Energy Transfer in Reactions

Step 1: Identify Bonds: List all the chemical bonds present in the reactant molecules and all the bonds present in the product molecules.

Step 2: Sum Bond Energies: Use a table of average bond energies to calculate the total energy required to break all reactant bonds ($E_{in}$) and the total energy released by forming all product bonds ($E_{out}$).

Step 3: Apply the Formula: Calculate the overall energy change using the relationship: $\Delta H = \sum (\text{Energy of bonds broken}) - \sum (\text{Energy of bonds formed})$

Step 4: Interpret the Sign: If $\Delta H$ is negative, the reaction is exothermic (more energy released than absorbed). If $\Delta H$ is positive, the reaction is endothermic (more energy absorbed than released).

Always check the sign: In calculation questions, a negative result for $\Delta H$ must be explicitly stated if the reaction is exothermic. Forgetting the sign is a frequent cause of lost marks.

Activation Energy Measurement: Ensure you measure $E_a$ from the reactant energy level to the peak of the curve, never from the product level or the x-axis.

Labeling Diagrams: When drawing reaction profiles, always include labels for the y-axis (Energy), x-axis (Progress of Reaction), Reactants, Products, Activation Energy ($E_a$), and Enthalpy Change ($\Delta H$).

Units Consistency: Ensure all bond energies are in the same units (usually $kJ/mol$) before summing them. If a molecule has multiple identical bonds (e.g., $CH_4$ has 4 $C-H$ bonds), remember to multiply the bond energy by that coefficient.