An energy profile diagram shows how the energy changes during a reaction, with the activation energy drawn as the height of the barrier the particles must overcome.
An energy profile diagram shows how the energy changes as a reaction proceeds, with the activation energy drawn as the height of the barrier the particles must overcome. Malay: gambar rajah profil tenaga · Chinese: 能量剖面图.
The diagram plots energy on the vertical axis against the progress of the reaction on the horizontal axis. Reactants start at one energy level and products end at another. In between, the curve rises to a peak, because bonds must partly break before new ones form. The vertical rise from the reactant level up to the peak is the activation energy, the minimum energy an effective collision must supply. Whether the products sit lower than the reactants (exothermic) or higher (endothermic) shows the overall energy change.
Example. For an exothermic reaction, the diagram climbs from the reactant level to a peak and then falls to a lower product level. If a catalyst is added, a second curve is drawn with a lower peak, showing that the alternative path has a smaller activation energy. The start and end levels are unchanged, because a catalyst does not alter the overall energy change, only the height of the barrier between.
Confusion to avoid. Do not confuse the height of the barrier (the activation energy) with the difference between reactant and product levels (the energy change of the reaction). They are two separate readings on the same diagram. Also, a catalyst lowers the peak but never moves the reactant or product levels.
The energy profile diagram makes activation energy visible and shows exactly how a catalyst works, tying the collision theory to the catalyst section, and drawing or labelling one correctly is a common SPM Chemistry task.
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