Chlorine Chemistry

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Chlorine Chemistry

I used this to direct a 2 lesson sequence aimed at teaching section 8.6 of the NEAB modular science syllabus to a top set Y11 group. It was also designed to reinforce prior work on bonding, atomic structure and reactivity.

I started with a brief recap (Q/A session for 2-3 min) on the properties of halogens from a previous lesson. This was then expanded to link with the electrolysis of brine. The practical was then duly carried out (I leave the risk assessment to yourself) and students were directed to start the worksheet when they had tidied their apparatus. They worked through at their own speed. At the end of the second lesson, students were asked to feed back one thing that they had learned in the lessons. Extension work was to research a number of other compounds of halogens such as iron chloride, copper chloride and calcium chloride.

The worksheet text and activities

Chlorine is commonly found in nature, but almost always in combination with other elements. The main industrial source of chlorine is sodium chloride – common salt. This is found in large quantities in the sea and in underground deposits such as those in Cheshire. Chlorine's structure makes it very reactive (it is in group VII of the periodic table and therefore needs to gain one electron in order to become stable). It is very useful to chemists, because it is so reactive. It is also used by engineers and other people involved in making things we use every day.

To be used in manufacturing, chlorine must first be separated from the other elements with which it is combined. Manufacturers use a process known as "electrolysis," which breaks down salt water (brine) into chlorine, sodium hydroxide and hydrogen. An electrical current passes through the salt water and splits apart the positive sodium and negative chloride ions. Since opposite charges attract, the negative chloride ions collect at the positive electrode (anode) and are discharged to form chlorine gas, Cl2 . At the negative electrode (cathode), the positive sodium ions are discharged, forming sodium atoms. Sodium atoms react readily with water, forming hydrogen gas and sodium hydroxide. A simple test for the presence of chlorine is that it bleaches damp Universal Indicator paper.

Task 1 – Answer the following questions about the information above.

1     Why is chlorine so reactive? What it the chemical formula of chlorine gas?

2     What is the name given to the process of breaking down brine to obtain chlorine? Where is salt found in large quantities?

3     At which electrode is chlorine formed? What is the name of the other electrode? What simple test can be used to detect chlorine?

4     Why is hydrogen and sodium hydroxide formed at the other electrode, instead of just sodium?

Task 2 – Use the available text books and the practical that you carried out in the lab to answer these questions.

5     Draw a diagram to show how brine could be electrolysed in a school laboratory. Describe how you would test for each of the materials formed in the experiment.

6     What pH value would you expect to find if you tested the electrolysed brine at the end of the experiment?  Explain your answer.

7     If chlorine has a proton (atomic) number of 17 and a mass number of 35, how many protons, neutrons and electrons does it have? Draw the electron configuration (arrangement) of a chlorine atom.

8     For higher paper candidates. Show, by means of diagrams, how a bond is formed between sodium and chlorine, to form sodium chloride. Explain what is going on.

9     For higher paper candidates. List the other members of the halogen group. Describe and explain the trend (pattern) of reactivity in terms of atomic structure.

Using Chlorine

                                                                          

Every day of your life, you use products that have been made using this important chemical. The following information outlines some of the uses of chlorine.

Chlorine guards against diseases caused by waterborne bacteria (e.g. cholera, typhoid) or parasites in drinking water. Places like hospitals, homes and swimming pools use chlorine-based compounds (e.g bleach) for disinfecting. The bulk of top selling medicines are manufactured using chlorine  chemistry (e.g. TCP). Chlorine is also used to manufacture plastics such as polyvinyl chloride. Farmers use pesticides that are based on chlorine help feed the world.  Chlorine helps ensure that products like disposable nappies and paper towels are strong and absorbent. Chlorine also plays a role in the classroom. For example, it is used to make the vinyl and polyester that is used to make school bags of all types and sizes, helps in the production of rubber for pencil erasers, and strengthens and brightens notebook paper.

Compounds of chlorine and the other halogens.

Silver chloride, silver bromide and silver chloride (i.e. silver halides) are reduced to silver by the action of X-rays and ionising radiation (radiation from radioactive substances). They can therefore be used to make photographic film and paper. Hydrogen halides dissolve in water to form acidic solutions. Hydrogen chloride, dissolved in water, is better known as “dilute hydrochloric acid“.

Task 3 – Make a spider diagram to show the uses of chlorine, chlorine compounds and compounds of other halogens. Use the text books that are available to find and add some other uses not mentioned above.