Rate of Reactions

Fast or Slow Reactions

• Extremely slow reactions– Iron rusting– Limestone weathering

• Extremely fast reactions– Explosion

Measuring Rate of Reactions

• Some rate of reactions have detectable change with respect to time

• Changes that are observable like– When a volume of gas is given off– When this is a change in mass during the

reaction– When there are temperature changes– When there are colour changes– When a precipitate forms– When there are pH changes

Collision Theory

• For a reaction to occur– The reacting particles must collide into

each other– The reacting particles must possess

enough activation energy

• Once products are formed, effective collisions have occurred

Factors affecting Rate of Reactions

• Temperature of the substances used (reactants)

• Concentration of the substances used (reactants)

• Pressure on the reaction

• Particle size (surface area) of the substances used (reactants)

• Presence of catalyst

Temperature

• Rate of reaction increases with increasing temperature

• High temperature, particles have greater heat energy

• Particles move faster with greater kinetic energy

• Leading to more collisions between particles

• Increased probability of effective collision• Reactions take place faster

• Speed of reaction doubles when the temperature

rises by 10 C

TemperatureAmt of product formed

Time

Higher temperature

Lower temperature

Concentration

• Rate of reaction increases with increasing concentration

• Higher concentration, more reacting particles are present

• Greater probability of an effective collision

• Faster rate of reaction

Concentration – Same no. of molesAmt of product formed

Time/s

Higher concentration

Lower concentration

Concentration – Different no. of molesAmt of product formed

Time/s

Higher concentration

Lower concentration

Pressure

• Rate of reaction increases with increasing pressure

• Higher pressures, reacting particles are closer together

• Increasing concentration per unit volume

• Greater probability of an effective collision

• Faster rate of reaction

PressureAmt of product formed

Time/s

Higher pressure

Lower pressure

Particle Size

• Rate of reaction increases when particle size decreases

• Smaller particles has greater surface area than larger particles of the same mass

• Greater surface area for collision by another reacting particle

• Greater probability of an effective collision• Faster rate of reaction

Particle sizeAmt of product formed

Time/s

Smaller particle size

Larger particle size

Catalyst

• Presence of catalyst increases rate of reaction

• (Presence of inhibitors decreases rate of reaction)

• Catalysts lower activation energy of reactants

• Aids the formation of unstable intermediate products

• Increases probability of formation of products

• Faster rate of reaction

CatalystAmt of product formed

Time/s

Use of catalyst

Absence of catalyst

CatalystEnergy

Time/s

Use of catalyst

Absence of catalyst

Ea

Ea

Catalyst

Definition: A substance which increases the rate of a chemical reaction by providing an alternative pathway with a lower activation energy but remains unchanged at the end of the reaction