Chemistry Paper 2, May-June 2020

Question 3

In an equilibrium reaction between gases Q2 and R2 to form QR, the energy content of the reactants is 100 kj and that of the product is 54 kJ. The energy content of the activated complex is 210 kJ.

Draw an energy profile diagram for the reaction.
Determine the:
activation energy of the reaction;
enthalpy change H of the reaction.
Write a balanced equation for the reaction.
State whether the reaction is exothermic or endothermic.
Give a reason for the answer given in 3 (a)(iv).

[11 marks]

Consider the following table:

Element	E	F	G	H
Atomic number	7	9	12	13

Write the electron configuration for each of the elements.
State:

two elements that are metals;
the element)s) most likely to form an ion with a charge of +3;
the element(s) which belong(s) to group (VII) on the periodic table;
the formula of the compound formed between F and G [9 marks]

Define the term isotopy.

Name the three building blocks of matter.

[3 marks]

Observation

This was a popular question among the candidates as majority of the responded to it.

In part (a), majority of the candidates drew a correct diagram, but could not correctly label the axis. Also, majority of them omitted the reversible sign that should be in equilibrium reactions; many candidates wrote the unit of energy as kg instead of kJ.

In part (b), majority of the candidates were able to write the electron configuration of the elements and stated elements that are metals and the ones most likely to form an ion.

In part (c), some candidates defined allotropy instead of isotopy

In part (d), majority of the candidates misinterpreted the question as the building block of elements.

The expected answers include:

(a)

(i)

(ii)

I. Activation energy = 210 – 100

                                                                  = 110 k J

II.       ΔH = H_P – H_R
= 54 – 100
= - 46 k J mol^-1

(iii)       Q_2(g) + R_2(g) 2QR_(g)
(iv)       The reaction is exothermic

(v) Because energy is given out / H is negative / energy of product is lower than energy of reactant.

(b)

(i)

E          -           1s² 2s² 2p³
F          -           1s² 2s² 2p⁵
G         -           1s² 2s² 2p⁶ 3s²
H         -           1s² 2s² 2p⁶ 3s² 3p¹

(ii)

I.          Elements H and G
II.         Element H
III.       Element F
IV        GF₂

(c)        Isotopy – The existence of atoms of the same element with the same atomic number but different mass numbers / numbers of neutrons

(d)

- Atoms
- ions

- molecules

Chemistry Paper 2 (Practical), WASSCE (SC), 2020

Question 3

Observation