Acids and Bases
A substance that can act as an acid or base is called an amphoteric substance. Water is the most well-known amphoteric substance, but there are others, such as ammonia and some oxides.
Arrhenius Definition of Acids and Bases
The Arrhenius definition for acids and bases is the most basic and widely used. According to this definition, an acid is a substance that increases the concentration of H+ ions when dissolved in water, while a base decreases the concentration of H+ ions.
You can remember this because the word “acid” contains the letter “A,” which stands for “increases.” Similarly, the word “base” contains the letter “B,” which stands for “decreases.”
The Arrhenius definition is helpful in that it gives us a simple way to identify acids and bases. However, it has some limitations. First, it only applies to solutions, not solids or gases. Second, it only applies to water-based solutions. Finally, it only applies to H+ ions, not other ions that might be present in a solution.
Brønsted-Lowry Definition of Acids and Bases
The Brønsted-Lowry definition of acids and bases is the most widely used definition today. An acid is a substance that donates a proton (H+) to another substance, and a base is a substance that accepts a proton. The conjugate base of an acid is the substance that is left after the acid has donated a proton. The conjugate acid of a base is the substance that is formed when the base accepts a proton.
Lewis Definition of Acids and Bases
The Lewis definition of acids and bases is one of several ways to identify these important molecules. Developed in the early 20th century by American chemist Gilbert N. Lewis, the model is based on the concept of shared electrons.
In the Lewis definition, an acid is a molecule that has a free electron pair that it can share with another molecule. A base is a molecule that has an unpaired electron that it can donate to another molecule.
The Lewis model is useful for identifying acids and bases in situations where the Arrhenius or Bronsted-Lowry definitions do not apply, such as in reactions between molecules in the gas phase.
Properties of Acids and Bases
An acid is a molecule or ion capable of donating a hydron (proton or hydrogen ion H+) to another compound (base). When an acid is diluted in water, it increases the concentration of H+ ions in the water. acids have a sour taste, can cause corrosion and react with metals and carbonates to release bubbles of carbon dioxide.
Common acids include hydrochloric acid, sulfuric acid, nitric acid and phosphoric acid.
Bases are compounds that can accept protons, or hydrogen ions (H+). Bases are found in aqueous solutions and have a pH greater than 7.0. The higher the pH of a base, the stronger it is. Bases can be either organic or inorganic.
Inorganic bases are found in nature as minerals or formed when metal oxides or hydroxides dissolve in water to form solutions with a slippery feel. Many common household products contain bases, such as ammonia (found in window cleaners), drain cleaners, and oven cleaners.
Organic bases are found in plants and animals and are often used as medicine.household products, such as soaps, detergents, and fabric softeners, also contain organic bases.
Acid-base reactions are extremely important in many aspects of chemistry. This type of reaction is also referred to as a neutralization reaction. Acid-base reactions are important in the production of many products, including but not limited to soaps, detergents, and pharmaceuticals.
Acid-Base Reactions in Water
Water is the most important solvent in chemistry, and it is essential for life. All chemical reactions occur in water, including those involved in digestion, metabolism, and many other biochemical processes.
Acid-base reactions are particularly important in water because they help to maintain the pH of the solution. The pH of a solution is a measure of its acidity or alkalinity. A solution with a pH less than 7 is considered to be acidic, while a solution with a pH greater than 7 is considered to be alkaline.
The most important acid-base reaction in water is the dissociation of water itself:
H2O (l) ⇌ H+ (aq) + OH− (aq)
This reaction is responsible for the fact that water is neither acidic nor alkaline. The concentration of H+ ions in pure water is very low (about 10−7 molar), which gives it a pH of 7.
The H+ and OH− ions produced by the dissociation of water can combine to form other acids and bases. For example, when an acid such as hydrochloric acid (HCl) is added to water, it dissolves to form H+ and Cl− ions:
HCl (aq) → H+(aq)+ Cl−(aq)
The H+ ions then react with water molecules to form hydronium ions (H3O+):
H2O(l)+ H+(aq)→ H3O+(aq)
Similarly, when an alkali such as sodium hydroxide (NaOH) is added to water, it dissociates to form Na+ and OH− ions:
The OH− ions then react with water molecules to form hydroxyl ions (OH−):
Acid-Base Reactions in Non-Aqueous Solvents
In non-aqueous systems, such as those using supercritical fluids or ionic liquids, the Bronsted acidity function is not well defined. In these systems, one typically uses the Lewis concept of acidity. The Lewis acid is defined as any species that can accept a lone pair of electrons. The following table shows some examples of Lewis acids in various solvents.
Aqueous Bromine Supercritical CO2 Ionic Liquids
HBr(aq) + H2O <–> H3O+(aq) + Br-(aq) CO32-(scCO2) + H2O <–> HCO3-(scCO2) + OH-(scCO2) [Bmim][PF6]
BF4-(aq) + H2O <–> F- (aq) + B(OH)4-(aq) [Bmim][TfO]
AlCl3(aq)+ H2O <–> Al(OH)3(s) + 3 Cl-(aq))
The bases in these systems are typically Lewis bases, which are defined as any species that can donate a lone pair of electrons. The following table shows some examples of Lewis bases in various solvents.
Aqueous Bromine Supercritical CO2 Ionic Liquids
OH-(aq) HCO3-(scCO2) [Bmim]Br
F- (aq) [Bmim]Cl
Applications of Acid-Base Reactions
The ability of substances to act as both an acid and a base has many applications in industry and the home. For example, many cleaning products contain acids or bases, which react with and remove dirt or stains. In the food industry, acid-base reactions are used to control the pH of foods.
An acid-base titration is the determination of the concentration of an acid or base by exactly neutralizing it with a standard solution of base or acid. The point at which neutrality occurs is called the equivalence point and is determined by endpoint detection. The most common method for determining the endpoint is by monitoring the pH change during the reaction with a pH meter. Other methods that can be used include visual indicators, potentiometric titration, or conductometric titration.
The major advantage to using acid-base titrations to determine concentrations is that they are very accurate and can be performed quickly. In addition, they can be used to determine the concentration of weak acids and bases that cannot be accurately determined by other methods.
Preparation of Salts
The most common way to prepare a salt is to react an acid with a base. For example, when hydrochloric acid reacts with sodium hydroxide, NaOH, the product is sodium chloride, NaCl.
When an acid and a base react together in aqueous solution (that is, in water), they neutralize each other’s effect on the solution’s pH. The reaction between most acids and bases produces water and a salt as products. For strong acids and bases that are completely ionized in solution (that is, that exist as ions), the products are water and H+ or OH− ions.
In general, the reaction of an acid with a base can be written as:
Acid + Base → Salt + Water
However, this equation is valid only for strong acids and bases. When weak acids or weak bases are involved in the reaction, the products will contain both the salt and the conjugate acid or conjugate base of the reactants.
A substance that is capable of acting as both an acid and as a base is called a amphoteric substance. Water is the best example for an amphoteric substance. It can act as an acid as well as a base. When water acts as an acid, it donates a proton (H+) to the base. When water acts as a base, it accepts a proton from the acid.