Observation

This question was compulsory for all candidates, but majority of them could not answer it satisfactorily.

In part (a), majority of the candidates could not state two applications of titration in the laboratory.

In part (b), majority of the candidates could not write the reduction half-reaction equation to illustrate the reaction of acidified tetraoxomanganate (VII) ions with a reducing agent.

In part (c), majority of the candidates could not explain why there is a general increase in the first ionization energies of the elements across the period in the periodic table.

In part (d), majority of the candidates were able to name two greenhouse gases.

In part (e), majority of the candidates could not define standard electrode potential and could not state one factor that could affect the standard electrode potential.

In part (f), majority of the candidates were able to state two properties of gases that determine their methods of collection during preparation in the laboratory.

In par (g), only few candidates were able to name the reagents that could be used to test for the presence of tetraoxosulphate (VI) ions in aqueous solution.

In part (h), majority of the candidates could not distinguish between hydrolysis and hydration.

In part (i), majority of the candidates were able to state the sources of error during the titration of dilute HCl and aqueous NaOH.

In part (j), majority of the candidates were able to state Charles’s law.

The expected answers include:

(a)        -           determination of the concentration of aqueous solutions of acid or base
-           determination of the percentage purity / impurity of salts
-           estimation of the percentage composition of a mixture of two bases
-           determination of molecules of water of crystallization
-           determination of mole ratio of acid and base
-           determination of molar mass of base                                                                                                 
(b)        MnO₄^-_(aq) + 8H⁺_(aq) + 5e^- → Mn₂⁺_(aq) + 4H₂O_(l)

(c)        Across the period (from left to right) there is a gradual increase in the number of protons in the nucleus / (effective) nuclear charge. This increases the force of attraction between the nucleus and the electrons and hence more energy is required to remove        the valence electron.
                                                                                               
(d)       (i)        

-           carbon (IV) oxide
-           methane
-           nitrogen (I) oxide
-           water vapour
-           ozone
-           chlorofluorocarbons

(ii)       

-           global warming
-           climate change/extreme weather/wild fires
-           flooding/melting of ice caps/submerging of coastal regions
-           food supply disruptions
-           acidification of oceans / water bodies
-           Respiratory disease
           
(e)        (i)         It is the electromotive force / potential difference obtained when an electrode immersed in 1 mol dm^-3 solution of its ions at 298 K and 1 atmosphere is coupled with the standard hydrogen electrode.       

            (ii)       

-           concentration of the ions in solution
-           temperature
-           pressure
-           nature of metal / electrode                                         

(f)        (i)        

-           density           
-           solubility        

            (ii)        downward delivery / upward displacement of air

(g)       

-           a solution of barium chloride acidified with dilute hydrochloric acid
-           solution of barium trioxonitrate (V) acidified with trioxonitrate (V) acid

(h)        Hydrolysis is a process where there is a chemical reaction between salt or ions and water molecules while hydration is a process where water molecules surround salt or ions in aqueous solution                                                                      
OR
Hydrolysis is the (chemical) breakdown of a compound due to reaction with water while hydration is the addition of water molecule to a substance.
(i)       

-           inaccurate reading of volume on the burette
-           spillage of some of the reaction mixture in the conical flask
-           inaccurate reading of the volume of solution in the pipette
-           addition of too much indicator
-           exposing the stock solution of the base to air
-           shaking of the contents of the conical flask during titration instead of swirling the flask
-           leaving funnel on top of the burette
-           inaccurate observation / detection of colour change at the end point
-           blowing the last drop from the pipette into the conical flask
-           presence of air bubbles in the pipette or burette

(j)         The volume of a fixed mass of gas is directly proportional to the absolute temperature at   constant pressure.