Exothermic and endothermic reactions
| Enthalpy Change ($\Delta H$) |
|---|
| Negative (-) value. |
| Positive (+) value. |
In an exothermic reaction, energy is released from the chemical bonds into the surroundings. This extra thermal energy causes the particles in the surroundings to move faster, resulting in a higher temperature.
In an endothermic reaction, energy is absorbed from the surroundings to break bonds. This removal of thermal energy causes the particles in the surroundings to slow down, resulting in a lower temperature.
Enthalpy change is the term used to describe the transfer of thermal energy during a reaction at constant pressure.
- For exothermic reactions, \Delta H is negative (e.g., -286 \text{ kJ mol}^{-1}). This indicates that the system has lost energy to the surroundings.
- For endothermic reactions, \Delta H is positive (e.g., +50 \text{ kJ mol}^{-1}). This indicates that the system has gained energy from the surroundings.
A reaction pathway diagram (or energy profile) shows how the energy of the system changes as reactants turn into products.
Axes:
- Y-axis: Energy level (usually in kJ or kJ mol⁻¹).
- X-axis: Reaction progress (time/extent of reaction).
Interpreting the Diagrams:
- Reactants and Products: The horizontal lines represent the energy levels of the starting materials (reactants) and the final materials (products).
- The Curve: The curve shows the energy change during the reaction. It must start at the reactant level, go up to a peak, and come down to the product level.
- Activation Energy (E_a): This is the minimum energy colliding particles must have to react. On the diagram, it is the vertical distance from the reactant line to the highest point (peak) of the curve.
- Enthalpy Change (\Delta H): This is the difference in energy between the products and the reactants. It is the vertical distance between the product line and the reactant line.
Drawing Reaction Pathway Diagrams:
When asked to draw a diagram from provided information, follow these steps:
- Draw Axes: Label Y-axis 'Energy' and X-axis 'Reaction Progress'.
- Draw Reactant Line: Draw a horizontal line at the starting energy level. Label it 'Reactants'.
- Determine Product Level:
- If exothermic: Draw the product line lower than the reactant line.
- If endothermic: Draw the product line higher than the reactant line.
- Label this line 'Products'.
- Draw the Curve: Start at the reactant line, curve upwards to a peak, and curve downwards to meet the product line. Ensure the curve is smooth and does not cross the axes.
- Label E_a: Draw a vertical arrow starting from the reactant line pointing up to the peak. Label it 'E_a'.
- Label \Delta H: Draw a vertical arrow starting from the product line pointing up to the reactant line (or vice versa, but clearly indicating the gap). Label it '\Delta H'. Note: For exothermic, the arrow points down; for endothermic, it points up.
| Endothermic Diagram |
|---|
| Products are higher than reactants. |
| Starts low, ends high. |
| Positive (+) |
Even in exothermic reactions (which release net energy), energy is required initially to break the bonds in the reactant molecules. This initial 'energy barrier' is the activation energy.
- Breaking bonds always requires energy input. Therefore, bond breaking is an endothermic process.
- Making bonds always releases energy. Therefore, bond making is an exothermic process.
The overall enthalpy change (\Delta H) depends on the balance between these two processes:
\Delta H = \text{Energy required to break bonds} - \text{Energy released when bonds form}
Calculating Enthalpy Change from Bond Energies:
- If Energy in (breaking) > Energy out (making), the reaction is endothermic (\Delta H is positive).
- If Energy out (making) > Energy in (breaking), the reaction is exothermic (\Delta H is negative).
Question: Calculate the enthalpy change for the reaction: H_2 + Cl_2 \rightarrow 2HCl
Given Bond Energies:
- H-H: 436 \text{ kJ mol}^{-1}
- Cl-Cl: 242 \text{ kJ mol}^{-1}
- H-Cl: 431 \text{ kJ mol}^{-1}
We need to break 1 mole of H-H and 1 mole of Cl-Cl.
\text{Energy in} = 436 + 242 = +678 \text{ kJ mol}^{-1}
We form 2 moles of H-Cl bonds.
\text{Energy out} = 2 \times 431 = +862 \text{ kJ mol}^{-1}
(Note: We use positive values for the calculation and apply the sign in the final formula)
\Delta H = \text{Energy in} - \text{Energy out}
\Delta H = 678 - 862
\Delta H = -184 \text{ kJ mol}^{-1}
Since \Delta H is negative, the reaction is exothermic.
Correction: In an endothermic reaction, energy is taken from the surroundings. Therefore, the temperature of the surroundings (and the reaction mixture) decreases. Only exothermic reactions cause a temperature increase.
Mistake: Stating 'the reactants have more energy than the products' without specifying that this means the reaction is exothermic, or confusing the height of the peak with the overall energy change.
Correction: Always compare the final energy level (products) to the initial energy level (reactants).
- If Products < Reactants: Exothermic.
- If Products > Reactants: Endothermic.
The height of the peak only tells you about the activation energy (E_a), not whether the reaction is exo- or endothermic.
When to use: When asked to explain why a reaction is exothermic or endothermic based on a diagram or temperature data.
Why examiners accept this: Examiners look for the specific link between energy transfer and temperature change. Simply saying 'energy is released' is often insufficient without mentioning the effect on the surroundings.
Correct Phrasing Example:
- For Exothermic: "The reaction is exothermic because thermal energy is transferred to the surroundings, causing the temperature of the surroundings to increase."
- For Endothermic: "The reaction is endothermic because thermal energy is taken from the surroundings, causing the temperature of the surroundings to decrease."
When to use: When drawing or labeling a reaction pathway diagram.
Why examiners accept this: Precision in labeling is critical. The arrow for \Delta H must clearly span the gap between reactant and product levels, and E_a must start from the reactant level.
Correct Phrasing/Labeling Example:
- Ensure the arrow for \Delta H connects the reactant line to the product line.
- Ensure the arrow for E_a starts at the reactant line and ends at the peak of the curve.
- Do not label the peak as 'activation energy'; label the arrow or the distance as E_a.