Science · Solutions & reactions
Reaction Rates, Equilibrium, and Catalysts
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Reactions occur when particles collide effectively.
- Higher temperature usually increases reaction rate.
- Higher concentration and greater surface area can increase rate.
- Catalysts lower activation energy and are not used up.
- At equilibrium, forward and reverse rates are equal.
What you’ll be able to do
- Use collision ideas to predict rate changes.
- Explain how catalysts work.
- Tell reaction rate from equilibrium.
Predict what changes reaction speed and understand catalysts and dynamic equilibrium.
Optional review
If you’d like to review the basics, visit Chemical Reactions and Equations. You can start this lesson without completing those first.
Why reaction rates change
Particles must collide with enough energy and the right orientation to react. More effective collisions per second mean a faster reaction rate.
Higher temperature makes particles move faster. Higher concentration puts more particles in a space. Greater surface area exposes more particles. Each change can increase collision frequency and rate.
Worked example. A whole tablet reacts more slowly than a crushed tablet in the same liquid. Crushing exposes more surface particles, so collisions happen more often. The chemical identity and final amount possible do not change.
Average rate compares change with time: rate = amount changed ÷ time. If 48 mL of gas forms in 4 minutes, the average rate is 12 mL per minute.
Lower activation barrier
A catalyst gives a reaction a path with lower activation energy, the minimum starting energy needed. More collisions can then lead to reaction. The catalyst is not used up overall.
An enzyme is a biological catalyst. A catalyst speeds both forward and reverse reactions, so it helps a system reach equilibrium sooner but does not move the final equilibrium position.
An energy profile shows a lower peak with a catalyst. The reactant and product energy levels stay the same. Therefore, a catalyst changes the path and speed, not whether products are favored at equilibrium.
Cooling, lowering concentration, or using larger solid pieces usually reduces effective collisions and slows a reaction.
A catalyst changes the pathway
The lower curve shows an easier reaction route. A catalyst lowers the energy needed to get started. It does not change the starting or ending energy, or the final equilibrium balance. Read the horizontal axis as progress through the reaction, not time.
A catalyst changes the pathway — OpenStax College, Chemistry (CC BY archive). CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
Dynamic balance is not a stop
In a reversible reaction, products can change back into reactants. Dynamic equilibrium occurs when forward and reverse reaction rates are equal.
Particles still react at equilibrium. The amounts remain steady because the two directions occur at the same rate. Equal rates do not require equal amounts.
Worked example. In A ⇌ B at equilibrium, A still becomes B and B still becomes A. If ten particles change each way per second, the totals stay steady. The amounts of A and B need not be equal.
If product is removed, fewer product particles are available for the reverse reaction. The reverse rate drops at first, so the forward rate is faster until the system settles at a new equilibrium. Adding product causes the opposite first change.
Optional equilibrium extension
Separate rate from yield. For an exothermic forward reaction, write A ⇌ B + heat. Warming usually makes reactions proceed faster but shifts equilibrium toward A, so B can be made faster initially yet be less favored at equilibrium. Cooling favors the heat-producing direction, but generally slows reactions. For an endothermic forward reaction, heat is a reactant and warming favors products.
Optional equilibrium extension
Compressing a gaseous equilibrium mixture at constant temperature favors the side with fewer gas particles when the sides differ. For N2(g) + 3H2(g) ⇌ 2NH3(g), four gas particles on the left versus two on the right means compression favors the right. Do not apply a gas-particle rule to solids or to equal gas counts. A catalyst does not shift equilibrium.
Terms to remember
- reaction rate
- How quickly reactants become products.
- collision
- An encounter between reacting particles.
- activation energy
- The minimum energy needed to start a reaction.
- catalyst
- A substance that speeds a reaction without being used up.
- equilibrium
- A dynamic state with equal forward and reverse reaction rates.
Your quick summary
- Effective collisions allow reactions.
- Temperature, concentration, and surface area can change rate.
- Catalysts lower activation energy.
- Catalysts do not change the equilibrium position.
- Equilibrium is dynamic.
Lesson quiz
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