1. Chemical Equilibrium

2. I can describe a reversible reaction.
I can identify the forward and reverse reactions.
I can describe reaction rates.
I can describe the conditions required for chemical equilibrium.

3. Some reactions do not go to completion.
In other words, the reactants are not used up.
Some chemical reactions are also reversible.

4. A reversible reaction occurs when the products reform the original reactants.
In theory, all reactions are reversible.
Some reactions are reversible on their own.

5. Other reactions must be forced to reverse.
When writing reversible reactions, two half arrows are used instead of the traditional .

6. The forward reaction points to the right.
Example:NH4Cl  NH3 + HCl
The reverse reaction points to the left.
Example: NH4Cl  NH3 + HCl

7. The two reactions are opposite processes.
The reaction rate is the speed at which a chemical reaction occurs.
Reaction rates are proportional to the concentrations.

8. Reaction rates are faster when the concentration of the reactants is higher.
When a reaction starts, the concentration of reactants decreases as the products form in the forward reaction.

9. In reversible reactions, the products will start to reform the reactants in the revers reaction.
Chemical equilibrium occurs when the rate of the forward reaction = the rate of the reverse reaction.

10. In chemical equilibrium, the concentrations of the reactants and products remains constant.
It occurs when opposing reactions are proceeding at equal rates.

11. Equilibrium does not mean that the reactions stop.
It is a dynamic, or changing process.

12. The Equilibrium Constant

13. I can describe equilibrium constants.
I can write equilibrium constant expressions.
I can describe the law of chemical equilibrium.

14. The equilibrium constant expresses the relative concentrations of the reactants and products at equilibrium.
The symbol for the equilibrium constant is Keq

15. The equilibrium expression is the concentration of the products divided by the concentration of reactants.
The coefficients represent the power you raise the concentration to.

16. Concentrations are represented by square brackets.
Example: aA + bB cC + dD
Equilibrium Equation:

17. The equilibrium ratio is always a constant value for a given reaction, no matter how much you start with.
Every reversible reaction obeys this relationship and has a specific Keq.
This is called the law of chemical equilibrium.

18. According to the law, every reversible reaction proceeds to an equilibrium state that has a specific ratio of the concentrations of reactants and products.
The equilibrium constant must be determined by experiments.

19. The value of the equilibrium constant does not depend on initial concentrations.

20. The value of the equilibrium constant is a measure of the extent to which a reaction goes to completion.
Homogenous equilibria describes reactions where all of the reactants and products are in the same state of matter.

21. Heterogenous equilibria describes reactions where more than one state of matter exists.
The concentrations of pure solids and liquids are relatively constant.
Therefore, the concentrations of a pure solid or liquid does not change during a reaction, no matter how much is present.

22. The concentrations of pure liquids and solids can be omitted from the equilibrium equations.
Even though solids and liquids are left out of the equation, they are still important.

23. Example: 2C2H6(g) + 2Cl2(g) <---> C2H5Cl(s) + 2HCl(g)
Equilibrium Equation:

24. The Reaction Quotient

25. I can predict which way a reaction will shift based on the reaction quotient.
I can write and solve reaction quotient equations.

26. When reactant and product are mixed, it is not obvious if they have reached equilibrium.
If the reaction is not at equilibrium, it is useful to know the direction the system has to shift to reach equilibrium.

27. The reaction quotient (Q) is used to determine if a reaction is at equilibrium.
The reaction quotient is calculated much like Keq.
For the reaction quotient, you use concentrations taken at the time of measurement, not the equilibrium concentrations.

28. Example: Reaction Quotient:
C2H6(g) + Cl2(g) <---> C2H5Cl(s) + HCl(g)
Reaction Quotient:

29. If Q is less than the equilibrium constant, there is too much of the reactants and not enough of the products.
Therefore, the reaction will proceed to the right.

30. If Q is greater than the equilibrium constant, there is too much of the products and not enough of the reactants.
Therefore, the reaction will proceed to the left.

31. If Q is equal to the equilibrium constant, the system is at equilibrium and no shift in direction will occur.

32. Le Chatelier’s Principle

33. I can identify factors that change equilibrium.
I can describe Le Chatelier’s Principle.
I can predict shifts in equilibrium.

34. Factors that change chemical equilibrium include: concentrations of reactants and products, pressure, and temperature.

35. Le Chatelier’s Principle states that if a change in conditions occurs on a system at equilibrium, the equilibrium position will shift in the direction that reduces the change.
In other words, a reaction will shift in the forward or reverse direction to “undo” the changes.

36. If more reactants are added to a system, the system will shift right.
If more products are added to a system, the system will shift left.
If reactants are removed, the system will shift to the left to produce more reactants.

37. If products are removed, the system will shift to the right.
Removing a substance drives the system to produce more of that substance.
These changes are the reactions way to maintain equilibrium.

38. Examples:
2NO N2O4
If NO2 is added, the reaction will shift:
If NO2 is taken away, the reaction will shift:
If N2O4 is added, the reaction will shift:
If N2O4 is taken away, the reaction will shift:

39. For some gases at equilibrium, the reaction can shift by changing pressure.
When the total pressure increases the system will shift to decrease pressure.
The reaction will shift to the side that has fewer molecules.

40. Remember, the coefficients represent the number of moles of a substance in a reaction.
You will only be looking at moles of gas, not liquids or solids.

41. Examples: What will happen to the following reactions if pressure increases?
2NO2(g) N2O4(g) The reaction will shift ___________.
NH4Cl(s) NH3(g) + HCl(g) The reaction will shift ____________.
H2(g) + Cl2(g) HCl(g) The reaction will:

42. An equilibrium reaction that has the same number of moles of gas on both sides will not be affected by changes in pressure.
The value of Keq for a particular reaction depends on temperature.

43. Heat can be thought of as a reactant.
Reactions will shift in the opposite direction of where the heat is in the reaction to maintain equilibrium if heat is added.

44. Examples: Heat is added to the following reactions
Examples: Heat is added to the following reactions. Predict how they will shift.
H2(g) + I2(g) 2HI(g) + heat Reaction will shift _______________.
Heat + NH4Cl (s) NH3(g) + HCl(g) Reaction will shift _______________.

45. Adding heat is like adding a product or a reactant.
Changing the temperature changes the Keq.
Concentration and pressure do not change the Keq.