We Actually Could Make Mathematical Expressions of the Reaction Rate

The Ch. 19. Day 1

- In the lab we see how different things change the reaction rate. Both temperature and concentration effect how quickly a reaction occurs. - We actually could make mathematical expressions of the reaction rate. - For the reaction aA + bB cC + dD the rate expression might be: Rate = k [A]a[B]b (if this reaction is one step) (Here things in [ ] are concentrations in molarity and k is a temperature dependent constant) - Here would be a great place for some tough mathematics on reaction rates … but we’re going to use this information to study equilibrium reactions. - Equilibrium reactions are reversible reactions - For an equation to be at equilibrium the rate of the forward reaction is equal to the rate of the reverse reaction.

- N2(g) + 3H2(g) ⇌ 2NH3(g)

What equilibrium does mean

What equilibrium doesn’t mean

- We derive an equilibrium expression - For the reaction aA + bB cC + dD

[C] c [D] d a b the equilibrium expression Keq = [A] [B]

brackets [] once again mean molarity a,b,c, and d came from the coefficients in the balanced equation to be included in the expression it must be a (g) or (aq) not a (s) or a (l) Now we can do lots and lots of incredibly fun problems!!!!!! 1) Write equilibrium expressions for the following reactions:

2SO3(g) ⇌ 2SO2(g) + O2(g)

4NH3(g) + 3O2(g) ⇌ 2N2(g) + 6H2O(l)

2) Hydrogen gas and carbon dioxide gas set up an equilibrium with carbon monoxide gas and water vapor. Calculate the value of the equilibrium constant, Kc , for the system, if 0.1908 moles of CO2, 0.0908 moles of H2, 0.0092 moles of CO, and 0.0092 moles of H2O vapor were present in a 2.00 L reaction vessel were present at equilibrium.

3) Given the balanced equation, the K value and the partial set of equilibrium concentrations shown: -2 CH3OH  CO + 2H2 K=1.59*10 Equilibrium Concentrations: CO = 0.105M, H2 = 0.212M What is the equilibrium concentration of CH3OH? The reaction quotient (Q) is used to determine if a reaction is at equilibrium, and if is not, which way the equilibrium must shift (side to favor) to reach equilibrium. - In these types of problems molarities of all the species in the equilibrium expression must be given. Also the value of K must be given. - Substitute the molarities into the equilibrium expression… the value you get is Q not K. - Then analyze: - If Q = K then the reaction is at equilibrium - If Q > K then the reaction must shift towards the left - If Q < K then the reaction must shift towards the right EX: More fun problems!

Example: 0.035 moles of SO2, 0.500 moles of SO2Cl2, and 0.080 moles of Cl2 are combined in an evacuated 5.00 L flask and heated to 100oC. What is Q before the reaction begins? Which direction will the reaction proceed in order to establish equilibrium?

o SO2Cl2(g) SO2(g) + Cl2(g) Kc = 0.078 at 100 C

- Le Chatlier’s Principle:

- Things that effect an equilibrium: - Concentration

- Pressure

- Heat

- Catalyst

Look one last time at the Haber process:

N2(g) + 3H2(g) ⇌ 2NH3(g) H = -92.4 kJ how could we increase the yield of ammonia? PROBLEM SET 19A

Write equilibrium expressions for the following reactions:

a) 2SO3(g) ⇌ 2SO2(g) + O2(g)

b) carbon and water vapor reach an equilibrium with carbon monoxide and hydrogen gases

c) 4NH3(g) + 3O2(g) ⇌ 2N2(g) + 6H2O(l)

d) dinitrogen tetroxide gas is in equilibrium with nitrogen dioxide gas

e) Fe3O4(s) reacts with hydrogen gas to form an equilibrium with solid iron and water vapor

1) What is the K of equation 1a) if at equilibrium [SO2] & [O2] = .020 M, and [SO3] = 1.46 M

2) At 600 C the K for equation 1c) is 80. What is the equilibrium concentration of ammonia if [O2] = 0.5 M, [N2] = 1.05 M, and there is 25 ml of water at equilibrium.

3) a) If the above equation had concentrations (not at equilibrium) of 1.0 M for all the relevant reactants and products which way would the equilibrium go to reach equilibrium.

b) What if all the relevant concentrations were 0.4 M?

4) If reaction 1c) had a H = -1530.4 kJ how would the following stresses shift the equilibrium (L-left, R- right, N-no effect) _____a) take away water _____b) adding water _____c) adding a catalyst _____d) cool the reaction _____e) increasing the pressure _____f) remove ammonia _____f) add nitrogen _____g) adding heat

5) The H for 1a) is 98.9 kJ. Name five ways to increase the amount of SO2 in the equilibrium. 1)______4)______2)______5)______3)______