Collision Theory & Factors affecting rate

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Collision theory

Reactions occur when particles collide, but only collisions with enough energy and the right conditions can form products.

Effective collision

A collision that results in the formation of products.

Activation energy (Ea)

The minimum energy colliding particles must have for a chemical reaction to occur.

Collision frequency

The number of collisions between particles occurring per unit of time.

Factors affecting reaction rate

The six factors are nature of reactants, particle size, concentration, temperature, catalysts, and pressure.

Nature of reactants

Ionic reactions tend to be very fast because they involve oppositely charged species coming together, while covalent reactions are generally slower because bonds must be broken and formed.

Ionic reactions

Reactions involving ionic substances that tend to be very fast because they usually involve oppositely charged species coming together.

Covalent reactions

Reactions that involve breaking and forming covalent bonds and are generally slower than ionic reactions.

Particle size

Smaller particles react faster because they have a greater surface area available for collisions. Directly proportional

Why smaller particles react faster

Smaller particles provide more surface area, causing more collisions and more effective collisions per unit time.

Calcium carbonate & hydrochloric acid example

Crushed calcium carbonate reacts faster with hydrochloric acid than large chunks because more particles are exposed to the acid.

Concentration

The number of particles of a reactant present per unit volume.

Effect of increasing concentration

More particles are present per unit volume, causing more collisions and more effective collisions per unit time. Directly proportinal

Effect of decreasing concentration

Fewer particles are present per unit volume, causing fewer collisions and fewer effective collisions per unit time.

Temperature

Increasing temperature increases the rate of reaction because particles move faster and more particles have enough energy to reach the activation energy.

Why higher temperature increases rate

Particles move faster, causing more collisions, and a greater proportion of collisions have enough energy to overcome activation energy.

Why lower temperature decreases rate

Particles have less energy, move more slowly, and fewer collisions have enough energy to overcome activation energy.

Temperature and rate relationship

Increasing temperature increases reaction rate, but the rate is not directly proportional to temperature.

Pressure

For reactions involving gases, increasing pressure has a similar effect to increasing concentration because it increases the number of particles per unit volume.

Effect of increasing gas pressure

Pressure increases, volume decreases, particles collide more often, and the number of effective collisions per unit time increases.

Effect of decreasing gas pressure

Pressure decreases, volume increases, and there are fewer collisions and effective collisions per unit time.

State symbols

(s) means solid, (l) means liquid, (g) means gas, and (aq) means dissolved in water.

Catalyst

A substance that alters the rate of a reaction but is not consumed by the reaction.

How catalysts increase reaction rate

A catalyst provides an alternative reaction pathway with a lower activation energy.

Catalyst properties

Catalysts are recovered unchanged, are often specific to particular reactions, and usually only need to be present in small amounts.

Catalyst and equilibrium

A catalyst allows equilibrium to be reached faster but does not change the position of equilibrium or final mixture composition.

Catalyst poison

A substance that can destroy or reduce the effectiveness of a catalyst by permanently adsorbing onto its surface.

Negative catalyst/inhibitor

A substance that decreases the rate of a reaction.

Enzyme

A substance produced by a living cell that acts as a biological catalyst.

Amylase

An enzyme that breaks down starch.

Lipase

An enzyme that breaks down lipids (fats).

Protease

An enzyme that breaks down proteins.

Types of catalysis

homogeneous catalysis, heterogeneous catalysis, and autocatalysis.

Catalyst saturation

When the catalyst surface is fully occupied by reactant molecules, increasing reactant concentration no longer increases the reaction rate.

Desorption

Products must leave the catalyst surface before more reactant particles can attach to it.

Poisoned catalyst

A catalyst whose surface has been permanently occupied by catalyst poison, making it less effective or stopping it from working.

Limitation of collision theory: orientation

Collision theory requires particles to collide in the correct orientation, but it does not fully explain reactions involving complex

Limitation of collision theory: catalysts

Catalysts only alter reaction rate while they are not saturated or poisoned.

Limitation of collision theory: concentration

Increasing concentration does not always increase reaction rate, such as when a catalyst is saturated or another reactant is limiting.

Limiting reagent and reaction rate

Increasing the concentration of one reactant may not change the rate if there is not enough of the other reactant available.