Conservation of Mass

• In a chemical reaction, mass is always conserved
• During a chemical reaction no atoms are destroyed and no atoms are created. This means there are the smae number and types of atoms on each side of a reaction equation. This is conservation of mass
• By adding up the relative formula masses of the substances on each side of a balanced symbol equation, you can see that mass is conserved. The total Mr of all the reactants equals the total Mr of the products
• Example

  Show that mass is conserved in this reaction: 2Li + F2 -> 2LiF

  1)Add up the Ar on the left-hand side of the equation

  > 2 x Mr(Li) + 2 x Mr(F) = (2 x 7) + (2 x 19) = 14 + 38 = 52

  2) Add up the Ar on the right-hand side

  > 2 x Mr(LiF) = 2 x (7 + 19) = 2 x 26 = 52

  = The total Mr on the left and right side of the equation equals the same mass, so mass is conserved
• In a chemical reaction, if the mass seems to change, there's usually a gas involved
• If the mass in a reaction increases, it's probably because one of the reactants is a gas that's found in air (e.g. oxygen) and all the products are solids, liquids or aqueous:

  • Before the reaction, the gas is floating around in the air. It's there, but it's not contained in the reaction vessel, so you can't account for its mass
  • When the gas reacts to form part of the product, it becomes contained inside the reaction vessel - so the total mass of the stuff inside the reaction vessel increases
• When a metal reacts with oxygen in an unsealed container, the mass of the container increases. The mass if the metal oxide produced equals the total mass of the metal and the oxygen that reacted from the air

  metal(s) + oxygen(g) -> metal oxide(s)
• If the mass decreases, it's probably because one of the products is a gas and all the reactants are solids, liquids or aqueous

  • Before the reaction, all the reactants are contained in the reaction vessel
  • If the vessel isn't closed, then the gas can escape from the reaction vessel as it's formed. It's no longer contained in the reaction vessel, so you can't account for its mass - the total mass of the stuff inside the reaction vessel will decrease
• A reaction vessel is a glass container designed for use in chemical reactions
• When a metal carbonate thermally decomposes to form a metal oxide and carbon dioxide gas, the mass of the reaction vessel will decrease if it isn't sealed. But in reality, the mass of the metal oxide and the carbon dioxide produced will equal the mass of the metal carbonate that decomposed

  metal carbonate(s) -> metal oxide(s) + carbon dioxide(g)