F.6/7 Chemistry Practical:
An Introduction to the Study of Chemical Equilibrium
Objective: To understand
(a) the idea of 'chemical equilibrium' by investigating two examples,
(b) chemical equilibrium that can be attained from the reactant side or the product side,
(c) temperature effect on the position of equilibrium,
(d) pressure effect on the position of chemical equilibrium and
(e) concentration effect on the position of chemical equilibrium
Group size: Individual
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.
Introduction
Most chemical reactions are reversible in close system. Once the forward reaction rate is equal
to the backward reaction rate, equilibrium is said to be established. However, position of
equilibrium of some reactions are too difficult to be studied because these positions may be far in
the L.H.S. or in the R.H.S.
In this practical positions of some chemical reactions are investigated and important features
of chemical equilibrium should be obtained from the experimental results. To explain the change of
position of equilibrium LeChatelier's principle should be used.
Materials required
50 cm3 glass syringe (the plunger should be tied with thread to the body of the syringe), 600 800 cm3 foam cup, 800 - 1000 cm3 beaker, pH paper with colour chart, sodium ethanoate (solid),
trichloromethane, crystals of iodine, 2 corks for boiling tubes, bismuth(III) chloride, bromine water,
Bench solutions
iron(III) nitrate solution, potassium thiocyanate solution, sodium phosphate solution, ethanoic
acid, , potassium iodide solution, sodium hydroxide solution, dilute hydrochloric acid, concentrated
hydrochloric acid in the fume-cupboard, potassium chromate(VI) solution,
Gas: 25 cm3 nitrogen dioxide gas in the 50 cm3 syringe,
Safety precaution
Trichloromethane is anaesthetic. Sodium hydroxide and hydrochloric acid are corrosive. bromine is
corrosive and its vapour is choking. Carry out experiment with bromine in the fume-cupboard or
with 'gas-collection set-up'. Potassium chromate(VI) is oxidizing, barium(III) chloride and other
chemicals are harmful. Note the use of glass syringe. Foam cup may catch fire easily. Do NOT
touch iodine crystals with bare hands.
Procedure A.
Investigating Some Reactions that can go 'either way'.
A1. The action of acid and alkali on bromine water
Caution: Bromine is poisonous and volatile. Do not inhale its vapour.
1. Place 3 cm3 of bromine water in a test tube. Observe colour of the solution against a white
tile. Add bench dilute sodium hydroxide solution from a dropper drop by drop, until there
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seems to be no further change. Shake the tube after each addition.
2. To the alkaline solution from step 1, add bench dilute hydrochloric acid in a similar way by
using another dropper.
3. Repeat the dropwise addition of alkali and acid in alternation once more.
4. Repeat part (a), using potassium chromate(VI) solution instead of bromine water, but add the
acid first. Record the observations.
A2. Hydrolysis of bismuth(III) chloride
Caution: Take care in handling concentrated hydrochloric acid.
1. Put about half a spatula measure [size of green bean] of bismuth(III) chloride in a test tube.
Add concentrated hydrochloric acid drop by drop from a dropper until the solid has just
completely dissolved. Shake after each addition. Add 4 to 5 drops of this solution to a boiling
tube half filled with water.
The hydrolysis of bismuth(III) chloride may be represented by
BiCl3(aq) + H2O(l)  BiOCl(s) + 2H+(aq) + 2Cl-(aq)
colourless
white
2. Pour off most of the mixture from the boiling tube, to leave only about 1 cm depth of
contents. Add concentrated hydrochloric acid dropwise from a dropper until there seems to
be no further change.
3. Now add water to the boiling tube again until it is half full. Record the observations.
Procedure B: The Study of distribution of iodine between two immiscible solvents
To investigate whether the same equilibrium position can be reached irrespective of starting point:
1. Transfer 5 cm3 of potassium iodide aqueous solution from a measuring cylinder into a boiling
tube. Put 5 cm3 of trichloromethane in another boiling tube. Select two very small crystals of
iodine of about equal size. Add one crystal to the potassium iodide solution and the other
crystal to the trichloromethane. Cork and shake the tubes vigorously until the crystal has
completely dissolved in each tube. Note the colour of each solution. Record your
observation.
2. To the solution of iodine in trichloromethane, add 5 cm3 of fresh potassium iodide aqueous
solution. Call this tube X. Shake the tube gently.
3. Add the solution of iodine in potassium iodide aqueous solution to 5 cm3 of trichloromethane
in a test tube. Call this tube Y. Shake the tube gently. Record colour change for step 9 and 10.
4. Cork both tubes X and Y and shake them vigorously for half a minute. Allow them to stand
until the two liquid layers have separated in each tube. Equilibrium is assumed to be reached
in each tube now.
Procedure C: Attaining a new equilibrium position
Use a dropper to remove about three-quarters of the upper layer in test tube X. Add fresh
potassium iodide aqueous solution up to the original level. Cork the test tube and shake vigorously
for half a minute. Allow the tube to stand until the two liquid layers have separated. Compare the
colour intensities of the two layers in tube X with those in tube Y. Record your observation.
Procedure D. The effect of concentration change on equilibrium position
When solutions containing iron(III) ions and thiocyanate ions are mixed, an intense blood-red
colour is produced. Using the following equation to interpret the results.
Fe3+(aq) + CNS-(aq)  [Fe(CNS)]2+(aq)
pale brown colourless
blood-red
Mix 10 cm of iron(III) nitrate solution with 10 cm3 of potassium thiocyanate solution in a beaker.
3
F.6/7 Chem.Prac./Eq1/p.2(4)
Divide the resulting red solution into four portions in four tests in a test-tube rack. Label them tube
1,2,3,4. To tube 1 add a few crystals of iron(III) nitrate. To tube 2 add a few crystals of potassium
thiocyanate. To tube 3 add half a spatula of sodium phosphate (which reacts with iron(III) ions to
form a stable complex.)
In procedure E and F, the following equilibrium is investigated:
H = +58 kJ
N2O4(g) <====> 2NO2(g)
pale yellow
dark brown
At room temperature and pressure, the so-called 'nitrogen dioxide gas' actually consists of a mixture
of dinitrogen(VI) tetroxide and nitrogen(VI) dioxide in equilibrium with each other. Any shift in the
position of equilibrium would be indicated by a corresponding change in colour intensity of gas.
Procedure E. The effect of pressure change on equilibrium position
(2 students form a group because of the limited supply of 100 cm3 syringe)
Introduce 25.0 cm3 of 'nitrogen dioxide gas' into a 50 cm3 gas syringe and close the end with a
rubber stopper. Hold the syringe in front of a white tile. Increase the pressure of the gaseous system
by quickly pushing the plunger until the volume of gas is approximately halved. Hold the plunger
steady in this position. Record your observations about
(a) what immediate colour change is observed?
(b) What is the colour change after several seconds?
Procedure F: The effect of temperature change on equilibrium position
Introduce 25.0 cm3 of 'nitrogen dioxide gas' into a 50 cm3 gas syringe and close the end with a
rubber stopper. Place the syringe in a large foam cup (ca. 600 cm3) containing ice-water. Leave the
syringe in the foam cup for about 10 minutes. Rotate the plunger every minute to prevent jamming.
Record the new volume and colour of the gas mixture. What is the change in the course of cooling?
Half fill the 800 cm3 beaker with tap water. Heat the beaker over a gauze and a tripod until the
water almost boils. Turn off the Bunsen flame and remove the beaker to the bench. Place the
syringe in the hot water. Record the new volume and colour of the gas mixture. What is the change
in the course of cooling?
Procedure G. Devise an experiment to demonstrate LeChatelier's principle.
You are given an aqueous solution of ethanoic acid, solid sodium ethanoate and pH paper. By
using these compounds together with simple apparatus supplied in your laboratory locker write
procedure to demonstrate LeChatelier's principle. Carry out your proposed procedure and give your
experimental results in your report.
Data and discussion: In this section you should give explanation or discussion immediately after
your results /observations for the experiment. The followings are guidelines for you.
For procedure A
1. Is the reaction between bromine water and sodium hydroxide solution reversible? Write an
equation for the reactions.
2. The action of acid and alkali on potassium chromate(VI) solution. [Caution : Acidified
chromate(VI) solution is strongly oxidizing.]
3. For step 1-4: Is the reaction reversible? Write an equation for the reactions.
For procedure B
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1.
Compare the colour intensities of the corresponding layers in the two tubes. Do you think the
2.
same equilibrium position is attained in each case? What can be concluded from this
experiment?
What is the purpose of shaking the test tubes vigorously?
For procedure C
Comment the experimental results.
For procedure D.
Compare the colours of solutions in tubes 1, 2 and 3 with that in tube 4. What is the effect of
concentration of one of the reactants on the position of equilibrium?
For procedure E, F
Give a full explanation for this experiment.
For procedure G
Propose a method that can be used to demonstrate LeChatelier's principle by using ethanoic
acid and sodium ethanoate solid.
The Report: Hand in your report to Mr. Lam (in the Preparation Room) before 4:30 p.m.
End
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