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C2 Salts and Electrolysis

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Salts
Electrolysis
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pH Scale

Indicators can be used to identify whether a substance is an acid, base or neutral. The scale runs from 0-14, and is a colour scale based on the reactions a substance has with universal indicator (UI).

  • Acids have a pH between 0-6
  • Neutral solutions have a pH of 7 (neither acidic/alkaline)
  • Alkaline/Basic solutions have a pH between 8-14

Acids and Bases

An acid is any chemical that produces H+ ions when dissolved in water.

A base is described as anything that can neutralise an acid. A subset of bases, called alkalis, can be dissolved in water - these produce OH- ions in solution.


Neutralisation

This is a chemical reaction when an acid and a base make a salt and water. If a carbonate is used as a base then carbon dioxide will be produced too.

If all of the acid and base react together, then it can be tested with UI to show the solution formed is neutral.


Acids + Metals

Salts can also be made using metals, but instead of making water - hydrogen gas is produced. Sodium is a very reactive metal, and so the reaction above wouldn't be carried out in a normal school laboratory.


Acids

There are three acids you need to know the names of, as well as their formula and the salts they form.


Insoluble Bases


Soluble Base

The technique shown in the video below is called titration. For C2 you do not need to worry about the apparatus, but for C3 you do. We will cover this in a later topic. You do need to know how to make a salt from a soluble base though!


Insoluble Salts

In this reaction, a soluble base is added to an acid to make a solid salt that is insoluble in water. This is called a precipitation reaction as a precipitate is formed.


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Electrolysis

Electrolysis is a process of separating ionic substances using electricity. Metals and gases usually are formed at the electrodes. Electrolysis can only work when the ionic compound is dissolved in water or molten (liquid) as the ions need to be able to move.

  • Positively charged ions (cations) will move to the negative electrode (cathode) where they lose electrons (oxidised).
  • Negatively charged ions (anions) will moved to the positive electrode (anode) where they are reduced (receive electrons).

Half Equations

Higher

A half equation shows you what happens at one of the electrodes during electrolysis. You need to be able to write and/or balance half equations.

The video below will show you how to write half equations using the example of lead bromide, and then will give you a few questions for you to have a go at:


Molten Compounds

Electrolysis can happen in solution, or when the ionic compound is molten. When in solution there are two extra ions present: hydrogen and hydroxide. It is easier to do an electrolysis in solution, but these ions can complicate what is formed.


Aluminium

Aluminium is the most abundant metal on Earth, but it is also very reactive so it is expensive to extract - due to the amount of electricity needed to complete electrolysis.

How to extract Aluminium:

  • Aluminium ore is called bauxite.
  • bauxite is purified to make aluminium oxide
  • aluminium oxide must be melted (m.p. over 2000°C) - it would be expensive to melt it by itself so is dissolved in molten cryolite which)
  • Aluminium metal forms at the cathode and sinks to the bottom of the tank
  • Oxygen forms at the anodes. This oxygen reacts with the carbon of the anodes, forming carbon dioxide. As a result, the anodes have to be replaced frequently, adding to the cost of the process.

Brine

Brine is a solution of sodium chloride (NaCl) in water. When electrolysed it produces sodium hydroxide (a strong alkali).

During electrolysis:

  • Hydrogen ions (from the water) are given off at the cathode as hydrogen gas
  • Chloride ions are given off at the anode as chlorine gas
  • Sodium ions and hydroxide ions (from the water) stay behind forming sodium hydroxide

Electroplating

The negative electrode should be the object that is to be electroplated

The positive electrode should be the metal that you want to coat the object with

The electrolyte should be a solution of the coating metal, e.g. its metal nitrate or sulfate


Rules of Electrolysis

When working through an electrolysis question it may be unclear at the start what is going to form at each electrode. Follow the steps below and it will help you when working through questions!

At the cathode (negative electrode)

Metal and hydrogen ions will gather here to try and recieve electrons. What will form is dependent on the reactivites. If the metal is less reactive than hydrogen it will form, but if hydrogen is less reactive, then it will form.

At the anode (positive electrode)

Negative ions (e.g. sulfate, oxide, chloride) will gather here wanting to be oxidised (lose electrons). In order of priority (1 being the highest), the following will form:

  1. halides (Cl-, F-, Br-) will form halogen gases
  2. hydroxide
  3. all other negative ions

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Page last updated: 14/04/2017