Download Equilibrium

Equilibrium
(Do Reactions Ever Really Stop?)
Model 1: Reaching Equilibrium: Are We There Yet?
Read section 17.4, pages 543 – 545, from your textbook.
1. After reading through this section write and discuss with your lab partner how physical
equilibrium and chemical equilibrium compare and contrast.
2. What is an example of a system that involves equilibrium without going through a chemical
change?
3. If you filled a container with rubbing alcohol and then sealed the container, how could you tell
when equilibrium in the container had been reached?
4. If you had an open container of water, could it reach a state of equilibrium? Explain.
5. How do chemists recognize a system that has reached a state of chemical equilibrium? When
writing chemical equations, how do we indicate reactions that come to a state of chemical
equilibrium?
You and your lab partner work together on #6.
6. Obtain 2 graduated cylinders (both the same size) and 2 beakers.
b. Label one of the beakers and cylinders as A and the other as B.
c. Fill cylinder “A” a little over half full with water and no water in cylinder “B”.
d. Record the volume of water in each cylinder. This will be trial 1.
e. Pour ½ of the water from cylinder A into beaker B and 1/3 of the water from cylinder B
into beaker A. Yes, I know there is initially no water in cylinder B.
f. Pour the water from the beakers into their respective cylinders (in other words water from
beaker B goes into cylinder B)
g. Record the volume in each cylinder.
h. Repeat steps e - g four more times. Keep the water in the cylinders when you finish
collecting the data.
i. Graph the data using volume versus trial. The graph can be put on the same paper as
questions 1 – 4. You will have 2 lines on the same graph (one line for A and one for B).
j. Describe what eventually happens to the volume of water in the 2 cylinders.
k. What point has now been reached?
l. If the cylinders A and B represented the reactants and products in a chemical reaction,
which of the cylinders represented the reactants and which would represent the products?
Explain how you came to this conclusion. Saying you guessed is not an acceptable
answer.
Model 2: Disturbing Equilibrium
Read section 17.5, pages 545 – 547. Then read section 17.8 pages 554 – 557 about how changing the
concentration (the amount of a substance) affects equilibrium, including example 17.4 on pages 557 and
558. Don’t worry about the mathematical calculations shown.
Now it is time to apply what you have just read.
(Over)
7. You will be disturbing a reaction at equilibrium. To do this you need safety goggles from the lab
room. The chemicals and equipment for this are located on the counter in the classroom.
a. To start with, you need to get the reaction to equilibrium. To do this, fill a small test tube
2/3 full with an aqueous solution of potassium thiocyanate (KSCN). Then add 10 drops
of ferric nitrate, Fe(NO3)3, and mix. Mix by pouring the combined chemicals back and
forth between two test tubes. The reaction is now at equilibrium as shown below
Fe3+(aq) + SCN1-(aq)  Fe(SCN)2+(aq)
b.
c.
d.
e.
f.
g.
Often reactions are written with only ions that are actually involved in the reaction. This
is why the nitrate and potassium ions have been left off of the equation. These ions that
are left off the equation are called spectator ions. Write this equation and below each
chemical list the solution color.
Pour the mixture from part A into 2 test tubes about 1/3 of the total into each test tube.
All 3 test tubes should now have the same amount of solution in them. The third test tube
will be used as a comparison when changes are made to test tubes 1 and 2 in parts c
through g.
To the first test tube add 8 drops of KSCN. Record your observations. Based on the
color observed what appears to happen to the amount of Fe(SCN)2+(aq)?
Next, add 8 drops of ferric nitrate to the second test tube. Record your observations.
Based on the color observed, what appeared to happen to the amount of Fe(SCN)2+(aq)?
Now add a few crystals of Na2HPO4 to the first test tube. Nake sure you shake for at least
20 seconds. Record your observations. Based on the color observed, what appeared to
happen to the amount of Fe(SCN)2+(aq)?
Add 4 drops of AgNO3 to the second test tube. Record your observations. Based on the
color observed, what appeared to happen to the amount of Fe(SCN)2+(aq)?
Finally, add 8 drops of HCl(aq) to the second test tube. Record your observations. Based
on the color observed, what appeared to happen to the amount of Fe(SCN)2+(aq)?
8. As you read in your textbook, changing the amount (or concentration) of a reactant or product
will shift an equilibrium reaction in a particular direction. If a substance is removed, the reaction
will shift to produce more of the substance to reestablish equilibrium. Likewise, when a
substance is added to the reaction, the reaction will shift in a direction to remove some of the
substance that was added. For example, in part 6c when the KSCN is added the reaction shifts to
form more Fe(SCN)2+(aq), thus removing some of the SCN1- that was added.
Use the reaction, for the formation of the ammonia from its elements, listed below to
describe which way the reaction will shift when equilibrium in a, b, and c.
N2(g) + 3 H2(g)  2 NH3(g)
a. Addition of ammonia to the reaction.
b. Addition of nitrogen gas.
c. Addition of oxygen gas that reacts with the hydrogen gas.