (i) NO2 is an oxidizing agent
   (ii) NO2 is an oxidizing agent as well as a reducing agent
   OR
   Accept acid anhydride or mixed anhydride
3. (i) Enthalpy of combustion is the amount of heat evolved or liberated when one mole of a substance is burnt completely/ in excess oxygen.
   (ii) Heat is always given out / reaction is exothermic
4. (i) Primary cells cannot be recharged while secondary cells can be recharged.
   OR
   Primary cells do not supply a steady current while secondary cells supply a steady current.
   OR
   Primary cells do not last long while secondary cells last long.
   OR
   Reactions in primary cells are not reversible while reactions in secondary cells are reversible.
   (ii) Examples of primary cells
   - radio battery
   - dry cell
   - alkaline battery
   - button battery
   - Le’clanché cell
   - Daniel cell
   - torch battery
   - clock battery
   - bicycle lamp battery
   Examples of secondary cells
   - lead-acid (storage) cell
   - lead- acid accumulator
   - lithium-ion battery
   - battery in rechargeable lamp
   - car battery
   - nickel – metal hydride cell
   - phone battery
   - nickel – cadmium cell
   - ( hydrogen – oxygen) fuel cell
   - Aluminium-air cell / battery
5. The amount of substance containing 6.02 x 1023 particles of that substance / the amount of substance containing as many ( elementary) particles / entities / units as there are atoms in 12g of carbon -12
6. Amount of hydrochloric acid
7. - carbon / graphite
   - platinum
8. - van der Waals’ forces / induced dipole – dipole forces
   - hydrogen bonds
   - (permanent) dipole-dipole interactions / dipolar forces
9. - carbon (II) oxide
   - water
10. 1s2 2s2 2p6 3s2 3p6 3d5