Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 107

Fundamentals of Chemistry

Fall 2009

Lecture Notes: 22 September

© R. Paselk 2005
 
     
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Solution Concentrations, cont.

Concentration Terms

Molarity:

The most commonly used concentration term in chemistry is molarity, M = 1 mole of solute dissolved in 1 L of solvent.

First recall that Chemical Equations involve descriptions of chemical processes in which mass is conserved and therefore atoms are conserved.

Also want to note what is observed when chemical reactions take place. Need to pay attention and be observant. Some common observations indicating a chemical reaction has taken place include:

I also want to look at reactions from the perspective of what is happening at the atomic and molecular levels where reactions occur because of the associations of ions and molecules with each other to form precipitates and complexes, or atoms and/or molecules exchange electrons in oxidation/reduction or redox reactions.

Ionic reactions - dissolving and precipitates

Much of the chemistry around us involves the dissolution of ionic solids in water to give aqueous solutions and the precipitation of ions from aqueous solution to give precipitates (solids). So what I would like to do first is to look a little at the process of dissolving and the nature of aqueous solutions.

First we need to look a bit a water itself. (models, overhead) The thing we need to keep in mind is that the ions in water are not independent - they dissolve because they substitute interactions with water molecules for interactions with counter ions. And they stay in solution because they are insulated from each other by the water "shells" around each ion. A couple of corollaries

Let's consider some chemical processes:

 

Na+(aq) + Cl-(aq) + Ag+(aq) + NO3-(aq)  right arrow AgCl(s) + Na+(aq) + NO3-(aq)

Net Ionic Equations

Notice that two ions don't change, so why show them. Instead we write a net ionic equation:

 Ag+ + Cl- right arrow AgCl(s)

Notice the ions that appeared on both sides are not shown (in mathematical terms they canceled)

Ba2+(aq) + 2 Cl-(aq) + 2 K+(aq) + SO42-  right arrow BaSO4(s) + 2 K+(aq) + 2 Cl- (aq)

Again, we want to write a net ionic equation showing only the ions which reacted:

Ba2+ + SO42-  right arrow BaSO4 (s)

Notice that net ionic equations are very general expressions. Essentially they are saying that any time we have these species present they will react, regardless of what else happens to be there! (Sometimes folks are confused when they add ions which should react and they don't. This is usually a case where something else reacted first, so the ions of interest really weren't there!).


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Last modified 24 September 2009