R.1 Chemical Reactions & Equations

UNIT SYLLABUS

MYP

A chemical reaction rearranges atoms into new substances — nothing is created, nothing destroyed. That single principle, conservation of mass, is why equations must balance: every atom entering a reaction must leave in some product. You will learn to spot the evidence of reaction (colour change, gas, precipitate, temperature change), to write word and symbol equations, and to recognise the classic reaction types — combustion, neutralisation, displacement, thermal decomposition — that appear in every MYP paper.

Guiding Questions

? How do we know a chemical reaction has happened rather than a physical change?
? If atoms are never created or destroyed, what does a chemical equation actually describe?

✓ What the IB expects you to master

Identify evidence of chemical change: new substance formed, colour change, gas evolved, precipitate, energy change.
State the law of conservation of mass and use it to explain why equations balance.
Write word equations and balanced symbol equations with state symbols (s, l, g, aq).
Recognise and give examples of combustion, neutralisation, displacement and thermal decomposition.
Describe the reactivity series of metals and use it to predict displacement reactions.
Describe the tests for hydrogen (squeaky pop), oxygen (relights glowing splint), and carbon dioxide (limewater turns milky).
Explain apparent mass changes in open systems (gases escaping or being absorbed).

1 Key Formulas

2 Exam Preparation & Topic Explanations

Balancing without panic (Criteria A & C)

Balance systematically: count each element left and right; adjust coefficients only (never subscripts); leave lone elements (O₂, H₂, metals) until last; then recount everything. If an odd number of oxygens appears, doubling everything often resolves it.

Conservation-of-mass questions in open systems are a favourite: name the gas that entered or escaped and the numbers reconcile.

Pro Exam Strategy

State symbols matter for full marks: (s), (l), (g), (aq) — especially in precipitation reactions.
The three gas tests are near-guaranteed marks: hydrogen pops, oxygen relights, CO₂ milks limewater.
Metals burning in air GAIN mass (oxygen joins); carbonates decomposing LOSE mass (CO₂ leaves).
Learn the short reactivity series: K, Na, Ca, Mg, Al, Zn, Fe, Cu, Ag, Au — displacement follows it exactly.

3 MCQ Practice

Q1. In the balanced equation $2Mg + O_2 \rightarrow 2MgO$ , the 2 in front of Mg means:

Magnesium has a charge of 2+
Two atoms of magnesium react for each oxygen molecule
Magnesium is in Group 2
The reaction happens twice

Q2. 10 g of calcium carbonate is heated in an open crucible and the remaining solid weighs 5.6 g. The "missing" mass:

Was destroyed by the heat
Escaped as carbon dioxide gas
Turned into energy
Shows the balance was faulty

Q3. Zinc displaces copper from copper sulfate solution but not magnesium from magnesium sulfate. This shows the reactivity order is:

Copper > zinc > magnesium
Zinc > magnesium > copper
Magnesium > zinc > copper
Zinc > copper > magnesium

4 Short Answer Questions

PDF

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