Chemistry · Unit 10

Energetics

Energy in chemical reactions

Every reaction is an energy transaction: bonds break, which costs energy, and bonds form, which releases it. The difference is what you observe as heat.

The unit covers exothermic and endothermic changes, enthalpy change, calculating from bond energies, calorimetry, and Hess's law - which lets you find an enthalpy change you cannot measure directly.

This unit breaks down into 20 short steps and 120 questions, starting at difficulty 1 and building to 3. Below you can see exactly what it covers, how the path is structured, and worked examples with explanations.

Steps: 20
Questions: 120
Difficulty: 1-3

What this unit covers

Exothermic and Endothermic
Enthalpy Change
Bond Energies
Calorimetry
Hess's Law

Where this fits

Needs Chemical Bonding and Moles and Calculations.

Where people slip

Breaking bonds always requires energy and forming them always releases it - both, in every reaction. Whether the reaction is exothermic depends on which total is larger.

How the unit is structured

Energetics runs as 20 short steps that unlock in order. 15 are practice rounds and 5 are challenge rounds that pull together everything before them. Questions start at difficulty 1 and climb to 3 as you progress.

Step 1 · easierStep 20 · harder

Challenge rounds

Example questions

30 real questions from this unit, with the answer and the reason behind it, grouped by what they practise. There are 120 in the unit altogether.

Bond Energies

Multiple choiceLevel 1
1. What happens to energy when new chemical bonds are formed?
- Energy is releasedcorrect
- Energy is taken in
- Energy is destroyed
- The bonds instantly break again
Forming new bonds releases energy, which is why bond making is an exothermic step.
Build the sentenceLevel 2
2. Build the sentence that describes what happens during bond making.
Answer: Energy is released when new bonds form
Energy is released whenever new bonds form between atoms.
Fact or fibLevel 2
3. Forming new chemical bonds takes in energy from the surroundings.
Answer: False
Forming bonds releases energy, so bond making is exothermic, not endothermic.
Fill the blankLevel 2
4. Breaking chemical bonds is an ____ step because energy must be supplied.
- endothermiccorrect
- exothermic
- explosive
- instant
Energy has to be put in to break bonds, so bond breaking is endothermic.
Guess the numberLevel 3
5. The H-H bond energy is 436 kJ/mol. How much energy, in kJ, is released when one mole of H-H bonds forms?
Answer: 436 kJ
Forming a bond releases the same amount of energy as its bond energy, so one mole of H-H bonds releases 436 kJ.
Sort into groupsLevel 3
6. Sort each ionic feature by its effect on the magnitude (size) of the lattice enthalpy.
Answer: Smaller ionic radius = Increases magnitude; Higher ionic charge = Increases magnitude; Larger ionic radius = Decreases magnitude; Lower ionic charge = Decreases magnitude
Smaller ions and higher charges sit closer with stronger attraction, increasing the lattice enthalpy magnitude; larger ions and lower charges decrease it.

Calorimetry

Multiple choiceLevel 1
7. Which piece of equipment is used to measure the temperature change during a reaction?
- A thermometercorrect
- A balance
- A measuring cylinder
- A stopwatch
A thermometer measures the temperature change, which tells you whether a reaction is exothermic or endothermic.
Fact or fibLevel 2
8. If the temperature of the water rises during a calorimetry experiment, the reaction being measured is exothermic.
Answer: True
A temperature rise means heat is released into the water, so the reaction is exothermic.
Fill the blankLevel 2
9. Recording the ____ change of a reaction mixture shows whether it is exothermic or endothermic.
- temperaturecorrect
- mass
- colour
- volume
A rise or fall in temperature tells you which way energy is being transferred in the reaction.
Match the pairsLevel 2
10. Match each item to what it means in a calorimetry experiment.
Answer: Thermometer = Measures the temperature change; Polystyrene cup = Insulates to reduce heat loss; Temperature rises = Reaction is exothermic; Temperature falls = Reaction is endothermic
A thermometer reads temperature, a polystyrene cup reduces heat loss, and a rising or falling temperature signals an exothermic or endothermic reaction.
Choose all that applyLevel 3
11. Which of these cause a measured enthalpy of combustion to be lower (less exothermic) than the true value? Select all that apply.
- Heat loss to the surroundingscorrect
- Incomplete combustion of the fuelcorrect
- Evaporation of fuel before it burnscorrect
- Using distilled water in the calorimeter
Heat loss, incomplete combustion and evaporation all reduce the energy transferred to the water; the type of water used does not lower the result.
Guess the numberLevel 3
12. 50 g of water is warmed by an exothermic reaction and its temperature rises by 10 C. Using q = mc(delta T) with c = 4.2 J/g/C, how much energy in joules is transferred?
Answer: 2100 J
q = 50 x 4.2 x 10 = 2100 J of energy is transferred to the water.

Enthalpy Change

Fact or fibLevel 2
13. For an exothermic reaction, the enthalpy change (delta H) is negative.
Answer: True
Exothermic reactions release energy, so their enthalpy change is a negative value.
Fill the blankLevel 2
14. On a reaction profile, the ____ energy is the size of the barrier the reactants must overcome.
- activationcorrect
- bond
- kinetic
- nuclear
The activation energy is the height of the barrier reactants must climb before they can react.
Guess the numberLevel 2
15. Standard enthalpy changes are measured at a fixed standard temperature. What is this temperature in kelvin?
Answer: 298 K
Standard conditions use a temperature of 298 K, which is 25 degrees Celsius.
Match the pairsLevel 2
16. Match each standard enthalpy change to what it describes (all per mole).
Answer: Enthalpy of formation = A compound made from its elements; Enthalpy of combustion = A substance burned fully in oxygen; Enthalpy of neutralisation = Water formed from acid and alkali; Enthalpy of reaction = The amounts shown in the equation
Each enthalpy term specifies the exact chemical change it refers to, always measured per mole under standard conditions.
Build the sentenceLevel 3
17. Build the Gibbs free energy equation.
Answer: delta G = delta H - T delta S
The Gibbs free energy change is delta G = delta H - T delta S.
Choose all that applyLevel 3
18. Which of these steps in the Born-Haber cycle for NaCl are endothermic? Select all that apply.
- Atomisation of sodiumcorrect
- First ionisation energy of sodiumcorrect
- First electron affinity of chlorine
- Lattice enthalpy of formation
Atomisation and ionisation absorb energy (endothermic), while the first electron affinity of chlorine and lattice formation release energy (exothermic).

Exothermic and Endothermic

Fact or fibLevel 1
19. During an endothermic reaction the temperature of the surroundings falls.
Answer: True
Endothermic reactions take in energy from the surroundings, so the surrounding temperature drops.
Fill the blankLevel 1
20. A reaction that transfers energy to the surroundings and makes the temperature rise is described as ____.
- exothermiccorrect
- endothermic
- reversible
- spontaneous
A reaction that releases energy and warms the surroundings is called exothermic.
Multiple choiceLevel 1
21. What happens to the temperature of the surroundings during an exothermic reaction?
- It risescorrect
- It falls
- It stays exactly the same
- It drops to absolute zero
Exothermic reactions transfer energy out to the surroundings, so the surrounding temperature rises.
Choose all that applyLevel 2
22. Which of these are exothermic processes? Select all that apply.
- Burning methanecorrect
- Respirationcorrect
- A self-heating coffee cancorrect
- Photosynthesis
- Thermal decomposition of limestone
Burning methane, respiration and a self-heating can all release energy, while photosynthesis and thermal decomposition take energy in.
Odd one outLevel 2
23. Three of these reactions are exothermic. Which is the odd one out (endothermic)?
- Thermal decomposition of limestonecorrect
- Combustion of a fuel
- Neutralising an acid
- Respiration
Thermal decomposition needs a constant supply of heat to break a compound apart, so it is endothermic.
Match the pairsLevel 3
24. Match each combination of signs of delta H and delta S to when the reaction is feasible.
Answer: delta H negative, delta S positive = Feasible at all temperatures; delta H positive, delta S negative = Feasible at no temperature; delta H positive, delta S positive = Feasible only at high temperature; delta H negative, delta S negative = Feasible only at low temperature
Because delta G = delta H - T delta S, the temperature dependence of feasibility is set by the signs of delta H and delta S.

Hess's Law

Fill the blankLevel 3
25. The law of conservation of energy says that energy cannot be created or ____.
- destroyedcorrect
- stored
- weighed
- coloured
Energy cannot be created or destroyed; in a reaction it is only transferred between the chemicals and their surroundings.
Guess the numberLevel 3
26. For CH4 + 2O2 -> CO2 + 2H2O, formation enthalpies are CH4 = -75, CO2 = -394, H2O = -286 kJ/mol (O2 = 0). What is delta H in kJ/mol?
Answer: -891 kJ/mol
delta H = [(-394) + 2(-286)] - [(-75) + 0] = -966 + 75 = -891 kJ/mol.
Multiple choiceLevel 3
27. What does Hess's law state about the enthalpy change of a reaction?
- It depends only on the start and end states, not the route taken
- It changes depending on which route the reaction follows
- It is always exactly zero
- It doubles every time the reaction is repeated
Hess's law says the overall enthalpy change depends only on the starting and finishing states, not the route taken, because energy is conserved.
Put in orderLevel 3
28. Put the conceptual energy-cycle steps of dissolving an ionic solid into the correct order.
Answer: Ionic lattice breaks apart into separate gaseous ions -> The gaseous ions become surrounded by water molecules (hydration) -> Hydrated aqueous ions spread through the solution
Dissolving can be modelled as breaking the lattice into gaseous ions and then hydrating those ions to give aqueous ions in solution.
Sequence recallLevel 3
29. Recall the order of enthalpy steps in a Born-Haber cycle for sodium chloride, starting from the elements.
Answer: Atomise sodium metal to gaseous sodium atoms -> Ionise sodium atoms to form Na+ ions -> Atomise chlorine molecules to gaseous chlorine atoms -> Add electrons to chlorine atoms to form Cl- ions -> Combine the gaseous ions to form the solid lattice
A Born-Haber cycle builds gaseous ions from the elements, then brings them together to form the ionic lattice.
Spell itLevel 3
30. Born-Haber cycles are often used to find the lattice enthalpy of common salt. Spell the chemical formula of sodium chloride.
Answer: NaCl
Sodium chloride is NaCl, the classic example used to illustrate a Born-Haber cycle.

Where these questions come from. Each unit starts as a plan of the concepts it should cover and the difficulty it should span. Questions are written against that plan with AI assistance, then checked by a validator that rejects anything without a single defensible answer, an explanation, or plausible wrong options. How we write questions sets out the whole process, and corrections are fixed in the bank and reach the site and the app the same day.

How you practise

This unit mixes 17 different question formats, so you are recalling and applying rather than recognising the same layout every time.

Build the sentence
Choose all that apply
Fact or fib
Fill the blank
Guess the number
Listen and choose
Match the pairs
Multiple choice
Odd one out
Picture question
Put in order
Sequence recall
Sort into groups
Spell it
Tap the pairs
True or false
Type the answer

Practise Energetics

120 questions across 20 steps. Start with step one and crawl at your own pace.

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Read about Energetics

Explainers from our blog on what this unit covers. Each one ends with real questions from the bank.

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