WJEC Chemistry for AS: Study and Rev Guide

Chemical equations are written to sum up what happens in a chemical reaction using chemical formulae. However, since atoms are neither created nor destroyed in a chemical reaction there must be the same number of atoms of each element on each side of the chemical equation. To balance a chemical equation, all you can do is to multiply a formula in the equation by putting a number in front of the formula. E.g. hydrogen burns in oxygen to form water H 2 + O 2 H 2 O Counting the number of atoms on each side gives: L.H.S. Hydrogen 2 atoms, oxygen 2 atoms R.H.S. Hydrogen 2 atoms, oxygen 1 atom To balance the equation we need 2 atoms of oxygen on the R.H.S. A simple solution would be to write H 2 + O 2 H 2 O 2 This is obviously incorrect since H 2 O 2 is hydrogen peroxide not water. You cannot change a formula, you can only put a number in front of the formula to multiply by it. Since 2 atoms of oxygen are needed on the R.H.S. multiply the water by 2. H 2 + O 2 2H 2 O However, we now have: L.H.S. Hydrogen 2 atoms, oxygen 2 atoms R.H.S. Hydrogen 4 atoms, oxygen 2 atoms So multiply the hydrogen on the L.H.S. by 2 2H 2 + O 2 2H 2 O and the equation is balanced. Many reactions involve ions in solutions. However, in these reactions not all of the ions take part in any chemical change. An ionic equation may help to show what is happening. It provides a shorter equation which focuses attention on the changes taking place. Any ions that do not change during a reaction are left out of an ionic equation. These ions are known as spectator ions. Ionic equations are frequently used for displacement and precipitaton reactions. E.g. barium sulfate is insoluble in water. When a solution of barium chloride is added to a solution of sodium sulfate a white precipitate forms. Write an ionic equation, including state symbols, for the reaction. The chemical equation for the reaction is: BaCl 2 (aq) + Na 2 SO 4 (aq) BaSO 4 (s) + 2NaCl(aq) Writing out all the ions gives: Ba 2+ (aq) + 2Cl – (aq) + 2Na + (aq) + SO 4 2– (aq) BaSO 4 (s) + 2Na + (aq) + 2Cl – (aq) The Na + (aq) ions and the Cl – (aq) ions do not change during the reaction. They are spectator ions and are omitted, giving the ionic equation: Ba 2+ (aq) + SO 4 2– (aq) BaSO 4 (s) Chemical and ionic equations Grade boost Remember that all the elements in the mnemonic HOFBrINCl exist as diatomic molecules. Pointer The following state symbols can be included in an equation: (s) for solid, (l) for liquid, (g) for gas and (aq) for a solution. QUICKFIRE ICKFIRE ICKFIRE  Write a balanced chemical equation for the reaction between: a) sodium carbonate and hydrochloric acid b) nitrogen dioxide (NO 2 ) and water to give nitric oxide (NO) and nitric acid. QUICKFIRE  Write an ionic equation, including state symbols for the following reactions: a) adding magnesium to hydrochloric acid b) adding a solution of lead(II) nitrate to a solution of potassium iodide to form the yellow precipitate lead(II) iodide. 10 AS Chemistry: Study and Revision Guide