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Representing Reactions (Higher)
 
Elements and Compounds | Chemical Reactions | Electrolysis
 

Elements and Compounds

Each element is represented by a different symbol.

For example C represents carbon.

Where there are two letters in a symbol the second letter is a small one, so you don't think it refers to two different elements.

For example, Co is cobalt. If it was written CO it would represent carbon and oxygen.

A compound is formed when different elements are joined together.

A compound can be represented by a formula. The formula shows the different elements present and the relative number of atoms of each element.

The formula for water is H2O. This means there are two atoms of hydrogen and one of oxygen in a molecule of water.

You can work out the formula of an ionic compound by looking at the charge on the ions.

A sodium ion, Na+, has one positive charge.

A chloride ion, Cl-, has one negative charge.

The formula for sodium chloride is NaCl, because the positive charge on the sodium ion is balanced by the negative charge on the chloride ion.

A calcium ion is Ca2+

A hydroxide ion is OH-

The formula for calcium hydroxide is Ca(OH)2 because two negative charges are needed to balance the two positive charges on the calcium ion.

 
Chemical Reactions

A chemical reaction can be represented by a word equation or a balanced chemical equation.

The word equation for burning carbon in air is:

carbon
+
 oxygen
—›
 carbon dioxide

Carbon and oxygen are the reactants. Carbon dioxide is the product.

The balanced chemical equation for this is as follows:

C(s) + O2(g)
—›
 CO2(g)


One atom of carbon reacts with one molecule of oxygen to form one molecule of carbon dioxide.

The symbol in brackets refers to the state of the substance.

(g) means it is a gas

(s) is a solid

(l) is a liquid

(aq) is an aqueous solution (solution in water)


Sometimes there is more than one product.

sulphuric acid + sodium hydroxide
—›
 sodium sulphate + water

The total mass of the products is always the same as the total mass of the reactants.

This is because the products are made up of the same atoms as the reactants.

Symbol equations must therefore be balanced. The total number of each type of atom in the reactants must equal the number of atoms of the same element in the products.

H2SO4(aq) + 2NaOH(aq)
—›
 Na2SO4(aq) + 2H2O(l)


 

Electrolysis

During electrolysis ions gain or lose electrons at the electrodes.

Electrically neutral atoms or molecules are released.

Half equations can be written to show what is happening at each electrode.

Aluminium is extracted from aluminium oxide by electrolysis.

The half reaction at the negative electrode is:

Al3+
+ 3e-
—›
 Al


Each ion gains 3 electrons to form an atom.

At the positive electrode:

2O2- - 4e-
—›
 O2

For every two oxide ions, four electrons are removed by the positive electrode forming one molecule of oxygen gas.

The number of electrons lost by the negative electrode must equal the number gained by the positive electrode, so each reaction needs multiplying in order to balance the electrons.

4Al3+ +12e-
—›
 4Al and 6O2- -12e-
—›
 3O2

The overall equation is:

2Al2O3(l)
—›
 4Al(l) + 3O2(g)