Lesson 1 - What is a "Dynamic Equilibrium"?
Typically when we think of what happens during a chemical reaction we think of the reactants getting totally used up and ending up with only products. Also, we generally consider chemical reactions as one-way events. You may well have learned during earlier science classes that this is one way to distinguish chemical change from physical changes - physical changes (such as the melting and freezing of ice) are easily reversed, but chemical changes cannot be reversed (pretty tough to un-fry an egg). (click below)
In this unit we will see that this isn't necessarily the case- it really depends on the feasibility of the reverse reaction (which is influenced by the activation energy, among other things).
Many chemical reactions are, in fact, reversible under the right conditions. And because many reactions can be reversed, our idea of a reaction ending with no reactants left- only products- needs to be modified.
Reversible reactions have both forward and reverse reactions that are happening at the same time. Here are some examples to whet your appetite:
Example #1 - Nitrogen Dioxide, NO2
Nitrogen dioxide, NO2, a reddish-brown gas, reacts with itself to form colourless dinitrogen tetroxide, N2O4:
But the reaction can also go the other way - dinitrogen tetroxide also readily breaks down to form nitrogen dioxide:
We typically write a reaction that can go in both directions by using a double arrow (which will sometimes appear as ⇔, though not technically correct):
Because the reaction continues in both directions at the same time, we never run out of either NO2 or N2O4. NO2 is continually being used up to form N2O4, but at the same time N2O4 is forming more NO2.
(Credit: © 2002 Prentice-Hall Inc., "General Chemistry: Principles and Modern Applications 8e")
Example #2 - Iron and Iron Oxide
Okay, now another example. When hydrogen gas is passed over heated iron oxide, iron and steam are produced:
The reverse reaction can occur when steam is passed over red-hot iron:
We can write these two equations together as:
As written above, we would say that in the forward reaction iron oxide and hydrogen gas are reactants, and iron and steam are products.
In the reverse reaction, iron and steam are reactants, and iron oxide and hydrogen gas are products.
Take note of how the double-arrows are written. The proper equilibrium double-arrow is hard to get to on a computer, so you'll often see it misrepresented like this: ⇔, but this is not actually the right way to do it. Write the reversible equations in the examples above in your notes with a proper double arrow:
Give it a few tries in your notes.
In both of the examples above the rates of both forward and reverse reactions become equal to one another (balance out) after a certain amount of time, and once that happens the amount of reactants and products will remain constant. Both forward and reverse reactions are still going on, but have the same rate. The overall system is balanced, but in a dynamic way. The system has reached DYNAMIC EQUILIBRIUM.
DYNAMIC EQUILIBRIUM - an equilibrium situation in which microscopic changes occur, but macroscopic changes do not
Imagine yourself on an escalator that is going down.
You start at the top (reactants) and end up at the bottom (products).
But when you are partway down you start walking up the escalator as it continues going down. If you match your rate of walking up to the same rate that the escalator is going down, you make no progress and appear to be at a standstill.
To an observer it would look as if you and the escalator had come to a stop, but actually both upward and downward movements continue. Dynamic equilibrium!
You can play with the applet below to see the same principle in action. Instead of a person and an escalator, it's a creature (which kind?) on a conveyer belt. You can control the rate of running and the rate of the conveyer belt.
Your Turn:
- Write reversible reactions for each of the following situations (be sure to balance your equations):
- Hydrogen iodide gas (HI) decomposes into its elements.
- Hydrogen and nitrogen gases combine to form ammonia gas, NH3.
- Describe two different mixtures of starting materials that can be used to produce the following equilibrium:
A + B ⇔ C + D
1a. 2HI(g) ⇔ H2(g) + I2(g)
1b. 3H2(g) + N2(g) ⇔ 2NH3(g)
2. You could start with either a mixture of A and B or with a mixture of C and D. Both starting materials will produce the same equilibrium.
Next Time...
Throughout this unit we’re going to take up 3 major tasks:
- Understand what dynamic equilibrium is, and what kinds of reactions will reach equilibrium.
- Use Le Chatelier’s Principle to get a qualitative understanding of how factors like temperature, pressure, volume, and changes in concentration shift an equilibrium.
- Be able to describe an equilibrium quantitatively, using the equilibrium constant expression Keq.
We continue with task #1 in the next lesson! See you then.