Chemical Equilibrium

1. Often we talk about reactions as if you mix a couple of things together to produce one or more new substances. For example, we say that Hydrogen and Oxygen can combine to make Water and write the chemical equation as:
   2 H₂ + O₂ 2 H₂O
   
   
   
2. Why don't we write the equation as: 2 H₂ + O₂ 2 H₂O
   In other words, why is it less likely that water breaks up to form hydrogen and oxygen than it is for hydrogen and oxygen to form water?
3. In fact, there is a very small chance that water could break up to form hydrogen and oxygen. Maybe we should write the chemical equation as:
   
   
   2 H₂ + O₂ 2 H₂O
   to show that it is mostly water that is formed.
   
4. However, so little water breaks up to form hydrogen and oxygen that it is OK, and considered correct to write the equation as:
   2 H₂ + O₂ 2 H₂O
   
5. However, now consider the following reaction in which the potential energy of the reactants and the products are almost the same, making the activation energy almost equal from left to right as it is from right to left:
   

6. Which do you think is the best way to write the equation for the above chemical reaction?

   2 NO₂ N₂O₄
   2 NO₂ N₂O₄
   2 NO₂ N₂O₄

7. If we start with all NO₂ , then they will collide and start to form N₂O₄. At first the likelyhood of two N₂O₄ molecules colliding will be small, but eventually the N₂O₄ will build up in concentration until it will be common for them to collide. Because they can almost as easily break up into NO₂ as NO₂ can combine to form N₂O₄ the reaction will start to go to the left. Eventually, you will have just as much N₂O₄ forming as NO₂ forming. In other words, the reaction will go left and right at the same rate, reaching an equilibrium.
8. All reactions are equilibrium reactions. However, many (like the formation of water from hydrogen and oxygen) are much more likely to happen one direction that we think of these reactions as only going in that direction. For those reactions it is OK to use a single arrow.
9. For reactions in which a significant amount of product and reactants are constantly being formed we use the double arrow to indicate the behavior of this equilibrium.