Unit 7: Chemical Equilibrium The Equilibrium Constant Before discussing equilibrium do the following demo with two students: PART ONE Set up a small table up front with two students across from one another Fill two 5000 mL beakers with blue-tinted water Have students transfer water to the other beaker using small Dixie cup Result: Water amounts will be roughly equal. Tell students this does NOT mean the system is in equilibrium – EQUILIBRIUM DOES NOT MEAN EQUAL AMOUNTS OF PRODUCTS AND REACTANTS. PART TWO Give one student a larger plastic cup Have students transfer water again, but do not allow the student with the larger cup to touch the beaker to get more water into the cup Result: Eventually, the amount of water in each beaker will stay roughly constant because of the amount the student with the larger cup can transfer. Amounts are not equal, but the amount into and out of each beaker is equal – THIS is equilibrium.

Chemical Equilibrium Many reactions are reversible – this means they can go in forward and reverse directions Chemical Equilibrium: the forward rate = the reverse rate (NOT when the concentrations are equal) Speed  Time  equilibrium Forward reverse

Equilibrium Constant Keq For a reaction at equilibrium: aA+bB ↔ cC + dD coefficients from balanced equation molarity Note: experiments show that equilibrium position does not depend on the amounts of pure solids and liquids present because these concentrations cannot change Write them as “1” in the Keq expression or leave them out!

Equilibrium Constant Keq Example Write the equilibrium constant expression for the following reversible reaction: H2 (g) + I2 (g) ↔ 2HI (g)

Equilibrium Constant Keq Practice #1 Write the equilibrium constant expression for the following reversible reaction: CaCO3 (s) ↔ CaO (s) + CO2 (g)

Equilibrium Constant Keq Practice #2 Write the equilibrium constant expression for the following reversible reaction: hydrogen gas + carbon dioxide gas ↔ carbon monoxide gas + water vapor

Equilibrium Constant Keq Practice #3 Write the equilibrium constant expression for the following reversible reaction: ammonia gas (NH3) + oxygen gas ↔ water vapor + nitrogen monoxide gas 4NH3 + 5O2 <-> 6H2O + 4NO

Equilibrium Constant Keq Practice #4 Write the equilibrium constant expression for the following reversible reaction: ammonia gas (NH3) + oxygen gas ↔ water vapor + nitrogen monoxide gas 4NH3 + 5O2 <-> 6H2O + 4NO

Equilibrium Constant Keq What does the equilibrium constant tell us? Remember that the equilibrium constant is roughly: Bigger value of Keq = product formation favored more Smaller value of Keq = reactant formation favored more

Equilibrium Constant Keq Worksheet – Additonal Problems 1. Which reaction favors product formation the most? 2. Which reaction favors reactant formation the most? 3. For problem #3: Determine [H+] if [OH-] = 0.037 M. 4. For problem #4: Determine [CO] if [O2] = 1.50 M and [CO2] = 0.250 M. 3. [H+] = 0.0065 M 4. [CO] = 0.577 M

Equilibrium Visualizations http://www.chem.arizona.edu/~jpollard/fido/fido.html

Equilibrium Book Problems Read Ch. 15 (some parts we won’t cover) Assigned: 15.2, 15.4, 15.6, 15.9, 15.16, 15.52