Thermal energy in chemical reactions

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Lesson notes

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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Practice questions

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1.What is the sign of the enthalpy change (ΔH) for an exothermic reaction?
Easy
- ANegative
- BPositive
- CZero
- DCannot be determined
2.Which of the following is an example of an exothermic reaction?
Easy
- ACombustion of methane
- BMelting of ice
- CPhotosynthesis
- DDissolving ammonium nitrate in water
3.In a chemical reaction, the total bond energy of the reactants is 2000 kJ/mol and the total bond energy of the products is 1500 kJ/mol. What is the enthalpy change (ΔH) for the reaction?
Medium
- A+500 kJ/mol
- B-500 kJ/mol
- C+3500 kJ/mol
- D-3500 kJ/mol
4.Which of the following statements correctly describes the energy change in an endothermic reaction?
Medium
- AThe products have higher enthalpy than the reactants
- BThe products have lower enthalpy than the reactants
- CThe reaction releases heat to the surroundings
- DThe temperature of the surroundings increases
5.A student mixes two solutions in a beaker and observes that the beaker feels cold. Which type of reaction is occurring?
Hard
- AExothermic
- BEndothermic
- CCombustion
- DNeutralization
6.What type of reaction is the burning of natural gas (methane)?
Easy
- AExothermic
- BEndothermic
- CEndergonic
- DEndothermic and exergonic
7.In a hand warmer, iron oxidizes according to the reaction: 4Fe + 3O2 → 2Fe2O3, ΔH = -1648 kJ/mol. This reaction is:
Medium
- AExothermic
- BEndothermic
- CEndergonic
- DIsothermic
8.Which of the following is an endothermic process?
Easy
- ABurning wood
- BRusting of iron
- CMelting ice
- DNeutralizing an acid with a base

Revision notes

## 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.

Flashcards

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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