Rates of Reaction

In a chemical reaction, old bonds are broken and new bonds are formed

If the energy released in bond making is greater than that needed for bond breaking, the reaction is EXOTHERMIC. If not, it’s ENDOTHERMIC


H2 + Cl2 = 2HCl

is highly exothermic and produces a lot of heat.


  • Fast reaction: silver nitrate + sodium chloride = sodium nitrate + silver chloride (a white precipitate forms IMMEDIATELY the two liquids meet)
  • Slower reaction: concrete sets in a matter of hours
  • Very slow reaction: iron rusts (oxidises) in the presence of oxygen(air) and water


Concentration: If there is more of a substance in a fixed volume, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed. 


Surface area. If marble chips are reacted with hydrochloric acid the following happens:

Calcium Carbonate + Hydrochloric acid = Calcium Chloride + Water + Carbon dioxide

A gas can be seen to bubble from the marble surface. If the chip size is small there is more reactant area available to the acid, so the reaction goes faster. With larger chips, the reaction proceeds more slowly and the gas is formed less rapidly.

Temperature: When you raise the temperature of a system, the molecules bounce around a lot more (because they have more energy). When they bounce around more, they are more likely to collide. That fact means they are also more likely to combine. When you lower the temperature, the molecules are moving on average more slowly and collide less. That temperature drop lowers the rate of the reaction. 

Pressure: Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater concentration of molecules increases the number of collisions. When you decrease the pressure, molecules don’t hit each other as often. The lower pressure decreases the rate of reaction. 






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