An energy level diagram is a diagram with energy on the vertical axis that shows the energy of the reactants and of the products, with ΔH marked as the difference between the two levels.
An energy level diagram is a diagram with energy on the vertical axis that shows the energy of the reactants and of the products, with ΔH marked as the difference between the two levels. Malay: gambar rajah aras tenaga · Chinese: 能级图.
The diagram turns the words exothermic and endothermic into a picture you can read at a glance. Two horizontal lines are drawn, one for the reactant level and one for the product level, and a vertical arrow between them represents the heat change. When the products sit lower than the reactants, energy has been given out and the reaction is exothermic, so the arrow points down and ΔH is negative. When the products sit higher, energy has been taken in and the reaction is endothermic, so the arrow points up and ΔH is positive.
Example. For the combustion of a fuel, you draw the reactants at a high level and carbon dioxide and water at a lower level, then label the downward gap ΔH with a negative value. For the thermal decomposition of a carbonate, the products are drawn higher than the reactant, and the upward gap is labelled with a positive value.
Confusion to avoid. An energy level diagram is not the same as an energy profile diagram from the Rate of Reaction chapter. The energy level diagram shows only the two flat levels and their difference; it need not show the activation-energy hump. Adding a peak, or drawing the products on the wrong side, is a common way marks are lost.
Reading and drawing this diagram correctly links the sign of ΔH to the position of the products, which is the core skill SPM Chemistry tests when it asks you to sketch or interpret the energy of a reaction.
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