Types of chemical reactionIB MYP Chemistry: Revision notes
Section 1
Synthesis and decomposition
In a synthesis reaction two or more simple substances join to make one product: A + B → AB.
Example: 2Mg + O₂ → 2MgO, and N₂ + 3H₂ → 2NH₃.
In a decomposition reaction one compound breaks down into two or more simpler substances: AB → A + B. In thermal decomposition the break-down is caused by heating.
Example: CaCO₃(s) → CaO(s) + CO₂(g). The solid loses mass because the carbon dioxide escapes.
Section 2
Combustion
Combustion is burning: a substance reacts quickly with oxygen and gives out heat (it is exothermic), often with a flame.
- Metals form metal oxides: 2Mg + O₂ → 2MgO
- Hydrocarbons burn completely to carbon dioxide and water: CH₄ + 2O₂ → CO₂ + 2H₂O
A word equation pattern is: fuel + oxygen → oxides of the elements in the fuel.
Section 3
Single displacement
In a single displacement reaction a more reactive element takes the place of a less reactive element in a compound: A + BC → AC + B.
A more reactive metal displaces a less reactive metal from a solution of its salt. Reactivity order (most to least): magnesium, zinc, iron, copper.
Example: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). The zinc becomes coated with brown copper and the blue colour fades.
If the metal in the solution is more reactive than the metal added, there is no reaction.
Section 4
Neutralisation and precipitation
Neutralisation: an acid reacts with an alkali (or base) to form a salt and water.
HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
Precipitation: two solutions react to form an insoluble solid called a precipitate.
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
The (s) in a product after reactants that are (aq) is the clue that a precipitate has formed.
Writing the precipitate as (aq). A precipitate is a solid, so it must be (s).
Section 5
Redox
A redox reaction is one in which oxidation and reduction happen together.
- Oxidation: gain of oxygen or loss of electrons
- Reduction: loss of oxygen or gain of electrons
Example: CuO + H₂ → Cu + H₂O. Copper(II) oxide loses oxygen, so it is reduced; hydrogen gains oxygen, so it is oxidised.
In displacement reactions metal atoms lose electrons: Zn → Zn²⁺ + 2e⁻ (oxidation), and Cu²⁺ + 2e⁻ → Cu (reduction).
Remember OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain.
Section 6
Identifying the type of reaction
Look at the pattern of the equation:
- One product from two or more reactants: synthesis
- One reactant, several products (heated): thermal decomposition
- A substance reacts with oxygen and gives out heat: combustion
- An element and a compound swap partners: single displacement
- Acid + alkali → salt + water: neutralisation
- Two solutions → an insoluble solid (s): precipitation
Some reactions fit more than one type. Magnesium burning is synthesis, combustion and redox at the same time.
If a question asks for 'the' type, choose the one that matches the description given in the question.
Must Know
- Synthesis A + B → AB; decomposition AB → A + B
- Combustion: reaction with oxygen that releases heat
- Single displacement: more reactive element replaces a less reactive one
- Neutralisation: acid + alkali → salt + water
- Precipitation: solution + solution → insoluble solid
- Redox: oxidation (loss of electrons) and reduction (gain) together
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Types of chemical reaction
- A teacher in Ghana heats green copper(II) carbonate powder, CuCO₃, in a test tube. The powder turns black as copper(II) oxide forms, and a gas that turns limewater milky is given off.Explain what is meant by thermal decomposition, and describe how the mass of the solid in the test tube changes.2 marks
- A metalworker in Peru places a clean iron nail in blue copper(II) sulfate solution. After some minutes the nail is coated with a brown solid and the blue solution fades to pale green as iron(II) sulfate forms.Write the balanced symbol equation, with state symbols, for the reaction.2 marks
- A student tests three metals, magnesium, zinc and copper. She places a strip of each metal into separate solutions of the sulfates of the other two metals and records what happens. Magnesium in zinc sulfate solution: a grey coating forms. Magnesium in copper(II) sulfate solution: a brown coating forms and the blue colour fades. Zinc in copper(II) sulfate solution: a brown coating forms and the blue colour fades. Zinc in magnesium sulfate solution: no change. Copper in zinc sulfate solution: no change. Copper in magnesium sulfate solution: no change. Each test used the same volume of solution at room temperature.State a testable hypothesis for the student's investigation, give a scientific reason, and identify the independent 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).