Question 5
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- What is meant by the term allotropy?
- Name two substances that exhibit allotropy.
- Give three differences between the two crystalline allotropes of sulphur
. -
- What is meant by the term neutralization?
- Describe briefly how crystals of sodium trioxonitrate (V) could be prepared in the laboratory using 50cm3 of 2.0 mol dm-3 trioxonitrate (V) acid.
[6 marks]
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- Concentrated sodium chloride solution was electrolyzed using graphite electrodes.
- State the ions present in the solution before electrolysis..
- Name the product formed at each electrode..
- Write a balanced chemical equation for the reaction at each electrode..
- What would be the effect of the solution left after electrolysis on methyl orange?.
- State one large scale use of the solution left after electrolysis..
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- Why is water referred to as a universal solvent?
- State two differences between evaporation and boiling.
- What name is given to the change of state from liquid to solid?
[6 marks]
[6 marks]
[6 marks]
Observation
This question was popular among the candidates and their performance in the question was above average.
In part (a), majority of the candidates were able to explain the term allotropy and named the substances that that exhibit allotropy.
In part (b), majority of the candidates were able to explain the term neutralization. However, they could not describe how crystals of
NaNO3 could be prepared in the laboratory.
In part (c), majority of the candidates were able to identify the ions in solution during electrolysis of NaCl solution. However, majority of them could not write a balanced chemical equation for the reaction at each electrode.
In part (d), majority of the candidates were able to state the differences between evaporation and boiling.
The expected answers include:
-
- Is the existence of two or more forms of an element in the same physical
state OR
It is the ability or phenomenon of an element to exist in two or more forms in the
same physical state
- Is the existence of two or more forms of an element in the same physical
-
- carbon
- antimony
- tellurium
- Iron
- cobalt
- polonium
- arsenic
- phosphorus
- sulphur
- tin
- silicon
- oxygen
- selenium
- boron
- germanium
- neutralization is the reaction between H+ ions and OH- ions to form water.
OR
neutralization is the reaction between an acid and a base/alkali to form salt and water
prepared solution to the given solution and stir.
Evaporate the mixture to obtain saturated solution . Leave saturated solution for some time to crystallize .Filter the crystals and dry them between filter papers.
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Na+ and Cl-(1) (from NaCl)
H+ and OH- (1) (from H2O)
- at the anode - chlorine
- at the cathode - hydrogen
- at the anode - 2Cl-(aq) → C l2(g) + 2e-
- at the cathode - 2H+(aq) + 2e- → H2(g)
- for the manufacture of soap / paper / rayon/fabric
- purification of bauxite
- refining of petroleum
- water treatment
- metal processing
- oven and drain cleaners
- Because it dissolves most substances
- Freezing/solidification