Relative Stability of Reactants and Products, and Energy Profile Diagrams
Relative Stability of Reactants and Products: The Key to Energy Flow
Why Stability Matters in Reactions
- At the heart of every chemical reaction is a change in energy.
- Bonds in the reactants must be broken, and new bonds in the products must be formed.
- The energy required to break bonds and the energy released when new bonds form dictate whether a reaction absorbs or releases energy.
- But there’s more to the story: the relative stability of the reactants and products plays a crucial role.
- Reactants and Products as Energy Containers:
- Imagine reactants and products as vessels holding chemical potential energy.
- If the products hold less energy than the reactants, the "excess" energy is released, resulting in an exothermic reaction.
- Conversely, if the products hold more energy, the reaction absorbs energy from the surroundings, making it endothermic.
- Stability and Energy Levels:
- Stability and energy are inversely related.
- Molecules at lower energy levels are more stable, while those at higher energy levels are less stable.
Hint
- In an exothermic reaction, the products are more stable (lower energy) than the reactants.
- In an endothermic reaction, the reactants are more stable (lower energy) than the products.
Tip
Exothermic reactions often result in more stable products, but spontaneity also depends on entropy, a concept covered in later topics.
Example
Combustion of Methane
- The combustion of methane (CH₄) illustrates an exothermic reaction:
$$
\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(g) \quad \Delta H = -890 \, \text{kJ mol}^{-1}
$$ - In this reaction, the reactants (methane and oxygen) are higher in energy and less stable compared to the products (carbon dioxide and water).
- The energy difference, 890 kJ per mole, is released as heat and light.
Energy Profile Diagrams: Visualizing Energy Changes
- An energy profile diagram is a powerful tool to visualize the energy changes in a reaction.
- It shows the relative stability of reactants and products, the activation energy, and the overall enthalpy change ($\Delta H$).
Key Features of an Energy Profile Diagram
- Axes:
- The x-axis represents the reaction coordinate, tracking progress from reactants to products.
- The y-axis represents the potential energy of the system.
- Reactants and Products:
- The starting point corresponds to the energy of the reactants.
- The endpoint corresponds to the energy of the products.
- Activation Energy ($E_a$):
- The highest point on the curve represents the transition state, the moment of highest energy during the reaction.
- The energy difference between the reactants and the transition state is the activation energy, $E_a$, the energy barrier that must be overcome for the reaction to proceed.
- Enthalpy Change ($\Delta H$):
- The vertical difference between the reactants and products represents the enthalpy change, $\Delta H$.
Tip
- If the products are lower in energy than the reactants, $\Delta H$ is negative (exothermic).
- If the products are higher in energy than the reactants, $\Delta H$ is positive (endothermic).
Example
Exothermic Reaction
Consider the reaction between zinc and copper(II) sulfate:
$$
\text{Zn}(s) + \text{CuSO}_4(aq) \rightarrow \text{Cu}(s) + \text{ZnSO}_4(aq) \, \quad \Delta H = -217 \, \text{kJ mol}^{-1}
$$
An energy profile diagram for this reaction would show:
- Reactants (Zn and CuSO₄) at a higher energy level.
- Products (Cu and ZnSO₄) at a lower energy level.
- A peak representing the activation energy ($E_a$).
Example
Endothermic Reaction
Now, consider the dissolution of ammonium nitrate:
$$
\text{NH}_4\text{NO}_3(s) \rightarrow \text{NH}_4^+(aq) + \text{NO}_3^-(aq) \, \quad \Delta H = +25 \, \text{kJ mol}^{-1}
$$
An energy profile diagram for this reaction would show:
- Reactants (solid ammonium nitrate) at a lower energy level.
- Products (aqueous ions) at a higher energy level.
- A peak representing the activation energy ($E_a$).
Common Mistake
- Students often confuse $\Delta H$ with $E_a$.
- Remember, $\Delta H$ represents the overall energy change, while $E_a$ is the energy barrier that must be overcome for the reaction to occur.
Self review
- Can you sketch and label an energy profile diagram for both exothermic and endothermic reactions?
- How do the differences in energy levels reflect the relative stability of reactants and products?
