Before discussing water equilibrium, the student needs to remember the significance of the value of an equilibrium constant. The magnitude of the equilibrium constant tells if the reaction favors the reactants or products. If the equilibrium constant is greater than zero, the products are favored. If the equilibrium constant is less than zero, the reactants are favored.
Some examples from solubility data will illustrate how to evaluate K values. The values were determined at 20o C.
The constant for sodium chloride (table salt) is greater than zero (37.7). It is completely soluble in water. The constant for calcium sulfate (gypsum) is considerable less than zero (0.0000710). It is only slightly soluble and most of it remains as CaSO4 .
A primer on water.
Rarely will a person encounter pure water. The term “pure” means it is only water, nothing but water. Bottled water, drinking water from the tap, and water in nature all have some dissolved chemicals in them. Distilling water can remove a high percent of dissolved chemicals in water. Reverse osmosis can also remove most of the dissolved chemicals. Neither of these processes yield absolutely pure water.
One way to indicate there are dissolved chemicals in water is to test how well the sample of water conducts electricity. Water that contains dissolved ions will conduct electricity better than water without dissolved ions. This ability to conduct electricity is called conductance.
The two links below discuss preparation of absolutely pure water
Even absolutely pure water that would be expected to consist only of H2O molecules has a very small conductance. How can this be explained? The explanation is a very small number of water molecules dissociate (break apart) to form H+ ions and OH– ions. There is an equilibrium established between the water molecules and the H+ and OH– ions. The reaction is:
The arrows pointing both ways designate an equilibrium between the reactant water and the products H+ and OH–.
The equilibrium constant expression is:
The molar concentration of the H+ and OH– ions are equal at equilibrium. Their molar concentration is 1 x 10 -7 M. The molar concentration of water is 55.5 M ( see Interactions (Solutions) and scroll to the bottom of the page). Entering these values in the equilibrium constant expression and calculating gives the K value of 1.8 x 10-16.
When a solute is added to water to form a solution, the molar concentration of water changes very little. The molar concentration of water can be considered a constant. Therefore the molar concentration of water can be multiplied by the Kw value to get a simpler expression shown below.
What does this mean? An equilibrium constant value of 1 x 10-14 is very small. Therefore the equilibrium definitely favors water molecules. A very high percent of water is in the form of water molecules, and a very small percent of water molecules will break apart (dissociate) to form H+ ions and OH– ions in equal concentrations.
The equilibrium of water and the ions H+ and OH– is the foundation of acid and base chemistry that will be discussed in the following sections.
View the following videos to see an animation of what liquid water might look like at the molecular level.
The first video provides a review of the concept of dipoles and interaction presented in earlier sections. The bonds between two water molecules are called hydrogen bonds.