Click on the green flashing lights to follow links within this site

Rates of reaction (chemical kinetics).
The Kinetic Theory of matter.
The Collision Theory of chemical reactions.
Changing the rate of a chemical reaction.
Techniques for measuring the rate of a chemical reaction.
Catalysts.
The use of enzymes (biological catalysts) in the baking, brewing and dairy industries.
How the rates of enzyme-catalysed reactions vary with temperature.

Rates of reaction laboratory.
 


Techniques for measuring the rate of a reaction

Basically, we need to be able to measure how much a reactant is used up or how much of a product is formed as time passes. In the school laboratory, we usually choose reactions that are not too slow or too fast.

Even so, the reactions we can do can have a range of rates and we need to choose the technique which will be best for our reaction. We also have to think carefully about how much chemicals we can use so it is
safe and as cheap as is possible! Here we go:

Technique A: The test tube or conical flask

This technique is the easiest to carry out, but will only work if one of the reactants disappears (loses its colour or forms a soluble product) or a product appears (gives a colour or precipitates) during the reaction:
e.g.  2HCl(aq) + Mg(s) MgCl2(aq) + H2(g)

  • The acid is measured into the test tube or conical flask.
  • The magnesium is then added to the test tube or conical flask. This is time zero for the reaction; you should start the clock now.
  • You should see the change in the reactant or the solution and you record what you see at regular time intervals, say every 5, 10 or 20 seconds until there is no further change.

You can also use data-logging equipment with this technique, particularly when a coloured reactant disappears or a coloured or solid product is formed. You need to set up the data-logger with a light source and photocell sensor.

This is useful for any amount of chemicals (e.g. between 1 - 50 cm3 of liquids and 0.1 - 3.0 g
of solid
s).
Ideal reactions: sodium thiosulphate and acid, metals and acids, metal oxides and acids, metal carbonates and acids, metal displacement reactions.

Technique B: The conical flask and upside down measuring cylinder

This technique will only work if one of the products is a gas:
e.g.  HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O(l)

  • The acid is measured into the conical flask and the cylinder is filled with water from the trough.
  • As the cylinder is turned upside down, so long as its mouth is kept under the water, the water will not fall out.
  • The calcium carbonate is then added to the conical flask and the bung is replaced. This is time zero for the reaction; you should start the clock now.
  • Bubbles should come out of the tube under the cylinder and they will push the water out of the cylinder and you record the water level at regular time intervals, say every 5, 10 or 20 seconds.
  • Some people hold the cylinder in a clamp, but if you are careful, it is easier to manipulate by one person holding it. Don't worry, these reactions rarely last longer than 3 days!
Not very useful for small amounts of chemicals. You need to work on quite a large-scale for this technique to work (e.g. between 10 - 50 cm3 of liquids and 0.5 - 3.0 g of solids). Ideal reactions: metal carbonates and acids, manganese (IV) oxide and hydrogen peroxide.

Technique C: The conical flask and gas syringe

This technique is a more accurate version of Technique B. Instead of the measuring cylinder, a high precision glass gas syringe is used. In this case, you must hold the syringe in a clamp (not too tight!) or a specially designed syringe holder.

Try not to get any liquids inside the syringe, it should only have gas going into it. Otherwise, just follow the instructions as for Technique B.

Not very useful for small amounts of chemicals. You need to work on quite a large-scale for this technique to work (e.g. between 10 - 50 cm3 of liquids and 0.5 - 3.0 g of solids). Ideal reactions: metal carbonates and acids, manganese IV oxide and hydrogen peroxide.

Technique D: The conical flask and top-pan balance

This technique will only work if one of the products is a gas:
e.g.  2HCl(aq) + CaCO3(s) CaCl2(aq) + CO2(g) + H2O(l)

  • The acid is measured into the conical flask.
  • The calcium carbonate is then added to the conical flask and a plug of cotton wool is placed in the hole to let the gas out, but to stop any acid which may splash around get out. This is time zero for the reaction; you should start the clock now.
  • The reading on the balance should go down as the gas escapes and you record the reading at regular time intervals, say every 5, 10 or 20 seconds.
  • When the reading stops changing, you have finished.

The problem here is that you probably don't have many top-pan balances in your school! For this reason, the balance is sometimes connected to a computer which will plot graphs automatically for a whole group. It is also possible to do it normally as a whole group activity and then share the results, but it is usually more fun to do it yourself!

Not very useful for small amounts of chemicals. You need to work on quite a
large-scale for this technique to work (e.g. between 10 - 50 cm3 of liquids and
0.5 - 3.0 g of solid
s).
Ideal reactions: metal carbonates and acids, manganese IV oxide and hydrogen peroxide, metals and acids.