The Equilibrium Constant

1. Mr. Chapman Chemistry 30 The Equilibrium Constant

2. In Case You Weren’t Here Wednesday... • We spent the entire class period working on the Dance Analogy to Chemical Equilibrium Assignment. It is due on Thursday, November 18. • The reaction kinetics assignment was due on Wednesday. If you missed class, hand it in now, no worries. • There are certain values you should have at the start of the second and third tables. Those values are on the following page.

3. Entries for Tables 2 and 3 @ 55 minutes Table 2: Table 3:

4. A Brief Review to Now... • Chemical reactions are all theoretically reversible. • In a closed system (one in which no reactants can be added and no products can be removed), certain reactions may reach equilibrium. • Chemical equilibrium is the state at which the rate of the forward reaction is equal to the rate of the reverse reaction.

5. What the Dancing Analogy Should Have Shown You...

6. Dancing Assignment Debrief • Remember that couples getting up to dance was the forward reaction, and couples sitting down was the reverse reaction. • Initially, the rate at which couples got up (forward) was very high, and the rate at which couples sat down (reverse) was very low.

7. Dancing Assignment Debrief • Over time, the forward reaction started slowing down as reactants got used up. The reverse reaction started speeding up as more and more products were formed. • Eventually, the rate at which couples got up to dance EQUALLED the rate at which couples sat down. At this point, the reaction reached CHEMICAL EQUILIBRIUM.

8. Dancing Assignment Debrief • Although the forward and reverse reactions were occurring at the same rate, you could see that the products side of the reaction was favoured. This is because more people were dancing than sitting! • The graph originally looked something like this as a result of the equilibrium:

9. This is what the original shape of your graph should look like. The number of products is higher than the number of reactants, although both are constantly being formed. In our dancing analogy assignment, there were 104 dancers and 46 sitters at equilibrium. The ratio was 104/46, or approximately 2.26. Products are favoured

10. Dancing Assignment Continued... • When the Boy’s football team shows up at the dance, they add a total of 30 more couples that are originally sitting. • This throws off the chemical equilibrium a little bit, because now we have 104 couples dancing but 76 couples sitting. This ratio is NOT equal to 2.26.

11. Dancing Assignment Continued... • As a result of this, more couples start to get up to dance. Eventually, we end up with a new chemical equilibrium, with a certain number of couples dancing and a certain number of couples sitting. • Guess what the ratio is for couples dancing to couples sitting? That’s right, it’s about 2.26. No matter what we do to the number of products or reactants, eventually they will reach the same ratio to each other.

12. Big Idea: The Law of Chemical Equilibrium At a given temperature, a chemical system might reach a state in which a particular ratio of reactant to product concentrations has a constant value.