General Chemistry/Types of chemical reactions

Chemical reactions can be divided into several classes each having similar characteristics. These different types of reactions will be discussed in greater detail throughout the book.

Synthesis Reactions

The general form of a synthesis reaction is A + B → AB. The most well-known example is the formation of water via the fusion of hydrogen gas and oxygen gas (and energy):

$2{\text{H}}_{2(g)}+{\text{O}}_{2(g)}\to 2{\text{H}}_2{\text{O}}_{(l)}$

Synthesis reactions always yield one product.

Redox

Redox is an abbreviation of reduction / oxidation reactions. This is exactly what happens in a redox reaction, one species is reduced and another is oxidized. Reduction involves a gain of electrons and oxidation involves a loss, so a redox reaction is one in which electrons are transferred between species. Reactions where something is "burnt" (burning means being oxidised) are examples of redox reactions, however, oxidation reactions also occur in solution, which is very useful and forms the basis of electrochemistry.

Redox reactions are often written as two half-reactions showing the reduction and oxidation processes separately. These half-reactions are balanced (by multiplying each by a coefficient) and added together to form the full equation. When magnesium is burnt in oxygen, it loses electrons (it is oxidised). Conversely, the oxygen gains electrons from the magnesium (it is reduced).

$\begin{array}{cccc}\text{Mg}& \to & {\text{Mg}}^{2+}+2e^{-}& \times 2\\ {\text{O}}_2+4e^{-}& \to & 2{\text{O}}^{2-}& \times 1\\ 2\text{Mg}+{\text{O}}_2+4e^{-}& \to & 2\text{MgO}+4e^{-}& \end{array}$

Redox reactions will be discussed in greater detail in the [[../Redox Reactions|redox]] section.

Decomposition Reactions

These are the opposite of synthesis reactions, with the format AB → A + B. One example is the electrolysis of water (passing water through electrical current) to form hydrogen gas and oxygen gas:

$2{\text{H}}_2{\text{0}}_{(l)}\to 2{\text{H}}_{2(g)}+{\text{0}}_{2(g)}$

Just as synthesis reactions can only form one product, decomposition reactions can only start with one reactant.

Single Replacement Reactions

A component in a compound is swapped with another component, in the format AB + C → AC + B.

Double Replacement Reactions

In these reactions, two compounds swap components, in the format AB + CD → AD + CB.

Precipitation

A precipitation reaction occurs when an ionic substance comes out of solution and forms an insoluble (or slightly soluble) solid. The solid which comes out of solution is called a precipitate. This can occur when two soluble salts (ionic compounds) are mixed and form an insoluble one - the precipitate. An example is lead nitrate mixed with potassium iodide, which forms a bright yellow precipitate of lead iodide.

$\text{Pb}({\text{NO}}_3{)}_{2(aq)}+2{\text{KI}}_{(aq)}\to {\text{PbI}}_{2(s)}+2{\text{KNO}}_{3(aq)}$

Note that the lead iodide is formed as a solid. The above equation is written in molecular form, which is not the best way of describing the reaction. Each of the elements really exist in solution as individual ions, not bonded to each other (as in potassium iodide crystals). If we write the above as an ionic equation, we get a much better idea of what is actually happening.

${\text{Pb}}_{(aq)}^{2+}+2{\text{I}}_{(aq)}^{-}\to {\text{PbI}}_{2(s)}$

In the solution, there exist both lead and iodide ions. Because lead iodide is insoluble, they spontaneously crystallise and form the precipitate.

Acid Base

In simple terms, an acid is a substance which can lose a H⁺ ion (i.e. a proton) and a base is a substance which can accept a proton. When equal amounts of an acid and base react, they neutralize eachother forming species which aren't as acidic or basic. For example, when hydrochloric acid (HCl) and sodium hydroxide (NaOH) react, they react to form water and sodium chloride (common salt, NaCl).

$\text{HCl}+\text{NaOH}\to {\text{H}}_2\text{O}+\text{NaCl}$

Again, we get a clearer picture of what's happening if we write an ionic equation.

${\text{H}}^{+}+{\text{OH}}^{-}\to {\text{H}}_2\text{O}$

Acid base reactions often happen in aqueous solution, but they can also occur in the gaseous state (and perhaps other states). Acids and bases will be discussed in much greater detail in the acids and bases section.

Organic Reactions

Organic reactions occur between organic molecules (molecules containing the element carbon). Since there are a virtually limitless number of organic molecules, the scope of organic reactions is very large. However, many of the characteristics of organic molecules are determined by functional groups - small groups of atoms which react in predictable ways.

Another key concept in organic reactions is Lewis basicity. Parts of organic molecules can be electrophillic (electron-loving) or nucleophillic (nucleus, or positive loving). Nucleophillic regions have an excess of electrons - they act as Lewis bases - whereas electrophillic areas are electron deficient and act as Lewis acids. The nucleophillic and electrophillic regions attract and react with each other (needless to say, this has inspired many terrible organic chemistry jokes).

Organic reactions are beyond the scope of this book, and are covered in more detail in Organic Chemistry.

Combustion

Combustion, better known as burning, is the combination of a substance with oxygen.