Which is a very important acid

The concept of acid in everyday life

Acids in the household

Acids are common in everyday life: The vinegar in the household contains acetic acid. Milk contains lactic acid and wine contains tartaric acid. Acids are needed to preserve food. They give the food a sour taste. Pickles, lemon juice or sauerkraut taste sour. The Coca Cola contains phosphoric acid and carbonic acid. The stomach acid even contains hydrochloric acid.


Acids as irritating and corrosive substances

Ants and nettles use formic acid as a defense agent. The acid burns the skin, it can cause inflammation or even dissolve the skin. Acids are irritating and corrosive: if they get into the eyes, they can cause severe burns, in the worst case they can blind you. For this reason, you have to wear protective goggles and protective gloves when working with acids in the laboratory. The battery acid for car batteries contains sulfuric acid. This is particularly dangerous. Corrosive substances are marked with the GHS pictogram "Corrosive effect":

Excerpt from the film: Working safely with corrosive substances> Movie

Use of acids in the household

Acids are used in a variety of cleaning agents, for example in toilet cleaners. A descaler contains an acid that dissolves limescale residues on water pipes or in vessels. Working with protective goggles and protective gloves is also required here. For example, if you put dilute hydrochloric acid or a decalcifying agent on a calcareous fossil, it starts to foam. The experiment also works with the mineral calcite, which is made up of crystalline lime. The lime is chemically built up from calcium carbonate. This is dissolved by hydrochloric acid, producing the gas carbon dioxide.

Detection of acids

Acids can easily be detected in every household by dripping vinegar on red cabbage leaves. They turn red in the process. In the laboratory, acids are detected with dye solutions that change color with acids. Such detection reagents are called pH indicators. Red cabbage also contains a dye as an indicator. In the laboratory, for example, litmus is used as an indicator. If you mix an acid with water, you get one acidic solutionthat tastes sour and discolored an indicator.

The concept of acid in chemistry

1. Acid as a substance

For the Greeks and Romans, an acid was a substance that had properties similar to vinegar, tasted sour or had a corrosive effect on other substances. The Latin word acidus for sour derives from the Latin word acetum for vinegar. The alchemist Johann Rudolph Glauber (1604–1670) produced many new acids, he succeeded for the first time in the production of concentrated hydrochloric acid and its salts. Hydrochloric acid, sulfuric acid, or nitric acid have long been used in every laboratory.


The French chemist Antoine Lavoisier still believed in 1789 that oxygen was present in all acids. He saw the principle of “oxygène” as an acid-generating principle in oxygen. Humphry Davy refuted Lavoisier's theory and showed in 1812 that oxygen cannot be separated from chlorine. From this he concluded that hydrochloric acid cannot be an oxygen-containing compound, but that hydrogen must be the characteristic element of acids.

The term still valid today Acid as a substance goes back to Justus von Liebig. Around 1838 he described acids as substances that contain chemically bound hydrogen that can be replaced by metals. The naming of the substances and chemicals is mainly based on this definition.

With metals or metal oxides, an acid forms a typical salt in aqueous solution with the evolution of hydrogen. In the first example, sodium reacts with hydrochloric acid to form sodium chloride and hydrogen. In the second example, copper (II) oxide reacts with hydrochloric acid to form copper (II) chloride and water.

Base metal + acid Salt + hydrogen
2 Na + 2 HCl(aq) 2 Well+ + 2 cl + H2

Metal oxide + acid Salt + water
CuO + 2 HCl(aq)   Cu2+ + 2 cl + H2O

When a metal reacts with hydrochloric acid, the corresponding chlorides are always formed. With nitric acid the nitrates are obtained, with sulfuric acid the sulfates and with phosphoric acid the phosphates. The acids react with alkalis in a neutralization reaction. Acids can also react with salts if the acid that reacts with the salt is stronger than the acid from which the salt was formed.

Metals and metal oxides react with acids

Sodium reacts with hydrochloric acid to form sodiumchloride with the formation of hydrogen.

Copper (II) oxide reacts with hydrochloric acid to form copper (II) -chloride with the formation of water.

Copper reacts with nitric acid to form copper (II) -nitrate and brown nitrogen dioxide.

2. Arrhenius concept of acid

Pure water only conducts very little electricity. If you add table salt, the solution becomes electrically conductive. The chemists concluded from this that ions must be present as charge carriers in the aqueous solutions of the salts. When you mix an acid with water, the liquid also conducts electricity. The Swedish chemist Svante Arrhenius defined acids around 1887 as substances that produce protons H in water+ can give off and form an acid residue ion. According to Arrhenius' acid term, acidic solutions contain positively charged protons and negatively charged acid residue ions, while alkaline solutions contain positively charged base residue ions and hydroxide ions.

According to Arrhenius, hydrochloric acid is no longer an acid, but rather hydrogen chloride, which reacts with water to form "hydrochloric acid" and forms the protons. According to today's understanding, the Arrhenius definition cannot adequately explain what happens when an acid reacts with water. H+-Ions by themselves cannot exist in water, they always need a reaction partner, by which they can be absorbed. Arrhenius' concept of base is also inadequate, since there are other substances besides hydroxides that can form alkaline solutions.

3. Functional term of acid according to Brönsted

In 1923 the Danish chemist Johannes Nicolaus Brönsted (1879–1947) specified the term acid-base. According to Brönsted, acids are proton donors and transfer them to a reaction partner. Form with water Hydronium ions H3O+that too Oxonium ions to be named. The following example shows what happens when the Brönsted acid dissolves hydrogen chloride in water. The hydrogen chloride transfers a proton to the water molecule, which reacts to form a hydronium ion. In the case of the chlorine atom, the remaining electron from the hydrogen atom remains and thus forms a negative excess charge. It becomes a negatively charged chloride ion:

An aqueous solution containing chloride ions Cl and hydronium ions H3O+ is called "hydrochloric acid", but according to Brönsted the hydrogen chloride is the actual acid. The Brönsted base is the opposite of a Brönsted acid. Brönsted bases take over protons from their reaction partners. Therefore, in our example, the water acts as a Brönsted base.
  • Brönsted acids give protons and transfer them, they act as Proton donors.
  • Brönsted bases receive protons from reactants, they act as Proton acceptors.
  • The Acid-base reaction according to Brönsted with proton transfer is one Protolysis.

The Bronsted acid HA transfers a proton to the Brönsted base B.. The couples HA / A and B / HB+ are corresponding acid-baseCouples. The double arrow shows that the reaction takes place in both directions and that a chemical equilibrium is established. The acid-base reaction of hydrogen chloride with water described above is used for clarification:

The chloride ion Cl is the corresponding base to the acid hydrogen chloride HCl. The hydronium ion H3O+ the acid corresponding to the base is water H2O. In the water itself, a protolysis can take place without a reaction partner, it is then an autoprotolysis.
S.acid strength according to Brönsted

The strength of an acid depends on how well it donates protons and transfers them to a reaction partner. Strong acids such as hydrochloric acid or nitric acid are almost completely protolyzed. In aqueous solution, they transfer almost 100% of their protons to the water molecule with the formation of the H3O+-Ions. Weak acids like acetic acid only react partially. In the case of an acetic acid solution with a concentration of 1 mol per liter, for example, an equilibrium is established that is still strongly on the left-hand side of the equation. Only 0.4% of the acetic acid molecules reacted with the water molecules.

4. Acid-base term according to Lewis

The American chemist Gilbert Newton Lewis (1875-1946) published another acid-base theory. According to Lewis, acids can add electron pairs, they are E.electron pair acceptors. In a Lewis acid-base reaction, electron pair bonds are formed. Lewis acids are electrophilic particles, they are looking for a negative charge and have an incompletely occupied orbital in the valence shell. According to Lewis, for example, aluminum chloride AlCl3 act as an acid.

Accordingly, Lewis bases are nucleophilic particles that are looking for a positive charge and can provide electron pairs. They act as Electron pair donors. These include water, nitrogen or ammonia molecules.

The Lewis model is rarely used in chemistry classes because it is too complicated. According to Lewis, for example, hydrogen chloride is no longer an acid, only the proton H+ is an acid because it acts as an electron pair acceptor. However, the Lewis concept is indispensable for understanding the reaction mechanisms in organic chemistry, because it can explain a number of the reactions there.

additional Information
Autoprotolysis (of water)
Corrosive substances in the hazardous substances
Powerpoint presentations on the topic

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