Chemical reactions

A definition

A chemical reaction is a modification of the structure of molecules or ionic compounds. During a chemical reaction, there is a modification of the chemical bonds existing between atoms or ions.

In a chemical reaction, the chemical substances that react together are called the reagents or reactants, while those formed by the reaction are called the products.

A chemical reaction can be written under the form of a chemical equation. The reagents, separated by plus signs, are on the left of the equation. The products, separated by plus signs, are on the right. The reagents and products are preceded by their soichiometric coefficient, unless this coefficient is 1 (we will discuss that later). The reagents and the products are separated by a simple arrow, symbolizing the chemical transformation.

The combustion reaction of one molecule of methane is written CH4 + 2 O2 → CO2 + 2 H2O. One molecule of methane (CH4) reacts with two molecules of dioxygen (O2) to give one molecule of carbon dioxide (CO2) and two molecules of water (H2O).

During a chemical reaction, the nuclei of the atoms are inchanged. Reactions involving modifications of atoms nuclei are called nuclear reactions, and are not studied by chemistry.

The reaction 6
3
Li
+ 2
1
H
→ 2 4
2
He
is a nuclear reaction, because the nucleus of the atoms are changed.

Different types of chemical reactions

There exist four basic types of chemical reactions : synthesis reactions, decomposition reactions, single replacement reactions and double replacement reactions.

In a synthesis reaction, written A + B → A–B, two simple reagents combine to form a more complex product.

In a decomposition reaction, written A–B → A + B, one reagent is broken down into two simpler products.

In a single replacement reaction, written A–B + C → A–C + B, one part of a compound is replaced by another part.

In a double replacement reaction, written A–B + C–D → A–C + B–D, two compounds exchange one of their parts.

There also exists more complex reactions, which are combinations of these four basic reactions.

Stoichiometry, the art of proportions

Stoichiometry is the study of the proportions between the amounts of substance of reagents and products involved in a chemical reaction.

Stoichiometry is useful to calculate the quantities of reactants that are necessary for a reaction, and the quantities of products that will be obtained.

Let's look again at the chemical equation of combustion of methane : CH4 + 2 O2 → CO2 + 2 H2O. The stoichiometric coefficients of methane, dioxygen, carbon dioxide and water are respectively 1, 2, 1 and 2. This means that every methane molecule must react with two molecules of dioxygen, and that this will form one molecule of carbon dioxide and two molecule of water.

This can be extrapolated to any number of methane molecules that reacts : n molecules of methane must react with 2 x n molecules of dioxygen, and this reaction will produce n molecules of carbon dioxide and 2 x n molecules of water.

Since amounts of matter (expressed in moles) are equivalent to numbers of molecules, stoichiometric coefficients allow us to reason in moles.

n moles of methane must react with 2 x n moles of dioxygen, and this reaction will produce n moles of carbon dioxide and 2 x n moles of water.

Any chemical equation must be equilibrated. This means that there must be the same atoms, in the same quantities, at each side of the arrow. There must also be the same net charge at each side of the arrow.

The equation 2 S2O32− + I2 → S4O62– + 2 I is equilibrated, because each side of the arrow there are 4 atoms of sulfur, 6 atoms of oxygen, 2 atoms of iodine and a negative net charge of 4.

Thermodynamics and kinetics

Thermodynamics studies the different forms of energy involved in chemical reactions : enthalpy, free energy, entropy, etc. Here are some important concepts of thermodynamics :

  • Any chemical reaction must release energy in order to be possible.
  • The activation energy of a reaction is the energy that must be provided to the reactants in order to perform the reaction
  • An endothermic reaction absorbs heat, while an exothermic reaction releases heat.

Kinetics is the study of the rates of chemical reactions. The rate of a reaction is the amount of matter reacting per unit of time. It is expressed in mol/s. Here are the main factors that can influence the rate of a reaction :

  • The activation energy of the reaction. The smaller it is, the faster the reaction will be.
  • The concentrations of the reagents. Often, the more important the concentration of reagents is, the higher the reaction rate will be.
  • The temperature. The higher it is, the faster the reaction will be.
  • The presence of a catalyst in the reaction mixture. A catalyst is a substance added to the reaction medium in small quantity, that increases the rate of reaction, but is not consumed by the reaction. As a consequence, only small quantities of catalysts are needed to accelerate a reaction. A catalysts increases the rate of a reaction by decreasing its activation energy.

To sum up, thermodynamics tells us if a chemical reaction is possible or not. If the reaction if possible, kinetics tells us its rate.

Thermodynamics and kinetics will be studied in detail in an article that will be published in the "Physical chemistry" category.

comments powered by Disqus