1. Lets Review!!!: Chemical Equilibrium
By: Emma and Gabe

2. Chemical Equilibrium: An Introduction
Some chemical reactions DO NOT reach completion where all reactants are converted into products  reach a constant state instead
Chemical Equilibrium= the state where concentrations of all reactants and products remain constant over time
Terminology:
Far to the right = at one time, more products exist than reactants
Far to the left = a small amount of the reactants are converted into products

3. 13.1: The Equilibrium Condition
Remember: equilibrium is DYNAMIC on the atomic level… the net result is no change
Why does equilibrium happen?
As the reactions progresses, the concentrations of the products build up, initiating the reverse reaction
Eventually, the concentrations reach levels where the forward and reversion reaction rates are EQUAL
Factors affecting equilibrium: initial concentrations, energies of reactants/products, & “organization”
*NOTE*= nature tries to achieve MAXIMUM disorder and LOWEST energy state

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5. 13.2: The Equilibrium Constant
Law of Mass Action: aA + bB  cC + dD
Applies to systems in solution AND gas phase
Remember that equilibrium constant is a RATIO and the concentrations can vary
Equilibrium Expression:
Use the equilibrium concentrations in this expression
Do you guys remember how to use it?

6. Just for Grins… Let’s Practice
Write the equilibrium expression for the following reaction:
N2(g) + 3H2(g)  2NH3(g)

7. 13.3: Equilibrium Expressions Involving Pressures
Equilibrium expression in terms of pressure:
Remember that these are the partial pressures of the gases
The problem-solving process is still the same

8. 13.4: Heterogeneous Equilibria
Homogeneous Equilibria: both the reactants and products are in the same state
Heterogeneous Equilibria: the reactants and products exists in different states
For equilibrium calculations, the position does NOT include the amounts of pure solids or liquids  the concentrations of these substances can’t change

9. Let’s Practice… Again
Write the equilibrium expression for this chemical reaction:
2H2O(l)  2H2(g) + O2(g)

10. 13.5: Applications of the Equilibrium Constant
Larger K value  reaction system will form mostly products
Smaller K value  reaction system will form mostly reactants
Remember: the size of K and the time required to reach equilibrium are NOT directly related
Reaction Quotient (Q) = used to determine shift in a reaction
Apply the law of mass action using initial concentrations rather than those at equilibrium
Q equal to K: the system is already at equilibrium
Q greater than K: the system shifts to the left
Q less than K: the system shifts to the right
Also remember how to do equilibrium pressure calculations…

11. Third Times a Charm… Let’s Practice
For the synthesis of ammonia at 500°C, the equilibrium constant is 6.0 x Predict the direction in which the system will shift to reach equilibrium at the following initial concentrations:
[NH3]= 0.01 M; [N2]= M; [H2]= .02 M
At a defined temperature, a 1.00-L flask initially contained moles of PCl3 gas and moles of PCl5 gas. After the system had reached equilibrium, moles of Cl2 gas was found in the flask. Calculate the equilibrium concentrations of all species and the K value.
Hint: PCl5  PCl3 + Cl2

12. 13.6: Solving Equilibrium Problems
Problem-Solving Steps for Equilibrium Problems;
Balance the equation.
Using law of mass action, write the equilibrium expression.
ICE chart it out.
Calculate Q to find the direct of shift to reach equilibrium.
Apply the change to the initial concentrations in the ICE chart.
Solve for Keq with these new equilibrium concentrations & solve for the unknown.

13. Last Time… Everybody Practice
Assume that hydrogen iodide gas is synthesized from hydrogen and iodine gas at a temperature with the equilibrium constant 100. In a reaction system, HI is at atm, H2 at atm, and I2 at atm in a 5.00 L flask. Calculate the equilibrium pressures of all of the species.

14. 13.7: Le Châtelier’s Principle
Le Châtelier’s Principle: if a change is imposed on a system at equilibrium, the position of the equilibrium will shift in a direction that tends to reduce that change
If the concentration of a component increases, the equilibrium position will shift in the opposite direction
Addition of an inert gas does NOT effect the equilibrium position
When the volume of the system is reduced, the system will decrease its total number of gas molecules
Remember: The above changes do NOT affect the equilibrium constant… only temperature alters the K value