Chemical Energetics
9. Chemical Energetics
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The core idea
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Learning objectives
- describe the term exothermic as a process or chemical reaction which transfers energy, often in the form of heat, to the surroundings and may be detected by an increase in temperature, e.g. the reaction between sodium hydroxide and hydrochloric acid
- describe the term endothermic as a process or chemical reaction which takes in energy, often in the form of heat, from the surroundings and may be detected by a decrease in temperature, e.g. the dissolving of ammonium nitrate in water.
In this course, the system is the chemical change being studied. The surroundings are everything that can exchange energy with that change. In a cup experiment, the thermometer measures the bulk solution around the reacting particles; that reading provides evidence about energy transferred between the chemical change and its surroundings.
1. Definition
Exothermic change
An exothermic change transfers energy, often as heat, from the reacting system to the surroundings. The surroundings may become warmer, so their measured temperature rises.
Endothermic change
An endothermic change takes in energy, often as heat, from the surroundings. The surroundings may become cooler, so their measured temperature falls.
System and surroundings
- System: the chemical reaction or dissolving process whose energy change is being classified.
- Surroundings: everything outside that defined process that can exchange energy with it—for example, the bulk solution, container, thermometer and nearby air.
2. Key Ideas
Use temperature as evidence
| Measurement | What happened to the surroundings? | Transfer and classification |
|---|---|---|
| Temperature rises | They gained energy | System → surroundings; exothermic |
| Temperature falls | They lost energy | Surroundings → system; endothermic |
Temperature change is evidence, not the definition. Insulation, quantities, thermometer resolution and heat exchange with the room can change the size of the measured result. The direction of energy transfer determines whether the change is exothermic or endothermic.
State the measured temperature change, identify whether the surroundings gained or lost energy, state the direction of energy transfer, then classify the change.
3. Detailed Explanations
Investigating a temperature change
In a school laboratory, measure the initial temperature, mix the substances in an insulated container with a lid, stir, and record the highest or lowest temperature reached. Keep quantities constant and repeat the measurements. The chemicals named below must be handled only with the laboratory controls specified by your teacher.
4. Common Mistakes
The thermometer records the surroundings. Write: “The surroundings warmed because the reacting system transferred energy to them.” This connects the observation to the classification.
If there is no obvious temperature change, do not automatically conclude that no energy was transferred. Heat exchange with the room or a change smaller than the thermometer resolution may hide the result.
5. Exam Tips
Write the observation, identify whether the surroundings gained or lost energy, state the direction of energy transfer, and then classify the change.
Exam question 1: Complete the energy-transfer explanationCore
A student adds a solid to water. The temperature changes from 25 °C to 20 °C. Complete the explanation.
Show Answer
- The surroundings become cooler, so they lose energy.
- Energy is transferred from the surroundings to the system.
- The change is therefore endothermic.
6. Worked Examples
Modelled example 1
Sodium hydroxide and hydrochloric acid
Problem
Study the worked solution
Describe the measured change
Method
Calculate the temperature rise of the surroundings.Reason
The thermometer measures the bulk solution around the reacting particles.Working
28.5-22.0 = 6.5 °C rise.Infer the transfer direction
Method
State that the surroundings gained energy.Reason
Their measured temperature increased, so energy reached them from the reacting system.Working
System → surroundings.Classify the change
Method
Classify the reaction as exothermic.Reason
An exothermic change transfers energy from the system to the surroundings.Working
The neutralisation is exothermic.
Guided practice 2
Use a temperature fall as evidence
Problem
Separate the measurement from the explanation
Hints
Hint 1: what cooled
Hint 2: follow the loss
View solution step by step
Calculate the evidence
Method
Subtract the lower temperature from the higher one.Reason
The question asks for the magnitude of the fall.Working
26.5-21.0 = 5.5 °C.Follow and classify
Method
Trace energy from surroundings to system and call the reaction endothermic.Reason
The surroundings cooled because the reaction took in energy.Working
Surroundings → system; endothermic.
Common misconception 3
Ammonium nitrate dissolving in water
Learner claim
Follow energy into the defined system
View solution step by step
Identify what cooled
Method
State that the measured surroundings cooled by 6.0 °C.Reason
The water and thermometer are outside the defined dissolving process.Working
23.0-17.0 = 6.0 °C fall.Reverse the learner's inference
Method
Trace energy from the surroundings into the system.Reason
Energy lost by one region is gained by the other; the student classified the surroundings instead of the process.Working
Surroundings → dissolving system.Classify the process
Method
Call the dissolving process endothermic.Reason
An endothermic change takes in energy from its surroundings.Working
Ammonium nitrate dissolving is endothermic.
Examiner practice 4
Plan a useful temperature-change measurement
Examination question
Write four specific method features
View solution step by step
Reduce unwanted transfer
1 markMethod
Use an insulated cup with a lid.Reason
This reduces energy exchange with the room.Working
insulated cup + lidEstablish a start
1 markMethod
Measure the initial temperature before mixing.Reason
A temperature change needs a reliable starting value.Working
Record Tᵢₙᵢₜᵢₐₗ.Capture the extreme
1 markMethod
Stir and record the highest or lowest temperature reached.Reason
The mixture may start exchanging energy with the room immediately.Working
Record T_highest or T_lowest.Check reliability
1 markMethod
Repeat using the same quantities and calculate a justified mean.Reason
Repeats reveal anomalies and improve confidence.Working
Repeat → identify anomalies → mean of valid results.
Self-mark with the mark scheme
Compare your response with each mark point. Select a point only when your response contains that evidence.
Award only specific actions linked to useful evidence.
Challenge 5
Follow energy through a hand warmer
Everyday-context transfer
Define the system before following energy
Hints
Hint 1: temperature evidence
Hint 2: classification
View solution step by step
Define the boundary
Method
Treat the chemical process inside the pack as the system.Reason
This keeps “system” and “surroundings” consistent.Working
System: reaction; surroundings: packet, hand and air.Follow the energy
Method
Trace energy from the process into the packet and hand.Reason
The surroundings warm because the system transfers energy to them.Working
System → surroundings; exothermic.
7. Mind Stretchers
Use these as independent practice. Each question requires a complete explanation, not just a classification.
Mind stretcher 1: Cold-pack energy transferExtension
A sealed cold pack falls from 24 °C to 10 °C after its contents are mixed. It then feels cold against a student’s hand. Trace the energy transfer from the hand to the process inside the pack and classify the change.
Show Answer
The process inside the pack takes in energy from its surroundings. The student’s hand transfers energy through the packet to the colder material, so the hand cools. Because the process takes in energy from the surroundings, it is endothermic.
Mind stretcher 2: Compare measurements without overclaimingExtension
The same reaction raises the temperature by 8.0 °C in an insulated cup and by 3.0 °C in an open beaker. A student says the beaker reaction is “less exothermic”. Evaluate the claim.
Show Answer
Both temperature rises are evidence that the reaction transfers energy to the surroundings, so both trials are exothermic. The smaller rise in the open beaker may be caused by greater energy transfer to the room. The measurements alone do not show that the chemical reaction has a different energy change.
Mind stretcher 3: Choose the correct temperature readingExtension
A neutralisation mixture starts at 21.5 °C, reaches 29.0 °C, then falls to 27.0 °C while the student is still recording. Which reading should be used to calculate the temperature change, and what does the later fall show?
Show Answer
Use the highest temperature, 29.0 °C, so the measured rise is 7.5 °C. The later fall shows that the warmer mixture is transferring energy to the room. It does not change the exothermic classification of the reaction.
Mind stretcher 4: Judge repeated measurementsExtension
A solid is added to water in three repeated trials. The temperature changes are −5.0 °C, −1.0 °C and −4.8 °C. Identify the anomalous result, state the supported classification, and suggest one check before repeating the experiment.
Show Answer
−1.0 °C is anomalous because it is far from the other two results. The consistent temperature falls support an endothermic classification. Before repeating, check that the same quantities were used and that the container was insulated and covered in every trial.
8. Quiz
Check 1: Check your understandingCore
Check that you can identify the system and surroundings, follow the direction of energy transfer and use temperature change as evidence without treating it as the definition.
Check your understanding