Rates of Reaction

Different Speeds of Reaction

Different chemical reactions take place at different speeds.

Very fast:

• explosion of a petrol-air mixture
• precipitation reactions
• fireworks going off
• coal burning

Moderately fast:

• reaction of metals or carbonates with dilute acids

Slow: 

• rusting of iron in air
• reaction of magnesium with cold water
• oil forming
• silver tarnishing

How do we calculate the speed of reaction? 

During a chemical reaction, the reactants get used up as products are formed. We can measure the speed of reaction by measuring the amount of a reactant used up per unit time, i.e.

Speed of reaction= amount of reactant used up/time taken

The speed of a reaction can also be measured in terms of the amount of a product formed/obtained per unit time, i.e. 

Speed of reaction= amount of product formed/time taken

For a chemical reaction that produces a gas, the speed of reaction can be found by measuring the volume of gas produced per unit time, i.e.

Speed of reaction= volume of gas produced/time taken

Measuring Speed of Reaction from Changes in Volume

The speed of a reaction can be found by measuring the following quantities at regular time intervals:

• volume of gas produced by the reaction
• the mass of the reactant that remains

Measuring Speed of Reaction from Changes in Mass

The speed of a reaction can also be found by measuring the changes in mass of a reaction mixture. This method works best for reactions which produce gases such as carbon dioxide. 

Factors Affecting Speed of Reaction

Many factors affect the speed of a chemical reaction. These include:

• the concentration of the reactants
• the pressure of the reactants (if the reactants are gaseous)
• the particle size or surface area of the reactants
• the temperature at which the reaction is occurring

Concentration:
Increasing the concentration means there's more of the reactants in the same volume. Thus with more particles there will be more collisions, and there will be higher chance of effective collisions, thus increasing the rate of reaction.

Pressure:
This is just like concentration, but it's more to do with gaseous reactants. With higher pressure--more particles--more collisions--higher chance of effective collisions--increase rate of reaction.

Surface area:
The bigger the surface area, the more chance of collisions, ditto above. Same thing. It's all about having more collisions, thus more effective collisions which increase rate of reaction. E.g. if you had a piece of metal, and you cut it into many smaller pieces, it would react faster with acid because the acid will have more area of the metal to collide with.

Temperature:
The higher the temperature, the more thermal energy is transferred to kinetic energy for the particles, thus they move faster and collide more often. If they have more energy, it is likelier that they will have minimum activation energy therefore there will be more effective collisions. And for a change, the rate of reaction increases... :P


For a reaction to occur between 2 particles: 

1. the reacting particles must collide with each other
2. they must collide with a certain minimum amount of energy known as the activation energy

In this way, collisions between reacting particles result in the formation of product particles. These collisions are known as effective collisions. 

Thus in a reaction between hydrogen and chlorine, only fast-moving molecules with energies equal to or greater than the activation energy will react on collision to form hydrogen chloride. 

• In general, when any factor increases the rate of effective collisions between reacting particles, it will also increase the speed of reaction.