While studying organic chemistry, you’ll often see acids and bases being discussed throughout your course. It’s because these two substances are an essential part of organic chemistry. And studying them might seem complex initially.
But worry not.
We’ll simplify the concepts and strengths of acids and bases. So, studying the subject of acids and bases organic chemistry will be fun for you.
So, let’s dive in.
What are Acids and Bases?
Let’s first understand what acids and bases are.
Three definitions explain both of these substances. Let’s explore all these in detail:
Arrhenius Theory
The Arrhenius theory is the first modern definition of acids and bases.
Presented by Svante Arrhenius, this theory explains how acids and bases work when dissolved in water. Acids produce hydrogen ions (H⁺) in water while bases produce hydroxide ions (OH⁻).
To explain it precisely, hydrogen ions make water more acidic, with a lower pH value, while hydroxide ions make it less acidic, with a higher pH value. Less acidic water is safe to drink.
When mixed in water, acids and bases neutralize each other. They combine the hydrogen ions with hydroxide ions to form H₂O.
H⁺ + OH⁻ → H₂O
Having discussed that, we understand that the Arrhenius definition only explains how acids and bases work after mixing in water.
However, it doesn’t explain how they work with other solutions or if they even exist outside water.
To understand this complexity, we’ll look at the Bronsted-Lowry definition of acids and bases.
Bronsted-Lowry Theory
J. N. Bronsted and T. M. Lowry take the Arrhenius definition one step further by differentiating between acids and bases.
According to Bronsted-Lowry, acids donate a proton while bases accept a proton.
Example: Hydrochloric acid (HCI) donates a H⁺ hydrogen ion to ammonia (NH₃).
This makes hydrochloric acid a Bronsted-Lowry acid and ammonia a Bronsted-Lowry base.
The following chemical equation explains it perfectly:
NH₃ + HCl → NH₄Cl
Together, they form ammonium chloride, a white water-soluble salt.
Having understood the Bronsted-Lowry definition, we find that it discusses protons, but doesn’t explore the equation of electrons. The Lewis theory steps in here.
How, let’s discuss!
Lewis Theory
Gilbert Lewis defines acids as substances that accept a pair of electrons and bases as substances that donate a pair of electrons.
Example: Methenium ion (CH₃⁺) is electron-deficient with a total of just six electrons, making it a strong Lewis acid. Ammonia (NH₃) is electron-rich as its nitrogen atom has a pair of electrons that it can donate.
When ammonia donates its pair of electrons to the methenium ion, it forms the methylammonium ion (CH₃NH₃⁺).
Here is the chemical equation to understand the reaction:
CH₃⁺ + NH₃ → CH₃NH₃⁺
These varying definitions of acids and bases can confuse.
So, to clarify, acids and bases are substances that donate or accept electrons and protons.
A Bronsted-Lowry acid donates protons and a Bronsted-Lowry base accepts protons, while a Lewis acid accepts electrons and a Lewis base donates electrons.
Now let’s discuss ways to identify the strengths of acids and bases.
Strength of Acid and Base
The strength of an acid and base lies in their ability to donate or accept a proton or electron, depending on the Bronsted-Lowry and Lewis definitions.
In the Bronsted-Lowry theory, an acid is strong if it can donate H⁺ easily, and a base is strong if it can accept H⁺ with ease.
On the contrary, in the Lewis theory, an acid is strong if it accepts an electron easily, and a base is strong if it donates an electron effortlessly.
But how will you identify the strength? Let’s find out.
pKa and pKb Values
Logarithms are used to identify the strength of acids and bases. These logarithms are pKa and pKb.
If the pKa has a lower value, it indicates that the acid is strong, and if pKb has a lower value, it suggests that the base is strong.
Example: The hydrochloric acid has an estimated pKa value of -7, which makes it a strong acid. On the contrary, ethanol has a very high pKa value of 16, making it a weaker acid.
Similarly, we can understand the strength of bases with the pKb value.
Example: Methoxide (CH3O-) has a pKb value of -1.5, indicating that it is a strong base, while pyridine (C₅H₅N) has a value of 8.77. This higher pKb value means that pyridine is a weaker base.
Now, if you’re wondering how to determine the pKa and pKb values of acids and bases, let’s break down the process.
Determining pKa of Acids
First, we’ll see the process of determining the pKa of an acid.
The easiest way to do so is by titration, adding an acid and a conjugate base to water. Here, we’ll explore the process step-by-step:
- Put an acid in water
- Titrate it by adding a base
- Measure it with a pH meter and see the changes in the pH level as you add the base.
- Draw a titration curve on a graph. Here, the X-axis equals the amount of base added, and the Y-axis shows the pH you’ve measured.
- Find the halfway point at the titration curve where you’ve added enough base to neutralize half the acid.
The pH value at the halfway point equals pKa, allowing you to determine the pKa value of the acid.
Example: Add acetic acid (CH₃COOH) to a beaker and titrate it by adding sodium hydroxide (NaOH) as the base. But, add it slowly to measure the pH and see the pH level at the halfway point.
Determining pKb of Bases
This is a step-by-step titration process that explains how to find the pKb value of a base.
- Put a base in water inside a beaker
- Titrate it by adding an acid to it
- Measure it with pH water and see the changes in pH level as you slowly add acid to the base.
- Draw a titration curve on a graph. Here, the X-axis equals the amount of acid added, and the Y-axis shows the pH you’ve measured.
- Find the halfway point at the titration curve where you’ve added enough base to neutralize half the acid.
Example: Add sodium hydroxide as a base to the beaker and titrate it by adding hydrochloric acid. It should be slow, so you can easily measure the pH and see the pH level at the halfway point to determine the pKb value of sodium hydroxide.
Conclusion
The acids and bases organic chemistry subject is essential in learning chemical reactions, helping you to perform your lab experiments and other practical problems with ease.
You can apply this study in different fields, including manufacturing fertilizers and pharmaceuticals, maintaining the PH levels in living organisms, and preparing beverages and food items.
So, an in-depth knowledge of acids and bases can help you opt for a wide range of career choices and excel in them.
