Chemistry 30 Equilibrium - Reversible Reactions

1.1 Reversible Reactions

Typically when we think of what happens during a chemical reaction we think of the reactants getting totally used up so that none are left 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).

In this unit we will see that this isn't always the case. We will see that 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, will need to be modified.

Here are some examples of reactions that can be reversed:

Nitrogen dioxide, NO₂, a reddish-brown gas, reacts to form colourless dinitrogen tetroxide, N₂O₄:

2 NO_{2 (g)} → N₂O_{4 (g)}

But the reaction can also go the other way - dinitrogen tetroxide also readily breaks down to form nitrogen dioxide:

N₂O_{4 (g)} → 2 NO_{2 (g)}

We typically write a reaction that can go in both directions by using a double arrow (which will sometimes appear as ↔ in these online notes):

2 NO_{2 (g)} N₂O_{4 (g)}

Because the reaction continues in both directions at the same time, we never run out of either NO₂ or N₂O_4. NO₂ is continually being used up to form N₂O₄, but at the same time N₂O₄ is forming more NO₂

When hydrogen gas is passed over heated iron oxide, iron and steam are produced:

(1)	Fe₃O_{4 (s)} + 4 H_{2 (g)} → 3 Fe_(s) + 4 H₂O_(g)
The reverse reaction can occur when steam is passed over red-hot iron:
(2)	3 Fe_(s) + 4 H₂O_(g) → Fe₃O_{4 (s)} + 4 H_{2 (g)}
We can write these two equations together as:
(3)	Fe₃O_{4 (s)} + 4 H_{2 (g)} 3 Fe_(s) + 4 H₂O_(g)

When we have a reversible reaction written in this way, we need to be able to distinguish between which way the reaction is headed. As written above in Reaction (3), we would say that in the forward reaction iron oxide and hydrogen gas, the reactants, produce the products iron and steam.

During the reverse reaction, iron reacts with steam to produce the products iron oxide and hydrogen gas.

It is important to understand the terminology, and to use the terms correctly.

Does it matter which way we write our reversible reaction? Could we write it as

3 Fe_(s) + 4 H₂O_(g) Fe₃O_{4 (s)} + 4 H_{2 (g)}

Yes, we could. Now iron and steam are reactants of the forward direction, and iron oxide and hydrogen gas would be the reactants of the reverse direction.

Chemistry 30

Chemical Equilibrium

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1.1 Reversible Reactions