Main content
Chemistry library
Course: Chemistry library > Unit 13
Lesson 1: Acids, bases, and pH- Arrhenius acids and bases
- Arrhenius acids and bases
- pH, pOH, and the pH scale
- Brønsted-Lowry acids and bases
- Brønsted–Lowry acids and bases
- Autoionization of water
- Water autoionization and Kw
- Definition of pH
- Strong acid solutions
- Strong base solutions
- Acid strength, anion size, and bond energy
- Identifying weak acids and strong acids
- Identifying weak bases and strong bases
- Introduction to acid–base reactions
© 2023 Khan AcademyTerms of usePrivacy PolicyCookie Notice
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.
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:
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:
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:
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:
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:
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.
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:
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:
- 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:
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:
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:
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.
Want to join the conversation?
- what is the difference between Aqueous and liquid(67 votes)
- Aqueous means it's in a solution, liquid is just a state.(135 votes)
- are hydrogen atoms the cause of the acidic taste(24 votes)
- 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(56 votes)
- 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(24 votes)
- 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)
- What is meant by "dissociation?"(18 votes)
- It means "to fall apart" or the opposite of parts coming together (opposite of association). Basically, when A-B are together then they can dissociate by forming A separated from B.(23 votes)
- 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)
- 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).(16 votes)
- if we are able to separate cl- ion from hcl solution what do we get ? if we get hydromium ions , is it is acidic ?(5 votes)
- HCl + H₂O → H₃O⁺ + Cl⁻
Yes, we get hydronium ions, and the solution is acidic because the HCl increases the concentration of hydronium ions in the water.(9 votes)
- 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)
- Water contains H⁺ ions, which are acidic, and an equal number of OH⁻ ions, which are basic.
The two effects cancel each other, so water is neutral.(5 votes)
- 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)
- 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)
- What other definitions are there?(3 votes)
- 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)
- what is the difference between an 'arrhenius acid' and a 'normal acid' ?(1 vote)
- 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)