Salts and electrolysis

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  • Created by: Chiara
  • Created on: 08-04-13 11:07

Acids and alkalis

When we dissolve a substance in water we create an aqueous solution. It may be acidic, alkaline or neutral. Soluble hydroxides are called alkalis. Their solutions are alkaline. Eg sodium hydroxide solution. Bases which include alkalis, are substances that neutralise acids. Metal oxides and metal hydroxides are bases. Eg iron oxide and copper hydroxide. Acids include critic, suffuric and ethanoic acid. They all taste very sour, but many are too dangerous to put in your mouth.
Hydrochloric acid is formed when hydrogen chloride dissolves in water. All acids form the ion h+ , when we add them to water.
Alkalis are bases that dissolve in water. All alkalis form OH- (hydroxide) ions when we add them to water.

Indicators are substances that change colour when we add them to acids and alkalis.
Litmus paper is one of them. We use the pH scale to show how acidic or alkaline a solution is. We can use universal indicator to find the pH of a solution.

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Making salts from metals or bases

Acids and metals

This makes salts by reacting the two. The is what happens: acid + metals = a salt + hydrogen.

Acids and insoluble base

When we react the two things we get a salt and water formed. It can be described as a neutralisation reaction. This is what happens: acid + base = a salt + water. The salt that we make depends on the metal to the base that we use, as well as the acid. So bases that contain sodium ions will always make sodium salts. In terms of acid used:

  • the salts formed when we neutralise Hydrochloric acid are always chlorides.
  • suffuric acids always make salts which are sulfates.
  • nitric acids always makes nitrates.
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Making salts from solutions

There are two other important ways of making salts from solutions:

  • we can react an acid and an alkali together to form a soluble salt.
  • we can make an insoluble salt by reacting solutions of two soluble salts together.

Acid and alkali
When an acid reacts with an alkali a neutralisation this is what happens: acid + alkali = a salt + water. To tell whether they have completely reacted we can use an indicator.

Making insoluble salts
We can sometimes make slats by combining tow solutions contain different soluble salts. When the soluble salts react they form an insoluble salt. We call the reaction a precipitation reaction. That's because the insoluble solid formed is a precipitate.

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The electrolysis means ' splitting up using electricity'. In electrolysis we use an electric current to break down an ionic substance. We call the substance that is broken down the electrolyte.
To set an electric circuit for electrolysis, we have to electrodes which dip into the electrolyte. These are conducting rods. One is connected to the positive and the other the negative terminal of a power supply. The electrodes are often made of unread five substances, often graphite. So that they do not react with anything. During electrolysis + ions move to the negative electrode, and - ions move to the positive one. When they move to the electrode they lose their charge and become elements. Gases may be given off or metals deposited.
Electrolysis of solution: Many ionic substances have ver high melting points, this can make electrolysis difficult. But some dissolve in water, when this happens the ions are free to move around. However, it is more difficult to predict what will happen, as water also forms ions. So the products at the electrode are not always what we expect.

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Changes at the electrodes

During electrolysis ions move to the electrodes. Where they go depends on their charge. When they reach the electrode they either lose or gain electrons, and this also depend on their charge. Negative lose and positive gain to become neutral. Gaining electrons is called reduction, and losing is called oxidation.

We can represent what is happening at each electrode by using half equations.

The effect of water

In aqueous solutions, electrolysis is more complex due to the ions from the water. The less reactive element, between hydrogen and the metal, is usually produced at the negative electrode. At the positive electrode, we often get oxygen gas given off from discharged hydroxide ions.

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The extraction of aluminium

It is used for pans, aeroplanes, drink cans etc. It is useful because it is strong but also have a low density. Aluminium is quite reactive, so we need to use electrolysis to extract it. We get aluminium oxide from bauxite ore, the ore is mined by open cast mining. It is mainly aluminium oxide but is mixed with other rocky impurities.

Electrolysis of aluminium oxide

We need to melt it first, so that the ions move to the electrodes. But it has avery high melting point, so molten cryolite is mixed. This can then be electrolysed at a lower temperature saving money and energy.

At the negative electrode: each ion gains 3 electrons.

At the positive electrode: each ion loses 3 electrons.

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Electrolysis in brine

Brine is a solution of sodium chloride in water. The solution contains sodium ions, Na+, chloride ions, Cl-, hydrogen ions, H+ and hydroxide ions, OH-. When brine is electrolysed, hydrogen is produced at the negative electrode from hydrogen ions. Chloride is produced at the positive electrode, from the chloride ions. This leaves a solution of sodium ions and hydroxide ions.

Sodium hydroxide is a strong alkali and has many uses including making soap. Chlorine is used to kill bacteria in drinking water and in swimming pools. Hydrogen is used to make margarine and Hydrochloric acid.

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An electroplated object is coated with a thin layer of metal by electrolysis. There are several reasons for electroplating; to protect the metal beneath from corroding, to make the object look more attractive, to increase the hardness of the surface and it resistance to scratching and to save money by using a thin layer of precious metal instead of pure expensive metal. Electroplating saves money unmaking cheaper jewellery.

Explaining electroplating

The metal object to be plated is used as the negative electrode. The positive electrode is made from the plating metal. The electrolysis takes place in a solution containing nickel ions.

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