Writing and balancing equationsIB MYP Chemistry: Revision notes
Section 1
Word equations
A word equation shows the names of the reactants (the substances you start with) on the left and the products (the substances made) on the right, joined by an arrow.
Example: zinc + hydrochloric acid → zinc chloride + hydrogen
The arrow means 'reacts to make'. Word equations tell you what is happening but not how much of each substance is involved.
Section 2
Symbol equations and state symbols
A symbol equation uses formulae instead of names. Remember that seven elements exist as diatomic molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂. Write them with the 2 when they are on their own.
State symbols go after each formula:
- (s) solid
- (l) liquid
- (g) gas
- (aq) aqueous: dissolved in water
Example: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)
Writing oxygen as O instead of O₂. The element oxygen exists as O₂ molecules, so a single O is wrong.
Section 3
Balancing equations
A balanced equation has the same number of each type of atom on both sides.
- Write the correct formulae first. Never change them.
- Count the atoms of each element on each side.
- Put large numbers in front of formulae to balance one element at a time.
- Recount every element to check.
Worked example: methane burning. CH₄ + O₂ → CO₂ + H₂O
Carbon is balanced (1 and 1). Hydrogen: 4 on the left, 2 on the right, so write 2H₂O. Oxygen is now 2 on the left and 2 + 2 = 4 on the right, so write 2O₂.
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)
Check: 1 C, 4 H and 4 O on each side.
Changing the small number in a formula to balance. Changing H₂O to H₂O₂ makes a different substance. Only change the large numbers in front.
Balance the elements that appear in only one substance on each side first, and leave oxygen or hydrogen until last.
Section 4
Conservation of mass
In any chemical reaction atoms are not created or destroyed; they are only rearranged into new substances. So the total mass of the products always equals the total mass of the reactants. This is the law of conservation of mass, and it is why equations must balance.
In a closed system (nothing can enter or leave) the total mass stays exactly the same.
The mass can seem to change in an open container:
- Mass decreases if a gas escapes. Marble chips and acid in an open flask lose the carbon dioxide made.
- Mass increases if a gas from the air joins the solid. Magnesium burning in air gains mass because it combines with oxygen.
In both cases the atoms are still there; some are simply outside the container or have come in from outside.
Saying mass is 'lost' when a gas escapes. The mass is not destroyed; the gas has simply left the container.
Must Know
- Word equations use names; symbol equations use formulae
- State symbols: (s), (l), (g), (aq)
- Balance with large numbers in front only; never change a formula
- Seven diatomic elements: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂
- Mass is conserved because atoms are only rearranged; apparent changes are due to gases entering or leaving
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Writing and balancing equations
- In a school laboratory in Dubai, students add small pieces of zinc to dilute hydrochloric acid in a test tube. Bubbles of hydrogen gas form and the zinc slowly disappears, leaving a colourless solution of zinc chloride.Explain why the number 2 is written in front of HCl in the balanced equation.2 marks
- A student in Lagos places a conical flask containing marble chips (calcium carbonate) and dilute hydrochloric acid on a balance. Cotton wool is pushed into the neck of the flask. As the reaction takes place the balance reading falls by 0.64 g, even though no solid or liquid has left the flask.A different student does the same reaction in a strong sealed container. Explain why the total mass would not change.2 marks
- A student investigates whether mass changes during a reaction. She places a small test tube of sodium carbonate solution inside a conical flask containing calcium chloride solution, seals the flask with a stopper and weighs it. She tips the flask so that the two solutions mix and a white precipitate of calcium carbonate forms. She then weighs the sealed flask again. Her balance reads to 0.01 g. The mass of the sealed flask before and after mixing was: trial 1, 215.32 g then 215.32 g; trial 2, 187.45 g then 187.45 g; trial 3, 201.18 g then 201.17 g.State a testable hypothesis for this investigation, give a scientific reason for it, and identify the dependent variable.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).