F.6 Chemistry Practical :
Objective: To determine the mole ratio of two ions in a precipitate by measuring temperature changes on mixing the aqueous ions and hence the ionic equation of the reaction is determined.
Size of the group: Individual work; but two students share two burettes
Introduction
Chemists are interested in the quantitative relation between two reacting species. In this experiment we assume that we do not know the charge the copper ion and hydroxide ion. If copper ion is completely reacted with exact amount of hydroxide ion, the reaction mixture should attain the maximum temperature. Conversely if temperature change of different reaction mixture is measured (that is, mole ratios of copper ion and hydroxide ion are different in these mixtures) , the mole ratio of these ions in the stoichiometric equation are determined. Suppose one of these ion is charged by one, charge of another ion can be determined.
---------------------------------------------------------------------------------------------------------------------
Theory: Give principle/reason to explain how is an ionic equation established by thermometric method in this experiment. The following questions should be useful to you.
1.
What is meant by the term 'stoichiometric equation'? Explain this term with a suitable example.
2.
Explain the term ‘exothermic reaction’ with the aid of an energy diagram.
3.
Explain the term ‘heat of reaction’.
4. In this practical, the reaction between copper(II) ion and hydroxide ion is studied.
(a) Outline the principle that you can determine the end point of this reaction
(b) Show how can you calculate mole ratio of copper(II) ion and hydroxide ion from the experimental results.
---------------------------------------------------------------------------------------------------------------------
Chemicals:
Solution H: 300 cm
3
of 1.0 M copper(II) sulphate(VI) solution,
Solution I: 300 cm 3 of 2.0 M sodium hydroxide solution.
Apparatus per group:
50 cm 3 burettes (2), burette holders/stand (2), polystyrene cup (1), thermometer (readable 0.2
o C).
Pre-laboratory work:
Answer questions in the ‘Theory section’ and get two graph papers before you go to the laboratory
Procedure
Record your results immediately after you get the readings. Wear goggle in the course of the practical. Distilled water may be replaced by de-ionized water.
1. Using a burette place 5.00 cm
3
of solution H in a 100 cm
3
beaker. By means of a second burette place 45.0 cm
3
of solution I in a plastic cup. Take the temperature of solution I to the nearest 0.2 o C, and record this temperature in the following table. Now add the measured portion of solution H to the solution I, stir continuously and measure the highest temperature attained to the nearest 0.2
o
C. Record this temperature. This is experiment 1.
2. After each experiment discard the contents of the plastic cup and wash out the cup twice with distilled water. Drain any liquid in the cup. Carry out experiment 2, 3, 4 and 5 using the quantities shown in the table and the procedure in (1). [Note: Pour the liquid waste into the sink and the solid waste into the litter bin.]
3. For experiment 6 to 9 work similarly except putting solution H in the plastic cup and solution I in the beaker and record the temperature of solution H.
Mole1 (Mole ratio from thermometric det’n)/p.1 of 2

Data and Results
Concentration of copper(II) sulphate(VI) solution =
Concentration of sodium hydroxide solution =
Give observations for step 1. Explain the chemical change with equation(s).
Exp. No. Volume of solution H/cm 3
Volume of solution I/cm 3
Initial temperature of H or I/ o
C
Final temperature of the mixture/ o
C
1
2
5.0
10.0
45.0
40.0
3
4
5
6
7
8
9
15.0
20.0
25.0
30.0
35.0
40.0
45.0
35.0
30.0
25.0
20.0
15.0
10.0
5.0
Temp.rise/ o
C
Data treatment and conclusion
Plot a graph of temperature rise (vertical axis, scale 2 cm to 1 o
C) against volume of solution H
(horizontal axis, scale 2 cm to 10 cm 3 ). You should draw two intersecting straight lines one originating from the point (0,0) and the other originating from the point (50, 0).
From the point of intersection of the straight lines, find the reacting volumes of the solutions H and I and hence the apparent mole ratio, negative ion : positive ion, in the precipitate formed.
Explain your work.
Questions for Discussion
1. Make conclusions for your results. Why does the maximum temperature change indicate that the all reacting species are reacted?
2. Discuss the sources of error in this practical.
3. Suppose you have some precipitates in the tip of your burette, it cannot be washed way and the flow rate of the liquid is thus very slow. Propose a chemical method to solve this problem.
4. What are safety precautions in this experiment?
Mole1 (Mole ratio from thermometric det’n)/p.2 of 2