Dynamic Equilibrium

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Many chemical reactions can easily run in both forward and reverse directions and are called reversible reactions. The consequences of this reversible behavior has become known as a chemical reaction "coming to equilibrium."

Showing it using a generic chemical equation, we have this:

A + B ⇌ C + D

A and B reacting to give C and D is called the 'forward reaction.' C and D reacting to give A and B is called the 'reverse reaction.'

In a chemical system that can come to equilibrium, both the forward reaction direction and the reverse reaction direction will run all the time. This is the meaning of the word "dynamic" in the title. J.H. van 't Hoff (on page 162) in his classic 1884 Études de dynamique Chimique used the phrase "principle of mobile equilibrium" to describe what we now use dynamic for.

The exact moment of equilibrium happens when the rate of the forward reaction equals the rate of the reverse reaction.

When a chemical system is at equilibrium, there are no visible changes in the system. The concentrations of every substance in the reaction will remain constant at equilibrium.

One thing. On the Internet, this symbol is often used: <===> with reactions that come to equilibrium. In your textbook, look for this symbol: ⇌. It is often called the equilibrium arrow.

You can also write the equilibrium arrow this way: ⇋. There appears to be a technical difference between ⇋ and ⇌. The ChemTeam will ignore the difference.

The style of equilibrium arrow used above was introduced in 1902 by H. Marshall (Proc. Edin. Roy. Soc., 24, 85 (1902)) as a modification of the original symbol, which was ⇆, introduced by J.H. van 't Hoff (on page 115) in his Études.

Just one more thing. Many other chemical reactions can only run in one direction, going only from the reactants on the left side of the arrow to the products on the right side of the arrow. These reactions are called "not reversible."

A good example of this might be burning some paper:

cellulose + O2 ---> CO2 + H2O

The reaction proceeds until all of either one of the reactants is used up and then it stops. You cannot make the reaction run in reverse. This is usually because of the complexity of one or more of the reactants. For example, imagine putting some carbon dioxide and water together in a beaker and trying to get starch or sugar or any number of other CHO compounds. It just does not happen!! Typically, reversible reactions are simple one-step reactions in both directions. The making of cellulose by a plant requires many steps, some with different requirements of temperature or time, whereas to break cellulose down to CO2 and H2O takes only one step.

One last point before going on to some equilibrium examples: the first two examples ARE NOT chemical reactions. However, they do establish situations which are truly in equilibrium

Example #1

Imagine a beaker with radioactive NaI solid at bottom. Carefully pour a saturated solution of non-radioactive NaI over the solid.

It's important that the solution is saturated. That means that the solution is holding the maximum amount of NaI it can at that temperature.

Allow to sit for several hours.

Remove solution and filter to get solid out.

Solution found to be radioactive. Accounting for radioactive decay, the solution increases in radioactivity until reaching a constant level.


Example #2

Naturally occuring iodine consists only of the single isotope I-127. However, I-131, a radioactive isotope is available commercially. Some methyl iodide, CH3I (which is a liquid at room temperature and one atmosphere pressure) was prepared using radioactive iodine and then used in the following experiment.

Side A is filled with radioactive CH3I while side B is filled with the same volume of non-radioactive CH3I and the beaker is left to sit after being tightly covered.

During the course of the experiment, the liquid levels in compartment A and B do not change. After several hours have elapsed, liquid in compartment B is removed and found to be radioactive.

Explain how the non-radioactive CH3I came to have some radioactive CH3I in it, even though the levels of liquid in both compartments did not change.

In both examples you could measure the growth in the radioactivity over time. You would find that both the non-radioactive portion in each example and the radioactive portion would eventually reach a point where there was constant amounts of radioactivity in each. (Notice: the word "constant" was used, NOT "equal.")

It's important to emphasize that once equilibrium is achieved, the two reactions (forward and reverse) continue to run. It's just at equilibrium, since the rates are equal, there is no more visible (or measurable) change to the system.

Example #3

Consider this chemical reaction (all three substances are gases):

H2(g) + I2(g) ⇌ 2HI(g)

Some H2 and I2 are allowed to react and HI is produced. This is the forward reaction. As the two reactants are used up, the reaction will go slower and slower. This is because there is less and less of H2 and I2.

However, keep in mind that the HI concentration is increasing.

Then, as HI is produced, it will begin to react with itself and will start to re-make some H2 and I2 that was used up in the forward. This is the reverse reaction.

Soon, the rate of the forward reaction and the rate of the reverse reaction will have come to be equal. This is the point at which dynamic equilibrium has been established.

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