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Weak Bases: Examples & Overview

Instructor: Danielle Reid

Danielle has taught middle school science and has a doctorate degree in Environmental Health

Did you know household ammonia, which is used as a cleaning product, can be classified as a weak base? Continue reading to learn about weak bases and discover more examples of these type of chemical substances.

Weak Bases: They're All Around Us

That scoop of baking soda you use when baking a cake, a household cleaner containing ammonia, and the bottle of antacid in your medicine cabinet all have something in common. Each of these products can be classified as a weak base. But what is a weak base anyway? Before we dive into a definition, let's first get a background on bases in general, and on one man whose discovery revolutionized the term.

'Fourscore and Seven Years Ago': The History of a Base

Many, many, years ago (late 1900's), a chemist by the name of Svante Arrhenius changed the landscape of physical chemistry. He not only proposed the theory that dilute solutions could conduct electricity, but that acids, bases, and salts could break apart and form ions in a solution. This theory, along with other scientific theories, revolutionized the concept of how certain molecules (charged and non-charged) behave in a solution. It paved the way for our current understanding on a topic we will be discussing, weak bases.

There are three broad ways to define a base. The first one is an Arrhenius base. Named after the man Svante himself, an Arrhenius base is a base that breaks apart (dissociates) in solution, forming hydroxide (OH) ions. The second definition is a Bronsted Lowry base. This defines a base that is willing to accept protons in a solution. Finally we have a Lewis base. A Lewis base states that a base is an electron pair donor in solution.

With three different definitions, you may be wondering how any of them are related to a weak base. Well, we know that Arrhenius theorized that substances in dilute solutions can conduct electricity. This led to the creation of a term called an electrolyte. An electrolyte is a substance that forms ions (charged molecules) in a solution. Bases are a type of electrolyte.

Arrhenius knew that electrolytes (i.e. bases) will dissociate (or break apart) into ions in a solution. However, a problem appeared when it was discovered that some of these guys don't completely follow this theory. Some electrolytes (i.e. bases) dissociate 100% to form ions in solution (diagram 1a), while others dissociate only partially (diagram 1b). This partial dissociation leads us to our next point, the definition of a weak base.

Diagram 1: Difference Between (a) Strong Bases and (b) Weak Bases
strong and weak base

Why Is A Base Weak?

Weak bases are substances that do not fully dissociate into ions in solution. Weak bases can also be referred to as weak electrolytes. The illustration is diagram 1b shows you exactly how a weak base would behave in solution. As you can see, some of the weak bases in solution break apart (i.e. dissociate) into ions while others appear to have a tough time breaking apart.

Keep in mind that the full dissociation of a strong base is roughly 100%. To put things in perspective regarding weak base dissociation, it is roughly 5-10%. Just by looking at these numbers you can see there is a drastic difference between a strong base and weak base dissociating. Thus, it makes perfect sense why diagram 1a and 1b illustrates the difference in behavior between a strong base and weak base present in a solution. Let's look at a few examples of weak bases.

Examples Of Weak Bases

When we first started on this journey to learn about weak bases we discovered that baking soda, household ammonia cleaner, and even antacid pills are all weak bases. How is this so? Using what we already know about weak bases, and partial dissociation, we can apply this to an example using baking soda.

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