Reaction Profiles (Edexcel GCSE Chemistry)
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Reaction Profiles
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
Examiner Tips and Tricks
You should be able to draw clear and fully labelled reaction profile diagrams for both types of reactions and you should be able to identify a reaction from its reaction profile.
Activation Energy
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 EA
Diagram shows that a catalyst increases the rate of a reaction by providing an alternative pathway which has a lower activation energy
Catalysts provide the reactants another pathway which has a lower activation energy
By lowering Ea, 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
Examiner Tips and Tricks
The activation energy is constant for a particular reaction. Reactions with a low activation energy occur readily as little energy is needed to break the bonds and initiate the reaction.
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