Energy Level Diagrams

• Conservation of Energy: According to the first law of thermodynamics, energy cannot be created or destroyed. Any change in the internal energy of the chemical system must be balanced by an equal and opposite change in the energy of the surroundings.

• Enthalpy Change ($\Delta H$): This is defined as the difference between the enthalpy of the products ($H_{products}$) and the enthalpy of the reactants ($H_{reactants}$).

Fundamental Equation: $\Delta H = H_{products} - H_{reactants}$

• Stability and Energy: Systems naturally tend toward lower energy states. In a reaction where the products are at a lower energy level than the reactants, the products are generally considered more energetically stable than the starting materials.

• Comparative Analysis: Use the following table to distinguish between the two primary reaction profiles shown on diagrams:

• Labeling Verification: Always ensure the y-axis is labeled as 'Energy' or 'Enthalpy' and the x-axis as 'Progress of Reaction'. Omitting these labels is a common way to lose easy marks.

• Arrow Precision: The enthalpy change arrow must clearly start at the reactant line and end exactly at the product line. An arrow that does not touch both lines may be marked as inaccurate.

• Sign Consistency: Double-check that the sign of your calculated $\Delta H$ matches the visual representation. If the products are lower, $\Delta H$ MUST be negative.

• Reaction Profiles vs. Level Diagrams: Be aware that 'Reaction Profiles' often include a curved path showing the activation energy, whereas 'Energy Level Diagrams' typically focus only on the initial and final states.

• Confusing System vs. Surroundings: Students often mistake a drop in the diagram's energy level for a drop in temperature. In an exothermic reaction, the system energy drops because heat is being pushed out into the surroundings, causing the surroundings' temperature to rise.

• Incorrect Arrow Start Point: A frequent error is drawing the arrow from the x-axis upward. The arrow represents the change from reactants to products, so its base must be the reactant level.

• The 'Zero' Energy Assumption: Do not assume the reactant level is at 'zero'. Diagrams show relative energy, not absolute energy values.

• Bond Energetics: These diagrams provide the visual context for bond energy calculations. Breaking bonds requires energy (upward movement), while making bonds releases energy (downward movement).

• Catalysis: While simple level diagrams don't show the transition state, they serve as the baseline for drawing reaction profiles where catalysts lower the peak without changing the initial and final energy levels.