Exothermic and endothermic reactions common mistakes

Common mistakes

• Mislabeling Components

  Students often label the reactants and products incorrectly on a reaction profile, confusing their positions.

  Fix itTo fix this, remember that reactants are always on the left side of the reaction profile and products are on the right side. Practice drawing and labeling reaction profiles to reinforce this concept.

• Misunderstanding Activation Energy

  Students often confuse activation energy with the overall energy change of the reaction, thinking they are the same.

  Fix itRemember that activation energy is the minimum energy required for reactants to collide and react, while the overall energy change refers to the difference in energy between reactants and products.

• Labeling overall energy change on a reaction profile

  Students often label the overall energy change as the height of the activation energy peak, rather than the difference between reactant and product energies

  Fix itExplain that the overall energy change is the vertical difference between the reactant and product energy levels; a negative value indicates an exothermic reaction and a positive value an endothermic reaction. The activation energy is the energy required to reach the transition state, not the overall ΔE.

• Misidentifying the overall energy change on a reaction profile

  Students often read the vertical drop or rise between the reactant and product baselines as the activation energy, rather than the overall energy change (ΔE).

  Fix itExplain that the activation energy is the height of the peak above the reactant baseline, while the overall energy change is the difference between the reactant and product baselines; a downward drop indicates an exothermic reaction and an upward rise indicates an endothermic reaction.

• Misinterpreting the reaction profile curve

  Students think the curved line on a reaction profile represents the speed of the reaction rather than the change in energy as the reaction proceeds

  Fix itExplain that the curved line shows the variation in potential energy of the system as reactants convert to products, not the reaction rate

• Confusing Reactant and Product Energies

  Students often confuse the energy levels of reactants and products in exothermic and endothermic reaction profiles, leading to incorrect conclusions about energy transfer.

  Fix itTo fix this, students should practice drawing and labeling reaction profiles, ensuring they clearly identify the energy levels of both reactants and products, and remember that in exothermic reactions, products have lower energy than reactants, while in endothermic reactions, products have higher energy.

• Confusion about Bond Breaking

  Students often state that energy is released when bonds in reactants are broken, rather than recognizing that energy must be supplied to break these bonds.

  Fix itRemember that breaking bonds requires energy input, while forming bonds releases energy. Focus on the distinction between these processes.

• Misidentifying bond formation energy

  Students often think that forming bonds always requires energy input, so they write that energy is absorbed when bonds in products are formed.

  Fix itExplain that bond formation releases energy; the energy released is the negative of the bond energy and should be subtracted when calculating the overall energy change.

• Misunderstanding Bond Energies

  Students often confuse bond energies with the energy changes in reaction profiles, thinking they are the same concept.

  Fix itClarify that bond energies specifically refer to the energy required to break bonds, while reaction profiles illustrate the overall energy changes during a reaction.

• Misunderstanding Bond Energies

  Students often confuse bond energies with the overall energy change of a reaction, thinking that bond energies alone determine whether a reaction is exothermic or endothermic.

  Fix itEmphasize that bond energies are used to calculate the energy released when bonds form, and this must be compared to the energy needed to break bonds to determine the overall energy change.