Reaction profiles

What you need to know

• Exothermic reactions transfer energy to the surroundings — products are lower energy than reactants, temperature rises.
• Endothermic reactions take energy in from the surroundings — products are higher energy than reactants, temperature falls.
• The activation energy is the minimum energy particles need to react — shown as the height from reactants to the top of the curve.
• A reaction profile shows: reactant energy level, product energy level, the overall energy change between them, the curved pathway, and the activation energy barrier.

The big picture

A reaction profile is an energy level diagram that shows the relative energies of reactants and products across a reaction. In an exothermic reaction, products sit at a lower energy level than reactants, so energy is transferred to the surroundings and the temperature rises. In an endothermic reaction, products sit at a higher energy level, so energy is taken in from the surroundings and the temperature falls. The activation energy is the minimum energy reacting particles must have for the reaction to occur — shown on the profile as the height of the peak above the reactant energy level.

Reading a reaction profile — step by step

Follow each stage to build up the full picture of what a reaction profile shows.

1. 1

   Draw the reactant energy level

   Start by marking a horizontal line for the reactants — this is your reference energy. Everything else on the profile is measured relative to where this line sits.

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2. 2

   Trace the curved pathway upward to the peak

   The reaction pathway always rises from the reactants to a peak — this is the energy barrier every particle must overcome. No particle can go straight from reactants to products; it must climb this hill first.

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3. 3

   Mark the activation energy

   The activation energy (Ea) is the vertical distance from the reactant energy level up to the very top of the peak — it shows the minimum energy particles must have to react.

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4. 4

   Trace the pathway down to the product energy level

   From the peak, the pathway curves back down to the product energy level. Where this line sits relative to the reactants tells you whether the reaction is exothermic or endothermic.

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5. 5

   Read the overall energy change

   The overall energy change is shown by the difference in height between the reactant and product energy levels — not by the peak height. The peak only tells you about activation energy.

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Predict, then check

Look at the description carefully before committing to your answer.

A reaction profile shows the product energy level drawn ABOVE the reactant energy level. The temperature of a thermometer placed in the reaction mixture drops during the reaction. Which statement correctly explains both observations?

Tap any cell to reveal it. Tap a column header to read one property down every item.

Memory hook

Think of exothermic as going DOWN a slide — products are lower, energy falls OUT to surroundings. Endothermic is climbing a hill — you have to take energy IN to reach a higher product level. Either way, you must get over the peak first: that peak height IS the activation energy.

Check yourself

Without looking — on a reaction profile for an endothermic reaction, are the products drawn higher or lower than the reactants, and where exactly does the activation energy arrow start and end?