Chemistry 2 Nov/Dec 2012

Chemistry Paper 2,Nov/Dec 2012

Questions:		1	2	3	4	7	8		Main

General Comments
Weakness/Remedies
Strength

Question8

Question 8

Potassium , hydrogen gas and potassium hydride exhibit different types of bonds.

Copy and complete the following table

Substance	Type of Bonding	Units which make-up this substance
K
H2
KH

(i) Arrange the following compounds in order of increasing boiling points:

                      H2O, CH4, H2S.
(ii)        Give reasons for your answer in 8(b)(i).
(c)       (i)        State two processes where the concept of oxidation number is applied.
           (ii)         Write the formula of the oxide in which nitrogen exhibits an oxidation number of:

(iii) Differentiate between a drying agent and a dehydrating agent.
Give an example of each agent.

(d) (i) State two chemical properties of an acid.
(ii) Classify each of the following solutions as acidic, basic or neutral;

NH4Cl(aq);
K2CO3(aq);
Na2SO3(aq);
CH3COONa(aq).

A fair attempt was made on this question. Majority of the candidates could not answer (a) part of the question especially the unit which make up the substance.
In (b), only few candidates could arrange the compounds in order of increasing boiling point.
In (c), candidates gave a fair attempt of application of oxidation number and also wrote formulae of compounds in which nitrogen exhibits oxidation number of +1, +2 and +3. Many candidates could not effectively differentiate between a drying agent and a dehydrating agent.
In (d), majority of the candidates that attempted the question correctly gave properties of an acid and they were also able to classify the following solutions NH4 C(aq), K2 CO3(aq);
Na2SO4(aq) and CH3COONa(aq) as either acidic, basic or neutral.

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OBSERVATION

The expected answers include:

8.
(a)

Substance	Type of Bonding	Units which make-up this substance
K	Metallic	K+ surrounded by mobile electrons
H2	Covalent	H2 molecules
KH	Ionic	K+ ions H- ions

(b)        (i)
                        Increasing boiling points

            (ii)        CH4 – weak van der Waal’s forces
                        H2S - Strong van der Waal’s forces than CH4
                        H2O – Hydrogen bond which is stronger than van der Waal’s forces

(c)        (i)         - Oxidation number is applied in the IUPAC system of naming
    compounds.
-   Balancing redox equations.
-   Oxidation number identified which element in a compound is oxidized or
     reduced /redox reactions.
            (ii)        I.          N2O
                        II.        NO
                        III.       N2O3
(iii)       Drying agent removes molecules of water or moisture from substances while dehydrating agent removes elements of water from compounds.
OR
Drying agent removes water from a substance without affecting the chemical composition while a dehydrating agent removes element of water from a substance and affects the chemical composition
            Drying agent – Quicklime/Calcium chloride/Copper (II) oxide/Silica gel

            Dehydrating agent – Conc. tetraoxosulphate (VI) acid

(d)       (i)         - Reacts with metals to liberate hydrogen.
                        - Reacts with bases to form salts and water (only).
                        - Reacts with trioxocarbonate (IV) to liberate carbon (IV) oxide.

            (ii)        I.          NH4Cl(aq)- Acidic
                        II.        K2CO3(aq) - Basic
                        III.       Na2SO4(aq) – Neutral
                        IV.       CH3COONa – Basic