Studio 9a

10/30/06

How do the Precipitation Reactions Differ?

• What assumption did you make about the precipitation reactions?

You assumed that all three reactions went to completion….

Equilibrium

• The third precipitation reaction was an example of a system in equilibrium.

Pb+2(aq) + 2I-

(aq) ↔ PbI2(s)

 

• What is the first thing that comes to mind when you think of the word equilibrium?

Equilibrium does NOT mean that ALL things are equal.  In chemical terms, it means that the rate at which products are formed from reactants and the rate at which reactants are formed from products are equal.

What's the rate of reaction?For a simple reactions A B, rate = k[A].

Most reactions slow down as they proceed (go to completion) and as the concentration/s of the reactant/s decreases (the rate approaches zero.)

k is the rate constant which varies with individual reaction (reactant and product)

Forward and Reverse RatesReactions in equilibrium have both a

significant forward and reverse rate of reaction. A B

When equilibrium is reached, the rates of the forward and reverse reaction are equal AND are NOT equal to 0.

Forward reaction:  A → B

Reverse reaction:  B → A

Forward Rate = Reverse Rate

Pb+2(aq) + 2I-

(aq) ↔ PbI2(s)

How do you results support the assignment of equilibrium to the reaction shown:

Pb+2(aq) + 2I-

(aq) ↔ PbI2(s)