A chemical reaction involves bond breaking and bond formation. In order to understand a chemical reaction, one needs to understand the meaning of a chemical bond. As an example we take the hydrogen molecule. This molecule is stable enough to exist and has one sigma bond between its hydrogen atoms.
The chemical bond is a quantum mechanical phenomenon that was discovered in 1927 by two scientists. These are called Heitler and london. Prior to their discovery the nature of the chemical bond was ambiguous. The explanation of the existance of the chemical bond was not possible. Heitler and london put a mathematical formula that explained the existence of the chemical bond through molecular orbitals in the hydrogen molecule.
There are three types of chemical bonds: the covalent bond and the ionic bond and the dative bond. The ionic bond is an electrostatic phenomenon in which a positively charged atom reacts with a negatively charged atom to form a neutral compound. An important condition to the ionic bond to exist is that the difference in electronegativity between the atoms involved in the bond must be significant. This condition is a prerequisite for the formation of ionic bond.
An example of an ionic bond is the bond in NaCl, the eating salt. The ionic bond is stronger than the covalent and the dative bonds. The covalent bond is formed between two atoms through contribution of one electron each atom to the bond.
The covalent bond comprises two electrons are located in a bonding orbital that is shared between the nucluae of both atoms. Thus forming an entity that is more stable than either atoms alone. The bonding orbital is called the molecular bonding orbital. The dative bond is found only in complexes of organometallic compounds.
After we explained the meaning of a bond, we return now to our discussion of chemical reactions which is called also a chemical transformation.
In a chemical reaction we change the identity of the compound that we have such as its structure and its physical properties such as its boiling point. An example of a chemical reaction is the formation of water from hydrogen and oxygen molecules. In this rection we start with two completely different compounds in the gas phase and ends up with water which is a liquid.
There are spontaneous reactions as well as non-spontaneous reactions. Spontaneous reactions have negative values of the free energy of Gibbs which its mathematical symbol is G. Non-spontaneous reactions have positive values of the free energy of Gibbs. A chemical reaction with zero value of the free energy of Gibbs means that the reaction is in equilibrium which means that the reaction is not complete.
In addition to the free energy of Gibbs, a chemical reaction has normally an energy barrier which it must overcome for the reaction to take place. A reaction with low energy barrier means it does not require much energy in order to proceed in the forward direction, while a reaction with high energy barrier means it requires a large amount of input of energy in the reaction in order to proceed in the forward direction.
Unstable compounds such as peroxides react normally with low energy barrier while stable compounds such as carbon monoxide or CO react with high energy barrier.
In addition to the characterstic functions of activation energy and free energy of Gibbs which are typical for chemical reactions, there is another important thermodynamic function which is called the enthalpy or the energy of the molecule. Each chemical reaction has an enthalpy of reaction associated with it. The enthalpy of a compound means the amount of energy this compound has in its bonds.
The reaction enthalpy is defined as the enthalpy difference between the reactants and the products. If this difference is positive we talk about an endothermic reaction or a reaction in which consumes energy i its process. On the other hand, if the reaction enthalpy is negative, one talks then about a reaction which releases energy.
An important principle of chemical reactions is called the Hammond principle. It states that chemical reactions with high energy barriers are normally endothermic, while chemical reactions with low energy barriers are normally exothermic or release energy upon reaction.
Another characteristic of chemical reactions is the thermodynamic function of entropy. Chemical reactions favor disorder over order. One talks then about chemical reactions which are entropically favored or disfavored depending on if there is an increase in order upon reaction or there is a decrease in order.
Reactions are favored entropically if there is an increase in disorder upon reaction. An example is the dissociation of NaCl to its ions. In this case the disorder is increased. An example of a decrease in disorder is the formation of water from hydrogen and oxygen molecules.
A phase transition is also accompanied by an increase in disoder or a decrease in disorder depending on which phase we are talking about. A transition from liquid to solid is accompanied by a decrease in disorder and thus is entropically disfavored. A transformation from liquid to gas is entropicaly favored due to the increase in entropy or disorder.
Finally a chemical reaction can be followed to completion through several ways. One is thin layer chraomatography or TLC in organic reactions and UV-VIS spectrum in the case of organometallic compounds. Crystallography is often used to identify the structure of inorganic compounds.
