Gas Equilibria

Unit 10

Equilibrium

• Reactions are reversible• Reactants are not consumed• Equilibrium mixture containing both

products and reactants is obtained.• @ Equilibrium both (forward and reverse)

reactions are taking place at the same time.

Equilibrium

• The rate of the forward reaction = rate of the reverse reaction.

• K = [products]^ coefficients

[reactants]^ coefficients

Equilibrium• K is the equilibrium constant; it shows the

relationship between partial pressures of different GASES present at equilibrium

• We need the partial pressure of each species– PV=nRT– Pressure must be in atms– R value will 0.0821– V in L– T in Kelvins

At equilibrium

• Partial pressures remain constant as long as volume of container and temperature remain the same.

• Kp is equilibrium using partial pressure• Kc represents concentration in moles/L

(This is when we are looking at aqueous solutions and ions)

• Kp= Kc(RT)∆ng ng= moles of gases

At Equilibrium

• For the most part, we are only going to be concerned about Kp so when asked to solve for K it is asking about Kp

• When writing K expressions, you only include gases and aqueous solutions or ions.

Writing K expressions• Let’s look at page 344

– #6– #8– #12

– HW#1

Solving for Kc

• N2 (g) + O2 (g) ↔ 2NO (g)

• Kp= 0.0255• T= 123oC

• Solver for Kc

• On problems 37-40 you are just solving for Kc. Do not follow the directions.

• HW#1

Solving for K

• N2 (g) + O2 (g) ↔ 2NO (g)– P N2= 0.345 atm

– P O2= 0.765 atm

– P NO = 1.34 atm

– HW#2

Le Chatelier’s Principle

• When a change (that is, a change in concentration, temperature, pressure, or volume, pH, etc) is imposed on a system at equilibrium, the system responds by attaining a new equilibrium condition that minimizes the impact of the imposed change.

• I.e., the system will shift to undo the change

Concentration

• If you increase the concentration of any species, the reaction goes in the direction to get rid of it. (Solids, liquids=no effect)

• A + B n C + D• Add A, • Add C, • Add E, since not in the equilibrium, no

effect!

Pressure

• When the pressure of a system increases, to reduce the pressure the equilibrium will shift toward the side with the least moles of gases.

• Decrease pressure, add more moles

Volume

• When the volume of the system increases, we need to fill it back up. The equilibrium will shift toward the side with more moles of gas.

• Decrease volume, decrease moles.

Temperature

• The effect of temperature depends on the sign of DH. Treat temperature like a chemical: add heat, go to the cool side.

• If DH is negative, products are exothermic (Hot side).

• If DH is positive, products are endothermic (Cool side)

Reaction Quotient

• Q = [products]^ coefficients

[reactants]^ coefficients

This relationship can determine if a system is at equilibrium.

It will also be used to predict direction to shift the system.

Q vs K

• If K < Q, the reaction R P• Because for Q to be too large, there is

presently too much product. To undo product, the reaction arrow shifts to make more reactant. Vice versa….

• If K > Q, the reaction R P• Q is too small because there isn’t enough

product.

CO + 3H2 n CH4 + H2O

• Kp = 102 at 773 K• The following are introduced into a flask at

773K: 0.902 atm CO, 1.794 atm H2, 18.38 atm CH4 and 27.57 atm H2O. In what direction will the reaction occur to reach equilibrium?

CO + 3H2 n CH4 + H2O

• Kp = 102 at 773K• The following are added to a 15.5 L

reaction vessel at 773K: 25.2 g CO, 15.1 g H2, 130.2 g CH4 and 125 g H2O. In what direction will a net reaction occur to reach equilibrium?

CO2 + CF4 n 2COF2

• Kc = 0.50 at 1000oC• If a 2.50 L reaction vessel at 1000oC

contains 0.525 mol CO2, 1.25 mol CF4 and 0.75 mol COF2, in what direction will a net reaction occur to reach equilibrium?

• HW#3

I C E• When an equilibrium shifts, it MUST do so

STOICHIOMETRICALLY! The coefficients determine the amount and the arrow determines the direction.

• I -- Initial concentration or pressure

• C -- Change by coefficients

• E -- Equilibrium is established• YouTube - Ice Ice Tables

CO + H2O n CO2 + H2

• Kp = 23.2 at 600K• If 0.250 mol of CO and H2O are introduced

into a 1 L flask and equilibrium is established, how many moles of each will be present at equilibrium?

C (s) + H2O n CO + H2

• Kp = 0.111 at 1100 K• If 0.100 atm H2O and 0.100 atm of H2 are

mixed with excess carbon in a 1 L flask initially, what will be the equilibrium concentration of each?

CO + Cl2 nCOCl2

• The Kp = 21.88 at 395oC.• Initially, 20.0 g CO and 35.5 g Cl2 are

placed in an 8.05 L reaction vessel at 395oC. What is the partial pressure of each at equilibrium?

H2 + I2 n 2HI

• At 430oC, the Kc = 54.3.• A 1.0 L vessel is charged at this

temperature with 1.4 atm of each. What is the partial pressure of each gas?

CO2 + CF4 n 2COF2

• The Kp at 1000oC is 0.50.• The vessel is charged initially with 0.45

atm of carbon dioxide and carbon tetrafluoride and 1.2 atm of COF2. What is the partial pressure of each gas?

NO + CO2 n NO2 + CO

• Initially a vessel is charged with 3.9 moles of NO and 0.88 moles of CO2. At equilibrium, there is 0.11 mol of CO2.

• What is the Kp?• What is Kc?• In another experiment, all gases are

initially at 1.5 atm. What is the equilibrium pressure?

Van’t Hoff Equation

• Ln K2 – ln K1 = -∆H/R (T2-1-T1-1)• lnK2/K1= -∆/R(T2-1- T1-1)

• R= 8.31

∆H= Standard Enthalpy

∆G = -RTlnK ***

Van’t Hoff Equation

• For the reaction, H2 + Br2 → 2HBr the Kp is 2.18 x 106 at 730oC. What is the Kp at 1000oC (you must calculate ∆H first)?

Van’t Hoff Equation

• The hydrogenation of octene to octane is an important process in refining gasoline. The K value for the reaction is 1.7 x 1015 at 298 K and is only 3.2 at 900K.

• (a) What is the ∆H for this reaction?• (b) What is the∆G for this reaction at

298K?

Adding Chemical Equations

• Reaction 1= K1• Reaction 2 = K2• Reaction 3= K3 (overall reaction)

• K3 = K1 * K2

Adding Chemical Equations

• Calculate K for the reaction:• SnO2 (s) + 2CO → Sn (s) + 2CO2• Given• SnO2 (s) + 2H2 → Sn (s) + 2H2O K = 8.12• H2 + CO2 → H2O + CO K = 0.771