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CHEMISTRY · Stage 9 · Chapter 12: Chemical Reactions 3 and Rates

Chemistry 12.1 Mass and energy in chemical reactions — Conservation of Mass and Energy

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12.1 Mass and energy in chemical reactions — Conservation of Mass and Energy — Alhussan International Academy — Chemistry (Stages 7–9) notes: Define conservation of mass and energy in reactions, explain apparent mass changes, and introduce symbol equations and balancing numbers.A…

  • Mass is conserved
  • Methane combustion
  • Balanced equation
  • Using conservation
  • Mass increases
  • Mass decreases

Part 1 of 5Conservation of mass

  • Atoms — in a chemical reaction no atoms are created or destroyed, they are rearranged and join together differently to make new substances.
  • Mass is conserved = the total mass of the products equals the total mass of the reactants.
  • Methane combustion — methane reacts with oxygen to make carbon dioxide and water, with 1 carbon, 4 hydrogen and 4 oxygen atoms before and after.
    • Bacteria make methane from human waste at a sewage works, and it can fuel buses.
  • Balanced equation — the number of atoms of each element is the same on both sides.
    • 24 g of magnesium reacting with 16 g of oxygen forms 40 g of magnesium oxide.
  • Using conservation — the mass of one product can be found by subtraction, for example 10.0 g of calcium carbonate giving 5.6 g of calcium oxide must give 4.4 g of carbon dioxide.

Part 2 of 5Apparent mass changes

  • Mass increases — when a solid reacts with a gas from the air, such as a metal burning in oxygen, because the oxygen is added to the solid.
  • Mass decreases — when a gas escapes from an open container, for example carbon dioxide leaving when a carbonate reacts with acid or is heated.
    • Mass of gas lost = starting mass - final mass.
  • Closed system — if the reaction is carried out in a sealed flask, the total mass is unchanged.

Part 3 of 5Experiments that show conservation

1Closed flask1 point
— a test tube of lead nitrate solution inside a flask of potassium iodide solution is stoppered and weighed, then tipped so they mix.
  • A yellow precipitate of lead iodide forms but the mass stays the same, because nothing has entered or left.
2Open container1 point
— marble chips with hydrochloric acid lose mass steadily as carbon dioxide escapes.
  • A graph of mass against time falls and levels off when the reaction finishes, and the total loss equals the mass of gas produced.
3Sealed bottle
— the mass stays constant, but the bottle may burst as pressure builds up, which is a safety hazard.

Part 4 of 5Conservation of energy

  • Energy is conserved = the total amount of energy does not change in a chemical reaction, whether it is exothermic or endothermic.
  • Exothermic — chemical energy stored in the reactants is transferred to thermal energy in the surroundings.
    • In the burning of methane, energy is transferred to the bus engine and in the end to the air.
  • Endothermic — thermal energy from the surroundings is transferred to the chemical store of the products.
  • Spreading out — some energy spreads out into the surroundings and becomes less useful.

Part 5 of 5Symbol equations

1Word equation
— methane + oxygen → carbon dioxide + water, with reactants on the left, products on the right and an arrow meaning "reacts to make".
2Symbol equation1 point
— CH4 + 2O2 → CO2 + 2H2O shows the chemical formula of each substance.
  • The formulae are CH4 for methane, O2 for oxygen, CO2 for carbon dioxide and H2O for water.
3Balancing numbers1 point
= numbers that show the relative numbers of particles of reactants and products.
  • They are written to the left of the formula, on the line and the same size as the letters.
4Meaning1 point
— one methane molecule reacts with two oxygen molecules to make one carbon dioxide molecule and two water molecules.
  • One billion methane molecules react with two billion oxygen molecules to make one billion carbon dioxide and two billion water molecules.
5Balancing and mass
— conservation of mass is the reason equations must be balanced, as in 2Mg + O2 → 2MgO.

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