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Chemical reactions · Topic 9 of 15

Reaction Rates, Catalysts & Equilibrium

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Chemistry · rates & equilibrium

Reaction Rates, Catalysts & Equilibrium

Reactions need activation energy; heat, concentration, surface area and catalysts speed them up. At equilibrium forward and reverse rates match, and a stressed equilibrium shifts.

01 · CatalystCatalyst lowers activation energy
02 · EquilibriumEquilibrium: forward rate = reverse rate
03 · Le ChatelierStress an equilibrium → it shifts to undo it
01

What it is

Activation energy = the minimum energy needed to start a reaction must know
Catalyst: lowers activation energy, speeds the reaction, not used up must know
Exothermic releases heat (ΔH −); endothermic absorbs heat (ΔH +) must know
Equilibrium: forward rate = reverse rate; amounts stay constant must know
Reactions need collisions with enough energy and the right orientation
Enzymes = the body's catalysts (speed metabolism, not used up)
Energy diagram: hump = Ea; products lower = exothermic, higher = endothermic
02

The rules

Le Chatelier: a stressed equilibrium shifts to undo the stress must know
Higher temperature → faster reaction (more and harder collisions)
Higher concentration → faster reaction (more collisions)
More surface area → faster reaction (powder beats a lump)
Add reactant or remove product → shift right; the reverse → shift left
Heating shifts away from heat: exothermic → left, endothermic → right
03

Worked examples

Burning fuel = exothermic; instant cold pack = endothermic
2SO₂ + O₂ ⇌ 2SO₃ + heat: remove SO₃ → right; heat it → left
04

Exothermic or endothermic?

Commonly confused
ExothermicEndothermic
HeatReleased to the surroundingsAbsorbed from the surroundings
Sign of ΔHNegative (ΔH < 0)Positive (ΔH > 0)
Energy diagramProducts end lower than reactantsProducts end higher than reactants
Everyday exampleBurning fuel (combustion)An instant cold pack
Heat in Le ChatelierTreat heat as a productTreat heat as a reactant
05

Common mistakes

A catalyst never shifts equilibrium (it only gets there faster)
At equilibrium amounts are constant, not equal (both reactions go on)
06

More detail

Show 3 more facts
More pressure → shift toward fewer gas moles (equal moles: no shift)
The reactants' own nature sets how fast they react
Top of the hump = activated complex (transition state)
Pearl

A catalyst lowers the hill but never moves the finish line: faster to equilibrium, same equilibrium.

Sources · OpenStax Chemistry 2e §12.2 Factors Affecting Reaction Rates · OpenStax Chemistry 2e §12.5 Collision Theory · OpenStax Chemistry 2e §12.7 Catalysis · OpenStax Chemistry 2e §5.1 Energy Basics (exothermic and endothermic) · OpenStax Chemistry 2e §5.3 Enthalpy (sign of ΔH) · OpenStax Chemistry 2e §13.1 Chemical Equilibria · OpenStax Chemistry 2e §13.3 Shifting Equilibria: Le Châtelier's Principle · Union Test Prep — HESI Chemistry study guide, page 2 (equilibrium, reaction rate, catalysts) · TSTC Library — HESI A2 guide (Chemistry: purpose of catalysts; enzymes) · NurseHub — HESI A2 Chemistry course outline (Reaction Rates, Equilibrium, and Reversibility lesson) · Mometrix — HESI A2 Chemistry practice test page (equilibrium listed as a tested concept) · Odessa College Library — HESI guide (Chemistry topics: Reaction Rates, Equilibrium, Reversibility) · Built from the verified NurseSavvy HESI A2 fact map (facts reviewed 2026-10-16). HESI is a trademark of Elsevier; NurseSavvy is not affiliated with Elsevier.

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