General Chemistry/Chemical equations

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Chemical equations are a convenient, standardized system for describing 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[edit]

\hbox{H}_{2 (g)} + \hbox{Cl}_{2 (g)} \to 2\hbox{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[edit]

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:


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. For ACS Style[1] the state is typeset at the baseline without size change.

  • (s) means solid
  • (l) means liquid
  • (g) means gas
  • (aq) means aqueous solution (i.e. dissolved in water)

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


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).

It is important that the Law of Conservation of Mass is not violated. There must be the same number of each type of atoms on either side of the equation. Coefficients are useful for keeping the same number of atoms on both sides:

2\hbox{H}_2 + \hbox{O}_2 \to 2\hbox{H}_2\hbox{O}

If you count the atoms, there are four hydrogens and two oxygens on each side. The coefficients allow us to balance the equation; without them the equation would have the wrong number of atoms. Balancing equations is the topic of the next chapter.

Other Information[edit]

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 to the right of the equation or above the reaction arrow. A simple example would be the melting of ice.

\hbox{H}_2\hbox{O}_{(s)} + heat \to \hbox{H}_2\hbox{O}_{(l)}, which could be written as \hbox{H}_2\hbox{O}_{(s)} \xrightarrow{heat} \hbox{H}_2\hbox{O}_{(l)}

Reactions commonly involve catalysts, which are substances that speed up a reaction without being consumed. Catalysts are often written over the arrow. A perfect example of a catalyzed reaction is photosynthesis. Inside plant cells, a substance called chlorophyll converts sunlight into food. The reaction is written:

6\hbox{CO}_2 + 6\hbox{H}_2\hbox{O} + sunlight \xrightarrow{chlorophyll} \hbox{C}_6\hbox{H}_{12}\hbox{O}_6  +  6\hbox{O}_2


\hbox{CH}_{4(g)} + 2\hbox{O}_{2(g)} \to \hbox{CO}_{2(g)} + 2\hbox{H}_2\hbox{O}_{(l)} This is the equation for burning methane gas (CH4) in the presence of oxygen (O2) to form carbon dioxide and water: CO2 and H2O respectively. Notice the use of coefficients to obey the Law of Conservation of Matter.
\hbox{Pb}_{(aq)}^{2+} + 2\hbox{I}_{(aq)}^- \to \hbox{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).
2\hbox{SO}_{2(g)} + 2\hbox{V}_2\hbox{O}_{5(s)} \to 2\hbox{SO}_{3(g)} + 4\hbox{V}_2\hbox{O}_{(s)}

4\hbox{V}_2\hbox{O}_{(s)} + \hbox{O}_{2(g)} \to 2\hbox{V}_2\hbox{O}_{5(s)}

These two equations involve a catalyst. They occur one after another, using divanadium pentoxide to convert sulfur dioxide into sulfur trioxide. If you look closely, you can see that the vanadium catalyst is involved in the reaction, but it does not get consumed. It is both a reactant and a product, but it is necessary for the reaction to occur, making it a catalyst.
2\hbox{SO}_{2(g)} + \hbox{O}_{2(g)} \xrightarrow{V_2O_5} 2\hbox{SO}_{3(g)} If we add both equations together, we can cancel out terms that appear on both sides. The resulting equation is much simpler and self-explanatory (although the original pair of equations is more accurate in describing how the reaction proceeds).
  1. Conventions in Chemistry. In The ACS Style Guide, 3rd ed.; Coghill, A. M.; Garson, L.R., Eds.; Oxford University Press: New York, 2006; p 294.