Introduction:
For this activity, you are going to simulate an equilibrium process, collect data, share data with your classmates, draw graphs, and answer some questions.
You will be investigating a simple equilibrium reaction: Head ⇌ Tail (this is exactly the same as A ⇌ B)
For the purpose of this exercise, we are going to use coins to denote whether we have a “Head” molecule (H) or a “Tail” molecule (T) – the coin will be Heads up or Tails up.
Head ⇌ Tail means that if a container holds H and T molecules, some H molecules will turn into T molecules and some T molecules will turn back into H molecules. At equilibrium, the number of H and T molecules remains constant because the rate of change will be the same in each direction (for every H that becomes T, a T will become an H).
In this activity, your system will have 10 molecules (coins) that can be either H or T. These will be numbered 1 – 10 (lay the coins on a piece of paper which has the numbers 1 – 10 written on it):
(Note: if you don’t have coins, you could use buttons or pieces of paper with “H” written on one side and “T” written on the other)
One way that molecules react and change is through collisions. As the molecules move around, these collisions are somewhat random, and will sometimes result in the molecule reacting and becoming something else, but not always. There is a probability that a collision will result in reaction: if the probability is 50%, there is a 50/50 chance that a molecule will react when it collides. If the probability is 90%, there is a 90/10 chance that a molecule will react when it collides. In the case of a simple equilibrium process, like the one considered here, the probability of the backwards reaction and the probability of the forwards reaction add up to 100%: if H  T is 90% likely when H experiences a collision, then T  H is only 10% likely when T experiences a collision. This preference for one direction over the other is what controls how much reactant and product is present at equilibrium.
In this activity, you will keep track of the number of H and T molecules over a 15-minute period. This doesn’t mean you’ll be timing anything – you will simply assume that five random collisions happen every minute, so that after you have completed five collisions, you will count the number of H and T molecules and record it as the number after 1 minute. This will be repeated to get the number after 2 minutes and so on for 15 sets of 5 collisions, which we will call 15 minutes.
Check list:
 Watch the Instruction Video (click or ctrl+click this link to see it)
 download the Excel RNG file from Files > Lab Activities > Week1b. Equilibrium Activity
 perform the four experiments below
 submit data to a Googledocs dataset by the deadline given in “Assignments”
 download the lab report sheet from Files > Lab Activities > Week1b. Equilibrium Activity then complete it (including 8 graphs) and turn it in by the due date given in “Assignments”

Instructions:
Expt. 1: 90% chance that a collision turns H into T, starting from all H (H = 10)
1. Set up the coins as shown in the picture above, with all ten Heads up (H = 10).
2. Using the Excel RNG (random number generator):
• Set the “% probability of H changing to T” to 90
• Enter any value into the yellow cell and hit “enter”
3. The RNG randomly chooses the molecules that will experience a collision with a minute. Using the probability you entered, it then randomly chooses whether the molecule reacts or not. For example, it may say that molecule 4 should “Change to T or remain T”. In this case you look at your molecule number 4: if it is currently H, you should turn it over so that it is now T. If it is already T, you should leave it as it is.
4. Do as instructed for each of the five molecules listed, in the order they appear (a molecule might be chosen more than once in the same minute, but that’s okay).
5. You have now completed “1 minute”. Next, create a table in Excel and enter the number of H and T that you have after “1 minute”.
6. Enter another value into the yellow cell and hit “enter” again. This will generate five more collisions and outcomes. Repeat the process of turning over the chosen molecules (or not, if that’s the instruction) for all five molecules, in order.
7. Repeat this process until you have completed “15 minutes” of reaction time. You can now create a graph in Excel. Your table and graph should look something like this:

8. Finally, post your “Number of H” data into a Googledocs document that you can find here: Monday/Wednesday Lab Section Data Entry
Tuesday/Thursday Morning Lab Section Data Entry
Tuesday/Thursday Morning Lab Section Data Entry
Note that all your Googledocs data MUST be submitted by midnight on Thursday (for Wednesday lab group) or by midnight on Friday (for Thursday lab group). You will lose points if your data is not submitted in time.

Expt. 2: 90% chance that a collision turns H into T, starting from all T (H = 0)
1. Set up the coins again, but this time with all ten Tails up (H = 0).
2. Using the Excel RNG, keep the “% probability of H changing to T” set to 90.
3. Enter any value into the yellow cell, hit “Enter” and then repeat steps 4-9 from Expt. 1.
(Make sure you enter your Expt. 2 data into the Googledocs document)

Expt. 3: 10% chance that a collision turns H into T, starting from all H (H = 10)
1. Set up the coins again, but this time with all ten Heads up (H = 10).
2. Using the Excel RNG, change the “% probability of H changing to T” to 10.
3. Enter any value into the yellow cell, hit “Enter” and then repeat steps 4-9 from Expt. 1.
(Make sure you enter your Expt. 3 data into the Googledocs document)

Expt. 4: 50% chance that a collision turns H into T, starting from all H (H = 10)
1. Set up the coins again with all ten Heads up (H = 10).
2. Using the Excel RNG, change the “% probability of H changing to T” to 50.
3. Enter any value into the yellow cell, hit “Enter” and then repeat steps 4-9 from Expt. 1.
(Make sure you enter your Expt. 4 data into the Googledocs document)

Group Data:
Everyone should have entered their data to Googledocs by midnight on Thursday (for Weds lab group) or midnight on Friday (for Thurs lab group). The Googledocs document will not allow entry after this time (or early the next morning, at the latest).
1. Once everyone has entered their data into Googledocs, copy the averaged group data (in the pink cells) to your Excel file containing your own data.
2. Remember that although only “Number of H” is given at each minute, you can easily calculate “Number of T”, because H + T = 10. Also note that the number of H and T in the averaged group data will not be whole numbers. In this case we can think of each H or T representing a mole of molecules rather than single, indivisible molecules.
3. For each of the 4 experiments, create a graph similar to the one you created for your own data.
4. You should now have 8 graphs in Excel. These can all be transferred into the lab report sheet by clicking on the graph, using the “copy” function (ctrl-C) and then once you are in your lab report sheet document, use the paste function (ctrl-V) to paste each graph into the file.