Today's class started with answering questions about why we need to maintain certain experimental conditions in Group I of Experiment #25. The general procedural details of this experiment are an excellent precursor to the topics we will discuss next week about manipulating solubility. Right now I want you to realize that the temperature, pH, and the concentration of the reagent used can play a big role on how soluble a substance is.
We have been discussing various aspects of solubility, but how can we predict whether a precipitate forms when we add two solutions together? In chemistry 121 the solubility rules were our guide for this, but these guidelines were not very specific about what concentration must be used to facilitate a precipitation reaction. We can selectively separate when various precipitates form by manipulating the concentrations of ions that are mixed together. The reaction quotient, Q, is obtained when concentrations of reactants are inserted into an equilibrium expression. If we are at equilibrium we have a special case of Q that we refer to as K. I performed an example of Q vs. K by analyzing solutions of silver nitrate and potassium chloride.
If you are asked to predict if a precipitate will form you need to calculate Q and compare it to K. In general, if:
Q < K no precipitate forms
Q = K the solution is at equilibrium
Q > K a precipitate will form
We can use this concept to analyze the ions which are contained in Group II and Group III. Over the weekend take a look over pages 51-52 of your lab manual and we will start of lecture on Monday by calculating the pH to determine in a particular ion will be contained in Group II or Group III.
Songs played before class:
Biz Markie - Just a Friend
Run This Town: Jay Z feat. Rihanna and Kanye West
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