Edward is an adjunct professor in several colleges across Michigan and has a Ph.D. in Biochemistry.
Introduction to the Acid Ionization Constant
When you wake up in the morning, you might start the day with a nice glass of orange juice. The delicious acid tang hits your tongue, and you know today is going to go well. But then you may find yourself wondering: why is it safe to drink orange juices while drinking a similar amount of hydrochloric acid would probably not end as well? The answer lies in the acid ionization constant. In this lesson, we will calculate the acid ionization constant, describe its use, and use it to understand how different acids have different strengths.
Definition of the Acid Ionization Constant
The acid ionization constant, also called the 'acid dissociation constant' or 'acidity constant,' is a measure of how much of the acid is deprotonated (which is when it's unbound to a hydrogen), compared to molecules still bound to a hydrogen. Let's look at a simple example of an acid:
In this example the acid HA becomes deprotonated by losing a hydrogen to the water and becoming A-. We know that not all acid HA will become deprotonated, so we can write an constant to establish a ratio of how much has and has not reacted with the water. Much like any other chemical equilibrium formula, we can write the equilibrium constant as:
Remember that a molecule in brackets such as this one...
...is the molar concentration of the molecule, the number of moles of HA per liter of solution in this case water. We're multiplying the molar concentration of the products, chemicals to the right of the equation, and dividing this by the concentration of the reactants, chemicals to the left of the equation. But, if we use the K above, this will almost always be an extremely small number because most of a water solution is water. Thus, we can modify the above equilibrium constant to this:
We are no longer looking at the amount of water in the solution, and the K changes from a standard equilibrium constant to a much more manageable value and becomes Ka, the acid ionization constant.
Real Example of an Acid Ionization Constant
As an example, let's look at hydrochloric acid, HCl. When we add solid HCl to water, we have the following reaction:
We then complete the equilibrium constant like we did above by dividing the concentration of the products by the reactants:
Next, we remove the concentra
Acid Ionization Constant Use
Thinking back to our breakfast and hydrochloric acid example, we can use the Ka to determine why orange juice is tangy, but our other acid is destructive. Hydrochloric acid's acid ionization constant is almost infinite. When hydrochloric acid is dissolved, almost all the molecules separate into Cl- and H+ and are ready to react with anything they come in contact with. Citric acid is another story. This acid is much weaker with its value much smaller than one. So, citric acid is slightly tangy as most of the molecules have not deprotonated, while HCl is mostly reactive ions.
But, what about acids with multiple protons? Citric acid is such a chemical. Citric acid has three separate acid portions of the molecule, and we can calculate the Ka of each just like above, but remember that each Ka is separate from the others.
Citric acid has a bit of a longer formula, so for this example we will abbreviate the chemical as citric and put the amount of hydrogens at the front of the chemical.
The first reaction of citric acid is:
and using our above method to calculate Ka we get:
We can then move to the second proton:
Each form of the acid has its own Ka that is found separately from each other Ka.
The acid ionization constant, Ka, is a way to measure the amount of deprotonated acid in a solution. Ka is calculated much the same way we calculate other equilibrium constants, but in this specific constant, the concentration of the solvent is left out of the equation. We can use this measurement to determine how strong each is compared to one another. The larger the value, the more of each acid molecule is deprotonated. We can also calculate several different Ka's for a molecule if the acid has multiple protons it can lose be separately calculating each value.
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