## Chem – College: How to Use the Reaction Quotient

How do you use the reaction quotient?

Now that we know how to calculate the reaction quotient (Q) we can talk about what it means and how to use it. Most books explain it in terms of comparing the reaction quotient (Q) to the equilibrium constant (K).

These are the classic phrases that people put in books:

If Q is LARGER than K then the reaction will proceed to the LEFT.

If Q is SMALLER than K then the reaction will proceed to the RIGHT.

Okay what the heck does that mean? It means if Q is LARGER than K then we have too many products to be at equilibrium. That means we have to take some of those products and turn them into reactants. What if Q is SMALLER than K? Then we have too many reactants to be at equilibrium and we need to take some of those reactants and turn them into products.

Lets say I have a party at my house. I invite 50 people because the backyard can hold 20 people and the inside of my house can hold 30 people.

Amount of people in backyard <——> Amount of people inside house

What is my equilibrium constant?

Answer:

 K = [ inside ] = [ 30 ] = 1.5 [ backyard ] [ 20 ]

As people start to arrive at the party they enter through the front door and start inside that house. 15 minutes after the party has started 25 people are inside the house and 5 people are in the backyard. What is the reaction quotient (Q) at that moment?

Answer:

 Q = [ inside ] = [ 25 ] = 5 [ backyard ] [ 5 ]

Is the Q LARGER or SMALLER than the K?

Answer: LARGER

What does that mean?

Answer: That there are too many people inside the house (on the right side of the equation). This will cause people to start moving outside (cause people to start moving to the left side of the equation).

After more time has passed more people start to move into the backyard and more people show up to the party. 2 hours after the party starts 10 people are inside the house and 40 people are in the backyard. What is the reaction quotient (Q) at that moment?

Answer:

 Q = [ inside ] = [ 10 ] = 0.25 [ backyard ] [ 40 ]

Is the Q LARGER or SMALLER than the K?

Answer: SMALLER

What does that mean?

Answer: That there are too many people in the backyard (on the left side of the equation). This will cause people to start moving inside the house (to the right side of the equation).

All we have to do now is think of molecules like we do the people in the example above.

If the chemical equation below has the equilibrium constant of 2.5 * 10-4. What will be the reaction quotient when the concentration of H2O is 25 M, the concentration of H2 is 0.3M, and the concentration of O2 is 0.5 M? Which side will have too high of a concentration REACTANTS or PRODUCTS? Which way will the reaction need to shift to return to the equilibrium?

2 H2O(g) <—-> 2 H2(g) + O2(g)

What is the Q?

Answer:

 Q = [ H2 ]2 [ O2 ]= [ 0.3]2 [ 0.5]= Q = 7.2 * 10-5 [ H2O]2 [ 25 ]2

How does the Q compare to the K?

Answer: Q is SMALLER than K

Which side has too much stuff (too high a concentration)?

Answer: REACTANTS

Which way will the reaction need to shift to return to the equilibrium?

Answer: It will need to shift to the right (away from the reactants and toward the products)

PRACTICE PROBLEMS: Which way the do reactions below need to shift to return to equilibrium?

If the chemical equation below has the equilibrium constant of 3.2 * 10-2. What will be the reaction quotient when the concentration of Fe3+ is 0.4 M and the concentration of CO32- is 0.8 M?  Which way will the reaction need to shift to return to the equilibrium?

2 Fe3+(aq) + 3 CO32-(aq) <—-> Fe2(CO3)3(s)

Answer:

 Q = 1 = 1= Q = 0.08 [ Fe3+ ]2 [ CO32-]3 [ 0.4 ]2 [ 0.8]3

Will shift to the right (products)

If the chemical equation below has the equilibrium constant of 15. What will be the reaction quotient when the concentration of H2SO4 is 4M, the concentration of H2 is 0.2M, and the concentration of O2 is 0.6M? Which way will the reaction need to shift to return to the equilibrium?

H2SO4(aq) <—–> H2(aq) + S(s) + 2 O2(aq)

Answer:

 Q = [ H2 ] [ O2 ]= [ 0.2] [ 0.6]= Q = 0.03 [ H2SO4] [ 4 ]

Will shift to the right (products)

If the chemical equation below has the equilibrium constant of 15. What will be the reaction quotient when the concentration of N2O3 is 0.6M, the concentration of N2 is 10M, and the concentration of O2 is 3M? Which way will the reaction need to shift to return to the equilibrium?

2 N2O3(g) <——> 2 N2(g) + 3 O2(g)

Answer:

 Q = [ N2 ]2 [ O2 ]3 = [ 10]2 [ 3]3 = Q = 12500 [ N2O3 ]3 [ 0.6]3

Will shift to the left (reactants)  