Survey of the Types of Chemical Reactions
There are many types of chemical reactions, and here a few are listed.
- Chain reactions
Decomposition is the breakdown of a compound into simpler molecules.
Combination reactions are the opposite of decomposition reactions.
Two elements can combine,
A compound and an element can combine
Two compounds can combine
Oxidation is an example of a combination reaction.
Replace one atom or group of atoms in a compound.
Sodium replaces hydrogen
Zn replaces H.
Copper replaces silver.
Notice how replacement reactions can be written generically. All the replacement reactions can be written this way.
This precipitation reaction is also a two component replacement reaction. Here both PbNO3 and KI are replaced.
In this reaction both HgCl2 and KI undergo replacement.
Both NaCO3 and HCl undergo replacement or, equivalently, displacement
Reactions can take several steps
A detailed classification shows that the reaction of the of sodium carbonate, NaCO3 with hydrochloric acid, HCl, is a two-step process with first being the formation of carbonic acid which then decomposes. Hence chemical reactions can take place in a series of steps,
When a reaction occurs in a series of steps, then the steps can be added and common terms cancelled. This leaves the overall equation for the process without displaying the steps.
Neutralization is a process of combining acids and bases in the correct proportions to leave the solution neutral. H+ and OH- are replaced and combine to form water.
The red and blue components of the two reactants below are displaced to form water in this neutralization process.
This can be summarized as follows:
Combustion reactions are burning or vigorous oxidation processes involving the liberations of heat and often associated with fire.
As an example acetylene can be produced from calcium carbine by the addition of water. Acetylene burns vigorously. It can also be used as a starting point to make many hydrocarbons.
Explosions are vigorous chemical reactions that take place very rapidly and produce large volumes of gas and a lot of heat. For the explosives shown here, all are solids or liquids with a small volume relative to the huge amount of gas that is released in the explosion. The rapid production of a large volume of gas causes damage.
Precipitations reactions are the formation of a solid in solution in a two component displacement reaction. Hence both lead nitrate and potassium iodide are replaced
In this reaction both mecury chloride and potassium iodide undergo replacement:
Many ionic salts dissolve in solution by dissociating into anions and cations. For example potassium chromate dissolves in water.
The reverse of the dissolving of ionic salts is precipitation. The anions and cations combine to form an insoluble solid. Here are some examples.
AgNO3 and KCl are soluble, but AgCl produces a white precipitate.
All nitrates are soluble, and all potassium salts are soluble, but lead chromate is insoluble.
All potassium salts are soluble, and copper sulphate dissolves to form a blue solutions. Cu(OH)2 is however, insoluble.
Acid and Base reactions
Acids and bases are electrolytes. That is they dissociate into ions in aqueous solutions. Acids turn litmus paper red and bases turn litmus paper blue
Acids taste sour, react with many metals to produces H2,, and turn litmus paper red. Acids neutralize bases.
The simplest definition of an acid and a base is that an acid produces hydrogen ions in aqueous solution and a base produces hydroxide ions in aqueous solution. This is called the Arrhenius definition.
Hydrogen ions and hydroxide ions combine to form water. The solution contains the anions and cations of a dissolved salt.
The Bronstead Lowry definition
The Bronstead Lowry definition of an acid and base is more general than the Arrhenius definition. In the Bronstead Lowry definition, an acid is a proton donor and a base is a proton acceptor. This definition allows for bases that don’t contain the hydroxide group and solvents that don’t contain water. It also allows for the definition of acids and base to be extended to the gas phase.
Lewis Definition of acids and bases
The most general definition of an acid and base is due to Lewis. In this definition, an acid is an electron pair acceptor and a base is an electron pair donor. In this definition, we are not restricted to type of solvents to the types that are being used and we are not restricted to compounds that contain proton.
REDOX reactions are oxidation and reduction processes. REDOX reactions always contain simultaneously as electrons lost are gained in reduction. Usually REDOX reactions are listed as reduction half reactions. To obtain the oxidation half reaction, reverse the chemical equation and change the sign of the voltage.
Most chemical reactions have fairly complex mechanisms, but the steps are usually the common ones we have already seen. Some are composed of parallel reactions. Some are consecutive. Others are opposing reactions. Some steps are rate determining.
Here is an example of a chain reaction showing the steps. The equation is a complicated expression and the reason it is shown here not for you to remember but to point out how all steps can come together to give an overall rate law of uncertain order and dependent upon all the rate constants of the individual steps.
Initiation by light–a photon adds energy to the bromine bond and causes it to break to from two free radicals,
A bromine atom collides with a hydrogen molecule in a replacement reactions:
The hydrogen atom so produced in “Propagation 1″ is reactive and replaces a Br atom in the bromine molecule,
However a hydrogen atom can also collide with a product molecule HBr, to form the initial reactants and so inhibits the process,
Two bromine atoms can also collide and this terminates the reaction.
This is an example of a mechanism. That is we know the overall reaction, but how do the reactants actually change? Mechanisms are suggested and tested. We measure the rate constants for each step and then solve the equations to get the overall rate equations,
Understanding rates is an important branch of chemistry called Chemical Kinetics.