SPI Chemistry · Formula sheet
Chemical Reactions formulas
2 formulas, 4 reference tables and 12 common traps for SPI Chemistry Chemical Reactions, grouped by subtopic.
Chemical Equations and Types of Reactions
Learn this subtopic in the notesReactants, products and balanced equations
Balancing rule
Combination and decomposition
| Reaction | Type | Equation |
|---|---|---|
| Quicklime and water | Combination, exothermic | |
| Heating ferrous sulphate | Decomposition by heat | Light green crystals, reddish brown residue, suffocating smell of sulphur dioxide. |
| Heating limestone | Decomposition by heat | |
| Silver chloride in sunlight | Decomposition by light | |
| Electrolysis of water | Decomposition by electricity |
Count the substances: many into one is combination, one into many is decomposition.
Displacement, double displacement and precipitation
| 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 |
| 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.
Common traps
Changing a formula to balance
Only the numbers in front of the formulae may change. Turning into balances nothing: it writes a different substance.
Writing the state at room temperature
State symbols give the state at the reaction temperature. Hydrogen burning in oxygen releases so much heat that the water formed is steam, (g), not liquid.
Mixing up the two names
Combination makes one product from two or more reactants. Decomposition starts from one reactant and gives two or more products.
Thinking decomposition happens by itself
A decomposition needs energy put in: heat for ferrous sulphate and limestone, light for silver chloride, electricity for water.
Expecting any metal to displace any other
Displacement goes one way only: a more reactive metal pushes out a less reactive one. Copper put into iron sulphate solution does nothing.
Calling a precipitate a crystal
The solid that settles out of a reacting solution is a precipitate, not a crystal. A precipitate is an insoluble product of the reaction; crystals grow slowly from a solution as it cools or evaporates.
Oxidation, Corrosion and Rancidity
Learn this subtopic in the notesOxidation and reduction
Redox in one line
Corrosion and rusting
| 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 it
| 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.
Common traps
Looking for oxidation on its own
Oxidation and reduction always happen together. If one substance gains oxygen, another in the same reaction has lost it.
Saying the oxidising agent is oxidised
The oxidising agent gives away oxygen, so it is reduced. The substance it oxidises is the one that gains oxygen.
Calling rust plain ferric oxide
Rust is hydrated ferric oxide, . Leaving out the water of hydration gives a different compound.
Thinking water alone rusts iron
Iron needs air and water together. In dry air, or in water with no dissolved air, it does not rust.
Blaming bacteria for rancidity
Rancidity is the oxidation of fats and oils by air, not the work of bacteria. Antioxidants stop it; antibiotics would not.
Thinking any gas will do in a chips packet
The gas must be unreactive. Nitrogen is used because it does not react with the oil and keeps oxygen out.