Curious about precipitates

[mburt]

1. The problem statement, all variables and given/known data

An example of a precipitation reaction: Aqueous silver nitrate (AgNO3) is added to a solution containing potassium chloride (KCl) and the precipitation of a white solid, silver chloride is observed. (Zumdahl, 2005)

The silver chloride (AgCl) has formed a solid, which is observed as a precipitate.

This reaction can be written emphasizing the dissociated ions in a combined solution. This is known as the ionic equation.

Ag+ (aq) + NO3− (aq) + K+ (aq) + Cl− (aq) → AgCl (s) + K+ (aq) + NO3− (aq)


2. Relevant equations

none.

3. The attempt at a solution

Okay, so I know that the AgCl(s) precipitate forms but how come the K+ and the NO3- do not combine to form KNO3(s)? And how would I know that KNO3(s) is not the formed precipitate?

[QuarkCharmer]

You know because of the "rules of solubility". They give you a general idea of how likely something is to form a precipitate, depending on it's ions. It's a little more complex than that, has to do with molecular geometry somewhat, but the rules of solubility are how you can predict that you will get Silver Nitrate from that etc.

[Borek]

Rules of solubility are kind of a rule of thumb, used to predict what will happen. Thermodynamics behind is sometimes rather complicated, but it is always result of a difference of stability - sometimes solid salt is more stable, sometimes its solution is more stable. Solids are stabilized by the interactions between ions (or molecules) they are composed of, solutions are stabilized by the increase of entropy and interactions between ions and water molecule dipoles. It is combination of these factors that yields final effect - precipitate, or lack of one.

[Mapes]

Okay, so I know that the AgCl(s) precipitate forms but how come the K+ and the NO3- do not combine to form KNO3(s)? And how would I know that KNO3(s) is not the formed precipitate?

If you're familiar with equilibrium constants: the value for AgCl is <10-9, while the value for KNO3 is >1. If follows that the first will barely dissolve (i.e., will precipitate when given a chance), while the second will readily dissolve. You can look up tabulated values for essentially any salt.