Rate of Reactions

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


• 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


• Rate of reaction increases with increasing concentration
• Higher concentration, more reacting particles are present
• Greater probability of an effective collision
• Faster rate of reaction

– Same no. of moles

– Different no. of moles


• 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

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


• 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


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

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