Revision (continued)
Francine Taylor-Campbell, Contributor

We will attempt the following questions as part of your revision programme for your upcoming examinations.

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 asked.
• If you are asked to explain your answer or to describe it, then it requires more information than just listing.
• Look at the marks assigned to each part of a question. More than likely the length of your answer to a 1-mark question should be significantly less than that required for a 5-mark question.

Question one

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

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 oxidised (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.

To ensure that you are fully preparing for your exams, try practising past paper questions under exam conditions. That is, try answering the questions in the time normally set for the exams. In this way you get used to working fast and with a time limit.

Kingston Technical athletes pose for the 'Youthlink' during the Multi-Care Sports Foundation Athletic Meet on Tuesday, May 6.

Francine Taylor-Campbell is an independent contributor.