Practical Questions

Question 1

A student is required to make 250 cm3 of an aqueous solution that contains an
accurately measured mass of sodium hydrogensulfate (NaHSO4).
Describe the method that the student should use to make this solution.

Answer 1

1. Put an empty weighing boat on a mass and zero it.
2. Add the mass amount you want, e.g. 10g, to the weighing boat.
3. Dissolve this into water and stir until dissolved.
4. Transfer this solution to a volumetric flask and make it up to 250cm3, make sure to use washings to ensure all the solution is transferred.
5. Gently shake the flask to ensure mixing.

Question 2

The initial rate of the reaction between sodium thiosulfate and hydrochloric acid can
be monitored by measuring the time taken for a fixed amount of sulfur to be produced.
Describe an experiment to investigate the effect of temperature on the initial rate of
this reaction.
Include
• a brief outline of your method
• how you will measure the time taken for a fixed amount of sulfur to be formed
• how you will present your results in graphical form
• a sketch of the graph that you would expect.

Answer 2

1. Add 25cm3 of sodium thiosulfate into a conical flask. Mark an x on the bottom of the conical flask
2. Add the flask to a water bath at a temperature of 10 degrees c.
3. Then add 25cm3 of HCL and begin a timer, recording the time taken for the cross to disappear.
4. Repeat this another two times to get repeat results for this temperature.
5. Repeat 1-4 with temperatures of 20, 30, and 40.
6. Make sure the concentrations and volumes of the sodium thiosulfate and HCL stay the same for each repeat.
7. Rate is 1/t. Plot the rate as the y axis and time as the x axis.
8. Curved line

Question 3

A general equation for a reaction is shown.
A(aq) + B(aq) + C(aq) → D(aq) + E(aq)
In aqueous solution, A, B, C and D are all colourless but E is dark blue.
A reagent (X) is available that reacts rapidly with E. This means that, if a small
amount of X is included in the initial reaction mixture, it will react with any E produced
until all of the X has been used up.
Explain, giving brief experimental details, how you could use a series of experiments
to determine the order of this reaction with respect to A. In each experiment you
should obtain a measure of the initial rate of reaction.

Answer 3

1. Measure volumes, e.g. 25cm3, of A, B, C and X all in separate containers. (Using known concentrations)
2. Mix A, B and X together in a conical flask. Make sure the conical flask is placed in a water bath at 25 in order to keep temperature constant.
3. Stir the solution to help it react faster.
4. Then add C, start a stopwatch, and time the amount of time taken for the solution to turn blue.
5. Repeat this, keeping all volumes constant and the concentrations of B, C and X constant, but changing the concentration of A. Do this several times.
6. Plot this in a graph (y axis 1/t, x axis concentration of A)
7. Horizontal = Zero order
   Straight line through origin = First order
   Curved = Second order

Question 4

Why might you not be able to determine the enthalpy change of a reaction via colorimetry?

Answer 4

Because a product may be soluble