Types of Reactions
In our universe, there are millions of different reactions that occur, all of which are different. However, most of these can be classified as one of 7 reactions. These 7 different reactions are very different from one another and can have very different outcomes.
The first and simplest of these is known as a synthesis reaction. When two chemical species react with one another, they can sometimes combine to form a new species.The standard form of a synthesis reaction is as follows:
A + B -> AB
One of the most common synthesis reactions is the combination of sodium, Na, and chloride, Cl, which combine to form sodium chloride, or table salt, NaCl, which releases energy. Usually, energy is released when the synthesis reaction takes place, which is known as an exothermic reaction.
Within a synthesis reaction, the bonds of the reactants break, and they form new ones together. For example, in this reaction:
2H₂ + O₂ -> 2H₂O
There are two H-H bonds that are being broken, as well as one O-O bond that breaks. Then, four O-H bonds are formed, creating 2H₂O (in the image below, a bond is depicted using overlapping molecules).
A decomposition reaction is the opposite of synthesis. A decomposition reaction is when one reactant decomposes into multiple compounds. Unlike synthesis reactions, decomposition reactions require an energy input to actually happen. This is known as an endothermic reaction. The standard form of a decomposition reaction is:
AB -> A + B
One of the common decomposition reactions is the decomposition of hydrogen peroxide, which turns into water and oxygen. Hydrogen peroxide decomposes slowly when exposed to light or minor heat, but very rapidly when exposed to organic compounds that act as a catalyst.
2H₂O₂ -> 2O₂ + 2H₂
Catalysts are chemical species that just speed up the reaction without actually interfering with any of the reactants or products. It is consumed and then remade after the reaction is over. An intermediate is the exact opposite, where it is formed during a reaction and consumed later, also having no effect.
A single replacement reaction is when an element acting as a reactant replaces an element in the other compound. This can only happen with metals or nonmetals, as long as the metal/nonmetal that is replacing the other is more reactive. The most reactive metals are lithium, potassium, barium, and calcium, and the least reactive metals are gold, platinum, mercury, and silver. This means that lithium will always replace any other metal, and gold will never replace any metal. The standard single replacement reaction is as follows:
AB + C -> AC + B
where B and C are either both metals or nonmetals.
A double replacement reaction is very similar to a single replacement reaction, except where two metals are exchanged rather than replaced. Also, double replacement reactions need metals and cannot happen with nonmetals. The standard form of a double replacement reaction is as follows:
AB + CD -> AD + CB
where B and D are both metals. A compound with a metal and a nonmetal is known as an ionic compound. Both ionic compounds exchange their ions, forming new compounds.
Both precipitate reactions and neutralization (Acid-Base) reactions are double replacement reactions. In a precipitate reaction, two soluble (dissolves in water) compounds react to form one insoluble solid and one soluble compound. One common reaction is the precipitation of silver nitrate:
AgNO₃(aq) + NaCl(aq) -> AgCl(s) + NaNO₃(aq)
Parentheses next to a chemical species denote its state of matter, (s) for solid, (l) for liquid, and (g) for gas. In precipitate reactions, (aq) will be shown which marks the compound as aqueous, or dissolved in a water solution. Some compounds will always be insoluble, which can help one to determine which compound is solid and which is aqueous. Sodium, Nitrate, Ammonium, and Potassium ions are always soluble, meaning they will never precipitate. Note: an ion is just an element or compound with a charge, however, the charges are required to balance between reactants and products, so there aren’t many effects in a standard reaction.
Neutralization reactions are double replacement reactions that contain an acid and a base. An acid is any compound with hydrogen, H⁺, and a base is any compound with hydroxide, OH⁻. Neutralization reactions produce water and a salt, which is any compound with the positive ion of a base and the negative ion of a base. To add the positive/negative ion, you either add or remove an H⁺ or an OH⁻ to change the charge. This is known as the conjugate. One common neutralization reaction is between hydrochloric acid (a strong acid) and sodium hydroxide (a strong base):
HCl(aq) + NaOH(aq) -> NaCl(aq) + H₂O(l)
The salt, NaCl, comes from HCl without hydrogen ion and NaOH without the hydroxide ion.
The final type of reaction is a combustion reaction. A combustion reaction involves the burning of a hydrocarbon with oxygen. When burned, this reaction creates carbon dioxide, water vapor, and, most importantly, energy. A hydrocarbon is a compound with any amount of hydrogen and carbon only. The combustion of fuel in cars involves the hydrocarbon known as octane or gasoline, and oxygen in the air:
2C₈H₁₈(l) + 25 O₂(g) -> 16 CO₂(g) + 18 H₂O(g)