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Francine Taylor-Campbell, Contributor

Principal of Hopewell High School, in Hanover, Mrs. Joyce Irving, discusses aspects of youth leadership with students, during a lunch break recently. They are (back from left) Randain Lawrence, vice-president, Key Club; Nicholas Powell, student rrepresentative on the school board; Duwayne Little, sub-prefect; Sheldon Lee, sub-prefect and student council representative. At front are Carlos Morris and Ricardo Holness. - Photo by Claudine Housen/Staff Reporter

Before we look at some questions please note that:

Scoring maximum marks from the questions on the exam paper will depend on how well you follow the instructions given.

If you are asked to explain your answer or to describe, this means that it requires more information than just listing.

More than likely, the length of your answer to a one-mark question should be significantly less than that required for a five-mark question.

Question 1

a) How many electrons, protons and neutrons are present in one atom of the chlorine isotope 37Cl?

b) When concentrated aqueous sodium chloride is electrolysed using graphite electrodes, hydrogen is collected at the cathode and chlorine at the anode.

i) Give the equations for the electrode reactions by which hydrogen and chlorine are formed.

ii) Name the product, other than hydrogen and chlorine, which is manufactured by the electrolysis of concentrated aqueous sodium chloride. Give a major use of this product.

iii) Why is concentrated hydrochloric acid not electrolysed for the manufacture of chlorine?

c) Household bleaches containing sodium chlorate (I), NaOCl, can be made by reacting chlorine with sodium hydroxide.

2NaOH + Cl2 ===== NaCl +NaOCl + H2O

Explain why this is a redox reaction.

ii) Give three uses of chlorine.

ANSWERS

1. (a) The isotope 37Cl contains 17 protons and 20 neutrons.

Comments: The mass number 37 is made up of the number of protons and neutrons, and since this is an isotope of chlorine, then it contains the same proton number as 35Cl.

b(i) At the cathode: 2H+ + 2e == H2 (g) hydrogen gas is liberated at the cathode

At the anode: 2Cl- == Cl2 (g) + 2e chlorine is produced at the anode.

(ii) Comment: As hydrogen and chlorine are removed from the concentrated aqueous sodium chloride solution, sodium Na+ and OH- ions remain, resulting in an alkaline solution of sodium hydroxide (NaOH). Sodium hydroxide is an important industrial chemical, which is used in the manufacture of soaps.

(iii) Concentrated hydrochloric acid is a very dangerous chemical to work with, especially as an electrolyte. It is much easier, safer and less expensive to use sodium chloride, which is abundant in the oceans as the electrolyte from which chlorine can be obtained. In most cases, the solution used in this electrolysis is concentrated brine (salt water) taken from the seas.

2NaOH + Cl2 ===== NaCl + NaOCl + H2O

(c) In a redox reaction, one substance is oxidised, while another is reduced. Based on the above equation, chlorine is acting as an oxidising agent and has been reduced in the same reaction. If one should look at oxidation numbers, chlorine is changing from an oxidation number of 0 in Cl2 gas to -1 in NaCl and +1 in NaOCl. This is an example of a reaction where a substance can be both oxidized (increase in oxidation number) and reduced (decrease in oxidation number) at the same time.

(ii) Chlorine is used as a domestic bleach, a disinfectant in pools, etc., and in many organic compounds to make antiseptics and insecticides.

Francine Taylor-Campbell is an independent contributor.