Understanding Exothermic and Endothermic Reactions
Analogy
- Consider you’re holding two beakers.
- In one, you mix hydrochloric acid with magnesium, and almost immediately, the beaker becomes warm to the touch.
- In the other, you dissolve ammonium nitrate in water, and the beaker feels cold.
- Why do these reactions behave so differently?
- The answer lies in how energy is transferred between the reaction system and its surroundings, defining the concepts of exothermic and endothermic reactions.
Energy Transfer in Reactions: The Basics
- Every chemical reaction involves breaking bonds in reactants and forming bonds in products.
- The overall energy change of a reaction depends on the balance between these two processes.
Hint
Breaking bonds requires energy (endothermic), while forming bonds releases energy (exothermic).
The System and the Surroundings
- System: The part of the universe where the reaction occurs (e.g., the reactants and products).
- Surroundings: Everything else, including the container and the air around it.
Energy transfer can occur in two directions:
Definition
Exothermic reactions
Energy is released from the system into the surroundings, increasing the temperature of the surroundings.
Definition
Endothermic reactions
Energy is absorbed by the system from the surroundings, decreasing the temperature of the surroundings.
- This transfer of energy is most commonly observed as heat.
- Let’s explore these two types of reactions in detail.
Exothermic Reactions: Energy Released
- In an exothermic reaction, the energy required to break bonds is less than the energy released when new bonds are formed.
- The excess energy is transferred to the surroundings, often as heat.
Key Features:
- Energy flow: From the system to the surroundings.
- Observable changes: The surroundings become warmer. For example:
- Combustion of fuels (e.g., burning wood or propane).
- Neutralization reactions between acids and bases.
- Respiration in living organisms.
Example
Combustion of Methane
$$
\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) + \text{energy}
$$
- In this reaction, the energy released when forming $\text{CO}_2$ and $\text{H}_2\text{O}$ is greater than the energy required to break the bonds in $\text{CH}_4$ and $\text{O}_2$.
- This results in a release of heat, making the surroundings warm.
Endothermic Reactions: Energy Absorbed
- In an endothermic reaction, the energy required to break bonds is greater than the energy released when new bonds form.
- The system absorbs this extra energy from the surroundings, causing the surroundings to cool.
Key Features:
- Energy flow: From the surroundings to the system.
- Observable changes: The surroundings become cooler. For example:
- Photosynthesis in plants.
- Dissolving ammonium nitrate in water.
- Melting ice into liquid water.
Example
Dissolving Ammonium Nitrate
$$
\text{NH}_4\text{NO}_3(s) \rightarrow \text{NH}_4^+(aq) + \text{NO}_3^-(aq)
$$
- When ammonium nitrate dissolves in water, energy is absorbed from the surroundings to break the ionic bonds in the solid lattice.
- This absorption of energy makes the solution feel cold to the touch.
Common Mistake
- Heat refers to the transfer of energy due to a temperature difference, while temperature measures the average kinetic energy of particles.
- An exothermic reaction releases heat, increasing the temperature of the surroundings, but they are not the same thing.
Self review
- What is the difference between an exothermic and an endothermic reaction in terms of energy transfer?
- Why does the temperature of the surroundings increase during an exothermic reaction?
- Can you think of an example of an endothermic reaction you’ve observed in daily life?
