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The Arrhenius Definition of Acids and Bases

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  • 0:01 Arrhenius Acids
  • 2:44 Arrhenius Bases
  • 5:00 Strong v.s Weak
  • 6:19 Lesson Summary
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Lesson Transcript
Instructor: Elizabeth (Nikki) Wyman

Nikki has a master's degree in teaching chemistry and has taught high school chemistry, biology and astronomy.

In this lesson, you will learn the definition of Arrhenius acids and bases, discover some of their chemical properties and learn some examples. You will also learn about the difference between strong and weak Arrhenius acids and bases.

Arrhenius Acids

In the 1880s, Swedish chemist Svante Arrhenius was busy studying the electrical properties of chemicals when put in water. In his studies, he noticed that certain compounds produced hydrogen ions, often called protons, when they were put into aqueous (or water-based) solution. These electrically-conductive compounds were termed Arrhenius acids. By definition, Arrhenius acids are compounds that produce hydrogen ions (H+) in solution.

Let's take a closer look at the chemical structure of an Arrhenius acid. Hydrobromic acid is an Arrhenius acid with the formula HBr. When HBr goes into an aqueous solution, the positively charged hydrogen ion breaks apart from the negatively charged bromide ion. The bromide ion roams around in the solution, but the hydrogen ion latches onto water to form the hydronium ion (H3O+).

The process is called acid dissociation. It is represented chemically two different ways. Sometimes it is written as HBr + H2O -> H3O+ + Br-; other times it is written in a simplified form that leaves water out of the equation, HBr -> H+ + Br-.

These dissociation expressions may be written for the generic acid HA, where H is the hydrogen ion and A is the negatively charged particle left over: HA + H2O -> H3O+ + A- or HA-> H+ + A-.

Arrhenius acids are all around us; in fact, there are some in us! Our stomach naturally produces hydrochloric acid (HCl) to help digest food. We often purposefully eat or drink acids for their delightfully sour taste. Vinegar, aka acetic acid (HC2H3O2), is used as both a preservative and a taste enhancer, and sodas often contain a combination of carbonic acid and phosphoric acid, sometimes even citric acid. Citrus fruits, like grapefruits, lemons and limes, contain citric acid.

Besides producing hydrogen ions in water, all Arrhenius acids have a few things in common. They have pH values anywhere from 0 up to 7, they taste and smell sour and they will turn pH paper pink, red or orange.

Arrhenius Bases

Bases are just about as common as acids, but they are certainly less well known. There are probably some bases in your medicine cabinet, kitchen cupboards and cleaning supplies. Bases are chemically opposite acids in many ways, but we'll talk about that later.

An Arrhenius base is a compound that produces hydroxide ions (OH-) when in water. Let's zoom into a bar of soap to understand more about the chemical nature of bases.

Sodium hydroxide (NaOH) is an Arrhenius base that is often used to make soap. When the NaOH goes into solution, the positively charged sodium ion breaks apart from the negatively charged hydroxide ion. Both ions roam free in solution. This process is shown chemically as NaOH -> Na+ + OH-.

This equation may be written for the generic Arrhenius base BOH, where OH is the hydroxide ion and B is the positive ion attached to the hydroxide: BOH -> B+ + OH-.

Have you ever had acid reflux or heartburn? This happens because the acid in your stomach is a little out of control. Chances are, you overcame your upset stomach with a little help from a base in the form of magnesium hydroxide or aluminum hydroxide. These Arrhenius bases are key ingredients in antacids, like Tums or Rolaids. The potentially dangerous Arrhenius base sodium hydroxide is the active ingredient in drain uncloggers. As mentioned earlier, sodium hydroxide is often used to make soap as well.

Arrhenius bases taste soapy, feel slippery and have pH values greater than 7 up to 14. They turn pH indicator paper green, blue or purple.

Arrhenius acids and bases combine to make water, a product that is neither acid nor base but in fact neutral. Bases are effective at quelling heartburn because the hydroxide from the base (OH-) reacts with the hydrogen ion from the acid (H+) to make water: H+ + OH- -> H2O.

Strong vs. Weak Acids & Bases

Not all Arrhenius acids and bases are created equal. Some are strong, while others are weak. Strong acids and bases dissociate (break apart) 100% in solution. This means that every acid or base compound that goes into solution breaks apart into its constituent ions. Every single one!

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