Endothermic & Exothermic Reactions — Practice · Grade 8 Science

Chapter: Chemistry

What this practice covers

By the end of Endothermic & Exothermic Reactions you should be able to:

11 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: exothermic reaction, endothermic reaction, activation energy, model, evidence, system boundary, assumption.

Problem 1

In an exothermic reaction, energy is…
  1. absorbed from the surroundings, cooling them
  2. released to the surroundings, warming them
  3. destroyed completely
  4. only stored, never released
Answer and explanation

released to the surroundings, warming them

Exothermic reactions release energy outward to the surroundings, which is why they feel warm.

Problem 2

A beaker of water at 24°C is used in a reaction. Afterward, the water measures 31°C. What is the temperature change, ΔT?
Answer and explanation

7 °C

ΔT = 31°C − 24°C = 7°C. The rise in temperature suggests an exothermic reaction.

Problem 3

Which of these is an example of an endothermic process?
  1. wood burning in a campfire
  2. an instant cold pack activating
  3. natural gas burning on a stove
  4. fireworks exploding
Answer and explanation

an instant cold pack activating

A cold pack absorbs heat from its surroundings to feel cold, which is endothermic.

Problem 4

A chemical reaction absorbs heat from surrounding water while forming new substances. Is it endothermic or exothermic? Explain using the reacting chemicals as the system.
Answer and explanation

Endothermic: the reacting chemical system takes energy as heat from the surroundings overall. Surrounding water can cool if other energy transfers are controlled.

Problem 5

A hand warmer raises the temperature of the air around it from 18°C to 42°C. What is ΔT?
Answer and explanation

24 °C

ΔT = 42°C − 18°C = 24°C.

Problem 6

A cold-pack process cools its surrounding water from 22°C to 15°C. Describe the direction of energy transfer. Explain the reasoning in your own words.
Answer and explanation

Energy entered the pack process; the process is endothermic under these conditions.

The surroundings lost thermal energy as they cooled.

The pack's process absorbed energy from those surroundings.

Response criteria

  • Use the stated conditions.
  • Show the evidence or calculation.
  • Explain the conclusion and any stated limitation.

Problem 7

A process lowers surrounding water from 24°C to 17°C. Classify it and justify your boundary. Show the reasoning, not just the conclusion.
Hints

State the system boundary before naming the direction.

Answer and explanation

Endothermic for the process: it takes energy from the water, which cools.

Problem 8

A process is exothermic but a poorly insulated cup shows little warming. Explain a possible reason.
Hints

State what the supplied observations or conditions establish, and separate that from any assumption.

Answer and explanation

Released energy may transfer to the room or container instead of remaining in the measured water; a small temperature rise alone does not measure all energy released.

Problem 9

Using the source table, choose a qualifying design, draw the containment/transfer model and justify one revision. Explain what a new test must measure and why selecting only the hottest design can fail. Complete either the supervised small-scale cooler construction/retest or the supplied-data design route. State which route you used and submit its matching evidence; do not claim actual construction from a paper model.
Hints

Use the illustrated study titled Design a chemical thermal device. Keep the observations, model and conditions separate.

Answer and explanation

A qualifies at 32 °C under the stated 40 °C maximum, while B’s 45 °C violates it despite longer duration. A revision could improve insulation or transfer geometry within the reagent limit, then retest target temperature over time and check containment. Supplied C records at 35 °C for 12 minutes support that simulated comparison; they do not substitute for actual construction evidence. Account for reaction, target and surrounding energy pathways. For a real cooler route, vinegar/bicarbonate interaction can lower the reaction-mixture temperature; target water can then transfer energy through the inner-cup wall. Record the target separately and compare repeated A/B curves with a no-reaction reference. Geometry and environmental exchange can obscure a small effect, so no precise decrease is promised. A model or example result is not a substitute for actual target readings.

A qualifies at 32 °C under the stated 40 °C maximum, while B’s 45 °C violates it despite longer duration. A revision could improve insulation or transfer geometry within the reagent limit, then retest target temperature over time and check containment. Supplied C records at 35 °C for 12 minutes support that simulated comparison; they do not substitute for actual construction evidence. Account for reaction, target and surrounding energy pathways. For a real cooler route, vinegar/bicarbonate interaction can lower the reaction-mixture temperature; target water can then transfer energy through the inner-cup wall. Record the target separately and compare repeated A/B curves with a no-reaction reference. Geometry and environmental exchange can obscure a small effect, so no precise decrease is promised. A model or example result is not a substitute for actual target readings.

Response criteria

  • Apply criteria and hard constraints.
  • Explain reaction energy, separate target/mixture readings and open containment.
  • Compare original/revised trial evidence and its limits.

Problem 10

Define the system before naming the energy-transfer directionDefine the system before naming the energy-transfer directionDefined reaction systemSpecified processExothermic: net outwardEndothermic: net inwardThe two arrows are separate alternative cases, not simultaneous net transfers.The same surroundings can include the container, water and nearby air.
The box marks a chosen reaction-system boundary. Two alternative supplied cases show net energy transfer outward for an exothermic process and inward for an endothermic process. These opposite alternatives do not mean the same reaction has both net classifications at once. Naming the surroundings makes the account explicit before discussing measured temperature or bond changes.
For this unit’s illustrated case, for the endothermic alternative, which side supplies net energy and why could the monitored surroundings cool?
Answer and explanation

Energy comes from the surroundings into the defined system overall. The surroundings can cool as they transfer energy, depending on the actual mass and other transfer paths.

A system is the chosen objects or process being studied. Surroundings are the interacting objects outside that boundary. Exothermic and endothermic name net transfer directions, so identify the boundary and interval first.

Energy comes from the surroundings into the defined system overall. The surroundings can cool as they transfer energy, depending on the actual mass and other transfer paths.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

Problem 11

A specified process needs 80 model energy-units for bond breaking and releases 55 through bond formation. Find the net requirement and explain why bond formation itself is not the energy-requiring step.
Answer and explanation

Net 25 units enter from surroundings under this model, so the overall reaction is endothermic. Forming bonds releases 55; breaking them requires 80.

Compare the two contributions with directions stated.

Classify the net result, not an isolated bond step.

Net 25 units enter from surroundings under this model, so the overall reaction is endothermic. Forming bonds releases 55; breaking them requires 80.

Response criteria

  • Use the named labels, directions or supplied quantities accurately.
  • Connect the visual evidence to the scientific mechanism or calculation.
  • State the relevant model conditions or limitation.

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