Conservation of Mass

• Conservation of Mass

  In a chemical reaction, the total mass of the products is equal to the total mass of the reactants
• Conservation of Mass

  • This idea is called the conservation of mass
  • Mass is conserved (kept the same) because no atoms are lost or made
  • Chemical symbol equations must always be balanced to show this, i.e. there must be the same number of atoms of each element on both sides of the equation
• Conservation of Mass

  • When solid iron reacts with copper sulfate solution, a reaction takes place, producing solid copper and iron sulfate solution: Fe + CuSO4 --> Cu + FeSO4
• Half equation

  1. The iron atoms lose two electrons to form Fe2+ ions: Fe --> Fe2+ + 2e-

  2. The Cu2+ ions gain two electrons to form copper atoms: Cu2+ + 2e- --> Cu
• Ionic equation

  Simplifies complicated equations by just showing the species that are involved in the reaction, without the spectator ions (ions not involved in the reaction)
• Ionic equation

  • When a silver nitrate solution is added to sodium chloride solution, a white precipitate of silver chloride is produced: Ag+ + Cl- --> AgCl
• Species

  The different atoms, molecules or ions that are involved in a reaction
• Key Point

  The total mass of the products of a chemical reaction is always equal to the total mass of the reactants. This is because no atoms are lost or made. The products are made from exactly the same atoms as the reactants
• Relative Formula Mass

  . The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in the numbers shown in the formula. It does not have a unit

  . The relative atomic masses of the atoms are shown in the periodic table

  . Due to conservation of mass, the sum of the relative formula masses of all the reactants is always equal to the sum of the relative formula masses of all the products
• Magnesium burned in air to produce magnesium oxide

  • Mass of the solid increases
  • Magnesium combines with oxygen from the air and the oxygen has mass
• When the mass of oxygen is included

  The total mass of all the reactants is equal to the total mass of all the products
• Calcium carbonate heated

  • Decomposes to form calcium oxide and carbon dioxide
  • Mass of the solid decreases because one of the products is a gas, which escapes into the air
• When the mass of carbon dioxide is included

  The total mass of all the reactants is equal to the total mass of all the products
• Key Point

  Some reactions appear to involve a change in mass. This happens when reactions are carried out in a non-closed system, so gases can enter or leave