Acid Base Chemistry

Introduction to Acid Base Chemistry

Acids and bases are essential for many processes such as: maintaining homeostasis in the body, sustaining plant growth, and preserving aquatic life. They are also used in products found in the home. Acids and bases are in household products and food. Sodium hydroxide, a strong base, is used in household cleaners and soaps. Baking soda, a basic compound, helps baked goods rise. Citric acid gives lemons their sour taste. Vinegar contains acetic acid, a common acid which has many uses, such as cleaning and preserving food, contains acetic acid. Acid base chemistry is everywhere.

Acid base chemistry controls fundamental biological processes in living things. Acidity controls The production of ATP, the energy currency of the cell. Chemical laboratories use Acid-base chemistry. Titrations are a common acid-base reaction. They determine the concentration of a compound.

History of Acids and Bases

Laurent Lavoisier introduced the first theory of acids and bases. His theory catalyzed the exploration of acids and bases. The first theory that reflects the properties of acids and bases was Svante August Arrhenius. The Arrhenius definition states that acids produce H+ ions in water, and bases produce OH- ions in water. There are some exceptions to this theory. For example, ammonia can act as a base, but it does not produce OH- ions when dissolved in water.

Along with Svante August Arrhenius, Bronsted and Lowry developed a theory which states that acids are proton donors and bases are proton acceptors. This theory expanded the definition of an acid or a base because water does not always have to be the solvent used. Gilbert N. Lewis hypothesized that acids are electron acceptors and bases are electron donors. Lewis’ theory expanded the definition of acids and bases even further by including the exchange of electrons.

Ions in Acid / Base Solutions

Aqueous solutions generally consist of some combination of water, acids, bases and salts. These compounds may be partially or fully ionized – which means the compound separates into it’s ions.

For example, in pure water, a small percentage of the water molecules ionize into H₃O⁺ (hydronium ions) and OH^– (hydroxide) ions.

$ch{2 H2O <=> H3O+ + OH-}$

We will use H⁺ as a shorthand for H₃O⁺ ions, even though technically H⁺ ions do not exists by themselves.

Hydronium and hydroxide ions have a love-hate relationship with each other in aqueous solutions. The more H⁺ ions there are, the less OH^– ions there, and vice-versa.

What is pH?

pH is a number from 0 to 14 that represents the strength of an acid or base. Read more about what is pH.

Acids

What is an acid?

An acid is a molecule capable of donating a proton or forming a covalent bond with an electron pair. Here are some more detailed definitions of an acid. In aqueous form, acids have many unique properties such as:

Sour taste
pH lower than 7
Produces H⁺ ions in water
Reacts with metals to form hydrogen gas
Turns blue litmus paper red

Acid Strength

Acids have different levels of strength. Acid strength is dependent on its ability to donate H⁺ to a base. Strong acids ionize entirely in water, meaning the ions completely separate. Weak acids are only partially separate in water. Strong acids are very corrosive and harmful when they contact the skin. Typically strong acids have a pH of 0-1 and weak acids have a pH of 2-6. Weak acids are not as corrosive and are found in the body and food. To learn more, read the articles Strong Acids & Bases, and What is pKa

Factors that affect acid strength are:

Electronegativity of element bonded to H
Bond Strength

As the electronegativity of the element bonded to H increases, the acidity of the molecule increases. The acidity increases because more electronegative atoms strongly pull electrons to themselves. Strong electronegativity causes the bond to be polar. If the bond is polar, the hydrogen is more likely to dissociate from the molecule. Bond strength depends on the size of the element bonded to H. The smaller the atom, the greater the bond strength. As the bond strength between the atoms in the acid increases, the acidity of the molecule decreases. For example, hydrofluoric acid, HF is a weak acid. Even though F is the most electronegative atom, the bond in HF is strong because fluorine is very small. The bond between hydrogen and fluorine does not dissociate easily. The hydrohalic acids in group 17 increase in strength the following order:

HF < HCl < HBr < HI

Important strong and weak acids are presented below. Common acids include nitric acid HNO3, sulfuric acid H₂SO₄, hydrochloric acid HCl and acetic acid C₂H₃O₂H.

Is HCl a strong acid or weak acid? Strong

Is HNO3 a strong acid or weak acid? Strong

Is Acetic acid a strong acid or weak acid? Weak

Bases

A base is a molecule that accepts protons or donates an electron pair for a covalent bond. In aqueous form, bases have many unique properties such as:

Bitter taste
pH higher than 7
Produces OH^– ions in water
Slippery to touch
Turns red litmus paper blue

Acid vs Base

Simply put, an acid produced hydrogen ions in a solution, and base produces hydroxide ions. You can get more in depth on the differences by reading about acid base theories.

Base Strength

Like acids, bases have different levels of strength. There are strong and weak bases. Base strength is dependent on its ability to accept H⁺ from an acid. Similar to strong acids, strong bases ionize entirely in water, meaning the ions completely separate. Strong bases typically have a pH of 10-14 and weak bases have a pH of 8-13.

Factors that affect base strength are:

Electronegativity
Size

As the atoms’ electronegativity in the same row increases across the periodic table, the basicity decreases. The atom wants to hold on to its electrons and not wish to donate them to a proton. As the atomic size of atoms increases down a group, the electron density decreases, which causes the basicity to decrease. Common bases include sodium hydroxide NaOH, potassium hydroxide KOH and ammonia NH₃.

Acid-Base Neutralization

Acids and bases can neutralize each other, which produces salt and water. This acid base reaction generates water from the combination of H⁺ ions and OH^– ions. When a strong acid and a strong base are combined, the pH is equal to exactly 7. The pH is equal to 7 is because the acid and base both dissociate completely. When a strong base is neutralized with a weak acid, the pH is greater than 7. Contrarily, when a weak base is neutralized with a strong acid, the pH is less than 7.