Reaction Profiles (Edexcel GCSE Chemistry: Combined Science)

Reaction Profiles

• Reaction profiles (sometimes called energy level diagrams) are graphical representations of the relative energies of the reactants and products in chemical reactions
• The energy of the reactants and products are displayed on the y-axis and the reaction pathway is shown on the x-axis
• Arrows on the diagrams indicate whether the reaction is exothermic (downwards pointing) or endothermic (upwards pointing)
• The difference in height between the energy of reactants and products represents the overall energy change of a reaction
• The initial increase in energy represents the activation energy required to start the reaction
• The greater the initial rise then the more energy that is required to get the reaction going e.g., more heat needed

Reaction profile of an exothermic reaction and an endothermic reaction

• Energy is given out in exothermic reactions
• The energy of the products will be lower than the energy of the reactants, so the change in energy is negative
• This is represented on the reaction profile with a downwards-arrow as the energy of the products is lower than the reactants
• Energy is taken in endothermic reactions
• The energy of the products will be higher than the energy of the reactants, so the change in energy is positive
• This is represented on the reaction profile with an upwards-arrow as the energy of the products is higher than the reactants

• The minimum energy the colliding particles need in order to react is known as the activation energy
• The activation energy can be lowered by the addition of a catalyst
• This means that a higher percentage of the particles will have the minimum energy required to react, hence there are a higher number of successful collision
• Therefore more product molecules are produced in a shorter time, thus increasing the rate of reaction
• Reaction profile graphs show the relative energy levels of reactants and products on a graph
• The reaction profile graph below shows the effect of catalysts on reactions

Reaction Profiles Showing ∆H and E_A

• Catalysts provide the reactants another pathway which has a lower activation energy
• By lowering E_a, a greater proportion of molecules in the reaction mixture have sufficient energy for an successful collision
• As a result of this, the rate of the catalysed reaction is increased compared to the uncatalyzed reaction

Molecules with the activation energy lead to successful collisions