C2 Salts and Electrolysis

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  • Created by: Phoebe
  • Created on: 03-04-13 11:36

5.1 Acids and Alkalis

When acids are added to water they produce hydrogen ions, H+ (aq).

Bases are substances that will neutralise acids.

An alkali is a soluble hydroxide. Alkalis produce hydroxide ions, OH- (aq).

When we dissolve a substance in water we make an aqeous solution.

All alkalis are bases, and all bases neutralise acids, but only bases that dissolve in water are alkalis. 

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5.2 Making salts from metals or bases

When an acid reacts with a base a neutralisation reaction occurs and produces a salt and water.

acid + base --> a salt + water

Salts can also be made by reacting a suitable metal with an acid. This produces hydrogen as well as salt.

A sample of this salt made can then be crystallised out of solution by evaporating off water

acid + metal --> a salt + hydrogen

  • the salts formed when we neutralise hydrochloric acid are always chlorides
  • sulphuric acid always makes salts which are sulphates
  • nitric acid always makes nitrates

HCl=hydrochloric acid   HNO3=nitric acid   H2SO4=sulphuric acid

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5.3 Making salts from solutions

We can make soluble salts by reacting an acid and an alkali:

acid + base --> salt + water   OR   H+ (aq) + OH- (aq) --> H20 (l)

An indicator or pH meter can show us when the reaction is complete.

Insoluble salts can be made by reacting two soluble solutions to produce a precipitate.

Precipitation is an important way of removing some metal ions from industrial waste water.

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5.4 Electrolysis

Electrolysis is the process that uses electricity to break down ionic compounds into elements.

Ionic compounds can only be electrolysed when they are molten or in solution because then their ions are free to move to the electrodes.

In electrolysis, positive ions move to the negative electrode while negative ions move to the positive electrode.

The substance that is broken down in called the electrolyte. 

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5.5 Changes at the electrodes

In electrolysis, the ions move towards the oppositely charged electrons.

Negative ions lose electrons and so are oxidised at the positive electrode (anode).

Positive ions gain electrons and so are reduced at the negative electrode (cathode). 

Ions with a single positive charge gain 1 electron, those with a 2+ charge gain two electrons.

When aqeuous solutions are electrolysed, oxygen gas is produced at the anode unless the solution contains halide ions.

When aqeous solutions are electrolysed, hydrogen gas is produced at the cathode unless the solution contains ions of a metal less reactive than hydrogen.

The less reactive element is usually produced at the cathode, so we get that metal

At the cathode: Pb2+ + 2e- --> Pb     At the anode: 2Br- --> Br2 + 2e-

OILRIG: Oxidation is loss, reduction is gain (of electrons)

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5.6 The extraction of aluminium

Aluminium is more reactive than carbon and so it must be extracted from its ire by electrolysis.

Aluminium oxide is mixed with molten cryolite to lower its melting point, which saves energy.

Aluminium forms at the negative electrode and oxygen at the positive electrode.

The positive carbon electrodes are replaced regularly as they gradually burn away.

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5.7 Electrolysis of brine

When we electrolyse brine, we get three products:

  • chlorine gas (formed at the positive electrode) which makes bleach and plastic
  • hydrogen gas (negative electrode) which makes margarine
  • sodium hydroxide solution, an alkali (formed in the cell) makes bleach, paper and soap                                      

Half equations:

2Cl- -->Cl2 + 2e- (at the positive electrode)

2H+ + 2e- --> H2 (at the negative electrode)

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5.8 Electroplating

Electroplating uses electrolysis to put a thin coating of metal onto an object in order to:

  • make it more attractive
  • protect it from corrosion
  • increase its hardness
  • reduce the costs

The object to be electroplated is made the negative electrode in an electrolysis cell.

The plating metal is made the positive electrode.

The electrolyte contains ions of the plating metal.

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