R1.1.3 Energy profiles

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:
${CH}_{4} (g) + 2 O_{2} (g) \to {CO}_{2} (g) + 2 H_{2} O (g) Δ H = - 890 {kJ mol}^{- 1}$
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 ( $Δ H$ ).

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 ( $Δ H$ ):
- The vertical difference between the reactants and products represents the enthalpy change, $Δ H$ .

Tip

If the products are lower in energy than the reactants, $Δ H$ is negative (exothermic).
If the products are higher in energy than the reactants, $Δ H$ is positive (endothermic).

Example

Exothermic Reaction

Consider the reaction between zinc and copper(II) sulfate:
$Zn (s) + {CuSO}_{4} (a q) \to Cu (s) + {ZnSO}_{4} (a q) Δ H = - 217 {kJ mol}^{- 1}$

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 ( $E_{a}$ ).

Energy profile of the exothermic reaction.

Example

Endothermic Reaction

Now, consider the dissolution of ammonium nitrate:
${NH}_{4} {NO}_{3} (s) \to {NH}_{4}^{+} (a q) + {NO}_{3}^{-} (a q) Δ H = + 25 {kJ mol}^{- 1}$

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 ( $E_{a}$ ).

Energy profile of the endothermic reaction.

Common Mistake

Students often confuse $Δ H$ with $E_{a}$ . Remember, $Δ H$ represents the overall energy change, while $E_{a}$ is the energy barrier that must be overcome for the reaction to occur.

Reflection

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?

Theory of Knowledge

To what extent should scientists prioritize environmental responsibility over energy efficiency?
How do values and ethics shape decisions about which chemical reactions to develop?

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R1.1.3 Energy profiles

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

Combustion of Methane

Energy Profile Diagrams: Visualizing Energy Changes

Key Features of an Energy Profile Diagram

Exothermic Reaction

Endothermic Reaction

Reflection

You've read 2/2 free chapters this week.

Energy changes in chemical reactions