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7. Calculate the pHof the solution, at which the formation of Cu (OH)
2
crystals
begins, if the concentration of Cu
2+
ions in the solution is 0.01 mol/l?
K
S
(Cu
(OH)
2
) = 8.3 ∙ 10
–20.
Answer
: pH> 5.46.
8. 20 ml of a 0.05M solution of Cd (NO
3
)
2
were mixed with 30 ml of a
0.02M solution of K
2
S? Is there a precipitate of CdS?
K
S
(CdS) = 1.6∙10
–28.
Laboratory work
Experiment 1. Precipitation conditions
In two tubes take 1 ml of 0.01 mol/l Pb (NO
3
)
2.
Add 1 ml of 0.01 mol/l NaCl
solution to the first tube, and 1 ml of 1 mol/l NaCl solution into the second
tube. What are you watching? Based on the calculated data, explain why in one
case a precipitate falls out, and in another – no?
K
S
(PbCl
2
) = 1.7∙10
–5.
Write the
reaction’sequation.
Experiment 2. Determination of the degree of completeness of precipitation
The precipitate obtained in the first experiment is filtered out. Add 1 drop
of NaCl solution 1 mol/l to the filtrate to determine the completeness of the
precipitation of lead (II) chloride, then add 5 drops of Na
2
SO
4.
Explain the
formation of a white precipitate of PbSO
4.
Write the reaction’s equation. What
other ions can precipitate lead ions after the precipitation of lead chloride?
Write a short explanation.
Experiment 3. Precipitation and dissolution conditions
Pour 1 ml of MgCl
2
solutionin a volumetric test tube, then add 1 ml of NaOH
solution. The obtained precipitate is divided into two test tubes. Add 1 ml of
HCl solution to the first test tube, in the other – 1 ml of NH
4
Cl solution. What
are you watching? Write the equations for the dissolution of precipitation.
Pour 1 ml of BaCl
2
solutionin a volumetric test tube, then add 1 ml of
Na
2
SO
4
solution. The obtained precipitate is divided into two test tubes. Add
1 ml of HCl solution to the first test tube, in the other – 1 ml of NH
4
Cl solution.
What are you watching? Give an explanation.
LESSON 6.
BASICS OF CHEMICAL THERMODYNAMICS.
THERMOCHEMISTRY. THE LAWOFHESS. ENTROPY. GIBBS ENERGY
Required base level.
Thermal effect of chemical reaction. Exothermic and endothermic reactions.
Types of thermodynamic processes: isothermal, isobaric, isochoric.