Acid-Base Definitions

Lesson Objectives Describe the properties of acids and bases. Define an acid and a base according to the Arrhenius theory. Define an acid and a base according to the Brønsted- Lowry theory. Be able to identify the conjugate acid-base pairs in a Brønsted-Lowry acid-base reaction. Define an acid and a base according to the Lewis theory.

Acids and bases are classes of compounds that you most likely encounter every day. Common household items such as fruits, juices, soaps, and detergents all contain various acids and bases. In this lesson, you will learn about the properties of acids and bases and several ways to define exactly what is considered an acid or base.

Acid-Base Properties

Acids are very common in some of the foods that we eat Acids are very common in some of the foods that we eat. Citrus fruits such as oranges and lemons contain citric acid and ascorbic acid, which is better known as vitamin C. Carbonated sodas contain phosphoric acid. Vinegar contains acetic acid. Your own stomach utilizes hydrochloric acid to digest food. Bases are less common as foods, but they are nonetheless present in many household products. Many cleaners contain ammonia, a base. Sodium hydroxide is found in drain cleaner. Antacids, which combat excess stomach acid, are comprised of bases such as magnesium hydroxide or sodium hydrogen carbonate.

Acids Acids are a distinct class of compounds because of the properties of their aqueous solutions. Those properties are outlined below. Aqueous solutions of acids are electrolytes, meaning that they conduct an electrical current. Some acids are strong electrolytes because they ionize completely in water, yielding a great many ions. Other acids are weak electrolytes that exist primarily in a non-ionized form when dissolved in water. Acids have a sour taste. Lemons, vinegar, and sour candies all contain acids. Acids change the color of certain acid-base indicators. Two common indicators are litmus and phenolphthalein. Litmus turns red in the presence of an acid, while phenolphthalein is colorless. Acids react with active metals to yield hydrogen gas. Recall that an activity series is a list of metals in descending order of reactivity. Metals that are above hydrogen in the activity series will replace the hydrogen from an acid in a single-replacement reaction, as shown below. Zn(s)+H2SO4(aq) → ZnSO4(aq)+H2(g) Acids react with bases to produce a salt and water. When equal moles of an acid and a base are combined, the acid is neutralized by the base. Water and an ionic compound called a salt are produced.

Bases Bases have properties that mostly contrast with those of acids. Aqueous solutions of bases are also electrolytes. Bases can be either strong or weak, just as acids can. Bases often have a bitter taste and are found in foods less frequently than acids. Many bases, like soaps, are slippery to the touch. Bases also change the color of indicators. Litmus turns blue in the presence of a base ( Figure 21.2), while phenolphthalein turns pink. Bases do not react with metals in the way that acids do. Bases react with acids to produce a salt and water.

Defining Acids and Bases An early way of classifying acids and bases was proposed by Svante Arrhenius, a Swedish chemist, in 1894. An Arrhenius acid is any compound that releases H+ ions when dissolved in water. An Arrhenius base is a compound that generates hydroxide ions (OH−) when dissolved in water. Some representative examples are given in the next table.

: Arrhenius acids and bases hydrochloric acid: HCl → H+ + Cl− sodium hydroxide: NaOH → Na+ + OH− nitric acid: HNO3 → H+ + NO3− potassium hydroxide: KOH → K+ + OH− hydrobromic acid: HBr → H+ + Br− calcium hydroxide: Ca(OH)2 → Ca2+ + 2 OH−

Many strong acids and bases can be identified based on the Arrhenius model. However, there are many compounds that share a number of common characteristics with acids and bases but do not fit the Arrhenius definitions. In the early 1920s, the Danish scientist Johannes Brønsted and the English researcher Thomas Lowry each published ideas that expanded the Arrhenius concept. According to this newer definition, a Brønsted-Lowry acid is any compound that can donate a proton (an H+ ion) to an appropriate acceptor. A Brønsted-Lowry base is a compound that can remove (or accept) a proton from a relatively Brønsted-Lowry acid.

Another more general definition of acids and bases was offered by the American chemist G.N. Lewis. A Lewis acid is any chemical species that accepts a pair of electrons, and a Lewis base is a chemical species that donates a pair of electrons. This is the broadest most commonly used definition, and all compounds that qualify as an acid or base under the previous definitions are also Lewis acids and bases.

In the diagram above, we see the same process illustrated multiple times, highlighting the ways in which the compounds are acting as an acid or a base according to each definition. According to the Brønsted-Lowry model, the protonated water molecule (the acid) is donating a proton to the OH− ion (the base). According to the Lewis model, the hydroxide ion has a pair of electrons (indicated by the black bar) that it donates to the protonated water molecule. "Pronated" refers to the extra hydrogen atom in the molecule, which increases its charge from neutral to +1. In both instances, the hydroxide ion serves as a base and the protonated water molecule is the acid.

An example of a Lewis acid-base reaction that would not fit the other definitions of acid and base is the formation of an adduct between boron trifluoride and ammonia shown above. The ammonia serves as a Lewis base by donating its lone pair of electrons to make a new bond with boron (the Lewis acid), which has an empty orbital (indicated by ◦) that can accept two electrons. Unless otherwise indicated, we will be using the Brønsted-Lowry model of acids and bases for the remainder of the semester.

Monoprotic and Polyprotic Acids Acids can further be categorized based on how many acidic hydrogen atoms they contain. Acidic hydrogen atoms are those which will be transferred to a base. A monoprotic acid has only one acidic hydrogen that would be transferred to a strong base, whereas a polyprotic acid has two or more. Common monoprotic acids include HCl, HBr, and HNO3. A common diprotic acid is sulfuric acid (H2SO4), and phosphoric acid (H3PO4) provides an example of a triprotic acid. In each case, all hydrogens are available to participate in acid-base reactions. However, that is not the case for all acidic molecules. For example, in acetic acid (CH3COOH), only the hydrogen bonded to the oxygen atom is acidic. The other three hydrogens are covalently bonded to carbon and cannot be removed by any of the bases that we will consider.

Lesson Summary Acids and bases were originally identified based on their physical and chemical properties. According to the Arrhenius model, acids and bases are compounds that release H+ or OH− ions when dissolved in water. In the Brønsted-Lowry model, an acid is a compound that can donate a proton (H+ ion), and a base is a compound that can accept a proton. A Lewis base is an electron-pair donor, whereas a Lewis acid is an electron- pair acceptor. This is the broadest of the three definitions. Acids can be classified as monoprotic or polyprotic based on the number of acidic hydrogens they contain.