Deducing Stoichiometry (Edexcel GCSE Chemistry: Combined Science)

Nothing created - nothing destroyed

• New substances are made during chemical reactions
  • However, the same atoms are always present before and after reaction
  • They have just joined up in different ways
  • Atoms cannot be created or destroyed, so if they exist in the reactants then they absolutely must be in the products!

• Because of this the total mass of reactants is always equal to the total mass of products
• This idea is known as the Law of Conservation of Mass

Conservation of Mass

• The Law of Conservation of Mass enables us to balance chemical equations, since no atoms can be lost or created
• You should be able to:
  • Write word equations for reactions outlined in these notes
  • Write formulae and balanced chemical equations for the reactions in these notes

Word Equations

• These show the reactants and products of a chemical reaction using their full chemical names
• The reactants are those substances on the left-hand side of the arrow and can be thought of as the chemical ingredients of the reaction
• They react with each other and form new substances
• The products are the new substances which are on the right-hand side of the arrow
• The arrow (which is spoken as “goes to” or “produces”) implies the conversion of reactants into products
• Reaction conditions or the name of a catalyst (a substance added to make a reaction go faster) can be written above the arrow
• An example is the reaction of sodium hydroxide (a base) and hydrochloric acid produces sodium chloride (common table salt) and water:

sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water

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
• During chemical reactions as 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

Higher Only

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

Higher Only

• Stoichiometry refers to the numbers in front of the reactants and products in an equation, which must be adjusted to make sure that the equation is balanced
• These numbers are called coefficients (or multipliers) and if we know the masses of reactants and products, the balanced chemical equation for a given reaction can be found by determining the coefficients
• First, convert the masses of each reactant and product in to moles by dividing by the molar masses using the periodic table
• If the result yields uneven numbers, then multiply all of the numbers by the same number, to find the smallest whole number for the coefficient of each species
  • For example, if the resulting numbers initially were 1, 2 and 2.5, then you would multiply all of the numbers by 2, to give the whole numbers 2, 4 and 5

• Then, use the molar ratio to write out the balanced equation