Energy Changes in Chemical Reactions
1. Chemical Energy
Learning outcomes
- I can define chemical energy as energy stored in chemical bonds.
- I can explain how chemical energy is released or absorbed during reactions.
- I can identify examples of chemical energy in everyday life.
- I can describe the relationship between chemical energy and bond energy.
- I can explain why chemical reactions involve energy changes.
What Is Chemical Energy?
Chemical energy is energy associated with the arrangement of atoms and the chemical bonds in substances.
Atoms join together by forming chemical bonds. Different arrangements of atoms have different amounts of stored chemical energy. When a chemical reaction occurs, atoms are rearranged: some bonds are broken and new bonds are formed.
This means chemical reactions can transfer energy between the reacting substances and their surroundings.
Examples of substances and systems involving chemical energy include:
- food
- gasoline and other fuels
- natural gas
- batteries
- wood
- coal
Chemical energy is a form of potential energy because it depends on the arrangement and interactions of particles within a substance.
What Happens During a Chemical Reaction?
In a chemical reaction, the atoms themselves are not created or destroyed. Instead, they are rearranged.
Consider the general reaction:
Reactants → Products
For the reactants to become products:
- Some bonds in the reactants must be broken.
- The atoms rearrange.
- New bonds are formed to produce the products.
Both bond breaking and bond formation involve energy.
Breaking bonds requires energy
Energy must be supplied to separate bonded atoms.
Breaking bonds → energy is absorbed
Forming bonds releases energy
When atoms form new bonds, energy is transferred to the surroundings.
Forming bonds → energy is released
This is one of the most important ideas when studying energy changes in chemistry.
| Bond change | Energy |
|---|---|
| Bonds are broken | Energy is absorbed |
| Bonds are formed. | Energy is released |
Why Do Chemical Reactions Involve Energy Changes?
Every chemical reaction involves both breaking old bonds and forming new bonds.
The overall energy change depends on the balance between:
Energy absorbed breaking bonds
and
Energy released forming bonds
If these amounts are different, there is an overall transfer of energy between the reaction and its surroundings.
This is why chemical reactions involve energy changes.
Example
Suppose:
- Breaking the reactant bonds requires 400 kJ.
- Forming the product bonds releases 550 kJ.
Overall:
400 kJ − 550 kJ = −150 kJ
The reaction releases 150 kJ of energy overall.
If the opposite occurred and breaking the bonds required more energy than was released when new bonds formed, the reaction would absorb energy overall.
Releasing Chemical Energy: Exothermic Reactions
A reaction that transfers energy to the surroundings is called an exothermic reaction.
In an exothermic reaction:
- energy is absorbed to break bonds
- more energy is released when new bonds form
- there is an overall release of energy
- the surroundings usually become warmer
More energy released than absorbed → exothermic
Example: Combustion
When a fuel burns, it reacts with oxygen.
For methane:
methane + oxygen → carbon dioxide + water
Energy is released because the bonds formed in the products release more energy than is needed to break the bonds in the reactants.
Some of the chemical energy is transferred to the surroundings as thermal energy and sometimes light.
Absorbing Energy: Endothermic Reactions
A reaction that takes in energy from the surroundings is called an endothermic reaction.
In an endothermic reaction:
- energy is absorbed to break bonds
- less energy is released when new bonds form
- there is an overall absorption of energy
- the surroundings usually become cooler
More energy absorbed than released → endothermic
For example, some instant cold packs contain chemicals that undergo an endothermic process when mixed. Energy is taken from the surroundings, causing the temperature of the pack to decrease.
Chemical Energy in Everyday Life
Chemical energy is involved in many processes that we experience every day.
Food
Food contains chemical energy that can be transferred during reactions in the body.
During cellular respiration, glucose reacts with oxygen:
glucose + oxygen → carbon dioxide + water
Energy released through respiration can be transferred and used by cells for processes such as movement, growth and maintaining body temperature.
Fuels
Fuels such as gasoline, natural gas and wood undergo combustion reactions.
Chemical energy is transferred mainly into:
- thermal energy
- kinetic energy
- electrical energy
For example, combustion in a car engine ultimately helps produce the kinetic energy that moves the vehicle.
Batteries
Batteries contain chemicals that undergo reactions.
These reactions allow stored chemical energy to be transferred into electrical energy, which can power devices such as:
- phones
- flashlights
- laptops
- electric vehicles
Wood and Other Biomass
Wood contains chemical energy originally stored through photosynthesis.
When wood burns, chemical reactions release energy to the surroundings as heat and light.
Chemical Energy and Bond Energy
Bond energy is the energy required to break a particular chemical bond.
Different bonds have different bond energies.
In general:
Stronger bond → more energy required to break it
Weaker bond → less energy required to break it
For example, if one type of bond requires 400 kJ/mol to break and another requires 200 kJ/mol, the first bond is stronger.
Bond energies help chemists predict whether a reaction will release or absorb energy.
Using Bond Energies
We can estimate the overall energy change of a reaction using:
Energy change = energy needed to break bonds − energy released when bonds form
Worked Example
During a reaction:
Energy required to break bonds = 750 kJ
Energy released when new bonds form = 900 kJ
Therefore:
Energy change = 750 − 900
Energy change = −150 kJ
The negative value tells us that energy has been released.
Therefore, the reaction is exothermic.
Now consider another reaction:
Energy required to break bonds = 600 kJ
Energy released forming bonds = 450 kJ
Therefore:
Energy change = 600 − 450
Energy change = +150 kJ
Energy has been absorbed.
Therefore, the reaction is endothermic.
Comparing Exothermic and Endothermic Reactions
| Feature | Exothermic | Endothermic |
|---|---|---|
| Overall energy transfer | Energy released | Energy absorbed |
| Energy moves | Reaction → surroundings. | Surroundings → reaction |
| Surroundings usually | Become warmer | Become cooler |
| Relative energy of products. | Lower than reactants | Higher than reactants |
| Example | Combustion | Some cold-pack reactions |
Energy Is Conserved
Energy is not created or destroyed during a chemical reaction.
Instead, energy is transferred or transformed.
For example, during combustion:
chemical energy → thermal energy + light
In a battery:
chemical energy → electrical energy
In photosynthesis:
light energy → chemical energy
These changes follow the law of conservation of energy.
The total amount of energy is conserved, even though the form and location of the energy may change.
Did You Know?
A fuel does not release energy simply because its bonds are "full of energy." Energy must actually be supplied to break the bonds in the fuel first. The overall release of energy occurs because forming the new bonds in the products releases more energy than was required to break the original bonds.
This is why the statement "breaking bonds releases energy" is incorrect.
Key Terms
- Chemical energy – energy associated with chemical bonds and the arrangement of atoms.
- Chemical bond – an attractive interaction that holds atoms together.
- Bond energy – the energy required to break a chemical bond.
- Reactant – a substance present at the beginning of a reaction.
- Product – a substance produced by a chemical reaction.
- Exothermic reaction – a reaction that releases energy to the surroundings.
- Endothermic reaction – a reaction that absorbs energy from the surroundings.
- Combustion – a reaction in which a substance reacts with oxygen and releases energy.
- Energy transfer – movement of energy from one system or energy store to another.
Key Takeaways
- Chemical energy is associated with the bonds and arrangement of atoms in substances.
- Chemical reactions involve breaking old bonds and forming new bonds.
- Breaking chemical bonds requires energy.
- Forming chemical bonds releases energy.
- Exothermic reactions release more energy during bond formation than they absorb during bond breaking.
- Endothermic reactions absorb more energy during bond breaking than they release during bond formation.
- Bond energy describes how much energy is required to break a bond.
- Food, fuels, batteries and wood are familiar examples of systems involving chemical energy.
- Chemical reactions involve energy changes because the bonds in the reactants and products have different energies.
- Energy is conserved during chemical reactions; it is transferred or transformed rather than created or destroyed.