CHAPTER 3. REACTION RATES 3.4
- Changes in mass:
The rate of a reaction that produces a gas can alsobe measured by calculating the
mass loss as the gas is formed and escapes fromthe reaction flask. Thismethod
can be used for reactions that produce carbondioxide or oxygen, butare not
very accurate for reactions that give off hydrogen because the mass is too low
for accuracy. Measuringchanges in mass may also be suitable for othertypes of
reactions.
Activity: Measuring reaction rates
Aim:To measure the effect ofconcentration on the rate of a reaction.
Apparatus:- 300 cm^3 of sodium thiosulphate(Na 2 S 2 O 3 ) solution. Prepare a solution
of sodium thiosulphateby adding 12 g of Na 2 S 2 O 3 to 300 cm^3 of water.
This is solution ’A’. - 300 cm^3 of water
- 100 cm^3 of 1:10 dilute hydrochloric acid. This is solution’B’.
- Six 100 cm^3 glass beakers
- Measuring cylinders
- Paper and marking pen
- Stopwatch or timer
Method:One way to measure the rate of this reaction isto place a piece of paper
with a cross underneaththe reaction beaker to see how quickly the cross is
made invisible by the formation of the sulphur precipitate.- Set up six beakers on a flat surface and mark them from 1 to 6. Under
each beaker you will need to place a piece ofpaper with a large black
cross. - Pour 60 cm^3 solution A into the first beaker and add 20 cm^3 of water
- Use the measuring cylinder to measure 10 cm^3 HCl. Now add this HCl
to the solution that is already in the first beaker(NB: Make sure that you
always clean out the measuring cylinder you have used before using it for
another chemical). - Using a stopwatch with seconds, record the time it takes for the precipitate
that forms to block out the cross. - Now measure 50 cm^3 of solution A into the second beaker and add 30
cm^3 of water. To this secondbeaker, add 10 cm^3 HCl, time the reaction
and record the results asyou did before. - Continue the experiment by diluting solutionA as shown below.