Energy Changes in Chemical Reactions
5. Everyday Examples of Energy Changes
Learning outcomes
- I can identify chemical reactions that involve energy changes in everyday life.
- I can explain energy changes in combustion reactions.
- I can describe energy transformations in respiration and photosynthesis.
- I can explain how hand warmers and cold packs work.
- I can relate chemical energy changes to real-world applications.
Everyday Examples of Energy Changes
Chemical reactions involving energy changes occur all around us. They happen when we cook food, burn fuels, use batteries, exercise, grow plants, or activate a hand warmer.
Chemical substances contain chemical energy associated with their arrangements and chemical bonds. During a reaction, energy may be transferred between the reacting system and its surroundings.
Some reactions are exothermic, meaning they transfer energy to the surroundings.
Others are endothermic, meaning they absorb energy from the surroundings.
Energy Changes Are Everywhere
Consider some familiar examples:
| Process | Energy Change | Type |
|---|---|---|
| Burning fuel | Chemical energy → thermal and light energy | Exothermic |
| Cellular respiration. | Chemical energy in glucose → usable cellular energy + thermal energy. | Overall exothermic |
| Photosynthesis. | Light energy → chemical energy | Endothermic |
| Hand warmer | Chemical energy → thermal energy | Exothermic |
| Instant cold pack | Energy absorbed from surroundings | Endothermic |
The important question is always:
Where does the energy come from, and where does it go?
Combustion
Combustion occurs when a fuel reacts rapidly with oxygen.
Common examples include:
- natural gas burning on a stove
- petrol burning in an engine
- candles burning
- wood burning
- fuels burning in power stations
Combustion reactions are exothermic.
They transfer chemical energy from the reacting system to the surroundings, mainly as thermal energy, and often as light.
Combustion of Methane
Methane is the main component of natural gas.
When methane burns:
Methane + Oxygen → Carbon dioxide + Water + Energy
The balanced equation is:
CH₄ + 2O₂ → CO₂ + 2H₂O
Energy is transferred to the surroundings.
This energy can be used for useful purposes such as:
- heating water
- cooking food
- heating buildings
- generating electricity
So the energy pathway can be simplified as:
Chemical energy → Thermal energy + Light
Combustion in a Car Engine
Cars powered by petrol or diesel also depend on combustion.
Fuel reacts with oxygen inside the engine.
Energy released by the reaction increases the temperature and pressure of gases inside the engine. The expanding gases push pistons, eventually causing the wheels to turn.
A simplified energy pathway is:
Chemical energy in fuel
↓
Thermal energy
↓
Mechanical energy
↓
Movement of the vehicle
Not all the energy becomes useful movement. Large amounts are transferred to the surroundings as thermal energy and sound.
This is why engines become hot.
Cellular Respiration
Your cells also depend on chemical reactions that release energy.
During cellular respiration, glucose reacts with oxygen.
The overall reaction is:
Glucose + Oxygen → Carbon dioxide + Water + Energy
The balanced equation is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy
Respiration is an overall exothermic process.
However, cells do not simply release all of this energy as heat. Much of it is captured in molecules such as ATP, which cells can use to perform biological work.
Where Does the Energy from Respiration Go?
Energy released during respiration supports many processes.
Cells use energy for:
- muscle contraction
- active transport
- growth
- protein synthesis
- cell division
- maintaining body functions
Some energy is eventually transferred to the surroundings as thermal energy.
The pathway can therefore be represented as:
Chemical energy in glucose
↓
Chemical energy transferred through ATP
↓
Cellular processes + thermal energy
Why Do We Get Warm During Exercise?
During exercise, muscle cells require more energy.
The rate of cellular respiration increases to help meet this demand.
As chemical energy is transferred through biological processes, some energy ultimately becomes thermal energy.
This contributes to an increase in body temperature.
The body responds through processes such as:
- sweating
- increased blood flow near the skin
- increased breathing
So the warmth you experience during exercise is closely connected to energy transformations occurring inside your cells.
Photosynthesis
Plants also carry out an important chemical reaction involving energy.
During photosynthesis, plants use light energy to produce glucose from carbon dioxide and water.
Carbon dioxide + Water + Light energy → Glucose + Oxygen
The balanced equation is:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Photosynthesis requires an input of energy.
It is therefore an endothermic process.
Energy Transformation in Photosynthesis
Plants capture light energy from the Sun.
Through photosynthesis, some of this energy is transformed and stored as chemical energy in organic molecules such as glucose.
The pathway can be simplified as:
Light energy
↓
Photosynthesis
↓
Chemical energy in glucose
Plants can later release usable energy from glucose through cellular respiration.
This creates an important connection:
Sunlight → Photosynthesis → Glucose → Respiration → Cellular work
Much of the chemical energy available to organisms in ecosystems can ultimately be traced back to energy from the Sun.
Photosynthesis and Respiration
Photosynthesis and respiration are closely connected but involve different overall energy transformations.
| Photosynthesis | Respiration |
|---|---|
| Requires an energy input | Releases energy overall |
| Uses light energy | Uses chemical energy in glucose |
| Produces glucose | Breaks down glucose |
| Stores energy chemically. | Transfers chemical energy for cellular use |
| Overall endothermic | Overall exothermic |
Hand Warmers
Have you ever opened a disposable hand warmer and noticed that it gradually becomes hot?
Many disposable hand warmers contain iron powder.
When the package is opened, oxygen from the air enters and reacts with the iron.
The iron undergoes oxidation.
A simplified representation is:
Iron + Oxygen → Iron oxide + Energy
The reaction is exothermic.
Energy is transferred:
Chemical system → Surroundings
The thermal energy released warms your hands.
Why Don't Hand Warmers Burn?
Combustion and the oxidation inside many hand warmers both involve reactions with oxygen.
However, the reactions occur at very different rates.
Combustion occurs rapidly and may produce flames.
The oxidation in a hand warmer occurs much more slowly.
Energy is therefore released gradually over several hours instead of very rapidly.
This demonstrates an important idea:
The same general type of energy transfer can occur at very different rates.
Instant Cold Packs
An instant cold pack produces the opposite sensation.
Many instant cold packs contain water and a substance that dissolves when the pack is activated.
When the substances mix, the overall dissolving process can absorb thermal energy from the surroundings.
The pack becomes colder.
This is an endothermic process.
Energy moves:
Surroundings → System
Hand Warmers vs Cold Packs
These products provide an excellent comparison of energy transfer.
| Hand Warmer | Cold Pack |
|---|---|
| Exothermic | Endothermic |
| Releases energy | Absorbs energy |
| Surroundings become warmer. | Surroundings become cooler |
| Energy moves out of system | Energy moves into system |
| Often uses oxidation | Often uses an endothermic dissolving process |
The useful effect of each product comes directly from the direction of energy transfer.
Cooking with Chemical Energy
Cooking provides many examples of chemical energy changes.
When natural gas burns on a stove:
Chemical energy → Thermal energy
The thermal energy is then transferred:
Flame → Cooking pan → Food
The combustion reaction is exothermic, but many of the changes occurring in the food itself require an input of energy.
This demonstrates how several energy transfers can occur as part of one everyday activity.
Batteries
Batteries provide another important application of chemical energy.
Chemical reactions inside a battery create conditions that allow electrons to move through an external circuit.
A simplified energy transformation is:
Chemical energy → Electrical energy
The electrical energy can then be transformed again.
For example, in a flashlight:
Chemical energy
↓
Electrical energy
↓
Light + Thermal energy
In an electric motor:
Chemical energy in battery
↓
Electrical energy
↓
Mechanical energy + Thermal energy
Glow Sticks
Glow sticks provide an interesting example because much of the energy is released as light rather than heat.
When a glow stick is bent, an inner container breaks and chemicals mix.
A chemical reaction occurs that transfers energy to molecules capable of producing visible light.
This process is called chemiluminescence.
The energy pathway is approximately:
Chemical energy → Light energy
This reminds us that chemical reactions do not always transfer most of their energy as heat.
Energy Changes in Food
Food contains chemical energy.
Carbohydrates, lipids, and proteins contain molecules that can participate in metabolic reactions.
Cells can release usable energy from food molecules through processes including cellular respiration.
That energy supports:
- movement
- growth
- repair
- active transport
- maintaining body temperature
- building biological molecules
Food is therefore not simply material for building the body. It is also an important energy source.
Energy Changes in Ecosystems
Energy changes in photosynthesis and respiration connect organisms throughout ecosystems.
Plants capture energy:
Sunlight → Chemical energy in organic molecules
Animals obtain chemical energy by eating plants or other animals.
Both plants and animals carry out respiration:
Chemical energy → Cellular work + thermal energy
Energy therefore flows through ecosystems:
Sun → Producers → Consumers → Surroundings
At each stage, some energy is eventually transferred to the surroundings as thermal energy.
Real-World Applications
Understanding chemical energy changes allows scientists and engineers to design useful technologies.
Heating
Exothermic reactions can provide thermal energy.
Examples:
- fuels
- hand warmers
- some industrial processes
Cooling
Endothermic processes can provide temporary cooling.
Example:
- instant cold packs
Transportation
Combustion reactions have traditionally provided energy for:
- cars
- aircraft
- ships
Electricity
Chemical reactions in batteries provide electrical energy for:
- phones
- computers
- electric vehicles
- medical devices
- portable electronics
Living Systems
Photosynthesis and respiration provide the foundation for energy transformations throughout much of the living world.
Identifying Energy Changes
When examining an unfamiliar situation, ask four questions:
1. What is the chemical process?
Is something burning, reacting, dissolving, respiring, or photosynthesising?
2. Where does the energy begin?
For example:
- chemical energy
- light energy
- thermal energy
3. Where does the energy go?
Look for:
- heating
- cooling
- movement
- light
- electricity
4. Is energy entering or leaving the system?
Energy leaves system → Exothermic
Energy enters system → Endothermic
Worked Example
A student activates two emergency packs.
Pack A
Temperature changes:
21°C → 39°C
The temperature increases.
Energy moves:
System → Surroundings
Therefore:
Pack A involves an exothermic process.
Pack B
Temperature changes:
21°C → 8°C
The temperature decreases.
Energy moves:
Surroundings → System
Therefore:
Pack B involves an endothermic process.
Putting It All Together
Consider four everyday situations:
A candle burns
Chemical → Thermal + Light
Exothermic
A plant photosynthesises
Light → Chemical
Endothermic overall
A hand warmer activates
Chemical → Thermal
Exothermic
A cold pack activates
Thermal energy from surroundings → Chemical system
Endothermic
All four examples involve energy transformations, but the direction and form of energy transfer are different.
Common Misconception
Energy is not created by combustion, respiration, batteries, or hand warmers.
Energy is transferred or transformed.
For example, a battery does not create electrical energy from nothing.
Instead:
Chemical energy → Electrical energy → Other energy forms
The total amount of energy is conserved.
Did You Know?
The chemical energy in many foods can ultimately be traced back to the Sun.
Plants capture light energy through photosynthesis and store some of it as chemical energy in organic molecules. Animals obtain this chemical energy by eating plants or other organisms.
So the energy used by your muscles while running may ultimately have begun as sunlight captured by a plant.
Key Terms
Chemical energy – Energy associated with chemical substances and their molecular arrangements.
Combustion – A reaction in which a substance reacts rapidly with oxygen and releases energy.
Respiration – Cellular reactions that transfer energy from molecules such as glucose for use by cells.
Photosynthesis – The process by which organisms such as plants use light energy to produce organic molecules.
Exothermic – Describes a process that transfers energy to the surroundings.
Endothermic – Describes a process that absorbs energy from the surroundings.
Oxidation – A chemical process involving loss of electrons; many everyday oxidation processes involve reaction with oxygen.
Chemiluminescence – Production of light as a result of a chemical reaction.
Energy transformation – Conversion of energy from one form to another.
Key Takeaways
- Chemical reactions involving energy changes occur throughout everyday life.
- Combustion is exothermic and transfers chemical energy to the surroundings.
- Fuels can provide energy for heating, transportation, and electricity generation.
- Cellular respiration transfers energy from glucose for use by cells.
- Photosynthesis transforms light energy into chemical energy stored in organic molecules.
- Hand warmers use exothermic processes to transfer thermal energy to their surroundings.
- Cold packs use endothermic processes that absorb thermal energy from their surroundings.
- Batteries transform chemical energy into electrical energy.
- Glow sticks demonstrate that chemical energy can be transformed into light.
- Energy transformations are important in living organisms, ecosystems, transportation, heating, cooling, and technology.
- Energy is never created or destroyed during these processes—it is transferred or transformed.
- Understanding energy changes allows us to explain how many familiar products and technologies actually work.