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CHEMISTRY · Stage 7 · Chapter 4: Chemical Reactions 1

Chemistry 4.2 Atoms in chemical reactions — Conservation of Atoms

5 min read 7 parts Lesson 2 of 6

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4.2 Atoms in chemical reactions — Conservation of Atoms — Alhussan International Academy — Chemistry (Stages 7–9) notes: Explain why mass is conserved in chemical reactions using atoms, particle diagrams and balanced equations.In a chemical reaction the bonds between atoms in the…

  • Rearranged only
  • Rocket example
  • Law of conservation of mass
  • Calculation
  • Explanation
  • Closed system

Part 1 of 7Atoms are rearranged

  • In a chemical reaction the bonds between atoms in the reactants break and new bonds form to make the products.
  • Rearranged only — atoms are not created or destroyed, so the number of each type of atom is the same before and after.
  • Rocket example — in a rocket flame, two hydrogen molecules react with one oxygen molecule to make two water molecules.
    • Four hydrogen atoms and two oxygen atoms are present before and after.

Part 2 of 7Conservation of mass

  • Law of conservation of massthe total mass of products equals the total mass of reactants.
  • Reasonthe numbers of atoms of each element do not change, so the mass does not change.
  • Calculation32 g of sulfur reacting with 32 g of oxygen makes 64 g of sulfur dioxide.

Part 3 of 7Magnesium in a crucible

  • Method — Halim weighed magnesium and heated it in a crucible, lifting the lid so air could get in.
  • Result — the mass of magnesium at the start was 0.24 g and the magnesium oxide at the end was 0.40 g.
  • Explanation — the product is heavier because magnesium reacts with oxygen from the air, and oxygen has its own mass.
    • 0.24 g of magnesium + 0.16 g of oxygen = 0.40 g of magnesium oxide.
    • The same number of magnesium atoms and of oxygen atoms are present before and after.

Part 4 of 7Apparent changes in mass

  • Gainmagnesium burning in air makes ash heavier than the metal because oxygen joins.
  • Lossa carbonate reacting with acid in an open flask loses mass because carbon dioxide escapes.
  • Closed systemin a sealed container where the gas is trapped, the mass stays the same.
  • Rulemass changes appear only when a gas is gained or lost, so check whether the system is open or closed.

Part 5 of 7Balanced equations

  • An equation is balanced when each type of atom has the same count on both sides.
  • Steps to balance:
    • Step 1: Write the correct formula of each substance.
    • Step 2: Count the atoms of each element on each side.
    • Step 3: Place numbers in front of formulae, never changing subscripts, until the numbers match.
    • Step 4: Check by recounting.
  • Examples — balanced equations show conservation of atoms.
    • 2Mg + O2 → 2MgO (Mg + O2 → MgO is unbalanced, as it has two O on the left and one on the right).
    • CH4 + 2O2 → CO2 + 2H2O has 1 C, 4 H and 4 O on each side.
    • H2 + Cl2 → 2HCl.
    • N2 + 3H2 → 2NH3 has two nitrogen and six hydrogen atoms on both sides.
  • State symbols — (s) solid, (l) liquid, (g) gas and (aq) dissolved in water, for example Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g).

Part 6 of 7Particle diagrams of reactions

  • Draw each reactant molecule on the left and rearrange the same atoms into the product molecules on the right.
  • Label each type of atom to show none has gone missing.

Part 7 of 7Hydrogen as a fuel

  • Hydrogen buses and carshydrogen reacts with oxygen in the engine or fuel cell and the one harmless product is water.
  • Diesel busesthey make waste substances that harm health and carbon dioxide, a greenhouse gas that makes the Earth's surface hotter and causes climate change.
  • Benefitusing hydrogen instead of diesel could reduce harmful substances and greenhouse gases.
  • Limitationshydrogen is difficult to store and there are few hydrogen filling stations.

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