An acid is a chemical substance that, when dissolved in water, produces hydrogen ions (H+) and has a pH value less than 7. Acids can also be defined as substances that donate a proton (a hydrogen ion) in a chemical reaction.

Acids - Understanding Their Properties, Types and Strength

History of acids

Acids have been known since ancient times, with the first recorded use of an acid being by the ancient Romans who used vinegar (acetic acid) as a cleaning agent. In the Middle Ages, alchemists began experimenting with acids, trying to find the "philosopher's stone" that would enable them to turn base metals into gold. These experiments led to the discovery of several new acids, including hydrochloric acid and nitric acid.

The Swedish chemist Svante Arrhenius was the first to propose a theory of acids and bases in 1884. He defined an acid as a substance that produces hydrogen ions (H+) in water and a base as a substance that produces hydroxide ions (OH-) in water. This theory helped to explain many of the properties of acids and bases and paved the way for the development of modern acid-base chemistry.

In the late 19th and early 20th centuries, many new acids were discovered, and their properties were studied in detail. This led to the development of many new applications for acids, including their use in the production of fertilizers, pharmaceuticals, and other chemicals.

Characteristics of acids

Acids have several characteristic properties, including:

1.Sour taste: Acids have a sour taste, which is why sour candies and citrus fruits taste tart.

2.Ability to dissolve metals: Some acids, such as hydrochloric acid, have the ability to dissolve metals, which can be useful in industrial applications.

3.Ability to react with bases: Acids react with bases to form salts and water in a process called neutralization.

4.Ability to turn blue litmus paper red: Acids have a low pH value, which means that they turn blue litmus paper red.

5.pH less than 7: Acids have a pH value less than 7 on the pH scale, which ranges from 0 to 14.

6.Release of hydrogen ions: When dissolved in water, acids release hydrogen ions, which can react with other substances.

7.Conductivity: Acids are good conductors of electricity when dissolved in water due to the presence of ions.

8.Corrosive: Some acids can be highly corrosive and can cause damage to living tissue and materials, which means that they need to be handled with care.

Types of acids

There are several types of acids, including:

1.Inorganic acids: Inorganic acids are derived from non-living sources and include hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), phosphoric acid (H3PO4), and carbonic acid (H2CO3).

2.Organic acids: Organic acids are derived from living organisms and include acetic acid (CH3COOH), citric acid (C6H8O7), lactic acid (C3H6O3), and oxalic acid (C2H2O4).

3.Strong acids: Strong acids are completely ionized in water and include hydrochloric acid, sulfuric acid, and nitric acid.

4.Weak acids: Weak acids are only partially ionized in water and include acetic acid, carbonic acid, and phosphoric acid.

5.Mineral acids: Mineral acids are a type of inorganic acid that are typically strong acids and are commonly used in industrial processes.

6.Fatty acids: Fatty acids are a type of organic acid that are important components of fats and oils and are essential for many biological processes.

7.Lewis acids: Lewis acids are a type of acid that can accept a pair of electrons in a chemical reaction.

8.Bronsted-Lowry acids: Bronsted-Lowry acids are a type of acid that donate a proton (a hydrogen ion) in a chemical reaction.

Determination of an acid

There are several ways to identify a substance as an acid:

1.Litmus paper test: Dip a strip of litmus paper into the substance being tested. If the litmus paper turns red, the substance is acidic.

2.pH test: Use a pH meter or pH indicator paper to test the acidity of the substance. Acids typically have a pH value of less than 7.

3.Reactivity with bases: Add a small amount of a known base, such as baking soda, to the substance being tested. If the substance fizzes or produces bubbles, it is likely acidic and is reacting with the base.

4.Sour taste: If the substance has a sour taste, it may be acidic. However, this method is not always reliable, as some substances that are not acids can also taste sour.

5.Corrosive properties: Acids are typically corrosive and can cause damage to living tissue and certain materials. If the substance is highly corrosive, it may be an acid.

Strength of acids

The strength of an acid depends on several factors, including:

1.Bond strength: The strength of the bond between the hydrogen ion (H+) and the rest of the molecule affects the acid strength. The weaker the bond, the stronger the acid.

2.Polarity: The polarity of the acid molecule affects its ability to donate a proton. More polar acids are generally stronger acids.

3.Size of the molecule: Smaller acids tend to be stronger acids because they can release a proton more easily than larger acids.

4.Resonance stabilization: If the acid molecule has resonance structures, this can stabilize the negative charge on the conjugate base, making the acid weaker.

5.Inductive effect: The presence of electronegative atoms or groups near the acidic hydrogen can withdraw electron density and stabilize the conjugate base, making the acid weaker.

6.Solvent effects: The strength of an acid can also be influenced by the solvent in which it is dissolved. In general, acids are stronger in more polar solvents.

7.Concentration: The concentration of the acid affects its strength, as higher concentrations of acid will have a higher concentration of H+ ions and thus a greater ability to donate protons.

Strength order of haloacids

Haloacids are a group of acids that contain a halogen atom (fluorine, chlorine, bromine, or iodine) bonded to a central atom, usually hydrogen. Here is a general order of the haloacids, from strongest to weakest:

Hydroiodic acid (HI)>Hydrobromic acid (HBr)>Hydrochloric acid (HCl)>Fluorohydric acid (HF)

The order of the haloacids depends on the electronegativity of the halogen atom. Halogens are highly electronegative and can stabilize the negative charge on the conjugate base of the acid. The more electronegative the halogen, the stronger the acid. As fluorine is the most electronegative halogen, it might be expected to produce the strongest acid. However, the fluorine atom is also relatively small, which makes it difficult to displace the proton, resulting in a weaker acid than hydroiodic acid, which has a larger iodine atom.

The acid strength  order of oxoacids of halogens 

HOF>HOCl>HOBr>HOI

Acid strength is based on the ease with which the hydrogen atom can leave as a proton (H+). For this to happen, clearly the partial charge on hydrogen should be as positive as possible. In other words, the oxygen must be able to pull the shared pair of electrons of the O−H bond towards itself. For this to happen easily, the electronegativity of the halogen atom should be as high as possible, so that it pulls the shared pair of electrons of the X−O bond towards itself, making oxygen partially electron deficient, resulting in it pulling the shared electrons of the O−H bond towards itself, making the liberation of the H+ ion easy.

Therefore, higher the electronegativity of the halogen atom, higher is the acid strength. As we know, as we move down a group, atomic size increases and this reduces the nuclear pull (attraction between the nucleus and the electrons), as electrons become further away from the nucleus. This leads to a decrease in electronegativity down a group. Hence, acid strength of oxoacids of halogens decrease as we move down the group.

The acid strength order of oxoacids

Oxoacids (acids that contain both oxygen and hydrogen) can vary in strength depending on several factors, such as the electronegativity of the central atom, the number of oxygen atoms, and the acidity of the hydrogen atoms attached to the oxygen atoms. Generally, the strength of oxoacids increases with the number of oxygen atoms attached to the central atom. Here is a general order of oxoacids from weakest to strongest:

Hypo- prefix oxoacids (e.g. hypochlorous acid, HClO) < Mono- prefix oxoacids (e.g. nitrous acid, HNO2) < Di- prefix oxoacids (e.g. carbonic acid, H2CO3) < Tri- prefix oxoacids (e.g. phosphoric acid, H3PO4) < Poly- oxoacids (e.g. sulfuric acid, H2SO4)

It is important to note that this is a general order and there may be exceptions depending on the specific oxoacid and the conditions under which it is tested.

The most dangerous acids and their effects

Acids are generally corrosive substances that can cause severe harm to living organisms and the environment. While many acids can be hazardous, some are particularly dangerous due to their high acidity, reactivity, or toxicity. Here are some examples of some of the most dangerous acids:

1. Hydrofluoric Acid (HF): HF is one of the most hazardous acids. It is highly corrosive and can penetrate the skin, causing severe burns and tissue damage. It can also cause systemic toxicity, affecting the heart, kidneys, and other organs.

2. Sulfuric Acid (H2SO4): Sulfuric acid is a strong acid that can cause severe burns and eye damage. It is also a powerful dehydrating agent that can cause materials to ignite or explode in contact with it.

3. Nitric Acid (HNO3): Nitric acid is a highly corrosive and toxic acid that can cause severe burns and eye damage. It is also a strong oxidizing agent that can react violently with organic compounds, causing explosions.

4. Hydrochloric Acid (HCl): Hydrochloric acid is a strong acid that can cause severe burns and respiratory damage. It is also a powerful corrosive agent that can dissolve metals and other materials.

5. Perchloric Acid (HClO4): Perchloric acid is a highly reactive and explosive acid that can cause severe burns and tissue damage. It is also a potent oxidizing agent that can react with organic materials and other chemicals, causing fires and explosions.

Applications

Industrial Processes: Acids are used in many industrial processes such as metal cleaning and pickling, oil refining, and chemical manufacturing. For example, sulfuric acid is used in the production of fertilizers, dyes, detergents, and plastics.

Food and Beverages: Acids are used as preservatives and flavor enhancers in the food and beverage industry. Citric acid, for example, is commonly used in soft drinks, candies, and jams.

Cleaning and Disinfecting: Acids are used in many cleaning and disinfecting products due to their ability to dissolve dirt and kill bacteria. Hydrochloric acid is often used in toilet bowl cleaners, while acetic acid is commonly used in vinegar-based cleaning solutions.

Medicine: Acids are used in medicine for a variety of purposes, such as in the formulation of certain drugs, as well as in diagnostic tests. For example, hydrochloric acid is used in the production of stomach acid supplements for individuals with digestive disorders.

Batteries: Acids are used in the construction of batteries, such as lead-acid batteries commonly used in cars and other vehicles.

Photography: Acids are used in the development of photographic film. For example, sulfuric acid is used in the production of the silver halides used in photographic emulsions.

Personal Care Products: Acids are also used in many personal care products, such as shampoos and skin care products. Salicylic acid, for example, is commonly used in acne treatments due to its ability to exfoliate and unclog pores.