Thermal energy in chemical reactions

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Lesson notes

Exothermic Reactions

An **exothermic reaction** releases thermal energy (heat) to the surroundings.
The enthalpy change ΔH is **negative** because the products have lower energy than the reactants.
Example: combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = –890 kJ/mol.
Other examples: hand warmers (oxidation of iron), thermite reaction, neutralization of acids and bases.
In exothermic reactions, the total bond energy of reactants is **greater** than that of products.

Endothermic Reactions

An **endothermic reaction** absorbs thermal energy from the surroundings.
The enthalpy change ΔH is **positive** because the products have higher energy than the reactants.
Example: photosynthesis (plants absorb sunlight to convert CO₂ and water into glucose).
Endothermic reactions often feel cold because they take in heat.

Energy Changes in Reactions

Chemical reactions involve breaking and forming **bonds**.
Breaking bonds requires energy (**endothermic**), forming bonds releases energy (**exothermic**).
If more energy is released forming bonds than absorbed breaking bonds, the reaction is exothermic overall.
The **enthalpy change** ΔH = total bond energy of reactants – total bond energy of products.

Calorimetry and Measurement

**Calorimetry** is used to measure the heat released or absorbed in a reaction.
A **bomb calorimeter** measures the heat of combustion accurately.
The heat transfer q is related to ΔH at constant pressure: ΔH = q.
ΔH is usually reported in kJ per mole of reaction (kJ/mol).

Real-World Applications

**Hand warmers** use the exothermic oxidation of iron: 4Fe + 3O₂ → 2Fe₂O₃, ΔH = –1648 kJ/mol.
**Combustion** of fuels (natural gas, gasoline) provides energy for heating, cooking, and engines.
**Aerobic respiration** is a controlled exothermic process that releases energy for cellular functions.
Endothermic reactions are used in **cold packs** (e.g., ammonium nitrate dissolving in water).

Key Vocabulary

**Exothermic**: releases heat; ΔH < 0.
**Endothermic**: absorbs heat; ΔH > 0.
**Enthalpy (H)**: the total energy of a system.
**ΔH**: change in enthalpy during a reaction.
**Calorimeter**: device to measure heat changes.

Particle arrangement in solids, liquids, and gases. In chemical reactions, particles must collide with enough energy (activation energy) to react.

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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

More questions

9.In an exothermic reaction, the enthalpy of the products is:
Medium
- ALess than the enthalpy of the reactants
- BGreater than the enthalpy of the reactants
- CEqual to the enthalpy of the reactants
- DUnrelated to the enthalpy of the reactants
10.A bomb calorimeter is used to measure:
Hard
- AThe heat released or absorbed in a chemical reaction
- BThe temperature of a flame
- CThe volume of gas produced
- DThe mass of reactants
11.Which of the following best describes the energy change in an exothermic reaction?
Medium
- AEnergy is released to the surroundings
- BEnergy is absorbed from the surroundings
- CEnergy is destroyed
- DEnergy is created
12.The reaction 2H2(g) + O2(g) → 2H2O(g) has ΔH = -483.6 kJ/mol. This means that:
Hard
- A483.6 kJ of heat is released per mole of reaction
- B483.6 kJ of heat is absorbed per mole of reaction
- CThe reaction is endothermic
- DThe products have higher energy than the reactants
13.The combustion of methane gas releases 890 kJ of energy per mole. If a student burns 0.5 moles of methane in a calorimeter, how much energy is released?
Medium
- A445 kJ
- B890 kJ
- C1780 kJ
- D0.5 kJ
14.In an experiment, a student adds 50 mL of hydrochloric acid to 50 mL of sodium hydroxide solution in a polystyrene cup. The temperature of the mixture rises from 22°C to 28°C. Which statement about the reaction is correct?
Hard
- AThe reaction is endothermic because the temperature increased.
- BThe reaction is exothermic because the temperature increased.
- CThe reaction is endothermic because heat was absorbed.
- DThe reaction is exothermic because the temperature decreased.
15.A student uses a bomb calorimeter to measure the heat released when 1 g of glucose is burned. The calorimeter contains 500 g of water, and the temperature rises by 3.5°C. The specific heat capacity of water is 4.18 J/g°C. How much heat is absorbed by the water?
Medium
- A7315 J
- B731.5 J
- C73150 J
- D2090 J
16.The bond energies for a reaction are: C–H: 413 kJ/mol, O=O: 498 kJ/mol, C=O: 799 kJ/mol, O–H: 463 kJ/mol. For the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, what is the total bond energy of the products?
Hard
- A2648 kJ/mol
- B3444 kJ/mol
- C2648 kJ
- D3444 kJ
17.A hand warmer contains iron powder that reacts with oxygen to produce iron(III) oxide, releasing heat. Which statement explains why the hand warmer feels warm?
Medium
- AThe reaction is endothermic, absorbing heat from the surroundings.
- BThe reaction is exothermic, releasing heat to the surroundings.
- CThe reaction is exothermic, absorbing heat from the surroundings.
- DThe reaction is endothermic, releasing heat to the surroundings.
18.The diagram shows a particle arrangement for a substance in a closed container. The particles are closely packed but can slide past each other. The container is then heated, and the particles move faster and spread apart. Which type of process is this?
Medium
- AExothermic chemical reaction
- BEndothermic physical change
- CExothermic physical change
- DEndothermic chemical reaction
19.Which of the following is an example of an endothermic reaction?
Easy
- ABurning wood
- BPhotosynthesis
- CRusting iron
- DNeutralising acid with alkali
20.A student measures the temperature change when 0.1 mol of ammonium nitrate is dissolved in water. The temperature drops from 22°C to 18°C. Which statement is correct?
Hard
- AThe process is exothermic, ΔH is negative.
- BThe process is endothermic, ΔH is positive.
- CThe process is exothermic, ΔH is positive.
- DThe process is endothermic, ΔH is negative.
21.In the reaction 2H₂(g) + O₂(g) → 2H₂O(g), the total bond energy of the reactants is 1370 kJ/mol and that of the products is 1850 kJ/mol. What is the enthalpy change ΔH?
Medium
- A+480 kJ/mol
- B-480 kJ/mol
- C+480 kJ
- D-480 kJ
22.Which of the following is true for an exothermic reaction?
Easy
- AThe products have higher enthalpy than the reactants.
- BHeat is absorbed from the surroundings.
- CThe enthalpy change ΔH is negative.
- DThe temperature of the surroundings decreases.
23.A student mixes 25 mL of hydrochloric acid with 25 mL of sodium hydroxide in a polystyrene cup. The initial temperature of both solutions is 20°C. After mixing, the highest temperature recorded is 26°C. Which of the following best describes the energy change?
Medium
- AThe reaction releases 6°C of heat.
- BThe reaction absorbs 6°C of heat.
- CThe reaction is exothermic, releasing thermal energy.
- DThe reaction is endothermic, absorbing thermal energy.
24.The table shows bond energies for some bonds. What is the enthalpy change for the reaction H₂ + Cl₂ → 2HCl? Bond energies: H–H: 436 kJ/mol, Cl–Cl: 243 kJ/mol, H–Cl: 431 kJ/mol.
Hard
- A-183 kJ/mol
- B+183 kJ/mol
- C-183 kJ
- D+183 kJ
25.Which piece of equipment is used to measure the heat released or absorbed in a chemical reaction?
Easy
- AA calorimeter
- BA thermometer
- CA balance
- DA barometer
26.In a bomb calorimeter, a sample is burned in excess oxygen. The heat released is calculated from the temperature rise of the water surrounding the bomb. If the temperature of 500 g of water rises by 4.0 °C, how much heat was released? (Specific heat capacity of water = 4.18 J/g°C)
Medium
- A8360 J
- B2000 J
- C4180 J
- D836 J
27.A student performs an experiment to determine whether a reaction is exothermic or endothermic. They mix two solutions in a polystyrene cup and measure the temperature every 30 seconds for 5 minutes. The temperature rises from 22.0 °C to 28.5 °C, then slowly decreases. Which conclusion is correct?
Hard
- AThe reaction is exothermic because the temperature increased.
- BThe reaction is endothermic because the temperature increased.
- CThe reaction is exothermic because the temperature decreased after the peak.
- DThe reaction is endothermic because the temperature decreased after the peak.
28.Which of the following describes 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 enthalpy change (ΔH) is negative.
29.The thermite reaction is: Fe2O3 + 2Al → 2Fe + Al2O3. This reaction is highly exothermic. Which statement explains why the reaction is used to weld railway tracks?
Hard
- AIt produces a large amount of heat that melts the iron, allowing it to fuse with the tracks.
- BIt produces a large amount of heat that solidifies the iron quickly.
- CIt absorbs heat from the surroundings, cooling the tracks.
- DIt produces oxygen gas that helps the tracks burn.
30.In a chemical reaction, if the total bond energy of the reactants is greater than the total bond energy of the products, the reaction is:
Easy
- AExothermic
- BEndothermic
- CNeither exothermic nor endothermic
- DCannot be determined
31.A student wants to design a hand warmer that releases heat slowly over several hours. Which of the following would be the best approach?
Medium
- AUse a reaction that is exothermic but has a slow rate, such as the oxidation of iron.
- BUse a highly exothermic reaction that occurs instantly, such as the thermite reaction.
- CUse an endothermic reaction that absorbs heat from the surroundings.
- DUse a reaction that produces a gas to inflate the pack.
32.In an experiment, 0.1 mol of ammonium nitrate is dissolved in 100 g of water. The temperature drops from 25.0 °C to 21.5 °C. The specific heat capacity of water is 4.18 J/g°C. What is the enthalpy change (ΔH) for dissolving 1 mole of ammonium nitrate? (Assume no heat loss to surroundings.)
Hard
- A+14.6 kJ/mol
- B-14.6 kJ/mol
- C+1.46 kJ/mol
- D-1.46 kJ/mol
33.Which of the following is a common example of an exothermic reaction?
Medium
- ACombustion of natural gas
- BDissolving ammonium nitrate in water
- CPhotosynthesis
- DMelting ice
34.A student uses a reaction calorimeter to measure the heat flow when 50 mL of 1 M hydrochloric acid reacts with 50 mL of 1 M sodium hydroxide. The temperature rises by 6.5 °C. The density of the solution is 1.0 g/mL, and its specific heat capacity is 4.18 J/g°C. How much heat is released by the reaction?
Hard
- A2717 J
- B1358 J
- C5434 J
- D679 J
35.In an exothermic reaction, the enthalpy change (ΔH) is:
Easy
- ANegative
- BPositive
- CZero
- DCannot be determined
36.A student claims that dissolving salt in water is an exothermic process because the salt dissolves. Which of the following would disprove this claim?
Medium
- AMeasuring the temperature of the solution and finding it is lower than the initial temperature of the water.
- BMeasuring the temperature of the solution and finding it is higher than the initial temperature.
- CObserving that the salt disappears.
- DObserving that the volume of the solution increases.
37.A student mixes 25 mL of hydrochloric acid with 25 mL of sodium hydroxide in a polystyrene cup. The initial temperature of both solutions is 20°C. After mixing, the temperature rises to 26°C. Which statement correctly describes the reaction?
Medium
- AThe reaction is exothermic because the temperature increased.
- BThe reaction is endothermic because the temperature increased.
- CThe reaction is exothermic because energy was absorbed.
- DThe reaction is endothermic because energy was released.
38.The bond energies for a reaction are: C–H: 413 kJ/mol, O=O: 498 kJ/mol, C=O: 799 kJ/mol, O–H: 463 kJ/mol. For the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, what is the enthalpy change (ΔH)?
Hard
- A-890 kJ/mol
- B+890 kJ/mol
- C-802 kJ/mol
- D+802 kJ/mol
39.In a bomb calorimeter, a sample is burned in excess oxygen. The heat released is calculated from the temperature rise of the water surrounding the bomb. If the temperature of 500 g of water rises by 3.2°C, how much heat was released? (Specific heat capacity of water = 4.18 J/g°C)
Medium
- A6688 J
- B668.8 J
- C66.88 J
- D6.688 J
40.A student uses a reaction calorimeter to measure the heat flow when 50 mL of 1 M hydrochloric acid reacts with 50 mL of 1 M sodium hydroxide. The temperature rises from 22°C to 28°C. The density of the solution is 1 g/mL and specific heat capacity is 4.18 J/g°C. What is the enthalpy change per mole of water formed?
Hard
- A-50.16 kJ/mol
- B-25.08 kJ/mol
- C+50.16 kJ/mol
- D+25.08 kJ/mol

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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