Understanding chemical reactions

Chemistry students often struggle to answer the fundamental question: why do chemical reactions happen? In organic chemistry, we primarily rationalise reactivity by the energy of reactants, intermediates and products. Although we teach energy as a core idea at school level, students can find it hard to understand – particularly in trying to connect energy to reactivity and structure. As a result, their knowledge can become fragmented, dampening their problem-solving abilities.

In a recent study, researchers sought to identify the knowledge students activate when reasoning about energy and chemical structure in the reaction of hydrogen and chlorine to make hydrogen chloride.

Learners often rationalise bonding in terms of the octet rule rather than by forces and energies

Previous studies indicate that students seldom use electrostatic attractions to relate energy to structure, and that they often don’t distinguish the energetic processes at the sub-microscopic level from macroscopic, everyday experiences with energy. They often interpret energy as a property of particles rather than a consequence of particle interactions. Electrostatic interactions – most prominently chemical bonds – determine the relationship between structure and energy. While students are typically aware of the different types of bonds, they struggle with the common nature of attractions between particles.

Learners often rationalise bonding in terms of the octet rule, rather than by forces and energies. Reaction coordinate diagrams, commonly referred to as energy profile diagrams (EPDs), are key representations which connect kinetic and thermodynamic aspects of reactions. They are used to relate energy to structure and reactivity.

The study involved semi-structured group interviews with 16 groups of post-16 students at a German school. Each group watched a video showing the reaction of hydrogen and chlorine in a syringe, where a piezoelectric igniter generated a spark, causing the reaction mixture to explode. Thermal imaging revealed the exothermic nature of the reaction.

Students had to consider what the spark was for, why the reaction mixture turned hot and why a reaction occurred at all. They also had to submit detailed explanations and graphic elements.

Activating knowledge

Most groups linked the spark to the activation energy, although responses indicated that some students understood the term differently. The authors warn against the interpretation that activation energy is energy that is added to the system. Rather, it should be defined as the energy that specific particles need to react, generally being the energy required to break bonds.

Many students thought the spark induced the reaction of all particles at the same time. Only one group identified that the initial reaction of a small portion of the reactants leads to a release of energy that stimulates further reaction.

The groups predominantly related the temperature increase to the exothermic nature of the reaction. Most students didn’t realise that this was determined by the energy released in the bonding process.

Almost all the groups reasoned that the reaction was driven by the octet rule, even though the atoms in the reactants had full outer shells. Five groups rationalised the reaction process using energetic arguments, citing entropy and free energy. All groups could draw an EPD, but often with flawed interpretations of its features.