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7.1: Arrhenius Acids and Bases

  • Page ID
    221508
  • Learning Objective

    • Identify an Arrhenius acid and an Arrhenius base.
    • Write the chemical reaction between an Arrhenius acid and an Arrhenius base.

    Historically, the first chemical definition of an acid and a base was put forward by Svante Arrhenius, a Swedish chemist, in 1884. An Arrhenius acid is a compound that increases the H+ ion concentration in an aqueous solution. Classic Arrhenius acids can be considered highly polarized covalent compounds which undergo dissociation in water, producing an anion (A-) and H+ as the cation. The H+ is usually referred to as a proton. The hydrogen atom contains one proton in the nucleus and one electron around it, so when the cation H+ is formed, only a proton is left, and therefore its name. 

    HA(aq) → A-(aq) + H+(aq)

    Typical Arrhenius acids are hydrochloric acid and nitric acid, since they undergo the following reactions in water:

    HCl(aq) → Cl-(aq) + H+(aq)

    HNO3(aq) → NO3-(aq) + H+(aq)

    The formation of the H+ cation in water is responsible for all the properties of an acid. 

    How can a covalent compound such as HCl generate ions in solution?

    In hydrogen chloride, the H-Cl single covalent bond is highly polarized, due to the difference in electronegativity between H and Cl. The chlorine atom carries a partial negative charge (ð-) while the hydrogen atom carries a partial positive charge (ð-). When hydrogen chloride is dissolved in water,  the polar nature of water molecules makes water interact with these partial charges in HCl, and it favors the rupture of the H-Cl, generating Cl- anion and H+ cation. This type of bond dissocation in which one of the elements keeps the shared electrons and ions are produced is called Heterolytic bond cleavage or ionization 

    H-Cl(aq) → Cl-(aq) + H+(aq)

    The formation of these ions is also stabilized by the water, by ion-dipole interactions

    In order to undergo ionization producing H+, the H atom must be attached to a highly electronegative element such as halogen (group 17 in the periodic table), or O, such as in the case of polyatomic acids, like HNO3 (the H is attached to one of the oxygen atoms, not the N. Remember Lewis structures?)

     

     

    Table \(\PageIndex{1}\) lists some Arrhenius acids and their names.

    Table \(\PageIndex{1}\) Some Arrhenius Acids
    Formula Name
    HC2H3O2 (also written CH3COOH) acetic acid
    HClO3 chloric acid
    HCl hydrochloric acid
    HBr hydrobromic acid
    HI hydriodic acid
    HF hydrofluoric acid
    HNO3 nitric acid
    H2C2O4 oxalic acid
    HClO4 perchloric acid
    H3PO4 phosphoric acid
    H2SO4 sulfuric acid
    H2SO3 sulfurous acid

    An Arrhenius base is a compound that increases the OH ion concentration in aqueous solution. Ionic compounds of the OH ion are classic Arrhenius bases. All hydroxides, such as NaOH and Ca(OH)2 are considered Arrhenius bases. Hydroxides are ionic compounds; therefore they undergo dissociation in water producing OH- (also sometimes written as HO- to indicate that the negative charge is carried by the oxygen atom) anion:

    NaOH(aq)→ Na+(aq) + OH-(aq)

    Ca(OH)2(aq) → Ca2+(aq) + 2 HO-(aq)

    This is in line with the general properties of ionic compounds. In this case, also water molecules stabilized the formation of cations and anions by ion-diple interactions.  

    Example \(\PageIndex{1}\):

    Identify each compound as an Arrhenius acid, an Arrhenius base, or neither.

    1. HNO3
    2. CH3OH
    3. Mg(OH)2

    Solution

    1. This compound is an ionic compound between H+ ions and NO3 ions, so it is an Arrhenius acid.
    2. Although this formula has an OH in it, we do not recognize the remaining part of the molecule as a cation. It is neither an acid nor a base. (In fact, it is the formula for methanol, an organic compound.)
    3. This formula also has an OH in it, but this time we recognize that the magnesium is present as Mg2+ cations. As such, this is an ionic compound of the OH ion and is an Arrhenius base.

    Exercise \(\PageIndex{1}\)

    Identify each compound as an Arrhenius acid, an Arrhenius base, or neither.

    1. KOH
    2. H2SO4
    3. C2H6

    Answer

    1. Arrhenius base
    2. Arrhenius acid
    3. neither (the H is attached to Carbon, the bond polarity is very low)

    Acids have some properties in common. They turn litmus, a plant extract, red. They react with some metals to give off H2 gas. They react with carbonate and hydrogen carbonate salts to give off CO2 gas. Acids that are ingested typically have a sour, sharp taste. (The name acid comes from the Latin word acidus, meaning "sour.") Bases also have some properties in common. They are slippery to the touch, turn litmus blue, and have a bitter flavor if ingested.

    Acids and bases have another property: they react with each other to make water and an ionic compound called a salt. A salt, in chemistry, is any ionic compound made by combining an acid with a base. A reaction between an acid and a base is called a neutralization reaction and can be represented as:

    acid + base → H2O + salt

    The stoichiometry of the balanced chemical equation depends on the number of H+ ions in the acid and the number of OH ions in the base.

    Example \(\PageIndex{1}\):

    Write the balanced chemical equation for the neutralization reaction between H2SO4 and KOH. What is the name of the salt that is formed?

    Solution

    The general reaction is as follows:

    H2SO4 + KOH → H2O + salt

    Because the acid has two H+ ions in its formula, we need two OH ions to react with it, making two H2O molecules as product. The remaining ions, K+ and SO42−, make the salt potassium sulfate (K2SO4). The balanced chemical reaction is as follows:

    H2SO4 + 2KOH → 2H2O + K2SO4

    Exercise \(\PageIndex{1}\)

    Write the balanced chemical equation for the neutralization reaction between HCl and Mg(OH)2. What is the name of the salt that is formed?

    Answer

    2HCl + Mg(OH)2 → 2H2O + MgCl2; magnesium chloride

    Key Takeaways

    • An Arrhenius acid is a compound that increases the H+ ion concentration in aqueous solution.
    • An Arrhenius base is a compound that increases the OH ion concentration in aqueous solution.
    • The reaction between an Arrhenius acid and an Arrhenius base is called neutralization and results in the formation of water and a salt.