Energy Profiles and Activation Energy

What Is an Exothermic Reaction?

An exothermic reaction is a chemical reaction that transfers energy from the reacting system to the surroundings.

This energy is commonly transferred as heat, although some reactions may also release energy as light or sound.

Common examples include:

• combustion
• many oxidation reactions
• acid-base neutralization
• cellular respiration
• some reactions between metals and acids

The key idea is:

chemical system loses energy → surroundings gain energy

On an energy profile diagram, this means the products are at a lower energy level than the reactants.

Recognizing an Exothermic Reaction Profile

An energy profile diagram usually has two axes.

The vertical axis represents:

Energy or Enthalpy

The horizontal axis represents:

Reaction progress or Reaction coordinate

The horizontal axis does not normally represent time. It simply represents the pathway from reactants to products.

For an exothermic reaction:

reactants start higher → curve rises to a peak → products finish lower

The easiest way to identify an exothermic profile is therefore to compare the beginning and end of the graph.

Products below reactants = exothermic reaction

Why Are the Products Lower in Energy?

Chemical reactions involve both breaking bonds and forming bonds.

Breaking existing chemical bonds requires energy.

Bond breaking → energy absorbed

Forming new chemical bonds releases energy.

Bond making → energy released

In an exothermic reaction, the formation of new bonds releases more energy than was required to break the bonds in the reactants.

For example, imagine:

Energy needed to break bonds = 300 kJ mol⁻¹

Energy released when new bonds form = 450 kJ mol⁻¹

The overall energy change is:

300 − 450 = −150 kJ mol⁻¹

Therefore, the reaction releases 150 kJ mol⁻¹ overall.

The products finish at a lower energy because energy has been transferred from the chemical system to the surroundings.

Enthalpy Change, ΔH

The overall energy change of a reaction is commonly represented by the enthalpy change, ΔH.

It can be calculated using:

ΔH = Hproducts − Hreactants

For an exothermic reaction:

Hproducts < Hreactants

Therefore:

ΔH < 0

The enthalpy change is negative.

The downward difference between the reactant and product energy levels represents the energy released by the reaction.

Worked Example 1: Identifying an Exothermic Reaction

Suppose an energy profile shows:

Reactants = 180 kJ mol⁻¹

Products = 110 kJ mol⁻¹

Because:

110 < 180

the products have less energy than the reactants.

Therefore, the reaction is:

exothermic

The enthalpy change is:

ΔH = 110 − 180

ΔH = −70 kJ mol⁻¹

The reaction releases:

70 kJ mol⁻¹

Notice the difference in wording:

ΔH = −70 kJ mol⁻¹

but:

energy released = 70 kJ mol⁻¹

The negative sign tells us the direction of the energy transfer.

Energy Released

The amount of energy released can be found from the vertical difference between the reactant and product energy levels.

For example:

Reactants = 250 kJ mol⁻¹

Products = 90 kJ mol⁻¹

Energy difference:

250 − 90 = 160 kJ mol⁻¹

Therefore:

160 kJ mol⁻¹ is released

and:

ΔH = −160 kJ mol⁻¹

A larger vertical drop between reactants and products represents a greater energy release.

Activation Energy Is Still Required

A common misconception is that an exothermic reaction should begin automatically because it releases energy.

That is not necessarily true.

Exothermic reactions still normally require an initial amount of energy called the activation energy, Eₐ.

Activation energy is measured from:

reactant energy level → peak of the reaction profile

It should not be measured from the bottom of the graph.

The peak represents the transition state, a very high-energy and unstable arrangement of atoms.

Worked Example 2: Calculating Activation Energy

Suppose:

Reactants = 100 kJ mol⁻¹

Peak = 260 kJ mol⁻¹

Products = 40 kJ mol⁻¹

Activation energy:

Eₐ = 260 − 100

Eₐ = 160 kJ mol⁻¹

Enthalpy change:

ΔH = 40 − 100

ΔH = −60 kJ mol⁻¹

Therefore:

• activation energy = 160 kJ mol⁻¹
• energy released = 60 kJ mol⁻¹
• ΔH = −60 kJ mol⁻¹
• reaction = exothermic

This illustrates an important point:

activation energy and energy released are not the same thing.

Why Does an Exothermic Reaction Need Activation Energy?

Consider the combustion of a fuel.

Fuel can be surrounded by oxygen without immediately bursting into flames.

An initial energy input may be needed from:

• a spark
• a flame
• electrical energy
• sufficient heating

That initial energy helps reacting particles overcome the activation-energy barrier.

Once the reaction begins, the energy released by the reaction may allow it to continue.

Combustion therefore provides a useful example of a reaction that is strongly exothermic but still requires activation energy.

Activation Energy vs Energy Released

These two quantities describe completely different parts of the profile.

Do not assume that a reaction releasing a large amount of energy must have a small activation energy.

It can have a large energy release and a large activation-energy barrier.

Comparing Exothermic Reactions

Energy profiles can be used to compare different exothermic reactions.

Suppose Reaction A has:

Reactants = 100 kJ mol⁻¹
Products = 40 kJ mol⁻¹

Then:

ΔH = −60 kJ mol⁻¹

Reaction B has:

Reactants = 100 kJ mol⁻¹
Products = −20 kJ mol⁻¹

Then:

ΔH = −120 kJ mol⁻¹

Both reactions are exothermic.

However, Reaction B releases more energy because its products finish much farther below its reactants.

Therefore:

larger downward energy difference → greater energy release

Comparing Activation Energies

Now suppose two exothermic reactions have the same reactant and product energy levels.

Reaction A:

Reactants = 80 kJ mol⁻¹
Peak = 150 kJ mol⁻¹
Products = 30 kJ mol⁻¹

Reaction B:

Reactants = 80 kJ mol⁻¹
Peak = 230 kJ mol⁻¹
Products = 30 kJ mol⁻¹

For Reaction A:

Eₐ = 150 − 80 = 70 kJ mol⁻¹

For Reaction B:

Eₐ = 230 − 80 = 150 kJ mol⁻¹

Both have:

ΔH = 30 − 80 = −50 kJ mol⁻¹

Therefore, the reactions release the same amount of energy, but they have different activation energies.

Under otherwise comparable conditions, Reaction A has the smaller energy barrier.

This shows why energy released and activation energy must be considered separately.

Exothermic vs Endothermic Profiles

It is useful to compare the two major types of energy profile.

For an exothermic reaction:

products below reactants

ΔH is negative

energy released

For an endothermic reaction:

products above reactants

ΔH is positive

energy absorbed

The position of the products relative to the reactants is therefore the quickest way to classify the reaction.

Exothermic Does Not Mean Fast

Another common mistake is to assume:

"A strongly exothermic reaction must be fast."

This is incorrect.

Exothermic describes the overall energy change.

Reaction rate describes how quickly the reaction occurs.

A reaction can release a great deal of energy but still proceed slowly if it has a large activation-energy barrier.

This is an important distinction between thermodynamics and kinetics.

Catalysts and Exothermic Profiles

A catalyst lowers the activation energy by providing an alternative reaction pathway.

However, it does not change:

• the reactant energy
• the product energy
• the amount of energy released
• ΔH

Therefore, on a catalyzed exothermic profile:

same starting level + same finishing level + lower peak

The reaction remains equally exothermic.

Only the activation-energy barrier changes.

Forward and Reverse Reactions

An exothermic reaction can also be considered in reverse.

Suppose:

Reactants = 120 kJ mol⁻¹

Peak = 280 kJ mol⁻¹

Products = 70 kJ mol⁻¹

For the forward reaction:

Eₐ = 280 − 120 = 160 kJ mol⁻¹

and:

ΔH = 70 − 120 = −50 kJ mol⁻¹

The forward reaction is exothermic.

For the reverse reaction, the products become the starting materials:

Eₐ(reverse) = 280 − 70

Eₐ(reverse) = 210 kJ mol⁻¹

and:

ΔH(reverse) = +50 kJ mol⁻¹

So if the forward reaction is exothermic, the reverse reaction is endothermic.

Reading an Exothermic Profile Step by Step

When you encounter an unfamiliar reaction profile, use this sequence.

First, locate the reactants.

Next, locate the products.

Ask:

Are the products below the reactants?

If yes, the reaction is exothermic.

Then determine:

ΔH = Hproducts − Hreactants

Next locate the highest point of the curve.

Determine:

Eₐ = Hpeak − Hreactants

Finally, compare these quantities with other profiles if required.

Worked Example 3

An energy profile has:

Reactants = 150 kJ mol⁻¹

Transition state = 310 kJ mol⁻¹

Products = 50 kJ mol⁻¹

Step 1: Identify the reaction.

Products are lower than reactants.

Exothermic

Step 2: Calculate ΔH.

ΔH = 50 − 150

ΔH = −100 kJ mol⁻¹

Step 3: Determine energy released.

100 kJ mol⁻¹

Step 4: Calculate activation energy.

Eₐ = 310 − 150

Eₐ = 160 kJ mol⁻¹

Worked Example 4: Comparing Two Profiles

Reaction X:

Eₐ = 90 kJ mol⁻¹
ΔH = −40 kJ mol⁻¹

Reaction Y:

Eₐ = 140 kJ mol⁻¹
ΔH = −100 kJ mol⁻¹

Which releases more energy?

Reaction Y, because its ΔH has the larger negative magnitude.

Which has the smaller activation-energy barrier?

Reaction X

Which reaction is exothermic?

Both

This demonstrates why a complete comparison should consider both Eₐ and ΔH.

Did You Know?

An exothermic reaction can sometimes release a large amount of energy but remain effectively dormant until something supplies its activation energy.

This is one reason fuels can be stored in the presence of oxygen without continuously burning. Once sufficient activation energy is supplied, however, combustion can proceed and transfer substantial energy to the surroundings.

Common Mistakes

"Products are lower, so activation energy must be negative."

Incorrect. Activation energy is the upward energy difference from the reactants to the transition-state peak and is normally positive.

"The energy released is measured from the peak to the products."

Incorrect. Overall energy release is determined from the difference between the reactant and product energy levels.

"A large ΔH means a reaction is fast."

Incorrect. ΔH describes overall energy change, not reaction rate.

"Exothermic reactions do not need activation energy."

Incorrect. They normally still need to overcome an initial energy barrier.

"The x-axis represents time."

Usually incorrect. It represents reaction progress or reaction coordinate.

Key Terms

Exothermic reaction – A reaction that transfers energy from the reacting system to the surroundings.

Energy profile – A diagram showing how the energy of a reacting system changes as a reaction progresses.

Reactants – The substances present at the beginning of a reaction.

Products – The substances formed by a reaction.

Enthalpy, H – A measure of the energy content of a system under constant-pressure conditions.

Enthalpy change, ΔH – The difference between product and reactant enthalpy.

Activation energy, Eₐ – The energy barrier that must be overcome for a reaction to occur.

Transition state – The unstable, high-energy arrangement associated with the peak of an energy profile.

Reaction coordinate – A conceptual representation of progress from reactants to products.

Key Takeaways

• An exothermic reaction releases energy to the surroundings.
• On an exothermic energy profile, the products are lower in energy than the reactants.
• Exothermic reactions have negative ΔH.
• ΔH = Hproducts − Hreactants.
• The magnitude of the vertical difference between reactants and products represents the amount of energy released.
• Products are lower in energy because bond formation releases more energy than is required overall to break bonds.
• Exothermic reactions still require activation energy.
• Activation energy is measured from the reactant level to the peak.
• Activation energy is not the same as the energy released.
• Two exothermic reactions can release different amounts of energy.
• Two reactions can release the same amount of energy while having different activation energies.
• A strongly exothermic reaction is not necessarily a fast reaction.
• A catalyst lowers activation energy but does not change ΔH.
• The reverse of an exothermic reaction is endothermic.