EXPERIMENT 1

CHEMICAL EQUILIBRIUM

Verification of the instructors

Name: Phạm Thị Phương Anh

MSSV: 20227361

I. SUMMARY
1.Chemical equilibrium.
Equilibrium is a state in which there are no observable changes as time goes
by. When a chemical reaction has reached the equilibrium state, the
concentrations of reactants and products remain constant over time, and there are
no visible changes in the system.
In thermochemistry, chemical equilibrium is a stable state, in which ⃤ Gsystem =
0.
The equilibrium constant is a dimensionless quantity; a certain ratio of reactant
and product concentrations at equilibrium and a constant temperature. For a
reversible reaction, in the solution, we often use Kc.
Ex: Equilibrium constant of follow equation:
Fe3+ + CNS - Fe(CNS)2+ (1)
Red color

[ ( ) ]
𝐾𝑐 =
[ ][ ]

Kc depends on temperature and the nature of substances, does not depend on

concentrations.
2. Factors that influence on chemical equilibrium.
* Le Chatelier’s principle:
If a change of conditions is applied to a system at equilibrium, the system shifts
in the direction that reduces the stress to more toward a new state of equilibrium.
a. Changes in concentration.
If we increase concentration of reactants, the system adjusts in such a way so
that its concentration decreases and the forward reaction favors.
Ex for (1): If we add FeCl3 or NH4CNS to the solution (1), the equilibrium
shifts from left to right (the red color of the solution (1) deepens). The forward
reaction favors, that means more Fe(SCN)3 is formed.
 b. Changes in temperature.
If we increase temperature, equilibrium shifts to endothermic reaction and if
we decrease the temperature, equilibrium will shift to exothermic reaction.
Ex: N2O4 2 NO2 (2)
The forward reaction is endothermic, the reverse reaction is exothermic so: if
we increase the temperature, equilibrium will convert to the forward reaction and
the dark brown color deepens and if we decrease the temperature equilibrium will
shift to the reverse reaction and the dark brown color lightens.
3. Equilibrium in weak acids and bases.
a. Equilibrium in weak acids.
For example, in CH3COOH solution, there is a balance:
CH3COOH + H2O CH3COO- + H3O+ (3)
Because the concentration of H3O+ is more than 10-7M so Methyl orange
changes to red – orange color.
If we add CH3COONa to this solution:
CH3COONa → CH3COO- + Na+
Equilibrium (3) will shifts to the reverse direction and the concentration of
H3O+ will decrease so the red – orange color will change to light orange.
b. Equilibrium in weak bases. For example, there is a balance:
NH3 + H2O NH4+ (4)
This is a base solution so phenolphthalein will change to pink color.
If we add NH4Cl to this solution
NH4Cl → NH4+ + Cl-
The equilibrium (4) will shift to the reverse direction and pink color lightens.
4. Equilibrium of poorly soluble electrolyte.
a. Solubility product constant (T or Ksp)
An electrolyte is even called poorly soluble or insoluble, however, it can
dissolve partially to form the corresponding ions and therefore to reach the
equilibrium state between precipitate and ions in the solution.
Ex: CaCO3↓ Ca2+ + CO32- (5)
BaSO4↓ Ba2+ + SO42- (6)
CaSO4↓ Ca2+ + SO42- (7)
In the saturated solution, the dissociation reaction reaches the equilibrium state,
the product of poorly soluble electrolytic ions con centration in the solution is a
constant, it is called solubility product and abbreviated to Tt
[Ca2+][SO42-] = TCaSO4 = 6,1.10-5
[Ba2+][SO42-] = TBaSO4 = 1,1.10-10
[Ca2+][CO32-] = TCaCO3 = 4,8.10-9
The less solubility of the electrolyte, the less value of Tt
 Tt depends on the nature of poorly soluble electrolyte and temperature, it does
not depend on the concentration.
b. Condition to form precipitate is the product of poorly soluble electrolyte
ions concentration in the solution must higher than solubility product.
Ex: [Ca2+][SO42-] > 6,1.10-5
[Ba2+][SO42-] > 1,1.10-10
[Ca2+][CO32-] > 4,8.10-9
c. Condition to dissolve poorly soluble electrolyte is the product of poorly
soluble electrolyte ions concentration must less than solubility product.
Ex: [Ca2+][CO32-] < 4,8.10-9
5. Hydrolysis of salt.
a. Definition: Hydrolysis of salt is a reaction of weak acid anion with water
or weak base
cation with water which changes the pH of solution.
b. Characteristic of salt hydrolytic reaction.
- Just weak acid anion and base cation in salt is hydrolyzed. Strong acid
anion and strong base cation in salt is not be hydrolyzed.
- Hydrolysis is a reversible reaction, it obeys chemistry equilibrium rules. c.
Types of hydrolysis.
- Salt which is made from weak acid anion and strong base cation, weak acid
anion will be hydrolyzed to form OH –
Ex: CH3COONa CH3COO- + Na+
CH3COO- + H2O CH3COOH + OH-
Na2CO3 2Na+ + CO32-
CO32- + H2O HCO3- + OH-
- Salt which is made from weak base cation and strong acid anion, weak base
cation will be hydrolyzed to form H3O+.
Ex: NH4C NH4+ + Cl-
NH4+ + H2O NH3 + H3O+
- Salt which is made from weak base cation and weak acid anion, both anion
and cation will be hydrolyzed.
II. EXPERIMENTAL SECTION.
1. Factors that affect chemical equilibrium.
1.1. Changes in concentration.
Look at this reaction:
Fe3+ + CNS - Fe(CNS)2+
Pour into a 100 ml beaker about 20 ml distilled water, add 1 drop of FeCl3
(saturated solution) and 1 drop of NH4CNS (saturated solution). Share the
solution to four test–tubes, each tube have about 1 ml (about 10 drops or 1 cm of
the height of solution in the test–tube).
The first test-tube is remaining to comparison.
 The second test-tube is added 1-2 drops of saturated FeCl3.
The third test-tube is added 1-2 drops of saturated Nh4CNS.
The forth test-tube is added some crystal of NH4Cl.
Observation and compare the color of solution in four test-tubes. Explain it.
1.2. Changes in temperature.
Let’s see the influence of temperature on this reaction:
2 NO2 N2O4
Using two connected test-tube which contain NO2 (the color is
brown), open the lock (K) to have the same color in two test-tubes
then close the lock. Put test-tube number 1 into ice water with
white salt. Test-tube number 2 is remained to observe. Look at the
changes in color of test-tube number 1 in the cold water, after that put it into hot
water and observe the color changes. Use Le Chatelier’s principle to consider
that reaction is endothermic or exothermic.

2. Equilibrium in electrolytic solution.

2.1. Color of indicators in various environments.
Take 3 test-tubes
N1: 10 drops of 2N H2SO4
N2: 10 drops of distilled water
N3: 10 drops of 2N NaOH
Put a piece of litmus paper into each test-tube. Observe the color in each test-
tube
Do the same but replace litmus paper by phenolphthalein and methyl orange.
Write down the result into the table:
Color indicator Color of indicator in various environments
Acid Neutral Base
Litmus paper
Methyl orange
Phenolphthalein
Note: use only one drop of indicators.

2.2. Equilibrium in week acids and weak bases.

a. Weak acids: Put 2 ml (20 drops) of Acetic acid 2N (CH3COOH) into test-
tube, add 1 drop of methyl orange then share the solution into 2 test-tubes.
N1: Remain to observe
N2: Add some CH3COONa crystals; shake the test-tube to dissolve them.
Observe and compare the color in 2 test-tubes. Explain it.
 b. Weak bases: Put 2 ml (20 drops) of 2N NH3 to test-tube, add 1 drop of
phenolphthalein. Observe the color then share the solution into 2 test-tubes.
N1: Remain to observe
N2: Add some NH4Cl crystals; shake the test-tube to dissolve them.
Observe the change in color of the solution and explain it.
2.3. Poorly soluble electrolyte
a. Condition to form precipitate.
Put into 2 test-tubes as follow:
N1: 5 drops of 0.1M CaCl2 and 5 drops of saturated BaSO4 solution.
N2: 5 drops of 0.1M BaCl2 and 5 drops of saturated CaSO4 solution
In which test-tube wil have precipitate? What is it? Explain it by calculating
each case in detail. (TBaSO4 = 1.1x10-10 ; TCaSO4 = 6.1x10-5)
b. Condition to dissolve precipitate
Prepare CaCO3 by adding into a test-tube 10 drops of 0.1M Na2CO3 and 10
drops of 0.1M CaCl2. Add slowly drop by drop of HCl 2N into precipitate which
we’ve just had. Observe and explain what happen did. Write down the chemical
equation.
2.4. Hydrolysis of salt
Take 2 test-tubes; put some NH4Cl crystals into N1 and some CH3COONa
crystal into N2. Add 2 ml (20drops) of distilled water into each test-tube.
Use pH paper to measure the pH values of these solutions, which environment
these solutions belong to (acid or base). Explain and write down the chemical
equation of hydrolysis reaction.