SPI Chemistry · Chemical Reactions
Chemical Equations and Types of Reactions
A chemical equation shows reactants turning into products with the same atoms on both sides, and most reactions fall into a few named types.
Why this matters
Six questions in the bank, three EASY, two MODERATE and one HARD, one of them from the 2022 paper (naming a combination reaction). The rest ask for the word reactants, the right state symbols, what colliding molecules need, the light green crystals that leave a reddish brown residue, and the name for a settled solid.
Concept 1 of 3: Reactants, products and balanced equations
In a reaction, atoms are not made or destroyed: they are only rearranged. So an equation must have the same number of each kind of atom on both sides. Balance it by changing the numbers in front of the formulae, never the formulae themselves.
Definition
- Reactants are the substances that react, written on the left. Products are the new substances, on the right.
- State symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water. They give the state at the temperature of the reaction.
- A balanced equation obeys the law of conservation of mass: each element has the same number of atoms on both sides.
- For a reaction to happen, reactant particles must collide with enough energy and with the right orientation.
Balancing rule
Worked example
Balance the equation .
Practice this conceptself-check · 5 quick reps
The same idea in a practice question:
Practice questionModerate
Example 1 · Chemical Reactions · Chemical Equations and Types of Reactions
In which of the following chemical equations, the abbreviations represent the correct states of the reactants and products involved at reaction temperature?
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.
Concept 2 of 3: Combination and decomposition
Combination and decomposition are opposites. In combination, two or more substances join into one. In decomposition, one substance breaks into two or more, and it needs energy to do it: heat, light or electricity.
Definition
- Combination: . Burning coal, , is one. Most give out heat (exothermic).
- Decomposition: . Most take in energy (endothermic).
- By heat (thermal): light green ferrous sulphate crystals first lose their water, then give reddish brown ferric oxide and two oxides of sulphur with a burning-sulphur smell.
- By light: silver chloride turns grey in sunlight. By electricity: water splits into hydrogen and oxygen in the volume ratio 2 : 1.
| 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.
Practice this conceptself-check · 4 quick reps
The same idea in a real exam question:
SPI · 2022 · SPI Entrance Examination 2022 · Q93Easy
Example 2 · Chemical Reactions · Chemical Equations and Types of Reactions
When two or more reactants in a reaction, form a single product, it is called____ reaction.
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.
Concept 3 of 3: Displacement, double displacement and precipitation
In displacement, a more reactive element pushes a less reactive one out of its compound. In double displacement, two compounds swap partners. When one new product does not dissolve, it appears as a solid in the solution and settles.
Definition
- Displacement: a more reactive metal takes the place of a less reactive one. Iron in copper sulphate: .
- Double displacement: two compounds exchange ions: .
- A precipitate is an insoluble solid formed in a solution during a reaction; it settles down. A reaction that gives one is a precipitation reaction.
- Neutralisation (acid + base) is also a double displacement.
| 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.
Practice this conceptself-check · 4 quick reps
The same idea in a practice question:
Practice questionEasy
Example 3 · Chemical Reactions · Chemical Equations and Types of Reactions
What is called ‘settled solid’ in chemical reaction?
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.
Summary — formulas & gotchas at a glance
A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.
Formulas (1)
- Reactants, products and balanced equations
Balancing rule
Reference tables (2)
Combination and decomposition5 rows
| 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 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 |
| 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
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.