SPI Chemistry · Free notes

Chemical Reactions: SPI Chemistry

Chemical Reactions has 8 questions in the SPI bank, and two of them are from the 2022 paper: one names a combination reaction, the other asks why oily food goes rancid. Five are EASY, two MODERATE and one HARD. Most ask for a name: reactants, a precipitate, rust, a type of reaction. The two that need more thought ask you to read a reaction: the state of water in a hot flame, and what is left when green crystals are heated.

The handout has every subtopic, worked example, formula and trap with answers shown. The formula sheet is the chapter’s formulas and traps alone, on one page.

SPI home All formulas on one page

Subtopic notes

Formula & revision sheet

2 formulas · 4 reference tables · 12 gotchas across all subtopics — the exam-eve cheat-sheet

Chemical Equations and Types of Reactions

Formulas (1)

Reactants, products and balanced equations · Balancing rule $\text{atoms of each element on the left} = \text{atoms of that element on the right}$

Reference tables (2)

Combination and decomposition5 rows

Reaction	Type	Equation
Quicklime and water	Combination, exothermic	$\mathrm{CaO + H_2O \rightarrow Ca(OH)_2}$
Heating ferrous sulphate	Decomposition by heat	$\mathrm{2FeSO_4 \rightarrow Fe_2O_3 + SO_2 + SO_3}$ Light green crystals, reddish brown residue, suffocating smell of sulphur dioxide.
Heating limestone	Decomposition by heat	$\mathrm{CaCO_3 \rightarrow CaO + CO_2}$
Silver chloride in sunlight	Decomposition by light	$\mathrm{2AgCl \rightarrow 2Ag + Cl_2}$
Electrolysis of water	Decomposition by electricity	$\mathrm{2H_2O \rightarrow 2H_2 + O_2}$

Count the substances: many into one is combination, one into many is decomposition.

Displacement, double displacement and precipitation4 rows

Reaction	Type	What you see
Iron nail in copper sulphate solution	Displacement	Brown copper coats the nail; blue solution turns pale green
Sodium sulphate and barium chloride	Double displacement, precipitation	White precipitate of barium sulphate $\mathrm{Na_2SO_4 + BaCl_2 \rightarrow BaSO_4 + 2NaCl}$
Lead nitrate and potassium iodide	Double displacement, precipitation	Yellow precipitate of lead iodide
Copper in iron sulphate solution	No reaction	Nothing: copper is less reactive than iron

A metal can push out only a metal less reactive than itself.

Watch out for (6)

Changing a formula to balance
→ Reactants, products and balanced equations
Writing the state at room temperature
→ Reactants, products and balanced equations
Mixing up the two names
→ Combination and decomposition
Thinking decomposition happens by itself
→ Combination and decomposition
Expecting any metal to displace any other
→ Displacement, double displacement and precipitation
Calling a precipitate a crystal
→ Displacement, double displacement and precipitation

Oxidation, Corrosion and Rancidity

Formulas (1)

Oxidation and reduction · Redox in one line $\text{oxidation} = +\,\mathrm{O} \text{ or } -\,\mathrm{H} \qquad \text{reduction} = -\,\mathrm{O} \text{ or } +\,\mathrm{H}$

Reference tables (2)

Corrosion and rusting3 rows

Metal	Coating that forms	Colour
Iron	Hydrated iron(III) oxide (rust)	Reddish brown Ferric means iron(III); rust carries water, so plain ferric oxide is not rust.
Silver	Silver sulphide	Black
Copper	Basic copper carbonate	Green

Galvanising coats iron with zinc; the zinc corrodes first and protects the iron.

Rancidity and how to prevent it4 rows

Method	How it helps
Adding antioxidants	They slow the oxidation of fats and oils
Filling packets with nitrogen	Nitrogen pushes out the air, so no oxygen reaches the oil Nitrogen, not oxygen: it is chosen because it does not react.
Airtight containers	Less air reaches the food
Refrigeration	Low temperature slows the reaction

Every method either keeps oxygen out or slows the oxidation down.

Watch out for (6)

Looking for oxidation on its own
→ Oxidation and reduction
Saying the oxidising agent is oxidised
→ Oxidation and reduction
Calling rust plain ferric oxide
→ Corrosion and rusting
Thinking water alone rusts iron
→ Corrosion and rusting
Blaming bacteria for rancidity
→ Rancidity and how to prevent it
Thinking any gas will do in a chips packet
→ Rancidity and how to prevent it

Question weightage by concept

6 concepts · 8 questions — where the marks actually sit, so you know what to drill first

Chemical Equations and Types of Reactions6 questions · 75%

Concept	Qs	Share
Reactants, products and balanced equations	3	38%
Combination and decomposition	2	25%
Displacement, double displacement and precipitation	1	13%

Oxidation, Corrosion and Rancidity2 questions · 25%

Concept	Qs	Share
Corrosion and rusting	1	13%
Rancidity and how to prevent it	1	13%
Oxidation and reductionfoundation	—	—

Test yourself on a real paper

Sit a past SPI paper, timed and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

Sit an SPI past paper