Question 3
(a) (i) State the important points put forward in Dalton’s atomic theory.
(ii) How does the theory explain the law of multiple proportion? [11 marks]
(ii) With the aid of equations, describe the chemical reaction involved in the transformations stated in 3(b)(i). [7 marks]
(c) Certain properties of Beryllium (Be) and its compounds differ from those of Magnesium (Mg) and its compounds, but rather resembles those of Aluminium and its compounds. Explain briefly why this is so.[5 marks]
(d) Write balanced equation for the reaction between iodine and aqueous sodium trioxothiosulphate (VI).[2 marks]
Observation
Few candidates responded to this question and majority of them did not understand the demands of the question.
In part (a), majority of the candidates were able to the important points put forward in Dalton’s atomic theory. However, they could state how the theory explains the law of multiple proportion.
In part (c), majority of the candidates could not explain why certain properties of Beryllium and its compounds differ from those of Magnesium and its compounds.
In part (d), majority of the candidates could not write balanced equation for the reaction between iodine and aqueous sodium trioxothiosulphate (VI).
The expected answers include:
(a) (i) - All elements consist of atoms which are indivisible
- Atoms can neither be created nor destroyed
- The atoms of an element are all alike but differ from those / atoms of other elements
- When atoms combine with other atoms they do so in simple ratios
- All chemical changes result from the combination or separation of atoms
(ii) If A and B form more than one compound, since the atoms are indivisible, combination will be in (simple) integral ratios, as mass of atom of each element is fixed.
(b) (i) - soap
- margarine
- propan – 1,2,3 - triol / glycerine
(d) I2 + 2Na2S2O3 → 2NaI + Na2S2O6
- Atoms can neither be created nor destroyed
- The atoms of an element are all alike but differ from those / atoms of other elements
- When atoms combine with other atoms they do so in simple ratios
- All chemical changes result from the combination or separation of atoms
(ii) If A and B form more than one compound, since the atoms are indivisible, combination will be in (simple) integral ratios, as mass of atom of each element is fixed.
(b) (i) - soap
- margarine
- propan – 1,2,3 - triol / glycerine
Alkaline hydrolysis gives soap / propan-1,2,3-triol
(c) Atomic radii increases down the group and decreases across the period hence the size of Beryllium (Be) is about the same as the size of Aluminium (Al). The charge densities of Be2+ and Al3+ cause almost the same effect.
OR
(d) I2 + 2Na2S2O3 → 2NaI + Na2S2O6