Summary Amount of Substance Notes - AQA Chemistry A Level

Amount of Substance
 Elements – A substance made from one type of atom
 Compounds – Two or more different atoms chemically bonded together
 Molecules – Two or more atoms covalently bonded together (atoms can be the same)

Equations
 Substance + Oxygen -> Oxide
 Metal + Water -> Metal Hydroxide + Hydrogen
 Metal + Acid -> Salt + Hydrogen
 Oxide + Acid -> Salt + Water
 Hydroxide + Acid -> Salt + Water
 Carbonate + Acid -> Salt + Water + Carbon Dioxide
 Hydrogencarbonate + Acid -> Salt + Water + Carbon Dioxide
 Ammonia + Acid -> Ammonium Salt
 Metal Carbonate -> Metal Oxide + Carbon Dioxide (On Heating)

Ionic Equations
 When an ionic substance dissolves in water, the positive and negative ions separate and
become hydrated (interact with water molecles instead of each other)
 Some of the ions may not be involved in the reaction – spectator ions
 Ionic equations can be written to only show the species involved in the reaction

Reactions of Acids
 Acid + Hydroxide
o H+(aq) + OH–(aq) → H2O(l)
 Acid + Carbonate
o H+(aq) + CO32–(aq) → H2O(l) + CO2(g)
 Acid + Hydrogencarbonate
o H+(aq) + HCO3–(aq) → H2O(l) + CO2(g)
 Acid + Ammonia
o H+(aq) + NH3(aq) → NH4+(aq)

Precipitation Reactions
 Some salts are insoluble in water
 If solutions containing these ions are formed, the insoluble salt forms as a solid as the
solutions are mixed – solid known as a precipitate and reaction as precipitation
 Most salts are soluble in water – often when two salts are mixed, no precipitation reactions
takes place and ions remain dissolved in water

Relative Mass
 Relative atomic mass (Ar) – The average mass of an atom of an element relative to 1/12 the
mass of a carbon-12 atom
 Relative formula mass (Mr) – Sum of the relative atomic masses of all the atoms in the
formula of a substance

, Moles
 One mole of anything contains 6.022x10 23 of those things – this number is known as the
Avogadro constant
 The mass of one mole of particles of a substance equals the Mr in grams



 1 ton = 1,000,000 g
 1 kg = 1,000 g
 1 mg = 0.001 g

Reacting Mass
 You can used balanced chemical equations to find out what mass of chemicals react or are
produced in a chemical reaction
 Calculate moles of substance with mass given in question
 Calculate moles of substance with mass/Mr needing to be calculated using balanced
equation
 Calculate mass/Mr of substance needed in question

Limiting Reagents
 It is rare that the exact right amount of chemicals are reacted together – normally one of the
reactants is in excess (more than needed)
 The reactant that is not in excess is called the limiting reagent
 Work out moles
 Use equation to work out how many moles of each substance are needed to completely
react
 See which substance is in excess and does not completely react
 If needed, work out change in moles of substances using moles of limiting reagent and
equation to work out mass of product

Percentage Yield
 When a new substance is formed by a chemical reaction, the expected amount of product
may not be formed
o The reaction may be reversible
o Some product may be lost when seperated from reaction mixture
o Some reactants react in other reactants
 % yield = (mass of product obtained/maximum theoretical mass of product) x 100

Atom Economy
 Measure of what proportion of the products of a reaction are the desired product and how
much is waste – higher the atom economy, the less waste produced
 % yield = ((mass/Mr of desired product)/(Total mass/Mr of products)) x 100

Empirical & Molecular Formulae
 Empirical formula – Simplest whole number ratio of stoms of each element in a substance
 Molecular formula – Number of atoms of each element in one molecule