Surface Area and Equilibrium

Surface area and equilibrium in a heterogeneous reaction

A heterogeneous reaction is one that takes place between two distinct states of matter. For example, a charcoal barbeque is an example of a heterogeneous reaction, between the surface of the solid carbon, and the oxygen gas in the air. One of the key characteristics of a heterogeneous reaction is that it can only take place where the surfaces of the different states of matter are in contact.

Sublimation, where a solid turns directly from the solid state to a gas, is another example of a heterogeneous reaction. If it is carried out in a closed system, sublimation is an example of a solid-gas equilibrium.

H₂O (s) H₂O (g) You may have noticed this process if you leave ice cream in the freezer too long. Ice crystals build up on the surface of the ice cream, even in a tightly sealed container. What is happening is that the solid water sublimes, and then recrystallizes in another location. In order for there to be an equilibrium, the rate at which the solid water sublimes must be the same as the rate at which the water vapor condenses¹. However, the two processes don't have to happen at the same place. Thus solid water gets moved around the container. Once ice crystals have started to form, the gas molecules start to selectively condense at these surfaces. This causes the ice crystals to grow in size. The large crystals don't have a very nice texture, so they change the palatability of the ice cream (if the container is not well sealed, then the crystals also pick up lots of unwanted food flavors).

Prepare a spreadsheet that will show how surface area changes a heterogeneous equilibrium. Use the spreadsheet for 2-butene as a guide. In creating your spreadsheet, use the following assumptions:

a.The equilibrium reaction is solid gas. The rate of sublimation depends only on the available surface (provided the temperature is constant). The rate of condensation depends on the available surface, and the amount of gas present.

b. The reaction rate is

The rate at which the solid sublimes (forward reaction)

R_f = k_f (area) where:

R_f = reaction rate for the forward reaction

k_f = rate constant in the forward direction

area = surface area of the solid

The rate at which the gas condenses (reverse reaction)

R_f = k_f (area)[gas] where:

R_f = reaction rate for the reverse reaction

k_f = rate constant in the reverse direction

area = surface area of the solid

[gas] = concentration of the gas

c. The surface area will not be significantly increased or decreased because of sublimation.

Step 1: Create an area on the spreadsheet to record values for k_f and k_r.²

Step 2: Create five columns on the spreadsheet

Step 3: In the Time column, enter values from 1 to 50, in intervals of 1. Hint: there is a formula you can use to do this rather than typing them in one at a time.

Step 4: In the cell below (Area), enter a surface area of 1. Now create a formula that will copy this value into the remainder of the column. Remember that we are assuming surface area is unchanged by the reaction.

Step 5: In the cell below [gas] enter a starting value of 0. In the next cell down enter a formula that will calculate the concentration of gas. Copy this formula to each cell in the [gas] column.

Step 6: The forward rate column equals the value of k(forward) times the (Area) for that row. Copy this formula to each cell in the Rate(forward) column.

Step7: The reverse rate column equals the value of k(reverse) times the (Area) times the [gas] for that row. Copy this formula to each cell in the Rate(reverse) column.

Step 8: Create an XY chart that plots time on the X axis, and both (Area) and [gas] on the Y axis.

Use your spreadsheet to do the following:

1. At what time does the reaction reach an equilibrium?

2. What happens to the concentration of the gas? What is the concentration of the gas at eauilibrium? What about the surface area? Explain what you see.

3. Double the surface area. At what time does the reaction now reach equilibrium?

4. Compare the concentration of gas when the surface area is doubled with the original value.

5. Try quadrupling the area, and halving it. Observe what happens to the time required to reach equilibrium, and to the concentration of gas at equilibrium.

6. Make a conclusion about how changing surface area affects chemical reactions at equilibrium

¹ The word "sublimation" is correct for both the solid

gas, and gas

solid transitions. Condensation actually means the change in state from gas

liquid, but is used here so that the forward and reverse processes are distinctly labelled.
² The units used are not real. They are chosen simply to make the graph show the change in a reasonable number of intervals of time.