Chemical and Ionic Equations
|A chemical equation describes what happens when a chemical reaction takes place. It uses chemical formulae to tell us what happens when the reactants (left) react to form the products.|
CH4(g) + O2(g) → CO2(g) + 2H2O(l)The arrow tells us that methane and oxygen react to form carbon dioxide and water. The reverse reaction, in which carbon dioxide and water react to form methane and oxygen does not occur so the arrow only points from left to right. Some reactions, however, can go either way. For example, N2(g) (nitrogen) and H2(g) (hydrogen) react to form NH3(g) (ammonia). However NH3(g) will also react with itself to form N2(g) and H2(g). Such reactions are said to be reversible. This is represented by either a double headed arrow, ↔, or the equilibrium arrow, ⇌:
N2(g) + 3H2(g) ↔ 2NH3(g) or N2(g) + 3H2(g) 2NH3(g)
Stoichiometry (pronounced "stoi-kee-om-i-tree",) is concerned with the relative amounts of reactants and products in a reaction. In the reaction of nitrogen and hydrogen to produce ammonia, the chemical equation has a number 3 in front of H2(g) and a number 2 in front of NH3(g). These are the stoichiometric coefficients and they indicate the number of molecules or moles of each kind among the reactants and products. When no number is written, as for N2(g), then the stoichiometric coefficient is 1. The chemical equations above thus should be read to mean:
CH4(g) + O2(g) → CO2(g) + 2H2O(l):
"1 molecule of methane reacts with 1 molecule of oxygen to make 1 molecule of carbon dioxide and 2 molecules of water", or
"1 mole of methane reacts with 1 mole of oxygen to make 1 mole of carbon dioxide and 2 moles of water".
N2(g) + 3H2(g) ↔ 2NH3(g):
"1 molecule of nitrogen reacts with 3 molecules of hydrogen to produce 2 molecules of ammonia", or
"1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia".
|1. Write the unbalanced equation:|
Write the reactants on the left, with plus signs between them, and the products on the right, with plus signs between them. Join the two sides with an arrow. Make sure the formulae are correct and the states (s, l, g or aq) are included!
|2. Change the coefficients:|
Follow the steps in order - keep checking the tally as you can stop as soon as it is balanced!
- Tally up the number of each atom on both sides on your unbalanced equation.
- Balance metallic atoms.
- Balance polyatomic ions or groups (like SO42-, NO3- etc) that appear unchanged on both sides of the equation.
- Balance any remaining non-metallic atoms and, of these, balance H and O last.
- Balance elemental substances (such as O2(g), H2, Al(s) etc) last.
- Divide the coefficients through to give the smallest whole numbers.
|Test your understanding using the quizzes below.|
Each quiz is made up of 10 questions randomly drawn from a large set so you can repeat the quiz many times over. The results are not stored!
Practice multiple choice questions (not an assessment)