Change in concentration in unit time.
unit: mol dm-3 s-1
For reactant, concentration decreased against time
For product, concentration increased against time
a) Average rate = change in concentration / time taken
b) Instantaneous rate = gradient of tangent line
a) Particles should collide each other;
b) with sufficient energy;
c) with correct collision orientation.
Collision with sufficient energy which is greater than activation energy (the minimum energy required for a reaction happening) and correct collision orientation can occur reaction, and called effective collision (or successful collision).
Concentration and pressure are similar. Concentration is used to describe the number of particles in unit volume. Pressure is the force over unit area. In a rigid volume, pressure is proportional to the number of particles. So concentration and pressure are similar concepts. Concentration is for solution, and pressure is for gas, respectively.
With increasing number of particles in unit volume, collision frequency is increased as well. Finally, more effective collision in unit time.
Surface area contacted can affect the rate of heterogeneous reaction, such as calcium carbonate (solid) reacts with hydrochloric acid (aquesou solution), carbon dioxide (gas) bubbled in to limewater (aqueous solution), and saponification (aqueous sodium hydroxide and vegetable oil).
With increasing surface area contacted, more surface are exposed for collision, and more collision in unit time.
Temperature is a dominant factor that affect the reaction rate. Temperature will affect rate from two aspects. With increasing temperature, particles have greater kinetic energy, so that more particles have sufficient energy to overcome activation energy. Meanwhile, particles will move faster, and more collision in unit time.
Catalyst can speed up reaction by lower the activation energy. When activation energy is lowered, more particles have sufficient energy to overcome activation energy, and then the reaction rate is increased (See the energy level diagram below).