Textbook assignment: Read Kotz and Triechel, Chemistry and Chemical Reactivity Chapter 3: Sections 4 to 5.
3.4 Reactions in Aqueous Solutions. Aqueous solutions are composed of solutes dissolved in a solvent to form a solution.
Chemists sometimes classify ionic reactions as
Depending on the actual products, an aqueous reaction may additionally be classified as gas-producing, or a precipitation (solid-producing) reaction.
The dissolution of the solute usually results in charged ions (electrolytes), which can then be used to conduct electricity through the solution. We'll come back to this important area when we study electric cells (batteries) later in the year. Not all ionic solids dissolve equally well in water. Solids that dissolve poorly produce few ions, so they conduct electricity weakly, and are called weak electrolytes with low solubility). Solids that dissolve more completely produce many ions that can conduct electricity, so these are strong electrolytes with high solubility. Solids that do not dissolve, or that dissolve to uncharged particles won't conduct electricity at all, and are considered non-electrolytes.
Solubility is an important factor in determining whether a chemical reaction will procede, and depends on many factors, including the temperature of the solution, and the concentration levels of different ions. There are some general guidelines for solubility represented in diagram 3.10, which you may want to copy off before you do the homework.
3.5 Precipitation reactions. Precipitation reactions occur when solutions of dissolved ions are combined and result in the formation of ionic bonds between pairs of ions that are stronger than the attraction of the water molecules for the separate ions, with the ionic compound "falling out" of the water as a solid. When not all of the types of ions available participate in a precipitation, the non-participants can be ignored, and only the participants are included in the net ionic reaction. Learn to use the solubility table so that you can recognize which of the available ions are "in" or "out" of the reaction. [In later chapters, we will have more precise rules for determining solubility.]
IMPORTANT NOTE! Remember when balancing reaction equations that you cannot change the formula for a specific molecule. Take the conversion of oxygen and hydrogen gas to water: H2 + O2 -> H2O. To get two oxygens on both sides, we double the water molecule and the hydrogen molecule, not the oxygen molecule in water:
2H2 + O2 -> 2H2O is correct
H2 + O2 -> H2O2 is wrong: this isn't water, but peroxide!
There are no formulae for these sections, but you should become familiar with which compounds are soluble and which are not.
|Solubile Compounds||Exceptions (Precipitates will form)|
|Salts of Na+, K+, NH4+||None|
|Salts of nitrates, chlorates, acetates||None|
|Salts of halogens (Cl, Br-, I-)||Ag+, Hg22+, PB2+|
|Salts of halogen F-||Mg2+, Ca2+, Sr2+, Ba2+, Pb2+|
|Salts of sulfate SO42-||Ca2+, Sr2+, Ba2+, Pb2+, Ag+|
|Insoluble Compounds||Exceptions (no precipitates will form)|
|Salts of carbonate, phosphate, oxalate, chromate, sulfide||All NH4+ and alkali metals (Group I)|
|Metal oxides and hydroxides||Alkali metal hydroxids, plus Ba(OH)2 and Sr(OH)2|
When in doubt, check!
Read the following weblecture before chat: Net Ionic Equations and Precipitation Reactions
Review the Videos at Thinkwell Video Lessons under REACTIONS IN AQUEOUS SOLUTIONS.
Determination of water in copper sulfate pentahydrate and sodium hydrogen carbonate
What is the Amount of Water in Common Hydrates?
For this week: Based on the references below, and your own equipment and materials, devise an appropriate dehydration method to determine the amount of water in a common hydrated salt such as copper sulfate pentahydrate, and justify formula predictions.
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