Tuesday 13 March 2012

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. 

9 comments:

  1. anything on mole calculations like examples please

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  2. Lolololololololololololol

    ReplyDelete
  3. This is so helpful! Thanks! Really helped in mY IGCSE course.

    ReplyDelete
  4. 2012 was so easy i mean i got 77 in p3 38 in p1 and 57 in p6

    ReplyDelete
  5. Doesn't a catalyst have an effect on the rate of a reaction

    ReplyDelete
  6. yes it has ,it just speeds up the reaction but cannot increase the yield.

    ReplyDelete

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Note: This blog will no longer be updated as I finished IGCSEs in 2012. Sorry! :( If you are interested in buying IB notes though, please contact me. :)