4.1 Writing and Balancing Chemical Equations

Chemical Equations

represent both the identities and the relative quantities of substances undergoing a chemical (or physical) change. (See image)

Reactants

The substances undergoing reaction. Their formulas are placed on the left side of the equation. (See image)

Products

The substances generated by the reaction. Their formulas are placed on the right side of the equation. (See image)

Plus signs (+)

Separate individual reactant and product formulas. (See image)

An arrow —>

separates the reactant and product (left and right) sides of the equation. (See image)

Coefficients

Numbers placed immediately to the left of each formula. The relative numbers of reactant and product species are represented by coefficients. A coefficient of 1 is typically omitted. (See image)

Balanced

Meaning that equal numbers of atoms for each element involved in the reaction are represented on the reactant and product sides. This is a requirement the equation must satisfy to be consistent with the law of conservation of matter. (It may be confirmed by simply summing the numbers of atoms on either side of the arrow and comparing these sums to ensure they are equal). (See image)

The numbers of atoms for a given element is calculated by…

multiplying the coefficient of any formula containing that element by the element’s subscript in the formula. If an element appears in more than one formula on a given side of the equation, the number of atoms represented in each must be computed and then added together.

To achieve balance…

the coefficients of the equation may be changed as needed. Keep in mind that the formula subscripts define (in part) the identity of the substance. Therefore, these cannot be changed without altering the qualitative meaning of the equation (identity of the compounds).

Sometimes, you can use what instead of whole numbers (integers) in the process of balancing a chemical equation…

fractions. When balance is achieved, all the equation’s coefficients may then be multiplied by a whole number to convert the fractional coefficients to integers without upsetting the atom balance. (See image)

If you have a balanced chemical equation, remember that you want the smallest whole number coefficients, so you can…

simplify the coefficients by their shared multiple. (see image)

The physical states of reactant and products in chemical equations…

Often are indicated with a parenthetical abbreviation following the formulas. S: Solid, L: liquid, G: gases, aq: substances dissolved in water (aqueous solutions).

(s)

solid

(L)

liquid

(g)

gases

(aq)

aqueous solutions (substances dissolved in water).

A reaction carried out by heating may be indicated by…

The uppercase Greek letter delta (Δ) over the arrow. Special conditions necessary for a reaction are sometimes designated by writing a word or symbol above the equations arrow.

Molecular Equation

Does not explicitly represent the IONIC SPECIES that are present in solution. (See image).

Complete Ionic Equation

Explicitly representing all dissolved ions. (Basically showing the charge of the ions involved) (See image).

Spectator Ions

Ions whose presence is required to maintain charge neutrality. These are neither chemically nor physically changed by the process, they may be eliminated from the equation to yield a clearer representation called a net ionic equation. [Spectator ions are ions that remain unchanged on both the reactant and product sides of a chemical equation and do not participate in it]. (see image, the ones crossed out are spectator ions).

Net Ionic Equation

A net ionic equation shows only the species that are directly involved in a chemical reaction, excluding spectator ions that do not change. (see image)