C2bL1 Rates of Reaction

Key Words

Catalyst - a substance that increases the rate of reaction, but does not react itself, nor does it affect the amount of product.

Concentration - how much substance there is dissolved in a volume of solvent.

Data-logger - a sensor connected to a computer to record changes.

Displace - push out of the way.

Insoluble - does not dissolve.

Mole - 6 × 1023 particles.

Precipitate - a product of the reaction of two solutions that is insoluble in water

Rate of reaction - how fast a reaction goes.

Reactant - a substance that undergoes chemical change.

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Grade E

You will have done an experiment to show how magnesium reacts with hydrochloric acid to liberate hydrogen:

magnesium + hydrochloric acid  ®  magnesium chloride + hydrogen

Mg (s) + 2HCl (aq)  ® MgCl2 (aq) + H2 (g)

Note the states given in brackets after symbols:

  • (aq) - aqueous, meaning dissolved in water;

  • (s) - solid;

  • (g) - gas;

  • (l) - liquid.

The hydrogen gas is displaced because magnesium is more reactive than hydrogen, and snatches the chloride away from the hydrogen.

We could speed up this reaction by:

  • Heating it up;

  • Making the acid more concentrated;

  • Grind the magnesium into a powder;

  • Use a catalyst.

We can measure the rate of reaction by:

  • Measuring how quickly it makes a gas;

  • Measure the loss in mass;

  • Time how long it takes for the magnesium to dissolve.

Grade C

For the reaction between magnesium and hydrochloric acid, the best way to measure the rate of reaction is to measure the volume of gas produced.  We can do this by:

  • Counting how many bubbles there are every ten seconds;
  • Collect the gas with a gas syringe;
  • Collect the gas in an upturned burette, or a measuring cylinder, displacing water.

In this case, we are measuring the rate of reaction by measuring how much product is made in a given period of time:

rate of reaction = amount of product

                               time

 

If we record the amount of hydrogen (dependent variable) given off against the time (independent variable), the graph would look like this:

 

 

 

The reaction starts off at a fast rate, but slows down as  the magnesium is used up.  When all the magnesium has reacted, the reaction stops.

 

We could measure the rate of reaction by seeing how fast the reactants are used up.  This is done by measuring the mass of the solution with a very sensitive top pan balance.   The graph would look like this:

 

 

If we have a precipitate, the solution goes cloudy.  We can use a light sensor with a data-logger to measure how cloudy the solution becomes.  Here is a typical precipitation reaction:

 

Sodium thiosulphate + hydrochloric acid  ®  Sulphur dioxide + sulphur + sodium chloride + water

 

Na2S2O3 (aq) + 2HCl (aq) ®  SO2 (g) + S (s) + 2NaCl (aq) + H2O (l)

 

The sulphur is the precipitate.

Grade A

The concentration of a dissolved substance is measured in moles per cubic decimetre (mol dm-3) where 1 dm3 is the same as 1 litre.  Only chemists seem to use the cubic decimetre!

If the reactants are gases, increasing the pressure makes collisions and reactions much more likely, so increases the rate of the reaction.

All the methods of measuring rates of reactions have uncertainties:

  • Counting bubbles only works if the gas is insoluble in water.  Even then, it's not very accurate.  Gases like carbon dioxide will dissolve in water, so you can't be sure how much has been given off. 
  • Displacement of water is not always accurate for the same reason as above.  Sulphur dioxide in the reaction above is very soluble in water, so measuring gas given off is not much use.
  • Timing a solid dissolving is difficult, as well as being as interesting as watching paint dry.
  • Measuring mass loss needs a sensitive balance.  Also if gas is given off, the fizzing can cause a spray mist.
  • Using a gas syringe has the difficulty of the plunger sticking.  Beautifully smooth syringes are expensive.