Thermal energy in chemical reactions

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Chemical Reactions Transfer Thermal Energy

Every chemical reaction changes some substances into new ones, and thermal energy always moves along with that change.
A reaction either releases thermal energy to its surroundings or absorbs thermal energy from its surroundings.
A thermometer placed in the surroundings before and after a reaction gives direct evidence of which way the thermal energy moved.
Temperature change is one kind of evidence a chemical reaction happened, alongside things like bubbles forming, a color change, or a new solid appearing.

Exothermic Reactions Release Thermal Energy

A reaction that releases thermal energy to its surroundings is called exothermic.
The surroundings warm up: a thermometer placed nearby reads a higher temperature after the reaction than before.
Burning fuel is exothermic — wood, natural gas, and gasoline all release thermal energy as they burn.
Iron reacting with oxygen (rusting) is exothermic too, and it happens slowly enough that commercial hand warmers use it to stay warm for hours.

Endothermic Reactions Absorb Thermal Energy

A reaction that absorbs thermal energy from its surroundings is called endothermic.
The surroundings cool down: a thermometer placed nearby reads a lower temperature after the reaction than before.
Dissolving certain solids in water, such as citric acid, is endothermic — the water gets noticeably colder as it dissolves.
Instant cold packs are built around an endothermic reaction: breaking an inner pouch lets a solid dissolve in water and pull thermal energy out of the skin.

Temperature Change Is the Evidence

Temperature change is the evidence scientists use to classify a reaction as exothermic or endothermic — not color, not smell, not bubbles.
A simple test: record the starting temperature, mix the substances in an insulated cup, stir, then record the highest or lowest temperature reached.
A rise in temperature after mixing shows the reaction released thermal energy (exothermic); a drop shows it absorbed thermal energy (endothermic).
An insulated cup, such as a foam cup with a lid, keeps thermal energy from leaking to or from the room, so the reading reflects the reaction itself.

Designing a Device That Releases Thermal Energy

A useful heat-releasing device needs an exothermic reaction that is safe, starts on demand, and releases thermal energy at a controlled rate.
Hand warmers package iron powder in a breathable pouch; opening it lets in air, and the iron slowly reacts with oxygen, releasing thermal energy for hours.
Design goals include how warm the device should get, how long it should stay warm, and staying below a temperature that could burn skin.
Testing a design means measuring temperature over time and comparing it to the goals, then changing one variable — such as the amount of iron powder — to improve it.

Designing a Device That Absorbs Thermal Energy

A useful heat-absorbing device needs an endothermic reaction that starts on demand and reliably cools its surroundings.
Instant cold packs keep water separate from a solid until the pack is squeezed; breaking the inner pouch lets them mix and absorb thermal energy from the pack's surface.
Design goals include how much the temperature should drop, how quickly it should get cold, and how long the cooling should last.
Testing a design means measuring the temperature drop and comparing it to the goals, then adjusting a variable — such as the amount of solid used — to improve the result.

Testing and Improving a Design

A fair test changes only one variable at a time — for example, the amount of a chemical — while keeping everything else, like the volume of water and starting temperature, the same.
Repeating a test and comparing results checks whether a design's temperature change is reliable rather than a one-time result.
Data recorded over several trials can be compared in a table to decide which design change worked best.
Engineers weigh trade-offs: a stronger reaction might change the temperature faster, but it could be less safe or use more material.

Everyday Devices and Safety

Many everyday products rely on controlled exothermic or endothermic reactions: hand warmers, self-heating meal packs, and instant cold packs.
A reaction that releases thermal energy too quickly can become a safety hazard, so devices are designed to release it slowly and evenly instead.
Following a device's instructions for activating and disposing of it keeps a chemical heating or cooling device safe to use.

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Soal latihan

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1.Which term describes a reaction that releases thermal energy to its surroundings?
Easy
- AExothermic
- BEndothermic
- CNeither
- DBoth
2.Which term describes a reaction that absorbs thermal energy from its surroundings?
Easy
- AExothermic
- BEndothermic
- CNeither
- DBoth
3.In an exothermic reaction, the temperature of the surroundings increases.
Easy
True or false?
4.In an endothermic reaction, the surroundings get warmer.
Easy
True or false?
5.A student mixes two chemicals in a cup, and the outside of the cup feels warm afterward. What can the student conclude?
Easy
- AThe reaction is exothermic — it released thermal energy to the cup and its surroundings.
- BThe reaction is endothermic — it released thermal energy.
- CThe reaction is exothermic — it absorbed thermal energy.
- DNo conclusion can be made from temperature alone.
6.A student mixes two chemicals in a cup, and the outside of the cup feels cold afterward. What can the student conclude?
Easy
- AEndothermic — the reaction absorbed thermal energy from the cup and surroundings.
- BExothermic — the reaction absorbed thermal energy.
- CEndothermic — the reaction released thermal energy.
- DNo conclusion can be made from temperature alone.
7.Which of these is the best evidence that a reaction is exothermic?
Easy
- AA rise in temperature measured with a thermometer
- BThe reaction happens quickly
- CBubbles form during the reaction
- DThe mixture changes color
8.Which of the following is an example of an exothermic reaction?
Easy
- ABurning natural gas (combustion)
- BA cold pack cooling down after being squeezed
- CAn ice cube melting on a counter
- DWater evaporating from a puddle

Soal lainnya

9.Which of the following is an example of an endothermic reaction?
Easy
- ADissolving certain salts, like citric acid, in water
- BBurning wood in a campfire
- CIron rusting in a hand warmer
- DStriking a match
10.Which of these are exothermic processes? (Select all that apply)
Easy
- ABurning candle wax
- BIron reacting with oxygen in a hand warmer
- CIce melting on a sidewalk
- DDissolving citric acid in cold water
11.Which of these are endothermic processes? (Select all that apply)
Easy
- AAn instant cold pack activating
- BDissolving citric acid in water
- CBurning natural gas in a stove
- DWood burning in a fireplace
12.A commercial hand warmer stays warm for hours because iron powder inside slowly reacts with oxygen from the air. Which statement explains why the hand warmer feels warm?
Easy
- AThe reaction is exothermic, releasing thermal energy into the pouch and the hand holding it.
- BThe reaction is endothermic, releasing thermal energy.
- CThe reaction is exothermic, absorbing thermal energy from the hand.
- DThe reaction is endothermic, absorbing thermal energy.
13.An instant cold pack is squeezed to break an inner pouch, letting a solid dissolve in water. The pack quickly feels cold. What type of reaction is happening inside the pack?
Easy
- AEndothermic — the reaction absorbs thermal energy from the pack's surroundings.
- BExothermic — the reaction absorbs thermal energy.
- CEndothermic — the reaction releases thermal energy.
- DExothermic — the reaction releases thermal energy.
14.A student records the temperature of water in a foam cup, adds a spoonful of baking soda, stirs, then adds vinegar. Bubbles form immediately and the temperature drops several degrees. What type of reaction occurred?
Easy
- AEndothermic
- BExothermic
- CNeither — no thermal energy was transferred
- DCannot be determined from a temperature reading
15.Why is temperature change, rather than bubbling or fizzing, used as evidence for whether a reaction is exothermic or endothermic?
Easy
- ABecause temperature directly shows whether thermal energy moved into or out of the surroundings
- BBecause bubbling only happens in endothermic reactions
- CBecause color change always means a reaction released heat
- DBecause a thermometer is required by law for all chemistry experiments
16.Put these steps in order for testing whether a reaction is exothermic or endothermic using a thermometer and an insulated cup.
Easy
- Measure and record the starting temperature of the liquid
- Add the second substance and stir
- Record the temperature again after mixing
- Compare the two temperature readings to decide if the reaction released or absorbed thermal energy
17.Match each description to the correct term.
Easy
- Reaction that releases thermal energy to its surroundings
- Reaction that absorbs thermal energy from its surroundings
- Exothermic
- Endothermic
18.Complete the sentence: A reaction that makes its surroundings colder is called ______ because it ______ thermal energy from the surroundings.
Easy
A reaction that makes its surroundings colder is called ____ because it ____ thermal energy from the surroundings.
19.Complete the sentence: A reaction that makes its surroundings warmer is called ______ because it ______ thermal energy to the surroundings.
Easy
A reaction that makes its surroundings warmer is called ____ because it ____ thermal energy to the surroundings.
20.What piece of equipment do scientists use to collect evidence about whether a reaction released or absorbed thermal energy?
Easy
21.A design team wants to build a reusable hand warmer for hikers. Which property should the chemical reaction they choose have?
Easy
- AIt releases thermal energy steadily over several hours without getting too hot to hold
- BIt releases all of its thermal energy in one second
- CIt absorbs thermal energy from the hiker's hand
- DIt changes color but does not transfer thermal energy
22.A design team is building an instant cold pack for sports injuries. Which reaction should they choose?
Easy
- AAn endothermic reaction that reliably absorbs thermal energy when activated
- BAn exothermic reaction that releases a large burst of thermal energy
- CA reaction that produces gas but causes no temperature change
- DA reaction that only changes color
23.A student is testing three different amounts of iron powder in a hand-warmer design to see which releases thermal energy for the longest time. Which one variable should they change between tests?
Easy
- AThe amount of iron powder, only
- BThe amount of iron powder AND the size of the pouch
- CThe starting time of day
- DThe color of the pouch
24.When testing a thermal-energy device, changing more than one variable at a time makes it easier to know which change caused the result.
Easy
True or false?
25.A cold pack is designed to drop the temperature of an injury by at least 10°C and stay cold for 20 minutes. After testing, it only drops the temperature by 4°C. What is a reasonable next step?
Medium
- AIncrease the amount of the endothermic chemical used, so more thermal energy is absorbed
- BSwitch to an exothermic reaction instead
- CStop testing — 4°C is close enough to any target
- DRemove the insulation so the pack warms up faster
26.Why might an engineer choose a slow exothermic reaction, like iron slowly rusting, instead of a fast one, like burning, for a hand warmer?
Easy
- AA slow reaction releases thermal energy gradually over hours instead of all at once, which is safer to hold
- BA slow reaction absorbs more thermal energy overall
- CA slow reaction produces more light
- DA slow reaction does not require any chemicals
27.Which of these is a safety reason for controlling how fast an exothermic reaction releases thermal energy in a device?
Easy
- AA reaction that releases thermal energy too quickly could get hot enough to burn skin
- BFast reactions always run out of chemicals before they can be used
- CFast reactions cannot be endothermic
- DFast reactions never produce any evidence of a chemical change
28.A cold pack feels cold within seconds of being squeezed, but a hand warmer takes minutes to feel warm and stays warm for hours. Using the words exothermic, endothermic, and thermal energy, explain what is different about the two chemical reactions inside them.
Medium
29.A student measures the temperature of water, then adds a chemical and stirs for several minutes. The temperature does not change at all. What can the student conclude?
Easy
- ANo thermal energy was transferred between the reaction and its surroundings, or no reaction occurred
- BThe reaction is strongly exothermic
- CThe reaction is strongly endothermic
- DThe thermometer must be broken
30.A group of students stirs baking soda into room-temperature water, then adds citric acid and stirs. They place a thermometer in the beaker throughout the test. What type of energy does the thermometer measure?
Easy
31.In the same investigation, the students record the temperature every 30 seconds for 2 minutes: it keeps dropping the whole time, while bubbles keep forming and the color never changes. What type of reaction is citric acid having with the baking soda in water, and why?
Easy
- AEndothermic — the reaction absorbed thermal energy from the water, cooling it down
- BExothermic — the reaction absorbed thermal energy from the water
- CEndothermic — the reaction released thermal energy, warming the water
- DExothermic — the reaction released thermal energy, warming the water
32.The students want to organize their observations into a data table, recording the time, temperature, bubbles, and color every 30 seconds for 2 minutes. Complete the table's row and column headers.
Easy
| ____ | 0 sec | 30 sec | 60 sec | 90 sec | 120 sec | |---|---|---|---|---|---| | ____ | | | | | | | Bubbles | | | | | | | Color | | | | | |

Catatan ulasan

## Thermal Energy in Chemical Reactions ### Exothermic vs. Endothermic - **Exothermic reaction**: releases heat; ΔH is negative (ΔH < 0). Products have lower energy than reactants. - Example: combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH° = –890 kJ/mol - Example: hand warmer (oxidation of iron): 4Fe + 3O₂ → 2Fe₂O₃, ΔH° = –1648 kJ/mol - **Endothermic reaction**: absorbs heat; ΔH is positive (ΔH > 0). Products have higher energy than reactants. ### Key Formula - ΔH = H(products) – H(reactants) - For exothermic reactions: ΔH < 0 (reactants have more energy than products). - Bond energy approximation: ΔH° ≈ (total bond energy of reactants) – (total bond energy of products) ### Measuring Heat Change - **Calorimetry**: measures heat flow during a reaction (e.g., bomb calorimeter). - ΔH is measured in kJ/mol (or J/mol) at constant pressure. - Standard conditions: 25°C, 1 atm. ### Real-World Applications - **Hand warmers**: use exothermic oxidation of iron. - **Combustion**: burning fuels (natural gas, gasoline) releases heat. - **Respiration**: controlled exothermic reaction in cells to produce ATP. - **Design challenge**: create a device that releases or absorbs thermal energy (e.g., hot pack or cold pack). ### Important Notes - Exothermic reactions are often also exergonic (ΔG < 0), but not always. - Uncontrolled exothermic reactions (fires, explosions) waste energy. - Endothermic reactions are driven by an increase in entropy.

Exothermic reaction: A chemical reaction that releases heat; the enthalpy change ΔH is negative.
Endothermic reaction: A chemical reaction that absorbs heat; the enthalpy change ΔH is positive.
Enthalpy change (ΔH): The heat energy transferred during a reaction at constant pressure; ΔH = H(products) – H(reactants).
Calorimetry: The measurement of heat flow in a chemical reaction, often using a calorimeter.
Combustion: A rapid exothermic reaction of a fuel with oxygen, producing heat and light.
Bond energy: The energy required to break a chemical bond; used to estimate ΔH° of a reaction.

Kartu flash

What is an exothermic reaction?

A chemical reaction that releases energy, usually as heat, and has a negative enthalpy change (ΔH < 0).

What is an endothermic reaction?

A chemical reaction that absorbs energy, usually as heat, and has a positive enthalpy change (ΔH > 0).

What does a negative ΔH indicate about a reaction?

A negative ΔH indicates an exothermic reaction where the products have lower enthalpy than the reactants, releasing heat.

Give an example of an exothermic reaction.

Combustion of methane: CH4 + 2O2 → CO2 + 2H2O, ΔH = -890 kJ/mol.

Give an example of an endothermic reaction.

Photosynthesis (simplified): 6CO2 + 6H2O → C6H12O6 + 6O2, absorbs energy from sunlight.

How is the enthalpy change ΔH calculated from bond energies?

ΔH = (total bond energy of reactants) − (total bond energy of products).

What is calorimetry?

The measurement of heat transfer during a chemical reaction, often using a calorimeter.

What is a bomb calorimeter used for?

To measure the heat released in a combustion reaction accurately.

In an exothermic reaction, is the enthalpy of products higher or lower than reactants?

Lower; Hproducts < Hreactants, so ΔH is negative.

What is the standard enthalpy change ΔH°?

The enthalpy change when reactants in their standard states (usually 25°C, 1 atm) react to form products in their standard states.

What units are used for ΔH?

Joules per mole (J/mol) or kilojoules per mole (kJ/mol).

Is a strongly exothermic reaction always exergonic?

Usually yes, because the negative ΔH makes a major contribution to a negative ΔG.

What is the difference between exothermic and exergonic?

Exothermic refers to heat release (negative ΔH); exergonic refers to negative Gibbs free energy change (ΔG < 0), meaning the reaction is spontaneous.

Give an everyday example of an exothermic reaction.

Hand warmers use the oxidation of iron: 4Fe + 3O2 → 2Fe2O3, releasing heat.

What is combustion?

A rapid exothermic reaction between a fuel and an oxidant, producing heat and light.

How does aerobic respiration capture energy from exothermic reactions?

Nature controls combustion in cells to release energy gradually, which is used to form ATP.

What happens to energy in an endothermic reaction?

Energy is absorbed from the surroundings, often as heat, causing the surroundings to cool.

What is the enthalpy change for the combustion of hydrogen?

2H2(g) + O2(g) → 2H2O(g), ΔH° = -483.6 kJ/mol.

What does ΔH = Hproducts − Hreactants represent?

The change in enthalpy; if negative, the reaction is exothermic; if positive, endothermic.

What is a reaction calorimeter?

A lab instrument that monitors heat flow into or out of a reaction vessel.

Why are uncontrolled exothermic reactions wasteful?

Because the released energy is difficult to capture and often leads to fires or explosions.

What is the thermite reaction?

An exothermic reaction between aluminum and iron oxide, producing molten iron and aluminum oxide.

What is the standard temperature for ΔH° values?

25°C (298 K).

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