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Definition. Occurs when opposing reactions are occurring at the same timeWe've had physical equilibria beforedissolvingvapor pressureWe will use partial pressure for gases We will use molarity for solutes. Why partial pressure?. A(g) ? B(g)A(g) ? B(g) rate = kf[A]A(g) ? B(g)
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1. Chemical Equilibrium Chapter 15 in Brown & LeMay
2. Definition Occurs when opposing reactions are occurring at the same time
We’ve had physical equilibria before
dissolving
vapor pressure
We will use partial pressure for gases
We will use molarity for solutes
3. Why partial pressure? A(g) ? B(g)
A(g) ? B(g) rate = kf[A]
A(g) ? B(g) rate = kr[B]
For ideal gas equation, what is the concentration of the gas?
n/v = P/RT
4. At Equilibrium: Rate of forward = rate of reverse
kfPA/RT = krPB/RT
R and T are constant
Kf/kr = PB/PA
This is called the equilibrium constant
5. The constant Use this as an example:
N2(g) + 3H2(g) ? 2NH3(g)
This is called the Haber Process. Why?
Equilibrium conditions can be reached from either direction
Equilibrium expression uses coefficients as exponents, products over reactants
Do it for above reaction
6. Good Website http://jersey.uoregon.edu/vlab/Thermodynamics/
7. Constant cont’ It is the value specific for that reaction under those conditions, no matter what you start with
animation
Keq greater than 1 – products favored
Keq less than 1 – reactants favored
Temperature dependent – defined for one temperature
Keq for a reaction is 1/Keq for the reverse
If reaction is multiplied by 2, Keq is squared, etc.
Dimensionless
Try some problems
8. Heterogeneous Equilibria Not just gases or solutions
Solids, liquids included in reaction
CaCO3(s) ? CaO (s) + CO2(g)
Pure solids, liquids, and solvents are not included in the equilibrium expression because their concentration is usually in excess and does not change significantly in the reaction.
9. Calculating Keq Easy when all concentrations are known – just substitute
Try PE
10. RICE Problems RICE is a table approach to solving equilibria problems
Reaction, Initial conc,Change in conc, Equilibrium conc
Used when there are unknowns in a problem
Try one together – bottom of page 589
PE
11. Q Problems Used when concentrations of reactants and products are given (or calculated by RICE) and Keq is given
Question asked is “Has equilibrium been reached?”
Called Q or “reaction quotient”
Calculated using Keq expression
If Q < Keq, reaction must go to right
If Q > Keq, reaction must go to left
12. “X” problems Used when amounts of reactants or products at equilibrium are unknown
You may or may not need to use RICE
Use “x” for unknown amounts
We will not have to use the quadratic equation
Try PE