REAZIONI CHIMICHE

CHEMICAL REACTION

In a chemical reaction, some initial substances, the reactants, react to each other and are transformed into different substances, the products, which have a different chemical identity than the reactants. A chemical reaction is a process in which bonds are formed and/or broken between atoms of the same or different elements.

Examples of chemical reactions are the burning of a piece of paper: the paper turns into ash and carbon dioxide and energy is produced; the formation of rust on a piece of iron is also a chemical reaction: if exposed to moisture and oxygen for a long time, an iron nail turns into a pile of reddish dust.

A chemical reaction is represented by a chemical equation.

In a chemical equation, the formulae of the reactants (the initial substances) are linked by the + sign if more than one, and are on the left; in the centre is the reaction arrow, which takes on the meaning ‘react to give’; finally, on the right are the product formulae (the final substances), linked by the + sign if more than one.

C + O2 CO2

Numerically, the atoms of each element present in the initial reactants must be the same in the final products.

All chemical reactions obey Lavoisier’s law (law of conservation of matter):

Nothing is created and nothing is destroyed

Numbers that multiply entire chemical formulae must therefore be introduced into the chemical equation to have an equal number of atoms of the same element among both reactants and products. It is not allowed to change the molecular formulae of the substances in a reaction, we can only vary the number of molecules with coefficients in front of them.

These coefficients are called ‘stoichiometric’ and the operation that defines them is called

balancing a chemical reaction.

Other symbols may also appear in chemical equations to give information about the reaction, or the substances involved; for example, symbols indicating the physical state are schematized by different subscripts: solid state (s); liquid state (l); aeriform state (g); or aqueous solution (aq).

The naming of a molecule in a chemical reaction is dictated by precise rules, drawn up by the International Union for Pure and Applied Chemistry (IUPAC), an international association that periodically meets to update the rules of ‘chemical syntax’ in the light of new knowledge. Chemical nomenclature is the set of such rules and allows compounds to be identified by a specific name, which is defined from the substance’s formula. The first to provide an arrangement for chemical nomenclature, making it similar to modern nomenclature, was the French chemist Antoine Lavoisier, around 1787. Some examples are sodium chloride: NaCl, zinc sulphate: ZnSO4, sulphuric acid: H2SO4

Another important aspect to know about chemical relationships is that there are four types:

synthesis reaction; this type of reaction allows a single product to be obtained from two or

more reactants: A + B → C

decomposition reaction; this type of reaction, also called analysis, is the reverse of the previous one, and a molecule can be split into two or more species: AB → A + B

exchange reaction; in this type of reaction, a species consisting of a single element can replace one or more atoms in another compound. In this way, two new molecules are formed, which often have profoundly different characteristics to those of the initial reactants: AB + C → AC + B

double exchange reaction; as the name suggests, this type of reaction allows two molecules to exchange elements with each other, forming two distinct species, different from the reactants: AB + CD → AC + BD

A particular type of ‘double exchange’ is the neutralization reaction, in which an acid and a base react to form salt and water.

WALL OF REACTIONS

To best understand chemical reactions, one must experience them!

The wall opposite features 12 chemical elements from the periodic table; each chemical element is represented by coloured balls:

Carbon (C) Hydrogen (H) Oxygen (O) Sodium (Na) Potassium (K) Iodine (I) Sulphur (S) Calcium (Ca) Chlorine (Cl) Manganese (Mn) Nitrogen (N)

Iron (Fe)

The wall is divided into two parts, each part has 8 boxes to be filled with coloured balls (a string of colours); the combination of the coloured balls will create a molecule that will be represented on the small monitor, both in chemical formula and with its nomenclature.

The objective is then to feed the coloured balls into the system through the auger and then with a game of logic and levers manage to create a combination from the ones below. If you get it wrong, no problem, just empty all the balls and start again.

Having created the first molecule, the first reactant, and then the second molecule, the second reactant, if these give rise to a chemical reaction, the reaction conducted in the laboratory will appear directly on the large monitor.

Once the video is finished, you can try a new reaction, resetting the two colour trings. Try imitating the combinations below: