4. Stoichiometry

Annotations:

• Stoichiometry is the study of measuring or predicting the amounts of reactants or products in a chemical reaction based on the variables such as the mass of reactants or products, the limiting reactant and the balanced chemical equation. The principles of Stoichiometry are based upon the law of conservation of mass. Matter can neither be created nor destroyed, so you must end up with the same number of atoms (products) of each element that you started with (reactants)

1. 4.1 Stoichiometry
   1. write the formulae of simple compounds using the symbols of the elements

      Annotations:

      • Learn the following symbols (to make your life easier)H     HydrogenHe   HeliumLi     LithiumN     NitrogenO     OxygenF      FluorineP      PhosphorousS      SulfurCl     ChlorineAr     ArgonFe    IronCo    CobaltCu    CopperZn    ZincBe    BerilliumB      BoronC     CarbonNe   NeonMg   MagnesiumAl     AluminiumK     PotassiumCa   CalciumV      VanadiumCr     ChromiumBr     BromineI       IodineAg    SilverPb    Lead
      • A rule of thumb for writing formulas and giving them names is to name elements as they appear on the periodic table from left to right, eg NaCl     Sodium Chloride NO        Nitrogen Oxide ZnS       Zinc Sulfide
   2. deduce the formula of a simple compound from...
      1. relative numbers of atoms present
      2. a model or diagram
   3. construct word equations and simple balanced chemical equations

      Annotations:

      • Examples of word equations: Oxygen reacts with hydrogen to form water Copper reacts with oxygen to form copper(II)oxide Aluminium reacts with oxygen to form aluminium oxide Iron reacts with sulfur to form iron(II)sulfide
   4. define relative atomic mass, Ar

      Annotations:

      • Relative atomic mass is the average mass of naturally occurring atoms of an element on a scale where the Carbon-12 atom has a mass of exactly 12 units
   5. define relative molecular mass, Mr

      Annotations:

      • The relative molecular mass is the sum of the relative atomic masses. (Relative formula mass (Mf) will be used for ionic compounds) (calculations involving reacting masses in simple proportions may be set. Calculations will not involve the mole concept)
   6. deduce the balanced equation for a chemical reaction, given relevant information

      Annotations:

      • In chemical reactions, atoms cannot be made or destroyed. You must end up with the same number of atoms that you started with. This means the chemical reactions are balanced. 
   7. determine the formula of an ionic compound from the charges on the ions present

      Annotations:

      • For ionic compounds you have to balance the charge of each part by multiplying each ion until the sum of the charges is 0 Ionic compounds always have two parts: cations (+) are always written first, anions (-) are always written second.  Metals always lose electrons. Non-metals always gain electrons Transition metals don't always lose the same number of electrons. If you see a roman numeral in the name, it tells you how many electrons the metal lost.
      • Isn't it ironic (ionic) that opposites attract :)
   8. construct equations with state symbols, including ionic equations

      Annotations:

      • You can work out a substance state by identifying the types of bonds it forms. Ionic bond (metal with non-metal), strong attractive forces, always solid (s), can dissolve in water (aq) Covalent bond (non-metal with non-metal), weak intermolecular forces, mostly gas (g) or liquid (l)Metallic bond (metal with metal), strong attractive forces, always solid (s)
      • Ionic equations show only the ions that react and the products of their reaction Only the aqueous (aq) reactants or products are split into ions Cross out the ions that are the same on both sides (the spectator ions)  Write down only the reactants and products that are left to get the ionic equation
2. 4.2 The mole concept

   Annotations:

   • A mole is a name for a number (just like a dozen is the name for the number 12) One mole of hydrogen weighs less than one mole of oxygen (just like one dozen of quail eggs weigh less than a dozen chicken eggs or ostrich eggs)
   1. define the mole

      Annotations:

      • Mole: The mass of substance containing the same number of fundamental units as there are atoms in exactly 12.000g of Carbon-12. One Mole is the amount of substance that contains 6 x 10 to the power of 23 particles (Atoms, molecules, formula units or ions) of the substance.
   2. define the Avogadro constant

      Annotations:

      • The Avogadro constant is the number of atoms, molecules, formula units or ions in one mole of substance. The number is 6 x 10 to the power of 23
   3. calculate stoichiometric reacting masses

      Annotations:

      • We can use chemical equations and relative formula masses to calculate the mass of product obtained from a given amount of reactant. mass of substance (grams) = Moles x Mr (or Ar)  Mr is relative molecular mass Ar is relative atomic mass (Calculations involving the idea of limiting reactants may be set)
   4. calculate volumes of
      1. solutions

         Annotations:

         • volume of solution (dm3) = amount of substance (mol) divided by concentration (mol/dm3)
      2. gases

         Annotations:

         • Volume of gas (dm3) = amount of gas (mol) x 24 (questions on the gas laws and the conversion of gaseous volumes to different temperatures and pressures will NOT be set)
   5. calculate concentration of solutions in
      1. g/dm3

         Annotations:

         • The mass of solute dissolved in a solution can be found if the concentration and volume of a solution are known as well as the relative formula mass. We can use titration to calculate an unknown solution concentration
      2. mol/dm3

         Annotations:

         • Concentration (mol/dm3) = amount of substance (mol)  divided by volume of solution (dm3)
   6. limiting reactants

      Annotations:

      • (calculations involving the idea of limiting reactants may be set.) The limiting reactant is the one that is NOT in excess. We can work out which reagent is limiting by comparing the number of moles of each reactant, taking into account the stoichiometry of the equation. 
   7. calculate .... formulae
      1. empirical

         Annotations:

         • An empirical formula shows the simplest whole number ratio of atoms in a compound. The empirical formula can be found using the mass of the elements present and their relative atomic masses. eg the empirical formula of hydrogen peroxide (H2O2) is HO (the 2s should be subscript)
      2. molecular

         Annotations:

         • A molecular formula shows the actual number of atoms present in a molecule of a compound eg hydrogen peroxide is H2O2 (the 2s should be subscript)
   8. calculate percentge
      1. yield

         Annotations:

         • The percentage yield in a chemical reaction is found by comparing the actual yield with the predicted yield.  % yield = (actual yield divided by predicted yield) x 100 The predicted yield is found using relative atomic masses and teh stoichiometry of the equation. 
      2. purity

         Annotations:

         • % purity = (mass of pure product divided by mass of impure product) x 100
   9. use the molar gas volume, taken as 24 dm3 at room temperature and pressure

      Annotations:

      • The molar gas volume is given at the bottom of the periodic table you are given in the exam) 24 dm3 is the volume that one mole of any gas (be it molecular such as carbon dioxide or monoatomic such as helium) will occupy.