Topic Overview

Common Acids and Common Bases in the Reference Tables for Physical Setting/Chemistry list a few of the most common acids and bases. But what exactly are acids and bases? Acids and bases are classes of compounds that can be recognized by their easily observed properties. In this topic you will learn about these properties, the definitions that are used to explain these properties, and the important reactions that occur between acids and bases.

Properties of Acids and Bases

Certain observable properties can be used to identify both acids and bases. Although these properties can indicate whether or not a substance is an acid or a base, they do not explain why acids and bases behave the way they do.

Characteristic Properties of Acids

• An Arrhenius Acid has H and releases H+

• Examples: HCL, HBr,

• Acids in aqueous solutions conduct electricity

• Blue litmus - turns red in an acid

• Phenolphthalein - colorless in an acid

• pH is less than 7

• Contains a high concentration of Hydronium Ions

• Taste sour

• Acids react with certain metals to produce hydrogen gas

• Acids react with bases to form water and a salt. This type of reaction is called a neutralization reaction.

Characteristic Properties of Bases

• An Arrhenius Base has OH (hydroxide) and releases OH- (hydroxide ion) in an aqueous solution

• Examples: NaOH, KOH

• The OH- ion is the only negative ion in these solutions

• Bases in an aqueous solution conduct electricity

• Red litmus - turns blue in a base

• Phenolphthalein - turns pink in a base

• pH is greater than 7

• Contains high concentration of Hydroxide Ions

• Taste bitter; feel slippery or soapy

• Caustic: dissolves skin

Characteristic Properties of Salts

• An ionic compound that has positive ions other than hydrogen and negative ions other than OH

• Example: NaCl

• Salts conduct electricity (electrolyte)

• Acids, bases, and salts are electrolytes

Arrhenius Theory

There have been several attempts to develop explanations for the observable properties of acids and bases. Svante Arrhenius, a Swedish chemist, proposed a commonality of all acids to explain their similar properties. An Arrhenius acid is defined as a substance whose water solution contains the hydrogen ion as the only positive ion. For example, hydrochloric acid ionizes in water to form hydrogen and chloride ions.

In a parallel manner to acids, the properties of bases are explained as properties of the hydroxide ion in solution. Each Arrhenius base produces hydroxide ions when dissolved in water. The presence of the hydroxide ion makes the base an electrolyte. It is also the presence of the hydroxide ion that produces the slippery feel and bitter taste common to Arrhenius bases.

Neutralization Reaction

In a neutralization reaction, an Arrhenius acid with an Arrhenius base to produce water and a salt. There are several ways that these reactions can be expressed. For example, consider the neutralization equation:

Acid + Base → Water + Salt

All neutralization reactions have the same net. The Arrhenius definition is able to explain the process of neutralization as a reaction between hydronium ions and hydroxide ions to form water with the spectator ions forming a salt.

Alternate Acid-Base Theory

There are other acid-base definitions that expand upon the Arrhenius definition of acids and bases, one of these, the Bronsted-Lowry theory, defines an add as any substance that donates a hydrogen เดn (H). As you know, a hydrogen ion is a hydrogen atom without an electron, that is, it is simply a proton. Thus, a Bronsted-Lowry acid is defined as a proton donor. All Arrhenius acids are also Bronsted-Lowry acids. Bronsted-Lowry theory expands upon the Arrhenius concept by including proton donors that are not in aqueous solution.

• Acids donate H+ (Proton donor)

• Bases accept H+ (Proton acceptor)

Conjugate Acid-Base Pairs

Two substances that are related to each other by the donating and accepting of a single proton. A conjugate base is what remains after the acid donates a proton. A conjugate acid is what remains after a base accepts a proton.

pH Scale

The pH scale is a logarithmic scale indicating the acidity of a substance. A pH less than 7 is acidic; the lower the number the more acidic. a pH more than 7 is basic; the higher the number the more basic. A pH of 7 is neutral.

• 0-2 = Strong Acid

• 3-5 = Medium-Strong Acid

• 6 = Weak Acid

• 7 = Neutral

• 8 = Weak Base

• 9-11 = Medium-Strong Base

• 12-14 = Strong Base

• Movement from one whole number to the next represents a change by a power of 10

Titration

Titration is the process of adding measured volumes of an acid or an base of known molarity to an acid or a base of unknown molarity until the neutralization process occurs. The solution of known concentrations/molarities is called the standard solution. Knowing the volume of acid and base used in the titration procedure, together with the known molarities it is possible to calculate the concentration of the unknown solution. Titration formula:

Vocabulary

Acidity - The level of acid present in a solution, typically measured by the concentration of hydrogen ions.

Electrolyte - A substance that conducts electricity when dissolved in water, typically dissociating into ions.

Alkalinity - The level of alkaline (basic) substances present in a solution, often measured by the concentration of hydroxide ions.

Arrhenius - A Swedish scientist known for his theory of electrolytic dissociation, which explains the behavior of acids and bases in aqueous solutions.

Acid - A substance that donates protons (hydrogen ions) or accepts electron pairs in chemical reactions.

Hydrogen ion - A positively charged ion (H+) formed when an acid dissolves in water.

Hydronium ion - The hydrated form of the hydrogen ion (H3O+), present when acids dissolve in water.

Arrhenius base - A substance that dissociates in water to produce hydroxide ions (OH-) in solution.

Indicator - A substance that changes color in response to changes in pH, used to determine the endpoint of a titration.

Neutralization - The chemical reaction between an acid and a base to form water and a salt.

pH scale - A scale used to specify the acidity or alkalinity of a solution, ranging from 0 to 14, with 7 considered neutral, values below 7 acidic, and values above 7 basic.

Salt - A compound formed from the reaction between an acid and a base, consisting of positive and negative ions other than hydrogen and hydroxide ions.

Titration - A laboratory technique used to determine the concentration of a substance in solution by reacting it with a known volume and concentration of another substance.