Exothermic and endothermic reactions common mistakes

Common mistakes

• Misunderstanding Bond Energy Calculations

  Students often confuse the total energy needed to break bonds with the energy released when bonds form, leading to incorrect calculations.

  Fix itTo fix this, students should clearly separate the energy needed to break bonds from the energy released when bonds form, ensuring they apply the correct formula for each part.

• Misunderstanding Bond Energies

  Students often confuse the total energy released when bonds form with the energy needed to break bonds, leading to incorrect calculations.

  Fix itTo fix this, students should clearly differentiate between the energy required to break bonds and the energy released when bonds form, ensuring they follow the correct steps in calculations.

• Mis‑applying the sign convention

  Students often add the energy released when bonds form to the energy required to break bonds, giving a positive value for an exothermic reaction.

  Fix itRemember the formula: ΔE = ΣE(bonds broken) – ΣE(bonds formed). For an exothermic reaction the second term is larger, so ΔE is negative, indicating energy is released.

• Misunderstanding Exothermic Reactions

  Students often think that exothermic reactions absorb energy from the surroundings instead of releasing it.

  Fix itRemember that exothermic reactions release energy to the surroundings, resulting in an increase in temperature.

• Misunderstanding Endothermic Reactions

  Students often think that endothermic reactions absorb energy without any comparison to the energy released from forming bonds.

  Fix itEmphasize that an endothermic reaction occurs specifically when the energy required to break bonds is greater than the energy released when new bonds form.

• Misunderstanding Energy Transfer

  Students often think that the energy change in a reaction is solely based on the reactants without considering the energy released when products form.

  Fix itEmphasize that the overall energy change is calculated by considering both the energy needed to break bonds in reactants and the energy released when bonds form in products.

• Sign Interpretation Error

  Students often forget to include the correct sign when interpreting energy changes, leading to incorrect conclusions about whether a reaction is exothermic or endothermic.

  Fix itAlways check if the energy change is positive or negative. A negative value indicates an exothermic reaction (energy released), while a positive value indicates an endothermic reaction (energy absorbed).

• Confusing bond‑energy calculations with reaction‑profile interpretation

  Students think that the shape of a reaction profile can be used to calculate the exact energy change of a reaction, or that the activation energy shown on the profile is the same as the bond energies used in calculations.

  Fix itExplain that a reaction profile is a qualitative diagram showing the relative energies of reactants, transition state and products, and that it cannot provide numerical bond‑energy values. Bond‑energy calculations use tabulated average energies for specific bonds to determine the overall energy change, while a reaction profile only indicates whether a reaction is exothermic or endothermic and the relative size of the activation energy.