Chemical Systems and Equilibrium

Learning Goals:

• Explain the concept of dynamic equilibrium, using examples of physical and chemical equilibrium systems.

• Explain the concept of chemical equilibrium and how it applies to the concentration of reactants and products in a chemical reaction at equilibrium.

• Create and complete an ICE table for an equilibrium system.

• Draw graphs of c vs. t to illustrate a chemical system approaching equilibrium.

• Use appropriate terminology such as dynamic equilibrium, reversible reaction, closed system, equilibrium concentrations, phase equilibrium, solubility equilibrium, chemical equilibrium, equilibrium position

Dynamic Equilibrium• rate of forward reaction = rate of reverse reaction• an observable macroscopic property remains constant

How do the following images represent

dynamic equilibrium?

Dynamic Equilibrium?•two kids balanced on a teeter-totter•player substitutions during a hockey game•outdoor pool•indoor pool•your own idea?

summer haze

Consider the following…

Over time, what happens to an open beaker of water?

Over time, what happens to the water level if we cover the beaker?

We can study equilibrium under closed systems!

Write an equation to describe the equilibrium system.

What conditions are required for a dynamic equilibrium to exist?

• reaction is reversible• closed system

What are the key characteristics of a dynamic equilibrium?

• forward rate = reverse rate• observable macroscopic property remains constant

3 types of Equilibrium

1. Solubility Equilibrium 2. Phase Equilibrium3. Chemical Equilibrium

NaCl(s) Na+ (aq) + Cl- (aq)

Chemical Equilibrium

Write an equation to describe this equilibrium system.

Initially, N2O4(g) is in the flask. Draw a graph to illustrate the changes in concentration of R and P as the system approaches equilibrium.

Does slope = 0 mean the reaction has stopped?

Are the concentrations of R and P equal at equilibrium?

Draw graphs to illustrate how equilibrium can be reached from either direction.

Use an ICE table to discover how concentration changes as the system approaches equilibrium.

Create and complete an ICE table for the following chemical system.

If 0.80 mol of HI are placed in a 0.50 L flask, predict how theconcentrations will change as the system approaches equilibrium. Write algebraic expressions for the [ ]eq.

Sample Problem 1 (p. 425)

• The initial concentrations of H2(g) and F2(g) are 2.0 M. What are the equil. conc. of H2(g) and HF(g) if the equil. conc. of F2(g) is 0.48 M?

Sample Problem 2 (p. 426)

• A chemist adds 4.0 mol of ammonia gas to a 2.0 L sealed container and heats it. Fig 7 shows the changes in the amount of NH3(g) observed over time. Determine the equil. conc. of N2(g)and H2(g).

• Practice Problems – p. 427 #1-3

Equilibrium Position• The relative concentrations of R and P at equilibrium

is called the equilibrium position.• Think of running up the down escalator. Can you

reach a state of dynamic equilibrium at different positions on the escalator?

• How does the equil. position relate to the solubility of a solute?

• What does a brown haze over a city in the summer indicate about the equil. position of this system? N2O4(g) 2 NO2(g)

• What happens in the winter?

Equilibrium Position lies… and favours …

extent of reaction

reaction is… arrow

very far to the right,products

100% complete

to the right,products

>50% reversible

in the middle,neither R or P

=50% reversible

to the left,reactants

<50% reversible

very far to the left,reactants

<1% no reaction NR

Draw a bar graph to illustrate the relative concentrations of R and P for each equilibrium position.

Dynamic Equilibrium

BEWARE! 3 Common Misconceptions: 1.Concentrations of reactants and products

are equal @ equilibrium – equilibrium position?

2. Equilibrium can only be approached from 1 side

3. The forward and reverse reactions stop @ equilibrium… Macroscopically there are no changes, but what about the molecular level?

Homework:

• Define key terms• p. 428 #1-6