Balancing Chemical Equations Study Pack

Law of Conservation of Mass

• •In any chemical reaction, the total mass of the reactants equals the total mass of the products because atoms are neither created nor destroyed.
• •This law requires that every atom present on the reactant side of an equation must also appear on the product side, in the same quantity.

Symbolic Representation of a Reaction

• •A chemical equation uses the arrow (→) to separate reactants on the left from products on the right.
• •A plus sign (+) between formulas indicates separate species that are all reacting together or all being produced together.
• •Phase labels — (s) for solid, (l) for liquid, (g) for gas, and (aq) for aqueous (dissolved in water) — are appended to each formula to give a complete picture of the reaction conditions.

Stoichiometric Coefficients

• •A stoichiometric coefficient is a whole number written in front of a chemical formula that indicates how many formula units of that substance participate in the reaction.
• •Coefficients can be freely adjusted during balancing — changing a coefficient scales the number of entire formula units without altering what compound is present.
• •If no coefficient is written, it is understood to be 1.

Subscripts Within Chemical Formulas

• •Subscripts are part of the chemical identity of a substance: the '2' in H₂O means water always contains two hydrogen atoms bonded to one oxygen atom.
• •Changing a subscript would create an entirely different compound (H₂O₂ is hydrogen peroxide, not water), which misrepresents the actual reaction.
• •During balancing, subscripts are never altered — only coefficients are adjusted.

How Coefficients and Subscripts Interact

• •To find the total number of a given atom in one term, multiply the coefficient by the subscript for that atom within the formula.
• •For example, 3 Ca(NO₃)₂ contains 3 calcium atoms, 6 nitrogen atoms, and 18 oxygen atoms.

• •Step 1 — Write the Unbalanced Skeleton Equation
• •Start by writing the correct formulas for all reactants and products as given; do not attempt to alter formulas at this stage.
• •An unbalanced equation is sometimes called a skeleton equation because it shows the identities of all species but not their correct proportions.
• •Step 2 — Tally Atoms on Each Side
• •Count every atom of each element on the reactant side and independently on the product side.
• •Keep a running tally as coefficients are adjusted, because changing one coefficient affects all atoms in that formula simultaneously.
• •Step 3 — Balance Complex Formulas and Uncommon Elements First
• •Begin with elements that appear in only one reactant and one product, and with the most structurally complex molecules.
• •Save diatomic elements like H₂, O₂, Cl₂, and N₂ for last, because their coefficients can be adjusted without disturbing the balance of other elements already addressed.
• •Step 4 — Reduce Coefficients to the Smallest Whole-Number Ratio
• •Once all atoms balance, check whether all coefficients share a common factor; if so, divide through to give the simplest whole-number ratio.
• •Fractional coefficients are mathematically valid as an intermediate step but should be converted to whole numbers in the final equation by multiplying all coefficients by the denominator.