Types of chemical reactionIB MYP Sciences: Revision notes
Section 1
Synthesis and decomposition
In a synthesis reaction, two or more substances join to make one new substance: iron + sulfur → iron sulfide.
In a decomposition reaction, one substance breaks down into two or more simpler substances. Many need heat: copper carbonate → copper oxide + carbon dioxide.
Section 2
Combustion, oxidation and reduction
Combustion is burning in oxygen and releases energy as heat and light: methane + oxygen → carbon dioxide + water.
Oxidation is the gain of oxygen. Reduction is the loss of oxygen. They happen together in one reaction. For example, in iron oxide + carbon → iron + carbon dioxide, the iron oxide is reduced and the carbon is oxidised.
Oxidation is gain of oxygen and reduction is loss of oxygen. A reaction that is one is always the other too.
Section 3
Displacement
In a displacement reaction, a more reactive element takes the place of a less reactive element in a compound.
iron + copper sulfate → iron sulfate + copper
Iron is more reactive than copper, so it displaces it. The solution turns paler and pink-brown copper coats the iron.
Section 4
Neutralisation and precipitation
Neutralisation: an acid reacts with an alkali to make a salt and water: hydrochloric acid + sodium hydroxide → sodium chloride + water. The mixture becomes neutral and warms up.
Precipitation: two solutions react to make an insoluble solid called a precipitate. For example, silver nitrate solution + sodium chloride solution → silver chloride (a white solid) + sodium nitrate solution.
Section 5
Evidence of chemical change
A chemical change makes new substances. Signs include:
- a colour change
- a gas given off (bubbles)
- a solid forming from a solution
- a temperature change
- a flame or light
A physical change, such as melting or dissolving, makes no new substance and can be reversed easily.
A colour change alone does not prove a chemical change. Look for the formation of a new substance.
Section 6
Exothermic and endothermic reactions
An exothermic reaction gives out energy to the surroundings, so the temperature rises. Examples are combustion and neutralisation.
An endothermic reaction takes in energy from the surroundings, so the temperature falls, or heating must continue. Examples are the decomposition of copper carbonate and photosynthesis.
Must Know
- Synthesis joins substances. Decomposition splits one substance.
- Oxidation is gain of oxygen and reduction is loss of oxygen.
- Displacement: a more reactive element replaces a less reactive one.
- Neutralisation: acid + alkali → salt + water. Precipitation forms an insoluble solid.
- Exothermic: temperature rises. Endothermic: temperature falls.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Types of chemical reaction
- In a school laboratory in Cairo, a student heats a spatula of green copper carbonate powder in a test tube using a Bunsen burner. After a few minutes the powder has turned black. A gas is given off that turns limewater cloudy. When the student stops heating, the reaction stops.Explain, using the observations, why this is a chemical change.2 marks
- A student in Johannesburg places a clean iron nail in a beaker of blue copper sulfate solution. After 20 minutes the nail is coated with a pink-brown solid and the blue colour of the solution has become paler.Write a word equation for the reaction.2 marks
- A student in Mexico City mixes 25 cm³ of dilute hydrochloric acid with 25 cm³ of dilute sodium hydroxide solution in a polystyrene cup. Both solutions start at 21.0 °C. After stirring, the highest temperature reached is 27.5 °C, and universal indicator added to the mixture turns green.Identify the type of reaction, name the two products and explain what the temperature rise shows.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).