In the previous post, we have discussed the strategy to determine if a particular chemical reaction is a Bronsted-Lowry Acid-Base reaction. In case you have missed it, you should check it out first to build up your fundamentals.
Today, we shall continue by looking at acids in terms of Bronsted-Lowry theory, specifically on weak acids.
Weak acids dissociate partially in water to form a low concentration of hydrogen ions, H+. The degree of dissociation for weak acids is always less than 1.
As such, the pH calculation for weak acids will be different from that of strong acids.
In order to determine the pH of weak acids, a useful table known as the I.C.E. table (which stands for Initial, Change and Equilibrium) is commonly used in Junior Colleges (JC) in Singapore.
Key to using this useful table is to appreciate that a weak acid will achieve a state of dynamic equilibrium and the acid constant, Ka , is used to represent its dissociation.
For example, a weak acid HA, would dissociate/ionise based on the following equilibrium equation:
HA + H2O <=> A– + H3O+
And from the equation, we can use the I.C.E. table to get the equilibrium concentrations for the terms involved in the equation.
With the Ka value given in the question, we can set up the Ka expression and then find the concentration of H3O+ which is equivalent to concentration of H+ ions. i.e. [H3O+] = [H+].
This will then allow you to find the pH of the weak acid HA.
And for degree of dissociation of acid, we just need to divide the amount of acid ionised/dissociated by the initial amount of acid.
Check out the video below which is an excerpt of Mr Sean Chua’s teachings in his JC2 H2 Chemistry Tuition Class, where he showed the step-by-step procedure to find the pH as well as the degree of dissociation of ethanoic acid, a weak acid.
I hope you find the content easy for your understanding and if you have any questions, leave me a comment below. Feel free to share this blog post with your friends.
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PS: Under related articles below, there are several blog post discussions and questions related to Acid-Base Equilibrium (also commonly known as Ionic Equilibrium). You can also do a keyword search using the search box at the top right hand corner. You will be able to find tips and notes on H2 Chemistry.
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