quantitive chemistry

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  • Created by: evarobo03
  • Created on: 07-02-18 11:47

Formulae

The chemical formula of a compound shows how many of each type of atom join together to make the units which make up the compound.

For example, in iron sulfide every iron atom is joined to one sulfur atom, so we show its formula as FeS. In sodium oxide, there are two sodium atoms for every oxygen atom, so we show its formula as Na2O. Notice that the '2' is written as a subscript, so Na2O would be wrong.

This diagram shows that one carbon atom and two oxygen atoms combine to make up the units of carbon dioxide. Its chemical formula is written as CO2.

Sometimes you see more complex formulae such as Na2SO4 and Fe(OH)3 :

  • a unit of Na2SO4 contains two sodium atoms, one sulfur atom and four oxygen atoms joined together
  • a unit of Fe(OH)3 contains one iron atom, three oxygen atoms and three hydrogen atoms - the brackets show that the 3 applies to O and H

Chemical equations

Word equations are useful to show which chemicals react together (the reactants) and which chemicals are produced (the products). Symbol equations allow chemists to work out the masses that will react or be produced.

Copper and oxygen reaction - getting a balanced equation

Balanced symbol equations show what happens to the different atoms in reactions. For example, copper and oxygen react together to make copper oxide.

Take a look at this word equation for the reaction:

copper + oxygen → copper oxide

  • copper and oxygen are the reactants because they are on the left of the arrow
  • copper oxide is the product because it is on the right of the arrow

If we just replace the words shown above by the correct chemical formulas, we will get an unbalanced equation, as shown here:

Cu + O2 → CuO

Notice that there are unequal numbers of each type of atom on the left-hand side compared with the right-hand side. To make things equal, you need to adjust the number of units of some of the substances until you get equal numbers of each type of atom on both sides.

Here is the balanced symbol equation:

2Cu + O2 → 2CuO

You can see that now there are two copper atoms and two oxygen atoms on each side. This matches what happens in the reaction.

Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide.

Two atoms of copper react with two atoms of oxygen to form two molecules of copper oxide

Remember: never change a formula to balance an equation

State symbols

Sometimes it is useful to know whether the reactants and products in a chemical reaction are solids, gases, liquids or dissolved in water. We can add state symbols to a symbol equation to show this.

State symbol and meaning

SymbolMeaning (s) Solid (l) Liquid (g) Gas (aq) Aqueous (dissolved in water)

For example, for the reaction between sodium and water, this is the…

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