How do I tell an endothermic reaction from an exothermic one?

An exothermic reaction releases energy, so the surroundings warm and the enthalpy change is negative; energy can be written as a product. An endothermic reaction absorbs energy, so the surroundings cool and the enthalpy change is positive; energy can be written as a reactant. Combustion and neutralization are exothermic; many decompositions and an instant cold pack are endothermic. Use the temperature change of the surroundings to decide the direction.

What does a potential energy diagram show?

It plots stored chemical energy along the reaction path. The activation energy is the height from the reactants up to the peak (the activated complex), and the enthalpy change is the difference between the reactant and product levels (products lower means exothermic, higher means endothermic). A catalyst lowers the peak (activation energy) without changing the reactant or product energies, so it speeds the reaction without changing the enthalpy change.

What does collision theory require for a reaction?

Particles must collide, and the collision must have at least the activation energy and the correct orientation. A collision meeting both conditions is an effective collision. Most collisions are too gentle or poorly aligned and simply bounce apart, so only the fraction that are effective produce a reaction. Anything that raises the number of effective collisions per second increases the rate.

Which factors change the rate of a reaction?

Five factors, all explained by collision theory: increasing concentration (more frequent collisions), raising temperature (more frequent and more energetic collisions), increasing surface area of a solid (more exposed particles), adding a catalyst (lower activation energy, so more effective collisions), and the nature of the reactants (some react faster by their bonding or state). Reversing each slows the reaction.

How do I use Le Chatelier's principle?

At dynamic equilibrium the forward and reverse reactions run at equal rates, so concentrations stay constant. Le Chatelier's principle says a stressed system shifts to oppose the change. Adding a substance shifts away from it; increasing pressure shifts toward the side with fewer gas molecules; raising temperature shifts in the endothermic direction (treat heat as a reactant or product). A catalyst speeds both directions equally and does not shift the position.

VirginiaChemistry

Virginia SOL Chemistry reaction energy and rates: a complete skills guide to endothermic and exothermic reactions, energy diagrams, collision theory, rate factors and equilibrium

A deep-dive Virginia SOL Chemistry guide to reaction energy and rates (CH.6): endothermic and exothermic reactions and enthalpy, potential energy diagrams and activation energy, collision theory, the factors that change reaction rate, and chemical equilibrium with Le Chatelier's principle, with worked examples and SOL exam technique.

Generated by Claude Opus 4.817 min readCH.6Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

Why energy and rate belong together
Reaction energy
Reaction rate and collision theory
Equilibrium and Le Chatelier
Check your knowledge

Why energy and rate belong together

The reporting category Solutions, Acids and Bases, Reaction Energy and Reaction Rates closes with the energy and speed of reactions (standard CH.6). Energy decides whether a reaction releases or absorbs heat; rate decides how fast it goes; and equilibrium describes reversible reactions that reach a steady balance. Collision theory is the thread that ties the rate ideas together. This guide ties together the matching dot-point pages, each with its own practice: endothermic and exothermic reactions, potential energy diagrams and activation energy, reaction rates and collision theory, factors affecting reaction rate, and chemical equilibrium and Le Chatelier's principle.

Reaction energy

An exothermic reaction releases energy (surroundings warm, negative enthalpy change, energy on the product side); an endothermic reaction absorbs energy (surroundings cool, positive enthalpy change, energy on the reactant side). On a potential energy diagram, the activation energy is the climb from reactants to the peak, and the enthalpy change is the difference between the reactant and product levels. A catalyst lowers the peak only, so it speeds the reaction without changing the enthalpy change.

Reaction rate and collision theory

The rate is the change in amount per unit time. Collision theory says particles must collide with enough energy (the activation energy) and the right orientation for an effective collision that leads to reaction. The factors that change rate, all through collision theory:

Factor	Increase it and the rate...	Reason
Concentration	increases	more frequent collisions
Temperature	increases	more frequent and more energetic collisions
Surface area	increases	more exposed particles to collide
Catalyst	increases	lower activation energy, more effective collisions
Nature of reactants	varies	bonding and state affect how easily particles react

Equilibrium and Le Chatelier

A reversible reaction reaches dynamic equilibrium when the forward and reverse rates are equal and concentrations hold steady. Le Chatelier's principle: a stressed system shifts to oppose the change. Add a substance and it shifts away from it; raise the pressure and it shifts toward fewer gas molecules; raise the temperature and it shifts in the endothermic direction. A catalyst does not shift the position.

A combined energy and equilibrium problem

The forward reaction $2\,\text{SO}_2 + \text{O}_2 \rightleftharpoons 2\,\text{SO}_3$ is exothermic. (a) Predict the shift when pressure increases. (b) Predict the shift when temperature increases.

step 1 Pressure: count gas molecules

Left: $2 + 1 = 3$ molecules; right: $2$ . Higher pressure shifts toward fewer molecules, so it shifts right.

step 2 Temperature: place heat

The forward reaction is exothermic, so heat is a product (on the right).

step 3 Apply Le Chatelier to temperature

Raising the temperature adds heat, so the system shifts away from heat, in the endothermic (reverse) direction, to the left.

step 4 Summarize

Higher pressure favors the product (right); higher temperature favors the reactants (left).

An exothermic reaction releases energy (surroundings warm, negative enthalpy change) and an endothermic reaction absorbs it (surroundings cool, positive enthalpy change). On a potential energy diagram, activation energy is the climb to the peak and the enthalpy change is the reactant-to-product difference; a catalyst lowers the peak only. Reaction rate is a change per unit time, and collision theory requires effective collisions (enough energy and right orientation). Concentration, temperature, surface area, a catalyst, and the nature of the reactants change the rate, all via collision frequency or effectiveness. At dynamic equilibrium the forward and reverse rates are equal; Le Chatelier's principle predicts shifts: away from an added substance, toward fewer gas molecules under higher pressure, and in the endothermic direction under higher temperature, with a catalyst leaving the position unchanged.

Check your knowledge

Attempt these under timed conditions, then check the solutions.

A reaction causes its surroundings to cool. Classify it and state the sign of the enthalpy change. (2 marks)
On a potential energy diagram, what does the height from reactants to the peak represent? (1 mark)
State the two requirements for an effective collision. (2 marks)
Explain, using collision theory, why increasing the temperature speeds a reaction. (2 marks)
For $\text{N}_2 + 3\,\text{H}_2 \rightleftharpoons 2\,\text{NH}_3$ , which way does the equilibrium shift when the pressure is increased? (1 mark)
Does a catalyst change the position of an equilibrium? Explain. (2 marks)

Sources & how we know this

2018 Science Standards of Learning - Chemistry — Virginia Department of Education (2018)
Chemistry Curriculum Framework — Virginia Department of Education (2018)