Chemical Reactions and Stoichiometry

Describing chemical reactions by writing chemical equations will help classify reactions by identifying patterns of reactivity, and will allow you to determine the quantitative relations between the amounts of substances involved in chemical reactions – that is, the reaction stoichiometry.

Writing Chemical Equations

An earlier chapter of this text introduced the use of element symbols to represent individual atoms. When atoms gain or lose electrons to yield ions or combine with other atoms to form molecules, their symbols are modified or combined to generate chemical formulas that appropriately represent these species. Extending this symbolism to represent both the identities and the relative quantities of substances undergoing a chemical (or physical) change involves writing and balancing a chemical equation.

The fundamental aspects of any balanced chemical equation are:

  1. The substances undergoing reaction are called reactants, and their formulas are placed on the left side of the equation.
  2. The substances generated by the reaction are called products, and their formulas are placed on the right side of the equation.
  3. Plus signs (+) separate individual reactant and product formulas, and an arrow
    (⟶) separates the reactant and product (left and right) sides of the equation.
  4. The relative numbers of reactant and product species are represented by coefficients (numbers placed immediately to the left of each substance).
  5. A coefficient of 1 is typically omitted.

To start we can use a food analogy of making a grilled cheese sandwich:

Two slices of bread sit beside one slice of cheese. At the top of the image, text reads: 1 sandwich equals 2 slices of bread plus 1 slice of cheese.

In order to make one sandwich, we need two slices of bread and one slice of cheese. From this standpoint, the bread and cheese are the two reactants and our product is the sandwich. So our equation would be:

2 slices of bread + 1 slice of cheese ⟶ 1 sandwich

It is common practice to use the smallest possible whole-number coefficients in a chemical equation, as is done in this example. Realize, however, that these coefficients represent the relative numbers of reactants and products, and, therefore, they may be correctly interpreted as ratios. In or example above we have a 2:1:1 ratio of bread to cheese to sandwich. If we were making two sandwiches we would have a 4-2-2 ratio and if we were making 12 sandwiches we would have a 24-12-12 ratio. However, because we want the smallest whole-number coefficients, we would still write the equation with a 2:1:1 ratio as shown above.

For a chemistry example, consider the reaction between one methane molecule (CH4) and two diatomic oxygen molecules (O2) to produce one carbon dioxide molecule (CO2) and two water molecules (H2O). The chemical equation representing this process is provided in the upper half of the image below. Use the information from this equation and the fundamental aspects of any chemical equation to drag and drop the corresponding space-filling molecular models into the correct place in the lower half of the figure.

Methane and oxygen react to yield carbon dioxide and water in a 1:2:1:2 ratio. This ratio is satisfied if the numbers of these molecules are, respectively, 1-2-1-2, or 2-4-2-4, or 3-6-3-6, and so on. Just like with our sandwich example above, these coefficients may be interpreted with regard to any amount (number) unit, and so this equation may be correctly read in many ways, including:

  • One methane molecule and two oxygen molecules react to yield one carbon dioxide molecule and two water molecules (1-2-1-2).
  • One dozen methane molecules and two dozen oxygen molecules react to yield one dozen carbon dioxide molecules and two dozen water molecules (12-24-12-24).
  • One mole of methane molecules and 2 moles of oxygen molecules react to yield 1 mole of carbon dioxide molecules and 2 moles of water molecules (1-2-1-2 mole ratio).

Additional Information in Chemical Equations

The physical states of reactants and products in chemical equations very often are indicated with a parenthetical abbreviation following the formulas. Common abbreviations include s for solids, l for liquids, g for gases, and aq for substances dissolved in water (aqueous solutions, as introduced in the preceding chapter). These notations are illustrated in the example equation here:

2Na(s) + 2H2O(l) ⟶ 2NaOH(aq) + H2(g)

This equation represents the reaction that takes place when sodium metal is placed in water. The solid sodium reacts with liquid water to produce molecular hydrogen gas and the ionic compound sodium hydroxide (a solid in pure form, but readily dissolved in water).

Special conditions necessary for a reaction are sometimes designated by writing a word or symbol above or below the equation’s arrow. For example, a reaction carried out by heating may be indicated by the uppercase Greek letter delta (Δ) over the arrow.


Other examples of these special conditions will be encountered in more depth in later chapters.