General Chemistry/Chemical equations

Chemical equations are a convenient, standardised system for describing [[../Chemical reactions|chemical reactions]]. They contain the following information.

• The type of reactants consumed and products formed
• The relative amounts of reactants and products
• The electrical charges on ions
• The physical state of each species (e.g. solid, liquid)
• The reaction conditions (e.g. temperature, catalysts)

The final two points are optional and sometimes omitted.

Anatomy of an Equation

${\text{H}}_{2(g)}+{\text{Cl}}_{2(g)}\to 2{\text{HCl}}_{(g)}$

Hydrogen gas and chlorine gas will react vigorously to produce hydrogen chloride gas. The equation above illustrates this reaction. The reactants, hydrogen and chlorine, are written on the left and the products (hydrogen chloride) on the right. The large number 2 in front of HCl indicates that two molecules of HCl are produced for each 1 molecule of hydrogen and chlorine gas consumed. The 2 in subscript below H indicates that there are two hydrogen atoms in each molecule of hydrogen gas. Finally, the (g) symbols subscript to each species indicates that they are gases.

Reacting Species

Species in a chemical reaction is a general term used to mean atoms, molecules or ions. A species can contain more than one chemical element (HCl, for example, contains hydrogen and chlorine). Each species in a chemical equation is written:

${\text{E}}_{x(s)}^y$

E is the chemical symbol for the element, x is the number of atoms of that element in the species, y is the charge (if it is an ion) and (s) is the physical state.

The symbols in parentheses (in subscript below each species) indicate the physical state of each reactant or product. (s) means solid (l) means liquid (g) means gas (aq) means aqueous solution (i.e. dissolved in water) (cr) means crystal state and is not commonly used

For example, ethyl alcohol would be written ${\text{C}}_2{\text{H}}_6{\text{O}}_{(l)}$ because each molecule contains 2 carbon, 6 hydrogen and 1 oxygen atom. A magnesium ion would be written ${\text{Mg}}^{2+}$ because it has a double positive ("two plus") charge. Finally, an ammonium ion would be written $[{\text{NH}}_4{]}^{+}$ because each molecule contains 1 nitrogen and 4 hydrogen atoms and has a charge of 1+.

Coefficients

The numbers in front of each species have a very important meaning - they indicate the relative amounts of the atoms that react. The number in front of each species is called a coefficient. In the above equation, for example, one H₂ molecule reacts with one Cl₂ molecule to produce two molecules of HCl. This can also be interpreted as moles (i.e. 1 mol H₂ and 1 mol Cl₂ produces 2 mol HCl).

Other Information

Occasionally, other information about a chemical reaction will be supplied in an equation (such as temperature or other reaction conditions). This information is often written above the reaction arrow. We will ignore this for now, as it only complicates matters (and it's hard to draw in TeX :).

Examples

${\text{CH}}_{4(g)}+2{\text{O}}_{2(g)}\to {\text{CO}}_{2(g)}+2{\text{H}}_2{\text{O}}_{(g)}$

Above is the equation for burning methane gas (CH₄) in the presence of oxygen (O₂) to form carbon dioxide and water: CO₂ and H₂O respectively.

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

This is a precipitation reaction in which dissolved lead cations and iodide anions combine to form a solid yellow precipitate of lead iodide (an ionic solid).