AQA AS Chemistry

Revision Notes

1.2.3 Balanced Equations

Deducing Formulae of Compounds

  • Ionic compounds are formed from a metal and a non-metal bonded together
  • Ionic compounds are electrically neutral; the positive charges equal the negative charges

Charges on positive ions

  • All metals form positive ions
    • There are also some non-metal positive ions, such as ammonium, NH4+, and hydrogen, H+
  • The metals in Group 1, Group 2 and Group 3 (13) have a charge of 1+ and 2+ and 3+ respectively
  • The charge on the ions of the transition elements can vary which is why Roman numerals are often used to indicate their charge
  • Roman numerals are used in some compounds formed from transition elements to show the charge (or oxidation state) of metal ions
    • E.g. in copper (II) oxide, the copper ion has a charge of 2+ whereas in copper (I) nitrate, the copper has a charge of 1+

Non-metal ions

  • The non-metals in group 15 to 17 have a negative charge and have the suffix ‘ide
    • E.g. nitride, chloride, bromide, iodide
  • Elements in group 17 gain 1 electron so have a 1- charge, eg. Br
  • Elements in group 16 gain 2 electrons so have a 2- charge, eg. O2-
  • Elements in group 15 gain 3 electrons so have a 3- charge, eg. N3-
  • There are also polytatomic negative ions, which are negative ions made up of more than one type of atom

 

Atoms, Molecules & Stoichiometry Ionic Charges, downloadable AS & A Level Chemistry revision notes

The charges of simple ions depend on their position in the Periodic Table

Formulae of ionic compounds table

Atoms, Molecules & Stoichiometry Formulae of Ionic Compounds Table, downloadable AS & A Level Chemistry revision notes

Worked Example

Formulae

Determine the formulae of the following ionic compounds

  1.    Magnesium chloride
  2.    Iron(III) oxide
  3.    Aluminium nitrate

Answer

Answer 1: Magnesium chloride

    • Magnesium is in Group 2 so has a charge of 2+
    • Chlorine is in group 17 so has a charge of 1-
    • Magnesium needs two chloride ions for each magnesium ion to be balanced so the formula is MgCl2

Answer 2: Iron (III) oxide

    • The Roman numeral states that iron has a charge of 3+
    • Oxygen is in group 16 so has a charge of 2-
    • The charges need to be equal so 2 iron ions to 3 oxide ions will balance electrically, so the formula is Fe2O3

Answer 3: Aluminum nitrate

    • Aluminium is in group 13 so has a charge of 3+
    • Nitrate is a polyatomic ion and has a charge of 1-
    • The polyatomic ion needs to be placed in a bracket if more than 1 is needed
    • The formula of aluminium nitrate is Al(NO3)3

Exam Tip

Remember: polyatomic ions are ions that contain more than one type of element, such as OH

Balancing Equations

  • A symbol equation is a shorthand way of describing a chemical reaction using chemical symbols to show the number and type of each atom in the reactants and products
  • A word equation is a longer way of describing a chemical reaction using only words to show the reactants and products

Balancing equations

  • During chemical reactions, atoms cannot be created or destroyed
  • The number of each atom on each side of the reaction must therefore be the same
    • E.g. the reaction needs to be balanced
  • When balancing equations remember:
    • Not to change any of the formulae
    • To put the numbers used to balance the equation in front of the formulae
    • To balance firstly the carbon, then the hydrogen and finally the oxygen in combustion reactions of organic compounds
  • When balancing equations follow the following the steps:
    • Write the formulae of the reactants and products
    • Count the numbers of atoms in each reactant and product
    • Balance the atoms one at a time until all the atoms are balanced
    • Use appropriate state symbols in the equation
  • The physical state of reactants and products in a chemical reaction is specified by using state symbols
    • (s) solid
    • (l) liquid
    • (g) gas
    • (aq) aqueous

Ionic Equations

Ionic equations

  • In aqueous solutions ionic compounds dissociate into their ions
  • Many chemical reactions in aqueous solutions involve ionic compounds, however only some of the ions in solution take part in the reactions
  • The ions that do not take part in the reaction are called spectator ions
  • An ionic equation shows only the ions or other particles taking part in a reaction, and not the spectator ions

Worked Example

Balance the following equation:

magnesium + oxygen magnesium oxide

Answer:

Step 1: Write out the symbol equation showing reactants and products

Mg + O2 → MgO

Step 2: Count the numbers of atoms in each reactant and product

Atoms, Molecules & Stoichiometry Worked example - Balancing equations table, downloadable AS & A Level Chemistry revision notes

Step 3: Balance the atoms one at a time until all the atoms are balanced

2Mg + O2 → 2MgO

This is now showing that 2 moles of magnesium react with 1 mole of oxygen to form 2 moles of magnesium oxide

Step 4: Use appropriate state symbols in the fully balanced equation

2Mg (s) + O2 (g) → 2MgO (s)

Worked Example

  1. Balance the following equation

zinc + copper(II) sulfate zinc sulfate + copper

     2. Write down the ionic equation for the above reaction

Answer 1:

Step 1: To balance the equation, write out the symbol equation showing reactants and products

Zn  + CuSO4  → ZnSO4 + Cu

Step 2: Count the numbers of atoms in each reactant and product. The equation is already balanced

Atoms, Molecules & Stoichiometry Worked example - Equations (balancing & ionic) table, downloadable AS & A Level Chemistry revision notes

Step 3: Use appropriate state symbols in the equation

Zn (s)  + CuSO4 (aq)  → ZnSO4 (aq) + Cu (s)

Answer 2:

Step 1:  The full chemical equation for the reaction is

Zn (s)  + CuSO4 (aq)  → ZnSO4 (aq) + Cu (s)

Step 2:  Break down reactants into their respective ions

Zn (s)  + Cu2+ +  SO42- (aq)  → Zn2++ SO42- (aq) + Cu (s) 

Step 3:  Cancel the spectator ions on both sides to give the ionic equation

Zn (s)  + Cu2+ +  SO42- (aq)  → Zn2+SO42- (aq) + Cu (s)

Zn (s)  + Cu2+(aq)  → Zn2+ (aq) + Cu (s)

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