CH095 – A Study of Equilibrium
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A STUDY OF EQUILIBRIUM
THEORY:
Many chemical reactions are reversible, i.e., a set of reacting materials form
certain products and these products react to form the original reactants. When
the rates of the forward and reverse reactions are equal, we say that there exists
a state of dynamic equilibrium. For a reversible reaction in which A + B combine
to form C + D or C + D combine to form A + B, we represent the equilibrium as
A+B
↔C+D
(1)
Associated with the equilibrium condition are equilibrium concentrations of A, B,
C, and D.
It is possible to affect the "position" of equilibrium and, therefore, the equilibrium
concentrations by altering the conditions of concentration, temperature, or
pressure that a system in equilibrium experiences. In this experiment we will
observe the effects of concentration changes on various equilibria.
Shifts in equilibrium are governed by Le Chatelier's Principle which generally
states that if we apply a stress to a system in equilibrium, the equilibrium will
shift so as to relieve that stress. For example, considering hypothetical A, B, C, D
equilibrium (1) above, addition of B results in a shift to the right (→) and addition
of D results in a shift to the left (←).
Many solutions have simultaneous equilibria operating, i.e., several
interdependent equilibrium systems. For example, in any carbonated beverage
the following equilibria exist:
CO2(g) + H2O(l)
H2CO3(aq)
↔
↔
H2CO3(aq)
H+ (aq) + HCO3-(aq)
(2)
(3)
A shift in one equilibrium will affect the other. In this example, the addition of H+
to the beverage (say from HCl --> H+ + Cl-) shifts the equilibrium (3) to the left
and the
equilibrium (2) is also shifted left because of the increase in H2CO3 as equilibrium
(3) shifts.
Like any other equilibrium, the solubility equilibrium in a saturated solution can
be shifted. For example, consider a saturated solution of BaCrO4(s):
BaCrO4(s) ↔ Ba2+ (aq) + CrO42- (aq)
BaCrO4(s) represents a precipitate. The addition of either Ba2+ ion or of CrO42- ion
CH095 – A Study of Equilibrium
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results in the formation of more precipitate. Conversely the removal of either ion
will cause the precipitate to dissolve at least in part.
PROCEDURE:
I.
Indicator Equilibria
Acid-base indicators are weak electrolyte dyes that undergo colour changes
as their ionization equilibrium shifts:
HInd (aq)
↔
Molecular
(unionized)
indicator
H+(aq) +
Ind-(aq)
Ionized
indicator
In general, the molecular form is a different colour from the anion. In the
case of the indicator, bromthymol blue, the molecular form is yellow and the
anion is blue:
HInd (aq)
yellow
↔
H+(aq) +
Ind-(aq)
(colourless)
blue
1. Add 3 drops of bromthymol blue to 5 mL of distilled water in a test tube.
Hold the test tube in front of a piece of white paper and note the colour.
2. Add 3 drops of 1 mol/L HCl(aq) solution to the test tube and note the
colour.
3. Add 1 mol/L NaOH(aq) drop wise until a colour change is observed, note
the colour.
NOTE: Volumes in this entire experiment can be estimated. The small
test tubes can hold approximately 10 mL of solution.
CH095 – A Study of Equilibrium
II.
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Chromate-Dichromate Equilibrium
Another equilibrium system which can be detected by colour changes is
the following:
2 CrO42-(aq)
+
2 H+(aq)
↔
chromate
yellow
Cr2O72-(aq)
+
H2O(l)
dichromate
orange
1. Place 3 mL of 1 mol/L K2CrO4(aq) in a test tube. Hold the test tube
against white paper and note the colour.
2. Add 3 mol/L H2SO4(aq) solution drop wise until you note a colour
change. Describe the colour.
3. Add 6 mol/L NaOH(aq) solution drop wise until a colour change is
noted. Describe the colour.
4. Discard the contents of all test tubes.
III.
Solubility Equilibrium and Simultaneous Equilibria
2 CrO42-(aq) + 2 H+(aq)
BaCrO4(s)
↔
↔
Cr2O72-(aq) + H2O(l)
Ba2+(aq) + CrO42-(aq)
1. Place 10 drops of 0.1 mol/L K2CrO4(aq) in a small test tube. Add 2
drops of 1 mol/L NaOH(aq). Next add 0.1 mol/L Ba(NO3)2(aq) one drop
at a time until a change is noted. Record your observations and retain
this tube for Step 3.
2. Place 10 drops of 0.1 mol/L K2Cr2O7(aq) in a small test tube. Add 2
drops of 1 mol/L HCl(aq) and 10 drops of 0.1 mol/L Ba(NO3)2(aq).
Record your observations and retain this tube for Step 4.
3. To the test tube from Step 1 add 1 mol/L HCl(aq) until a change is
noted. Record your observations. Retain this test tube for Step 5.
4. To the test tube from Step 2 add 1 mol/L NaOH(aq) until a change is
noted. Record your observations and retain this tube for Step 5.
5. Attempt to reverse the changes and reactions observed in Steps 3 and
4 using 1 mol/L HCl(aq) or 1 mol/L NaOH(aq). Record your procedure
and observations.
6. Place 10 drops of 0.1 mol/L K2CrO4(aq) in a small test tube and 10
drops of 0.1 mol/L K2Cr2O7(aq) in another small test tube. Add 5 drops
of 0.1 mol/L Ba(NO3)2(aq) to each. Record your observations including
the relative amounts of any product formed.
7. Discard the contents of all test tubes.
CH095 – A Study of Equilibrium
IV.
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Equilibrium in Ammonia Solution
Ammonia (NH3) is a gas. When dissolved in water, NH3 reacts with water in a
reversible reaction which results in the equilibrium:
NH3 (aq) + H2O(l)
↔
NH4+(aq) + OH-(aq)
In this experiment we will establish the existence of both reactant and
product species approaching the equilibrium from either side.
A. NH3(aq) + H2O(l)
↔
NH4+(aq) + OH-(aq)
1. Testing for NH3. Ammonia forms a deep blue complex ion in the
presence of Cu2+ ions:
Cu2+(aq) + 4NH3(aq)
light blue
↔
Cu(NH3)42+(aq)
deep blue
Place 4 mL of 0.1 mol/L Cu(NO3)2(aq) solution in a test tube and note
its colour. Add 1 mol/L ammonia solution drop wise until you see a
clear, deep blue solution. Record your observations.
2. Testing for OH-. With a stirring rod, touch a drop of ammonia solution
to a piece of red litmus paper. The paper's turning blue is a positive
test for the presence of hydroxide ion. Record your observation.
Place 2 mL of 1 mol/L ammonia solution in a test tube. Add 2 mL of
0.5 mol/L MgCl2(aq) solution. The formation of a white precipitate
Mg(OH)2(s) is a positive test for hydroxide ion. Record your
observation.
Label this test tube Case X and save it for a comparison to be done in
the next part.
CH095 – A Study of Equilibrium
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3. Testing for NH4+. The presence of this ion in the ammonia solution
can be established indirectly by a consideration of equilibrium shifts.
Addition of NH4+ to the ammonia solution shifts the equilibrium to the
left:
NH3(aq) + H2O(l)
↔
NH4+(aq) + OH-(aq)
That is if the system is stressed by adding NH4+, then the equilibrium
shifts ←.
This will also lower the OH-(aq) ion concentration.
Place 0.5 g of solid NH4Cl in a test tube. Add 2 mL of 1 mol/L ammonia
solution and stir to dissolve the solid. Add 2 mL of 0.5 mol/L
MgCl2(aq) solution. Label the test tube Case Y and carefully compare
its contents to Case X. Record your observations.
B. NH4+(aq) + OH-(aq)
↔
NH3(aq) + H2O(l)
Place about 1 g of NH4Cl in a test tube. Add 3 mL of 6 mol/L NaOH(aq)
solution and stir until the solid dissolves.
1. Testing for NH3. Carefully waft the vapours from the test tube toward
your nose. Do you detect the odour of NH3(g)? Record the results.
Add 1 mL of 0.1 mol/L Cu(NO3)2, stir and record your observations.
2. Testing for OH-. Test for hydroxide with litmus paper. Record your
results.
C. NH3(aq) + HCl(aq)
↔
NH4+(aq) + Cl-(aq)
1. Place 1 mL of 0.1 mol/L Cu(NO3)2(aq) in a test tube. Add 1 mol/L
NH3(aq) drop by drop until a change is observed. Add 0.1 mol/L
HCl(aq) drop by drop until a change is noted. Record your
observations and retain this test tube for Step 2.
2. Add 10 drops of 0.1 mol/L HCl(aq) to the above test tube. Add 10
drops of 0.5 mol/L MgCl2(aq). Record your observations.