You’ve probably been told your entire life that a pH of 7 is neutral. This isn’t actually true. A neutral solution has equal concentrations of hydrogen and hydroxide ions, regardless of its pH. We only think of 7 as the magic number because it is the pH value of water at room temperature. However, water’s pH can vary depending on the temperature. This is all to do with a value known as , which is the ionic product of water.
The ionic product of water, , is a modified equilibrium constant for the dissociation of water.
Let’s explore that term in more detail.
How does water dissociate?
You should be familiar with the actions of acids other bases in water. They both dissociate.
- Acids dissociate to form positive hydrogen ions, alongside a negative ion.
- Bases dissociate to form negative hydroxide ions, alongside a positive ion.
Water itself dissociates. However, it does something a little different – it behaves amphoterically.
An amphoteric substance behaves as both an acid and a base.
Regardless of whether it is pure or not, water always partially dissociates into hydronium ions, , and hydroxide ions, . One molecule of water acts as an acid by donating a proton, while a second water molecule acts as a base by accepting the proton. This is a reversible reaction and sets up the equilibrium shown below:
Note the state symbols used. Water is liquid, whilst the hydronium ions and hydroxide ions are aqueous – meaning they are dissolved in water.
The hydronium ion is a conjugate acid. You might remember from Bronsted-Lowry Acids and Bases that this is an acid formed when a base gains a proton. Likewise, the hydroxide ion is a conjugate base. This is a base formed when an acid loses a proton. Conjugate acids and bases behave just like standard acids and bases, and in fact, these two species are both quite strong. This means that once formed, they rapidly react with each other in the reverse reaction of our equation above, forming water again. Therefore, at any one time there are barely any hydronium ions and hydroxide ions in solution – most of the equilibrium consists of water molecules.
You can also represent the hydronium ion, as just a proton, . This simplifies our equation:
The dissociation of water. Anna Brewer, Study Smarter Originals
The equilibrium constant of water
You’ll notice that the dissociation of water is on reaction equilibrium, as we mentioned above. (We’d recommend checking equilibrium out.)
An equilibrium is a state of reaction where the rates of the forward and backward reactions are the same, and the concentrations of reactants and products remain constant.
You should remember that we can write equilibrium constants for reaction, known as . These relate the concentration of products to the concentration of reactants in a closed system at equilibrium. The formula is as follows:
For the reaction
What happens when we increase the temperature? The forward reaction is endothermic, meaning that it takes in energy. According to ‘Le Chatelier’s Principle’, changing the conditions of a reaction shifts the position of the equilibrium to oppose the change. Increasing the temperature means the forward reaction will be favored to absorb the extra heat, and the equilibrium will shift to the right. The concentrations of the products will increase. This in turn means that increases.
At room temperature, about 25℃, .
The effect of heat on the equilibrium reaction of the dissociation of water. Anna Brewer, Study Smarter Originals
How do we find the pH of water?
It’s all very well and good knowing what is and how it changes with temperature. Now we are going to learn how to use it to calculate the pH of water.
At the start of this article, we mentioned that water is neutralbut this doesn’t necessarily mean that it has a pH of 7.
A neutral solution has equal concentrations of hydrogen and hydroxide ions.
Remember, pH is a measure of hydrogen ion concentration in solution. As temperature increases, increases as the forward reaction in water’s equilibrium dissociation reaction is favoured. This means that the concentration of hydrogen ions increases. Therefore, as temperature increases, the pH of water decreases.
Now we can substitute this value into the equation for pH:
Remember that this solution is still neutral. It contains equal concentrations of hydrogen and hydroxide ions.
How do we find the pH of bases?
In pH, we explored how we can use to find the concentration of hydrogen ions in solution, and thus the solution’s pH. We’ll recap that now before exploring an alternative method using .
If we rearrange this, we get the following equation:
Similar to pH, pOH is a measure of hydroxide ion concentration in solution. We calculate it in the same way, taking the negative log of the hydroxide ion concentration. Let’s give it a go using the example above to see if we get the same answer:
We can now put this into the equation relating pH, pOH and pKw. First we need to find car:
This is the same answer as the one that we got before.
Both methods used to find pH work equally well. Find out which one your exam board wants you to know and practice using that one.
The following flow charts summarize how you find the pH of water and strong bases:
Finding the pH of water and strong bases. Anna Brewer, Study Smarter Originals