CHEMISTRY II
NAME
1
The Significance of Solubility to Chemical Reactions
Introduction
What do geothermal vents have in common with a bathtub ring? The vents spew intensely hot clouds
of mineral-rich water deep from inside the Earth into the ocean and help support an intriguing ecosystem. A
bathtub ring, the bane of clean-freaks around the world, is a deposit formed from the interaction of hard
water and soap. Both involve the process of precipitation, the formation of insoluble or only slightly soluble
solids. When oppositely charged ions come in contact, they attract each other, and if that attraction is
stronger than the ions’ attraction to water, they form crystalline solids.
When two different ionic solutions with concentrations below their saturation points are combined
and a precipitate forms, they have undergone a double replacement reaction in which one of the products is
insoluble. The reaction of aqueous solutions of calcium chloride and zinc sulfate, for example, combines Ca +2
ions and SO4- 2 ions in a concentration above the saturation point of calcium sulfate. The formation of the
precipitate is described by the following molecular and net ionic equations:
CaCl2(aq) + ZnSO4(aq) → ZnCl2(aq) + CaSO4(s)
Ca +2(aq) + SO4- 2(aq) → CaSO4(s)
Insoluble salts can be identified by their low Ksp values (equilibrium dissociation or solubility product
constants). The identity of precipitates can also be deduced from the results of combining pairs of salt
solutions, as you will do in this investigation. A comparison of the products from the combinations allows for
the identification of any precipitates that form. Trends, called solubility rules, can also be found for some ions
that tend to form precipitates more readily than others.
In this investigation, you will combine pairs of six given salt solutions and look for precipitates. You
will write molecular equations for the reactions and try to deduce which products comprise the precipitates.
You will write complete ionic and net ionic equations for the reactions that result in precipitates. And you
will familiarize yourself with several common solubility rules.
But before you begin, you will investigate the need for the salts to be in solution form. What is it about
solutions that make them more conducive to chemical reactions than the solid phase? The following online
resources will provide you with some insight before delving into the world of solutions.
http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/molvie1.swf
1. Be sure to click zoom. Draw an illustration of what happens when NaCl dissolves in water below.
http://www.mhhe.com/physsci/chemistry/animations/chang_7e_esp/crm3s2_3.swf
1. Click to mix NaCl and AgNO3. A precipitate will form and it will be AgCl. View the animation.
2. Investigate what happens when you mix NaCl and KOH. Does a precipitate form?
http://cwx.prenhall.com/petrucci/medialib/media_portfolio/text_images/015_ELECTANDNON.MOV
1. Why do you think solid NaCl is unable to conduct electricity while dissolved NaCl can? Be specific!
http://www.chem.vt.edu/RVGS/ACT/notes/Notes_on_Net_ionic_rxns.html
1. What is a net ionic equation?
2
Pre-Lab Questions
1.
How many products are there in a double replacement reaction?
2.
What visual clues can indicate the formation of a precipitate?
3.
Why were the solutions for the experiment prepared using distilled water?
CHEMISTRY II – UNIT 7 – LAB #1
NAME
3
Materials
chemical splash goggles
aprons
nitrile gloves
well plates with 6 and 24 wells
distilled water
stirrer
conductivity tester
pipettes
solutions:
potassium carbonate
magnesium chloride
copper(II) sulfate
sodium nitrate
silver nitrate
sodium phosphate
sodium sulfite
iron(II) sulfate
Procedures
1.
Put on your goggles. Obtain materials for the lab. Most of the materials are prepared for each group.
You will need to share the silver nitrate and the solids.
2.
Using the 6-well plate, put 2 small scoops of iron(II) sulfate in one well. Likewise, put 2 small scoops
of sodium sulfite in another well. Test each with the conductivity tester. Record your results.
3.
In a third well, put another 2 scoops of iron(II) sulfate and sodium sulfite. Mix well, and test the
mixture with the conductivity tester. Record your observations (both visual and conductivity).
4.
Add about 2-5 mL of distilled water to the wells of the individual salts. Stir. Test with the conductivity
tester. Record observations and sketch a representation of the intermolecular interactions taking place.
FeSO4
Na2SO3
5.
Add about 1-2 mL of distilled water to the third well with the mixed salts. Stir. Record observations.
6.
Rinse your well plate thoroughly.
4
7.
Using your data matrix as a guide for conducting your reactions in the wells of the 24-well reaction
plate, combine solutions by placing 4 – 5 drops of each of two solutions in a designated well.
Record your observations. Observations should include how you determined if a reaction occurred.
No evidence of reaction = NR
If a reaction occurred, record specific observations:
Color change
Precipitation formation (could be an obvious solid or a slight cloudiness)
AgNO3
NaNO3
Na3PO4
K2CO3
MgCl2
CuSO4
MgCl2
K2CO3
Na3PO4
NaNO3
Questions and Conclusions
1.
Why did the iron and sodium salts not react until water was present? Your answer should describe
what is happening on the atomic/molecular level.
2.
On another piece of paper, write molecular, complete ionic, and net ionic equations for the reactions
that occurred in the 24-well plate. Include phase symbols.
3.
What are the solubility rules for the ions examined in this experiment?
a.
List all metal ions that were not part of any precipitate.
b.
List all anions that were not part of any precipitate.
c.
List all metal ions that occur only in products as precipitates.
d.
List all metal ions that sometimes form precipitates.