How to Calculate Keq Given pKa

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Some chemical reactions are known as reversible reactions because they can go in two directions: forward and reverse. These reactions happen simultaneously and never stop, so they are also called dynamic reactions. A reaction is at equilibrium when the rate of both reactions is the same. However, while the concentrations of the reactants and products are constant, they are not necessarily equal. Equilibrium constants are sometimes called keq values. If you are experimenting with an acid-base reaction, the keq value is Ka, also known as acidity constant, which measures the strength of an acid in solution.

TL;DR (Too Long; Didn't Read)

In acid-base reactions, the equilibrium constant (keq value) is known as Ka. To work out Ka when you know pKa, use a calculator to find the antilog.

The pKa Value

When an acid dissociates in water, it releases a proton to make the solution acidic. However, only weak acids, which only partially dissociate in water, have both a dissociated state (A-) and undissociated state (AH). They exist together according to the equilibrium equation AH ⇌ A- + H+. The concentration ratio of both sides is constant provided analytical conditions are fixed. This is the Ka, which is defined by the equation Ka = [A-] [H+] ÷ [AH], where the square brackets indicate the concentration of the relative components. Because the Ka constants for acids can be long numbers (for example, the Ka for acetic acid is 0.000018), it is inconvenient to express acidity using the Ka constant alone. The pKa value was introduced as an index to describe the acidity of weak acids, defined as pKa = -log Ka.

Finding Keq From pKa

If you already have the pKa value of a compound, you can work out its Ka. For example, the pKa value of lactic acid is 3.86. The first thing you do is multiply the pKa value by negative one to invert its sign. In the case of lactic acid, this is 3.86 x (-1) = -3.86. Then use a calculator to raise 10 to the power of the negative pKa. In math, this is known as the antilog, and the key is normally marked 10x on scientific calculators. This means the Ka of lactic acid is 10(-3.86), which is 1.38 x 10-4 or 0.000138. The smaller the pKa value, the stronger the acid. This means lactic acid, with a pKa value of 3.86, is a stronger acid than acetic acid, which has a pKa value of 4.75.

References

About the Author

Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.

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