Cambridge IGCSE · Chemistry 0620
IGCSE Chemistry: Chemical energetics — Practice Questions & Answers
How chemical reactions release or absorb heat, why bonds breaking and forming change energy, and calculating energy from bond energies.
474 practice questions available for this topic — here are 10 with full answers and explanations.
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Practice questions with answers
1
Multiple choice · Easy
Which statement correctly describes an exothermic reaction?
- Thermal energy is transferred to the surroundings, raising their temperature
- Thermal energy is taken in from the surroundings, lowering their temperature
- No energy change occurs during the reaction
- Energy is only released as light, never as heat
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WhyExothermic reactions transfer thermal energy to the surroundings, so the temperature of the surroundings rises.
2
Multiple choice · Easy
What is the sign of the enthalpy change for an exothermic reaction?
- Negative
- Positive
- Always zero
- It has no sign
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WhyIn exothermic reactions energy is released, so the enthalpy change is negative.
3
Multiple choice · Easy
During an endothermic reaction, what happens to the temperature of the surroundings?
- It decreases
- It increases
- It stays exactly the same
- It first rises then returns to normal
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WhyEndothermic reactions absorb thermal energy from the surroundings, causing a temperature decrease.
4
Multiple choice · Easy
Which of the following is an example of an exothermic process?
- Combustion of methane
- Melting of ice
- Thermal decomposition of calcium carbonate
- Dissolving ammonium nitrate in water
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WhyCombustion (burning) releases energy to the surroundings and is exothermic.
5
Fill in the blank · Easy
In a reaction pathway diagram for an exothermic reaction, the energy of the products is than the energy of the reactants.
Answer:
lower / less
WhyFor exothermic reactions energy is released, so the products sit lower on the energy axis than the reactants.
6
Fill in the blank · Easy
The minimum amount of energy that colliding particles must have for a reaction to occur is called the energy.
Answer:
activation
WhyActivation energy is the minimum energy needed for a successful, reactive collision.
7
Multiple choice · Medium
On a reaction pathway diagram, the activation energy is the difference in energy between the reactants and which point?
- The peak (top of the energy barrier)
- The product energy level
- The lowest point on the curve
- The zero of the energy axis
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WhyActivation energy is measured from the reactant energy level up to the top of the energy barrier (the peak).
8
Multiple choice · Medium
In terms of bond breaking and bond making, why is a reaction exothermic?
- The energy released making bonds is greater than the energy absorbed breaking bonds
- The energy absorbed breaking bonds is greater than the energy released making bonds
- No bonds are broken during the reaction
- Bond making absorbs energy and bond breaking releases energy
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WhyIf more energy is released forming new bonds than is absorbed breaking old bonds, there is a net release of energy.
9
Multiple choice · Medium
Which process is endothermic?
- Breaking the bonds in reactant molecules
- Forming the bonds in product molecules
- Combustion of hydrogen
- Neutralising an acid with an alkali
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WhyBreaking chemical bonds always requires an input of energy, so bond breaking is endothermic.
10
Multiple choice · Medium
A reaction has an enthalpy change of +120 kJ/mol. What does this tell you?
- The reaction is endothermic and takes in 120 kJ/mol
- The reaction is exothermic and releases 120 kJ/mol
- The activation energy is 120 kJ/mol
- No net energy change occurs
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WhyA positive enthalpy change means the reaction takes in energy overall, so it is endothermic.
Key terms in Chemical energetics
Chemical energetics: The study of the energy changes that accompany chemical reactions.
Energy change: The difference in stored chemical energy between the reactants and products of a reaction.
Exothermic reaction: A reaction that transfers energy to the surroundings, usually as heat, causing the temperature to rise.
Endothermic reaction: A reaction that takes in energy from the surroundings, usually as heat, causing the temperature to fall.
Surroundings: Everything outside the reacting chemicals that can exchange energy with the reaction system.
System: The collection of reactants and products being studied in a reaction.
Heat energy: The form of energy most commonly transferred during chemical reactions, measured as a temperature change.
Temperature rise: An increase in temperature observed when an exothermic reaction releases energy to the surroundings.
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