Required Practical 5: Rates of Reaction

Question 1

How do we carry out an investigation to explore the effect of concentration on the rate of a reaction with the disappearing cross?

Answer 1

1) First we use a measuring cylinder to put 10cm³ of sodium thiosulfate solution into a conical flask.
2) Place the conical flask onto a printed black cross.
3) Next add 10cm³ of hydrochloric acid into the conical flask.
4) Swirl the solution and start a stopwatch.
5) Look down through the top of the flask and after a certain time the solution will turn cloudy.
6) Stop the clock when we can no longer see the cross.
7) Carry out the experiment again using lower concentrations of sodium thiosulfate solution.
8) Finally, repeat the whole experiment and calculate mean values for each concentration of sodium thiosulfate solution.

Question 2

What are the problems with the disappearing cross experiment?

Answer 2

-Different people have different eye sights which means that some people can see the cross for longer than others so they may not get the same results.
-However, because all the students use the same size printed cross, this problem may not be too great.

Question 3

How do we measure the volume of a gas produced by a reaction?

Answer 3

1) First we use a measuring cylinder to place 50cm³ of hydrochloric acid into a conical flask.
2) Attach the conical flask to a bung and delivery tube.
3) Now place the delivery tube into a container filled with water.
4) Then place an upturned measuring cylinder also filled with water over the delivery tube.
5) Add a 3cm strip of magnesium to the hydrochloric acid and start a stopwatch the reaction produces hydrogen gas which is trapped in the measuring cylinder.
6) Every 10 seconds, measure the volume of hydrogen gas in the measuring cylinder and continue until no more hydrogen is given off.
7) Repeat the experiment using different concentration of hydrochloric acid.