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Arrhenius acids and bases

Definition of Arrhenius acids and bases, and Arrhenius acid-base reactions 

Key points

  • An Arrhenius acid is any species that increases the concentration of start text, H, end text, start superscript, plus, end superscript in aqueous solution.
  • An Arrhenius base is any species that increases the concentration of start text, O, H, end text, start superscript, minus, end superscript in aqueous solution.
  • In aqueous solution, start text, H, end text, start superscript, plus, end superscript ions immediately react with water molecules to form hydronium ions, start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript.
  • In an acid-base or neutralization reaction, an Arrhenius acid and base usually react to form water and a salt.

Introduction

From the vinegar in your kitchen cabinet to the soap in your shower, acids and bases are everywhere! But what does it mean to say that something is acidic or basic? In order to answer this question, we need to examine some of the theories describing acids and bases. In this article, we will focus on the Arrhenius theory.

Arrhenius acids

The Arrhenius theory of acids and bases was originally proposed by the Swedish chemist Svante Arrhenius in 1884. He suggested classifying certain compounds as acids or bases based on what kind of ions formed when the compound was added to water.
Photograph of two Ruby Red grapefruits, one whole and one cut into three pieces.
Citrus fruits—such as grapefruit—contain high amounts of citric acid, a common organic acid. Image credit: Wikimedia Commons, CC BY-SA 2.5
An Arrhenius acid is any species that increases the concentration of start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54 ions—or protons—in aqueous solution. For example, let's consider the dissociation reaction for hydrochloric acid, start text, H, C, l, end text, in water:
start color #1fab54, start text, H, end text, end color #1fab54, start text, C, l, end text, left parenthesis, a, q, right parenthesis, right arrow, start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54, left parenthesis, a, q, right parenthesis, plus, start text, C, l, end text, start superscript, minus, end superscript, left parenthesis, a, q, right parenthesis
When we make an aqueous solution of hydrochloric acid, start color #1fab54, start text, H, end text, end color #1fab54, start text, C, l, end text dissociates into start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54 ions and start text, C, l, end text, start superscript, minus, end superscript ions. Since this results in an increase in the concentration of start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54 ions in solution, hydrochloric acid is an Arrhenius acid.

Hydrogen or hydronium ions?

Let's say we made a 2, M aqueous solution of hydrobromic acid, start text, H, B, r, end text, which is an Arrhenius acid. Does that mean we have 2, M of start text, H, end text, start superscript, plus, end superscript ions in our solution?
Actually, no. In practice, the positively charged protons react with the surrounding water molecules to form hydronium ions, start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript. This reaction can be written as follows:
start text, H, end text, start superscript, plus, end superscript, left parenthesis, a, q, right parenthesis, plus, start text, H, end text, start subscript, 2, end subscript, start text, O, end text, left parenthesis, l, right parenthesis, right arrow, start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript, left parenthesis, a, q, right parenthesis
Even though we often write acid dissociation reactions showing the formation of start text, H, end text, start superscript, plus, end superscript, left parenthesis, a, q, right parenthesis, there are no free start text, H, end text, start superscript, plus, end superscript ions floating around in an aqueous solution. Instead, there are primarily start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript ions, which form immediately when an acid dissociates in water. The following picture illustrates the formation of hydronium from water and hydrogen ions using molecular models:
Picture of a proton, represented by a dot, reacting with a water molecule to form hydronium.
When an acid dissociates in water to form start text, H, end text, start superscript, plus, end superscript ions, protons, the start text, H, end text, start superscript, plus, end superscript ions immediately react with water to form start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript. Thus, chemists talk about the concentrations of hydrogen ions and hydronium ions interchangeably. Image credit: UC Davis Chemwiki, CC BY-NC-SA 3.0 US
In practice, most chemists talk about the concentration of start text, H, end text, start superscript, plus, end superscript and the concentration of start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript interchangeably. When we want to be more accurate—and less lazy!—we can write the dissociation of hydrobromic acid to explicitly show the formation of hydronium instead of protons:
HBr(aq)+H2O(l)H3O+(aq)+Br(aq)        More accuratevs.HBr(aq)H+(aq)+Br(aq)    Shorter and easier to write!\begin{aligned}\text{HBr}(aq)+\text{H}_2\text{O}(l) &\rightarrow\text{H}_3\text{O}^+(aq)+\text{Br}^-(aq)~~~~~~~~{\text{More accurate}}\\ \\ &\text{vs.} \\ \\ \text{HBr}(aq) &\rightarrow\text{H}^+(aq)+\text{Br}^-(aq)~~~~\text{Shorter and easier to write!}\end{aligned}
In general, either description is acceptable for showing the dissociation of an Arrhenius acid.

Arrhenius bases

An Arrhenius base is defined as any species that increases the concentration of hydroxide ions, start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39, in aqueous solution. An example of an Arrhenius base is the highly soluble sodium hydroxide, start text, N, a, O, H, end text. Sodium hydroxide dissociates in water as follows:
start text, N, a, end text, start color #e84d39, start text, O, H, end text, end color #e84d39, left parenthesis, a, q, right parenthesis, right arrow, start text, N, a, end text, start superscript, plus, end superscript, left parenthesis, a, q, right parenthesis, plus, start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39, left parenthesis, a, q, right parenthesis
In water, sodium hydroxide fully dissociates to form start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39 and start text, N, a, end text, start superscript, plus, end superscript ions, resulting in an increase in the concentration of hydroxide ions. Therefore, start text, N, a, O, H, end text is an Arrhenius base. Common Arrhenius bases include other Group 1 and Group 2 hydroxides such as start text, L, i, O, H, end text and start text, B, a, left parenthesis, O, H, right parenthesis, end text, start subscript, 2, end subscript.
Beaker with water molecules, sodium cations, and hydroxide anions.
An aqueous solution of sodium hydroxide, an Arrhenius base, contains dissociated sodium and hydroxide ions.
Note that depending on your class—or textbook or teacher—non-hydroxide-containing bases may or may not be classified as Arrhenius bases. Some textbooks define an Arrhenius base more narrowly: a substance that increases the concentration of start text, O, H, end text, start superscript, minus, end superscript in aqueous solution and also contains at least one unit of start text, O, H, end text, start superscript, minus, end superscript in the chemical formula. While that doesn't change the classification of the Group 1 and 2 hydroxides, it can get a little confusing with compounds such as methylamine, start text, C, H, end text, start subscript, 3, end subscript, start text, N, H, end text, start subscript, 2, end subscript.
When methylamine is added to water, the following reaction occurs:
start text, C, H, end text, start subscript, 3, end subscript, start text, N, H, end text, start subscript, 2, end subscript, left parenthesis, a, q, right parenthesis, plus, start text, H, end text, start subscript, 2, end subscript, start text, O, end text, left parenthesis, l, right parenthesis, \rightleftharpoons, start text, C, H, end text, start subscript, 3, end subscript, start text, N, H, end text, start subscript, 3, end subscript, start superscript, plus, end superscript, left parenthesis, a, q, right parenthesis, plus, start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39, left parenthesis, a, q, right parenthesis
Based on our first definition, methylamine would be an Arrhenius base since the start text, O, H, end text, start superscript, minus, end superscript ion concentration increases in the solution. By the second definition, however, it would not count as an Arrhenius base since the chemical formula does not include hydroxide.

Acid-base reactions: Arrhenius acid + Arrhenius base = water + salt

When an Arrhenius acid reacts with an Arrhenius base, the products are usually water plus a salt. These reactions are also sometimes called neutralization reactions. For example, what happens when we combine aqueous solutions of hydrofluoric acid, start text, H, F, end text, and lithium hydroxide, start text, L, i, O, H, end text?
If we think about the acid solution and base solution separately, we know the following:
  • An Arrhenius acid increases the concentration of start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54, left parenthesis, a, q, right parenthesis:
start color #1fab54, start text, H, end text, end color #1fab54, start text, F, end text, left parenthesis, a, q, right parenthesis, \rightleftharpoons, start color #1fab54, start text, H, end text, start superscript, plus, end superscript, end color #1fab54, left parenthesis, a, q, right parenthesis, plus, start text, F, end text, start superscript, minus, end superscript, left parenthesis, a, q, right parenthesis
  • An Arrhenius base increases the concentration of start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39, left parenthesis, a, q, right parenthesis:
start text, L, i, end text, start color #e84d39, start text, O, H, end text, end color #e84d39, left parenthesis, a, q, right parenthesis, right arrow, start text, L, i, end text, start superscript, plus, end superscript, left parenthesis, a, q, right parenthesis, plus, start color #e84d39, start text, O, H, end text, start superscript, minus, end superscript, end color #e84d39, left parenthesis, a, q, right parenthesis
When the acid and base combine in solution, start text, H, end text, start subscript, 2, end subscript, start text, O, end text is produced from the reaction between hydrogen ions and hydroxide ions, while the other ions form the salt start text, L, i, F, end text, left parenthesis, a, q, right parenthesis:
H+(aq)+OH(aq)H2O(l)Formation of waterLi+(aq)+F(aq)LiF(aq)Formation of salt\begin{aligned} \greenD{\text H^+}(aq)+\redD{\text{OH}^-}(aq) &\rightarrow \text{H}_2 \text O(l)\quad&&\text{Formation of water} \\\\ \text{Li}^+(aq)+\text{F}^-(aq) &\rightarrow\text{LiF}(aq)&&\text{Formation of salt} \end{aligned}
If we add the reactions for the formation of water and the formation of salt, we get our overall neutralization reaction between hydrofluoric acid and lithium hydroxide:
start color #1fab54, start text, H, end text, end color #1fab54, start text, F, end text, left parenthesis, a, q, right parenthesis, plus, start text, L, i, end text, start color #e84d39, start text, O, H, end text, end color #e84d39, left parenthesis, a, q, right parenthesis, right arrow, start text, H, end text, start subscript, 2, end subscript, start text, O, end text, left parenthesis, l, right parenthesis, plus, start text, L, i, F, end text, left parenthesis, a, q, right parenthesis

Limitations of the Arrhenius definition

The Arrhenius theory is limited in that it can only describe acid-base chemistry in aqueous solutions. Similar reactions can also occur in non-aqueous solvents, however, as well as between molecules in the gas phase. As a result, modern chemists usually prefer the Brønsted-Lowry theory, which is useful in a broader range of chemical reactions. The Brønsted-Lowry theory of acids and bases will be discussed in a separate article!

Summary

  • An Arrhenius acid is any species that increases the concentration of start text, H, end text, start superscript, plus, end superscript in aqueous solution.
  • An Arrhenius base is any species that increases the concentration of start text, O, H, end text, start superscript, minus, end superscript in aqueous solution.
  • In aqueous solution, start text, H, end text, start superscript, plus, end superscript ions immediately react with water molecules to form hydronium ions, start text, H, end text, start subscript, 3, end subscript, start text, O, end text, start superscript, plus, end superscript.
  • In an acid-base or neutralization reaction, an Arrhenius acid and base usually react to form water and a salt.

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  • spunky sam blue style avatar for user ahmedhanif271
    what is the difference between Aqueous and liquid
    (67 votes)
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  • blobby green style avatar for user Khaled Tareq
    are hydrogen atoms the cause of the acidic taste
    (24 votes)
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    • primosaur ultimate style avatar for user Yellow Shiƒt»
      Yes, though to be more specific, it is the H+ ions that indirectly cause an acidic food to taste sour.

      To put it simply, when acids found in the food combine with saliva in your mouth, H+ ions are produced. These H+ ions react with the protein molecules on your tongue and cause them to change shape. The change in shape causes the protein molecules to send an electrical signal to your brain that you experience as a sour taste.

      Source: Introductory Chemistry(Third Edition) by Nivaldo J. Tro
      (49 votes)
  • piceratops seedling style avatar for user Caitlyn Limpioso
    Are certain compounds acid only when in aqueous solutions? For example, I found something saying, "The compound, H2S, is called hydrogen sulfide when it is in pure form but it is called hydrosulfuric acid when its acidic properties in an aqueous solution is being discussed." I am a little confused
    (3 votes)
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    • aqualine tree style avatar for user gargi
      The thing is an acid is any substance that increases the hydronium ion concentration of the solution or yields hydronium ions on dissociation. Thus, in aqueous solution only can a substance dissociate and form hydronium ions and be classified as an acid. Not in any other form can it yield a H+ ion. I hope it helps...
      (5 votes)
  • piceratops ultimate style avatar for user Kartikeye
    Are Arrhenius and Bronsted-Lowry just different ways to define an acid or a base or are they a different class of acids and bases all together??
    (14 votes)
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    • leafers seedling style avatar for user Ian N. Friskus
      It's a different way to define acids and bases. It's simply an extension to the theory of Arrhenius. Arrhenius defines acids and bases by the dissociation products that are formed when the acid or base is added to water, while Bronsted and Lowry define acids and bases by the reactions that occur when both are added together. Basically, acids in the Bronsted-Lowry theory donate protons (H+) to other substances while bases accept them to produce water (OH- + H+ = H2O).
      (2 votes)
  • old spice man green style avatar for user Jonathan Ziesmer
    What is meant by "dissociation?"
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  • blobby green style avatar for user Pooja Goyal
    if we are able to separate cl- ion from hcl solution what do we get ? if we get hydromium ions , is it is acidic ?
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  • leaf green style avatar for user vashist.ssy
    i have read somewhere that water is a bit acidic but H is neutral in nature?so what can we say about it?
    (6 votes)
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  • leafers ultimate style avatar for user rezatahmid
    I don't understand that when hydronium exists instead of hydrogen ions in water, why doesn't hydronium react with water instantly to form something else since it's an ion.
    (3 votes)
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    • piceratops seed style avatar for user RogerP
      Not all ions are reactive (think of dissolving salt in water to give Na+ and Cl- ions) - it depends on the circumstances.

      H+ ions are more reactive than H3O+ ions, so when an acid dissociates in the water, the protons immediately latch on to water molecules to give H3O+ ions which are more stable than H+ ions.
      (5 votes)
  • duskpin ultimate style avatar for user BHE
    What other definitions are there?
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    • mr pants purple style avatar for user Ryan W
      Bronsted Lowry acids see basically the same thing as Arrhenius acids, but it also introduces the idea of conjugate bases and acids.

      Lewis acids accept a pair of electrons and Lewis bases donate a pair of electrons. Lots of metal chemistry is based on them being Lewis acids.
      (5 votes)
  • duskpin ultimate style avatar for user ༺𝓢𝓲𝓭𝓭𝓱𝓪𝓷𝓽༻
    what is the difference between an 'arrhenius acid' and a 'normal acid' ?
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    • blobby green style avatar for user Toshaani Goel
      Arrhenius, Bronsted-Lowry and Lewis concept are basically ways of characterising acids and bases. The substance proclaimed an acid by Arrhenius theory is referred to as Arrhenius acid. there is nothing like normal acid. All acids are normal explained by different theories
      (1 vote)