Chemistry Unit 5: Electricity and Chemistry

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Unit 5. Electricity and Chemistry

1. State that electrolysis is the chemical effect of electricity on ionic compounds, causing them to break up into simpler substances, usually elements.

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From the diagram, you can see that the electrolyte (solution) is made up of negative and positive ions; these are called anions and cations, respectively. Through electrolysis, we can use electricity to separate and obtain the anions and cations. The electrolyte is a compound that can conduct electricity for the process to happen. 

2. Use the terms electrode, electrolyte, anode and cathode.

Electrode: the conducting material which passes an electrical current (carbon is commonly used)

Electrolyte: the solution being broken down

Anode: the positive electrode, attracts anions

Cathode: the negative electrode, attracts cations

3. Describe electrolysis in terms of the ions present and the reactions at the electrodes.

The ionic compound will only be able to be broken down if they are in liquid form, as in a liquid the ions can move freely. To obtain a liquid, water can be added to the compound to dissolve it. This way, the anions are free to be attracted to the anode and the cations to the cathode. 

4. Describe the electrode products, using inert electrodes, in the electrolysis of: 

• molten lead(II) bromide, 

• aqueous copper chloride, 

• dilute sulphuric acid.

Molten lead (II) bromide --> lead 2+ ion (to cathode) + bromine 1- ion (to anode)

Aqueous copper chloride --> copper 1+ ion (to cathode) + chlorine 2-  ion (to anode)

Dilute sulphuric acid --> hydrogen (to cathode) + sulphur (to anode)

5. State and use the general principle that metals or hydrogen are formed at the negative electrode (cathode), and that non-metals (other than hydrogen) are formed at the positive electrode (anode).

The term 'opposites attract' applies here: cathodes (-) attract cations (+), while anodes (+) attract anions (-). To help you with this, think about this trick:

Cations = pussytive --> positive

Even for me, this tip is weird, but in it's own weird way it works!

Metals and hydrogen are positive, so they are the cations in the equation. If the metal is more reactive than hydrogen on the reactivity scale, it will get replaced by hydrogen and hydrogen will be formed at the cathode. The less reactive substance will be formed at the cathode.

For example, in aqueous sodium chloride there is sodium (Na+) and hydrogen (H+). Hydrogen takes sodium's place in the reaction --> 2H+ + 2e- = H2 

This is called a reduction as the negatively charged ions in the circuit are attracted to the positive cations, therefore reducing the cation to neutral. 

Non-metals are negative, so they are the equation's anions. They attract the positive ions from the circuit and also become neutral; this is called oxidisation. 

6. Relate the products of electrolysis to the electrolyte and electrodes used, exemplified by the specific examples in the Core together with aqueous copper(II) sulphate using carbon electrodes and using copper electrodes (as used in the refining of copper).

Electrolysis can also be used to purify metals. When doing so, instead of using carbon rods as electrodes we use the metal we want to purify. 

Let's say we want to purify copper. We take a rod of impure copper as our anode, and pure copper as our cathode. The solution used is aqueous copper sulphate. When the circuit is switched on, copper ions are attracted to the anode as they are cations (all metals are cations!). The impurities of the anode become sludge and the anode decreases in size, while the cathode gains copper ions and increases in size. 

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7. Describe the electroplating of metals, using laboratory apparatus.

Electroplating a metal is to coat a metal with another metal. We do this because maybe the metal being coated is highly reactive and we want to protect them. Metals commonly used to for electroplating are tin, silver, copper and chromium. 

To electroplate, the cathode is the object to be plated and the anode is the metal you want to plate with. The electrolyte is a salt solution of the same metal as the anode. When the circuit is switched on, the anode ions are attracted to the cathode, wearing away the anode. The concentration of the solution stays the same because no ions are being taken from it. 

Say you want to coat a brass key in copper. With alligator clips that are connected to wires, you attach a rod or wire of copper as your anode and the key as your cathode. The wires will be connected to the power supply accordingly. In a beaker of copper sulphate, the anode and cathode are placed into the electrolyte and the circuit is switched on. With this lab equipment, this is a simple process of electroplating. 

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8. Predict the products of the electrolysis of a specified binary compound in the molten state, or in aqueous solution.

Let's use sodium chloride.

At the anode: 2Cl --> Cl2 + 2e-

At the cathode: Na+ + e- --> Na

9. Describe, in outline, the chemistry of the manufacture of 

• aluminium from pure aluminium oxide in molten cryolite, 

• chlorine, hydrogen and sodium hydroxide from concentrated aqueous sodium chloride.

Aluminium: aluminium ore, bauxite, is purified to obtain a white powder; this white powder is aluminium oxide. Aluminium can be extracted from aluminium oxide through electrolysis.

Chlorine, hydrogen and sodium: sodium chloride (brine) is used as a electrolyte for electrolysis, and chlorine, hydrogen and sodium can be obtained from there. 

Chemistry Unit 2: Experimental Techniques

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Unit 2.1 Methods of Separation and Purification

1. Describe paper chromatography.

Chromatography is used to separate two or more dissolved solids in a solution. Paper chromatography uses water to separate the solids. A line is draw on a piece of paper and the solution is dotted on that line. After labelling the dot, the piece of paper is placed into water (or another solvent), the water level meeting a place a little below the pencil line. As the water travels up the paper (this is called capillary action), it separates the solids in the solution. 

We can use chromatography to evaluate the contamination of food or water, which is part of health and safety procedures. 

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2. Interpret simple chromatograms.

The further a substance moves up the paper, the more soluble it is. 

3. Describe methods of separation and purification: filtration, crystallisation, distillation, fractional distillation.

Filtration: when you have a solid and solution, filtering the mixture can help you separate each from the other e.g. sand and water

Crystallisation: crystallisation allows you to separate a solute from a solvent. The solution is left to dry or heated up, evaporating the solvent and leaving the solute in the form of crystals e.g. salt and water

Distillation: distillation helps you obtain a pure liquid from a mixture of liquids. The mixture is heated up in a flask, and the liquid with the lower boiling point will evaporate first. The vapour is cooled in the Liebig condenser and collected in the form of a liquid e.g. ethanol and water

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Fractional distillation: this process is similar to distillation, but it is used to separate more parts in a mixture. The mixture is heated up in a flask, and attached above the flask is a column with beads. This helps to separate the different parts as they evaporate. The vapours are then cooled through the Liebig condenser and collected as liquids e.g. crude oil

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4. Understand the importance of purity in substances in everyday life, e.g. foodstuffs and drugs.

As mentioned above, checking the purity of things is important as contamination can result in health and safety issues, especially food or drugs. They can cause you to become very sick.

5. Identify substances and assess their purity from melting point and boiling point information.

Take water for example. We all know that the boiling point of water is 100°c. If the boiling point has changed, this indicates that the water is not pure. This is the same with the melting point.

6. Suggest suitable purification techniques, given information about the substances involved.

With the above methods of separation and purification, I have given some examples of what each method can be used for.