Past paper review
Francine Taylor-Campbell, Contributor

In this series of lessons we will examine a few past-paper questions that will help in your examination preparations.

Question 1

Metals and non-metals exhibit different physical and chemical properties. State TWO differences of metals and non-metals for EACH of the following properties:

a) (i) Physical properties (ii) Chemical properties

b) The electronic configuration of sodium is 2,8,1 while that of chlorine is 2,8,7. Which ONE of these elements will act as:

(i) An oxidising agent (ii) A reducing agent. Explain your answer.

c) The way nitrates of metals decompose depends on the position of the metal in the reactivity series. With the use of balanced equations, illustrate this statement by the action of heat on NaNO₃ and Cu(NO₃)₂.

d) Figure 1 below shows the position of three elements in the reactivity series.

Mg

H Decrease in reactivity

Cu

Fig 1 Position of selected elements in the reactivity series.

Based only on the position in the reactivity series, which element would you select for use in making containers to store acidic fruit juices? Explain your answer.

ANSWERS

a) (i) Physical properties

Metals conduct electricity, but non-metals do not (except for graphite).

Metals generally have high melting and boiling points, while non-metals have low boiling and melting points.

(ii) Chemical properties

Metals are reducing agents, while non-metals are oxidising agents.

Metals react with oxygen to form basic oxides, while non-metals form acidic oxides.

b) (i) Chlorine is a non-metal and will act as an oxidising agent since it gains electrons during chemical reactions (becomes reduced).

(ii) Sodium is a metal and will act as a reducing agent, losing electrons during reactions (becomes oxidised).

(c) 2NaNO₃ ====heat==== 2NaNO₂ + O₂

2Cu(NO₃)₂ ===heat===== 2CuO + 4NO₂ + O₂

(d) Based on Figure 1 shown, magnesium is the most reactive and copper is the least reactive. A reactive metal would be expected to react with acids hence the container must be made of a material that is not reactive towards acids. Copper would be the best element to be used in the container since it is the least reactive.

Question 2

A group of students added an excess of finely divided zinc metal to aqueous sulphuric acid. When no more gas was evolved they filtered the mixture and used the filtrate to obtain dry crystals of hydrated zinc sulphate. One mole of these crystals contained seven moles of water of crystallisation.

(a) Write a balanced chemical equation for the reaction between zinc and aqueous sulphuric acid.

(b) Write the chemical formula for hydrated zinc sulphate crystals.

(c) Describe three important steps the students should follow to obtain dry, hydrated zinc sulphate crystals from the filtrate.

ANSWERS

(a) Zn (s) + H₂SO₄ (aq) ==== ZnSO₄ (aq) + H₂ (g)

(b) ZnSO₄.7H₂O - hydrated zinc sulphate

(c) The steps to be taken to obtain dry hydrated zinc sulphate crystals are as follows:

(i) Boil the filtrate in an evaporating dish.

(ii) Do not evaporate to dryness, instead boil to a small volume.

(iii) Allow the solution to cool then collect the crystals formed.

Continue to prepare for your exams and remember to read the questions carefully!

Students at the Covenant Christian Academy and Children's Centre (from second left) Khori Bennett, Ashli Findlator and Robyn-Leigh Hearne, greet Carolyn Supersad (left), promotions manager for PediaSure, Cari-Med, which is the official drink of the Covenant/Globe Over-Distance Track Meet, now in its 10th year. They were at the launch of the meet, held on April 14, at Globe Insurance Company, Dominica Drive, New Kingston. - Contributed

Francine Taylor-Campbell is an independent contributor.