Getting Started
For a chemical reaction to occur, reactant particles must collide. However, not every collision results in the formation of products. This chapter explores the energetic requirements of a chemical reaction, visualizing the process at the molecular level as a journey over an energy "hill." We will examine why an initial input of energy is necessary to break bonds and how the overall energy changes as new, more stable bonds are formed.
What You Should Be Able to Do
By the end of this section, you should be able to:
Sketch a reaction energy profile for both energy-releasing (exothermic) and energy-absorbing (endothermic) reactions.
Identify the reactants, products, transition state, activation energy, and overall enthalpy change on a given profile.
Explain why increasing temperature increases the rate of a reaction.
Relate the height of the energy barrier on a profile to the relative speed of the reaction.
Key Concepts & Analysis
We can understand the energy changes during a reaction by tracking the system's potential energy as it transforms from reactants to products. This approach treats the reaction as a dynamic process of change over time.
Baseline Condition: The Reactants
A chemical system begins with reactants, which possess a certain amount of potential energy stored within their chemical bonds. At any given temperature, these reactant molecules are in constant motion, colliding with one another. However, for a reaction to be initiated, these collisions must be "effective," meaning they must have both sufficient energy and the correct geometric orientation. The initial energy level of the stable reactants serves as the starting point, or baseline, on our energy graph.
The Process: Traversing the Reaction Coordinate
As reactant molecules approach each other and collide effectively, they begin to transform. This transformation is plotted along a reaction coordinate, which represents the progress of the reaction from start to finish.
Bond Strain and Breaking: As reactants collide, existing bonds are stretched and begin to break. This process requires an input of energy, causing the potential energy of the system to increase.
The Transition State: The system reaches a point of maximum potential energy. At this peak lies the transition state, also known as the activated complex. This is a highly unstable, fleeting arrangement of atoms where old bonds are not fully broken and new bonds are not yet fully formed. It represents the highest energy barrier that must be overcome for the reaction to proceed.
Bond Formation: After the transition state, new, more stable bonds begin to form. Bond formation releases energy, causing the potential energy of the system to decrease as it moves "downhill" from the energy peak.
The Resulting Change: Products and Energy Change
The process concludes with the formation of products, which have their own characteristic potential energy. The key energetic quantities that describe this overall change are:
Activation Energy (Ea): This is the energy difference between the reactants and the transition state. It is the minimum amount of energy required to initiate the reaction—the height of the energy hill. A larger activation energy corresponds to a slower reaction rate, as fewer molecules will possess enough energy to overcome the barrier.
Enthalpy of Reaction (ΔH): This is the net energy difference between the products and the reactants.
If the products are at a lower energy level than the reactants, the reaction is exothermic, releasing energy into the surroundings (ΔH is negative).
If the products are at a higher energy level than the reactants, the reaction is endothermic, absorbing energy from the surroundings (ΔH is positive).
Crucially, the rate of a reaction depends on temperature. According to the principles described by the Arrhenius equation, increasing the temperature increases the average kinetic energy of the molecules. This means a larger fraction of the colliding particles will have sufficient energy to overcome the activation energy barrier, leading to a higher rate of reaction.
Key Models & Representations
The reaction energy profile is the central model for this topic. The table below compares the profiles for exothermic and endothermic elementary reactions.
| Feature | Exothermic Reaction | Endothermic Reaction |
|---|---|---|
| Relative Energy | Products have lower potential energy than reactants. | Products have higher potential energy than reactants. |
| Enthalpy (ΔH) | Negative (ΔH < 0). Energy is released. | Positive (ΔH > 0). Energy is absorbed. |
| Profile Sketch | The profile starts high, goes over the Ea hill, and ends at a lower energy level. | The profile starts low, goes over the Ea hill, and ends at a higher energy level. |
| Thermodynamic Favorability | Generally considered thermodynamically favorable. | Generally considered thermodynamically unfavorable. |