What Is a Reaction Profile Diagram?

A reaction profile diagram shows how the energy of a reacting system changes as a chemical reaction proceeds.

It allows us to see:

• the energy of the reactants
• the energy of the products
• the activation energy
• the highest-energy point of the reaction
• whether energy is released or absorbed overall

The vertical axis normally represents:

Energy

The horizontal axis represents:

Progress of reaction or Reaction pathway

The horizontal axis does not normally represent time.

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The Basic Shape of a Reaction Profile

A typical reaction profile starts with the reactants, rises to a peak, and then falls to the products.

The general pattern is:

Reactants

↓

energy increases

↓

highest-energy point

↓

energy decreases

↓

Products

The rise in energy shows that the reaction must overcome an energy barrier before products can form.

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Identifying the Reactants

The reactants are the starting substances in a chemical reaction.

On a standard reaction profile diagram, they are shown on the:

left side

For example:

A + B → C + D

A and B are the reactants.

Their position on the vertical axis tells us their starting energy.

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Identifying the Products

The products are the substances formed by the reaction.

They are normally shown on the:

right side

For:

A + B → C + D

C and D are the products.

The difference between the energy levels of the reactants and products tells us whether energy has been released or absorbed.

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Reading the Axes

Reaction profile diagrams usually contain two axes.

Vertical Axis

Energy

This may also be labelled:

• potential energy
• enthalpy
• energy of the system

Horizontal Axis

Reaction progress

or:

Reaction coordinate

It represents the progress from reactants to products.

It is not usually a direct measurement of:

• time
• distance
• concentration

This is an important distinction.

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The Energy Barrier

Chemical reactions do not usually move directly from reactants to products.

Reactant particles must first reach a higher-energy arrangement.

This is why the reaction profile rises to a peak.

The energy required to reach that peak from the reactants is called the:

activation energy

Symbol:

Eₐ

The activation energy is the minimum energy barrier that must be overcome for the reaction to occur.

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Activation Energy on the Diagram

Activation energy is measured vertically from:

the energy level of the reactants

to:

the highest point of the reaction profile

Therefore:

Activation energy = peak energy − reactant energy

The activation energy is not measured from zero.

It is the difference between the reactant energy and the top of the energy barrier.

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Why Is Activation Energy Needed?

During a chemical reaction:

• existing bonds may need to be weakened or broken
• atoms must rearrange
• new bonds must form

The reactants must therefore reach a temporary high-energy arrangement before they can become products.

This creates the activation-energy barrier.

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The Transition State

The top of the reaction profile represents a very unstable, high-energy arrangement called the:

transition state

It is sometimes also called the:

activated complex

The transition state exists for an extremely short time.

It is not usually isolated as a normal substance.

A useful sequence is:

reactants

↓

activation energy supplied

↓

transition state

↓

products

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Exothermic Reaction Profiles

An exothermic reaction transfers energy from the reacting system to the surroundings.

Examples can include:

• combustion
• many oxidation reactions
• many neutralization reactions

On an exothermic reaction profile:

products are lower in energy than reactants

The system has lost energy overall.

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Recognizing an Exothermic Diagram

Look at the starting and ending energy levels.

If:

product energy < reactant energy

then the reaction is:

exothermic

The profile generally looks like:

higher reactants

↓

peak

↓

lower products

Energy is released to the surroundings.

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Overall Energy Change in an Exothermic Reaction

The overall energy change is the difference between the energy of the products and the reactants.

This is often written:

ΔH = Hproducts − Hreactants

For an exothermic reaction:

Hproducts < Hreactants

Therefore:

ΔH < 0

The energy change is negative.

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Example: Exothermic Reaction

Suppose:

Reactant energy = 180 kJ mol⁻¹

Product energy = 120 kJ mol⁻¹

Then:

ΔH = 120 − 180

ΔH = −60 kJ mol⁻¹

The negative value shows that the reaction is:

exothermic

Energy released:

60 kJ mol⁻¹

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Endothermic Reaction Profiles

An endothermic reaction absorbs energy from the surroundings.

On an endothermic reaction profile:

products are higher in energy than reactants

The reacting system has gained energy overall.

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Recognizing an Endothermic Diagram

If:

product energy > reactant energy

then the reaction is:

endothermic

The profile generally looks like:

lower reactants

↓

peak

↓

higher products

Energy is absorbed from the surroundings.

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Overall Energy Change in an Endothermic Reaction

Again:

ΔH = Hproducts − Hreactants

For an endothermic reaction:

Hproducts > Hreactants

Therefore:

ΔH > 0

The energy change is positive.

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Example: Endothermic Reaction

Suppose:

Reactant energy = 90 kJ mol⁻¹

Product energy = 145 kJ mol⁻¹

Then:

ΔH = 145 − 90

ΔH = +55 kJ mol⁻¹

The positive value shows that the reaction is:

endothermic

Energy absorbed:

55 kJ mol⁻¹

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Exothermic vs Endothermic Profiles

The quickest way to identify the reaction type is to compare the reactant and product energy levels.

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Interpreting the Energy Change

The vertical distance between the reactants and products represents the overall energy change.

For an exothermic reaction:

ΔH points downward

because the products are lower in energy.

For an endothermic reaction:

ΔH points upward

because the products are higher in energy.

The size of this vertical difference tells us the magnitude of the energy change.

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Activation Energy Is Different from ΔH

Students often confuse:

activation energy

and:

overall energy change

They are not the same.

Activation Energy

Measured from:

reactants → peak

Overall Energy Change

Measured from:

reactants → products

Therefore:

Eₐ tells us about the energy barrier

while:

ΔH tells us about the overall energy change

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Example

Suppose a reaction profile has:

Reactants = 100 kJ mol⁻¹

Peak = 250 kJ mol⁻¹

Products = 60 kJ mol⁻¹

Activation Energy

Eₐ = 250 − 100

Eₐ = 150 kJ mol⁻¹

Overall Energy Change

ΔH = 60 − 100

ΔH = −40 kJ mol⁻¹

Therefore the reaction is:

exothermic

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A Useful Reading Method

When you are given a reaction profile, follow these steps.

Step 1

Find the reactants.

Usually on the left.

Step 2

Find the products.

Usually on the right.

Step 3

Compare their energy levels.

Products lower:

exothermic

Products higher:

endothermic

Step 4

Find the peak.

This represents the transition state.

Step 5

Measure from the reactants to the peak.

That is:

activation energy

Step 6

Measure from reactants to products.

That is:

overall energy change

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Comparing Two Reaction Profiles

Reaction profiles can be compared in several ways.

You can compare:

• reactant energy
• product energy
• activation energy
• energy change
• reaction type
• number of energy barriers

A reaction with a higher peak relative to its reactants has a larger activation energy.

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Example: Comparing Activation Energies

Reaction A:

Reactants = 50 kJ mol⁻¹

Peak = 130 kJ mol⁻¹

Reaction B:

Reactants = 50 kJ mol⁻¹

Peak = 200 kJ mol⁻¹

For Reaction A:

Eₐ = 130 − 50 = 80 kJ mol⁻¹

For Reaction B:

Eₐ = 200 − 50 = 150 kJ mol⁻¹

Therefore:

Reaction B has the larger activation energy.

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Activation Energy and Reaction Rate

A larger activation-energy barrier generally makes it harder for reacting particles to react at a given temperature.

A lower activation-energy barrier means that a greater proportion of collisions may have enough energy to react.

Therefore, under comparable conditions:

lower Eₐ → reaction can generally proceed more readily

However, reaction rate also depends on factors such as:

• temperature
• concentration
• pressure
• surface area
• catalysts
• molecular orientation

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Catalysts and Reaction Profiles

A catalyst increases reaction rate by providing an alternative reaction pathway with a lower activation energy.

The catalyst does not change the energy of:

• the reactants
• the products

Therefore, it does not change the overall:

ΔH

It only lowers the energy barrier.

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Catalyzed vs Uncatalyzed Reaction

On a reaction profile:

uncatalyzed pathway

has a higher peak.

catalyzed pathway

has a lower peak.

But both begin and end at the same energy levels.

Therefore:

Eₐ decreases

but:

ΔH remains unchanged

This is an important feature when comparing reaction profiles.

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Example: Catalyst

Without catalyst:

Reactants = 80 kJ mol⁻¹

Peak = 210 kJ mol⁻¹

Products = 40 kJ mol⁻¹

Activation energy:

210 − 80 = 130 kJ mol⁻¹

With catalyst:

Peak = 145 kJ mol⁻¹

Activation energy:

145 − 80 = 65 kJ mol⁻¹

The catalyst has lowered the activation energy from:

130 kJ mol⁻¹ to 65 kJ mol⁻¹

But:

ΔH = 40 − 80 = −40 kJ mol⁻¹

in both cases.

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Comparing Exothermic Reactions

Two reactions can both be exothermic but have different:

• activation energies
• amounts of energy released

For example:

Reaction A:

ΔH = −20 kJ mol⁻¹

Reaction B:

ΔH = −100 kJ mol⁻¹

Both are exothermic.

However, Reaction B releases more energy per mole according to these profiles.

This does not automatically mean Reaction B is faster.

Reaction rate depends strongly on activation energy and other kinetic factors.

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Comparing Endothermic Reactions

Two reactions can both be endothermic but absorb different amounts of energy.

For example:

Reaction C:

ΔH = +30 kJ mol⁻¹

Reaction D:

ΔH = +90 kJ mol⁻¹

Reaction D requires a larger net energy input.

Again, this does not automatically tell us which reaction is faster.

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Reverse Reactions

Reaction profiles can also be read backwards.

Suppose:

A → B

is exothermic.

Then:

B → A

must be endothermic.

The energy difference has the same magnitude but opposite sign.

For example:

Forward:

ΔH = −60 kJ mol⁻¹

Reverse:

ΔH = +60 kJ mol⁻¹

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Activation Energy of the Reverse Reaction

The activation energy for the reverse reaction is measured from:

products → peak

rather than:

reactants → peak

Therefore, the forward and reverse reactions usually have different activation energies.

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Example: Forward and Reverse Reaction

Suppose:

Reactants = 120 kJ mol⁻¹

Peak = 280 kJ mol⁻¹

Products = 70 kJ mol⁻¹

Forward activation energy:

280 − 120 = 160 kJ mol⁻¹

Reverse activation energy:

280 − 70 = 210 kJ mol⁻¹

Forward ΔH:

70 − 120 = −50 kJ mol⁻¹

Reverse ΔH:

+50 kJ mol⁻¹

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More Complex Reaction Profiles

Some reactions occur through several steps.

Their reaction profiles may contain:

• more than one peak
• valleys between the peaks

Each peak represents a transition state.

The valleys can represent intermediates.

A simplified multistep profile might look like:

reactants

↓

first peak

↓

intermediate

↓

second peak

↓

products

The largest relevant energy barrier can strongly influence the overall reaction rate.

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Reaction Intermediate

An intermediate is a species that:

• forms during one reaction step
• is used up in a later step
• does not appear in the overall chemical equation

On an energy profile, an intermediate is often represented by a valley between two peaks.

This is different from a transition state, which occurs at the top of a peak.

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Transition State vs Intermediate

This becomes especially useful when interpreting more complicated reaction mechanisms.

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Reaction Profiles Do Not Show Reaction Time

A common mistake is to assume that the horizontal axis represents time.

It usually does not.

The horizontal axis represents the:

reaction pathway

or:

reaction coordinate

A wider-looking curve therefore does not necessarily mean that the reaction takes longer.

Reaction speed cannot be determined simply from the horizontal width of the diagram.

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Reaction Profiles Do Not Directly Show Temperature

Reaction profiles show relative energy changes.

They do not directly show:

• temperature
• reaction time
• concentration

For example, an exothermic reaction may cause the surroundings to become warmer, but the vertical axis is not a temperature axis.

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Worked Example 1

A reaction profile has:

Reactants = 70 kJ mol⁻¹

Peak = 190 kJ mol⁻¹

Products = 20 kJ mol⁻¹

Activation Energy

Eₐ = 190 − 70

Eₐ = 120 kJ mol⁻¹

Energy Change

ΔH = 20 − 70

ΔH = −50 kJ mol⁻¹

Reaction Type

Because ΔH is negative:

exothermic

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Worked Example 2

Reactants = 40 kJ mol⁻¹

Peak = 160 kJ mol⁻¹

Products = 95 kJ mol⁻¹

Activation energy:

Eₐ = 160 − 40

Eₐ = 120 kJ mol⁻¹

Energy change:

ΔH = 95 − 40

ΔH = +55 kJ mol⁻¹

Therefore:

endothermic

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Worked Example 3: Comparing Profiles

Reaction A:

• Reactants = 60
• Peak = 140
• Products = 20

Reaction B:

• Reactants = 60
• Peak = 200
• Products = 20

Both have:

ΔH = −40 kJ mol⁻¹

But:

Reaction A:

Eₐ = 80 kJ mol⁻¹

Reaction B:

Eₐ = 140 kJ mol⁻¹

Therefore:

• both reactions release the same net amount of energy
• Reaction B has the larger activation-energy barrier

This demonstrates that activation energy and energy change describe different features of a reaction.

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Worked Example 4: Comparing Energy Changes

Reaction X:

Reactants = 100 kJ mol⁻¹

Products = 40 kJ mol⁻¹

Reaction Y:

Reactants = 100 kJ mol⁻¹

Products = 80 kJ mol⁻¹

For X:

ΔH = −60 kJ mol⁻¹

For Y:

ΔH = −20 kJ mol⁻¹

Both reactions are exothermic.

However:

Reaction X releases more energy overall.

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Did You Know?

A reaction can be strongly exothermic and still happen very slowly.

This is because:

thermodynamics tells us about the overall energy change,

while:

kinetics tells us how quickly the reaction occurs.

A reaction may have products that are much lower in energy but still require a very large activation energy to reach them.

This is one reason activation-energy diagrams are so useful: they connect chemical energetics with reaction rates.

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A Quick Visual Checklist

When looking at any reaction profile, ask:

1. Where are the reactants?
2. Where are the products?
3. Which is higher in energy?
4. Where is the highest point?
5. What is the activation energy?
6. What is the overall energy change?
7. Is ΔH positive or negative?
8. Is the reaction exothermic or endothermic?
9. Is there a catalyst shown?
10. Are there multiple peaks suggesting a multistep reaction?

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Connecting the Ideas

A reaction profile can be interpreted as:

reactants

↓

energy barrier

↓

activation energy

↓

transition state

↓

products

Then compare reactant and product energy:

products lower

→ exothermic

→ ΔH negative

or:

products higher

→ endothermic

→ ΔH positive

A catalyst changes:

activation energy

but does not change:

reactant energy, product energy, or ΔH

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Key Terms

Reaction profile diagram – A diagram showing the energy changes as a chemical reaction proceeds.

Reactants – Starting substances in a reaction.

Products – Substances formed during a reaction.

Reaction coordinate – A representation of the progress of a chemical reaction.

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

Transition state – The temporary highest-energy arrangement along a reaction pathway.

Activated complex – Another term often used for the high-energy arrangement at the transition state.

Enthalpy change (ΔH) – The difference between the energy of the products and reactants at constant pressure.

Exothermic reaction – A reaction that transfers energy to the surroundings.

Endothermic reaction – A reaction that absorbs energy from the surroundings.

Catalyst – A substance that increases reaction rate by providing an alternative pathway with lower activation energy.

Intermediate – A temporary species formed during one step of a multistep reaction and consumed during another.

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Key Takeaways

• Reaction profile diagrams show how the energy of a reacting system changes during a reaction.
• The reactants are normally shown on the left and the products on the right.
• The vertical axis represents energy.
• The horizontal axis represents reaction progress, not necessarily time.
• The highest point of the curve represents the transition state.
• Activation energy is measured from the reactant energy level to the peak of the profile.
• The overall energy change is:

ΔH = Hproducts − Hreactants

• If the products are lower in energy than the reactants, the reaction is exothermic and ΔH is negative.
• If the products are higher in energy than the reactants, the reaction is endothermic and ΔH is positive.
• Activation energy and overall energy change are not the same quantity.
• A catalyst lowers the activation energy but does not change the overall ΔH.
• Different reaction profiles can be compared by examining their reactant energy, product energy, activation energy, and overall energy change.
• Multistep reactions can contain several peaks and intermediates.
• Reaction profiles connect chemical energetics with the study of reaction rates and mechanisms.

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