Energy Level Diagrams

• The shape of the diagram is determined by the net balance of energy required to break existing bonds (endothermic) and energy released when forming new bonds (exothermic).

• The Activation Energy ($E_a$) represents the energy barrier that must be overcome; even energetically favorable (exothermic) reactions require this initial input to destabilize reactant bonds.

• The difference in energy between the reactants and products is the Enthalpy Change ($\Delta H$) of the reaction, which dictates whether the overall process is heat-absorbing or heat-releasing.

Drawing the Diagram

• Step 1: Define Axes: Label the y-axis as 'Energy' (usually in $kJ/mol$) and the x-axis as 'Extent of Reaction' or 'Reaction Progress'.
• Step 2: Plot Energy Levels: Draw horizontal lines for the relative energy of reactants and products based on the reaction type (exothermic or endothermic).
• Step 3: Sketch the Pathway: Connect the reactants and products with a smooth curve that peaks at the transition state.
• Step 4: Annotate Arrows: Draw a vertical arrow from the reactant level to the peak for $E_a$, and a vertical arrow between reactant and product levels for $\Delta H$.

Calculating Reverse Reaction Energy

• For a reversible reaction, the activation energy for the reverse process ($E_{a, reverse}$) can be calculated if the forward activation energy ($E_{a, forward}$) and $\Delta H$ are known.
• In an endothermic reaction: $E_{a, reverse} = E_{a, forward} - |\Delta H|$.
• In an exothermic reaction: $E_{a, reverse} = E_{a, forward} + |\Delta H|$.

• Thermodynamic vs. Kinetic Control: A reaction may be thermodynamically favorable (large negative $\Delta H$) but occur extremely slowly if the activation energy ($E_a$) is too high, making it kinetically controlled.

• Arrow Direction Matters: Always ensure the $\Delta H$ arrow starts at the reactant level and ends at the product level; for exothermic reactions, this arrow points down, and for endothermic, it points up.

• $E_a$ Reference Point: Activation energy is ALWAYS measured from the reactant level to the peak, never from the zero-axis or the product level.

• Labeling Units: Ensure the y-axis is labeled with specific units like $kJ/mol$ if values are provided in the question.

• Sanity Check: If a reaction is described as 'releasing heat', your diagram must show products at a lower energy level than reactants.

• Confusing $\Delta H$ and $E_a$: Students often mistake the total height of the curve for the enthalpy change, whereas $\Delta H$ is only the difference between start and end levels.

• Ignoring the Transition State: Forgetting to draw the 'hump' implies a reaction happens spontaneously without collision energy, which is physically inaccurate for most chemical processes.

• Incorrect Reverse $E_a$: When calculating the reverse activation energy, students often forget that the 'peak' remains the same height, but the starting point changes to the product level.