Chemical formulae and equationsIB MYP Sciences: Revision notes
Section 1
Formulae of elements and compounds
A formula uses chemical symbols and small numbers to show the atoms in a substance.
- Most elements are single atoms, such as Na, Fe, C and He.
- Some elements form diatomic molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂.
- A compound has two or more elements. The numbers show the ratio of atoms: H₂O has two hydrogen atoms to one oxygen atom.
Common formulae: CO₂, CH₄, NH₃, HCl, NaCl, MgO, CaCO₃ and H₂SO₄. Names ending in -ide usually have two elements, such as sodium chloride. Names ending in -ate also contain oxygen, such as calcium carbonate.
Do not write oxygen as O in an equation. Oxygen gas is O₂.
Section 2
Word equations
A word equation shows the reactants on the left and the products on the right, with an arrow showing the direction of the reaction.
magnesium + oxygen → magnesium oxide
calcium carbonate + hydrochloric acid → calcium chloride + water + carbon dioxide
Reactants are the substances you start with. Products are the new substances made.
Section 3
Balancing symbol equations
A symbol equation uses formulae. It must be balanced, with the same number of each type of atom on both sides.
Worked example: magnesium + oxygen
- Write the formulae: Mg + O₂ → MgO
- Count: two O on the left, one on the right
- Put 2 in front of MgO: Mg + O₂ → 2MgO
- Now two Mg are needed on the left: 2Mg + O₂ → 2MgO
Another: CH₄ + 2O₂ → CO₂ + 2H₂O
You change the big numbers in front only. Never change the small numbers inside a formula.
Never change a small number in a formula to balance an equation. Changing H₂O to H₂O₂ would make a different substance.
Section 4
Conservation of mass
In a chemical reaction, atoms are not created or destroyed. They are rearranged, so the total mass of the products equals the total mass of the reactants.
In an open system, the balance reading can seem to change:
- If a gas escapes, the mass seems to fall, such as carbon dioxide leaving a flask.
- If a gas from the air joins the solid, the mass seems to rise. For example, 0.24 g of magnesium burns with oxygen to give 0.40 g of magnesium oxide, and the extra 0.16 g is oxygen.
In a closed system, nothing can enter or escape, so the mass stays the same.
If a mass changes in an open container, ask yourself which gas has escaped or been taken in from the air.
Section 5
State symbols
State symbols show the physical state of each substance:
- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous, which means dissolved in water
Example: CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)
Must Know
- Write O₂, H₂, N₂ and Cl₂ for gases. Balance with big numbers only.
- Atoms are conserved, so mass is conserved.
- Open system: gas escapes (mass falls) or gas joins from the air (mass rises).
- State symbols: (s), (l), (g), (aq).
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Chemical formulae and equations
- A teacher in Lagos burns 0.24 g of magnesium ribbon in an open crucible. The shiny grey metal burns with a bright white flame and leaves a white powder, magnesium oxide. When the powder has cooled, she finds that its mass is 0.40 g.Explain why the mass of the white powder is greater than the mass of the magnesium, even though mass is conserved in chemical reactions.2 marks
- A student in Mumbai places an open flask containing calcium carbonate chips and dilute hydrochloric acid on a balance. The reaction forms calcium chloride solution, water and carbon dioxide gas. Bubbles are seen, and over three minutes the reading on the balance falls from 150.0 g to 148.9 g.Explain why the total mass of reactants and products is the same in this reaction, even though the balance reading fell.2 marks
- A student in Seoul heats 3.18 g of shiny pink-brown copper foil in an open crucible. The copper turns black. When the crucible has cooled, the black solid, copper oxide, has a mass of 3.98 g.Write a word equation for the reaction, then a balanced symbol equation with state symbols.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).