Chemistry: Two Reaction Directions

Two Reaction Directions

The melting of ice can be represented by the equation

and the freezing of water can be represented by the reverse equation

Either of these unilateral reactions is written with the implication that the reactant on the left is completely converted to the product on the right. The situation where both states of H₂O are in equilibrium (a stable, balanced situation) is shown by the reversible reaction

where the two arrows mean that some H₂O molecules are participating in the forward (melting) reaction and other molecules are simultaneously participating in the reverse (freezing) reaction. Therefore, equilibrium is the stable situation resulting from two offsetting reactions. At a pressure of 1 atmosphere and a temperature of 0° C, both solid ice and liquid water are stable and will coexist. Notice that this equilibrium condition can be reached from either side; it can begin with either pure ice at –10° C or pure water at 20° C.

Whether you warm such ice or cool such water, the second phase will appear at 0° C.

The second example demonstrates another aspect of equilibrium by using the transformations between dinitrogen tetroxide and nitrogen dioxide.

N₂O₄ is a colorless gas, whereas NO₂ is dark reddish-brown. Their relative abundances are a function of temperature because N₂O₄ dominates at room temperature and NO₂ dominates at higher temperatures. At any one temperature, both gases are present in a mixture, and the color of the mixture allows an estimation of the ratio of the two nitrogen oxides. If a glass vessel containing them is colorless or pale, N₂O₄ exceeds NO₂. Warming that container would cause the color to slowly darken as N₂O₄ is converted to NO₂ until the ratio of the two species is appropriate for the higher temperature.

Figure 1

Temperature and the N₂O₄-NO₂ mixture.

Then the color ceases to change and remains at the new, darker hue. The color change in this gaseous reaction allows you to readily imagine the pair of reactions involved. The experimental fact that warming the gas mixture causes the color to darken shows that temporarily the reaction

dominates over the reverse reaction. The additional consequence that the color stops getting darker shows that a new chemical equilibrium has been reached:

in which the forward and reverse reactions occur at the same rate. The rate of creation of NO₂ is precisely equal to the rate of its consumption forming N₂O₄.

The final example demonstrates that equilibrium may involve more than two substances. Sodium iodide will react with sulfuric acid until this chemical equilibrium is established: