SNC2D Word Equations and Balanced Chemical Equations: Notes and Practice

A recipe that says "flour + eggs → cake" tells you what goes in and what comes out, but not how much. A chemical equation written only with names, a word equation, has the same problem. Chemists write equations with formulas and numbers so the equation becomes an exact count of the particles involved. Because atoms are never created or destroyed (the law of conservation of mass from the last lesson), every atom on the left side of the arrow must show up on the right side. Making the counts match is called balancing the equation, and it is a skill you will use in every chemistry course from here on.

The parts of a chemical equation

Reactants are on the left and products on the right; the arrow means "produces" or "yields", and a plus sign separates substances.
A skeleton equation has the correct formula for every substance but is not yet balanced, e.g. $H_{2} + O_{2} \to H_{2} O$ .
A coefficient is a whole number placed in front of a formula, and it multiplies every atom in that formula: $3 H_{2} O$ means three water molecules, containing 6 H and 3 O.
A subscript is the small number inside a formula; it is part of the substance's identity and is never changed while balancing.
State symbols in brackets after each formula show the state: (s) solid, (l) liquid, (g) gas, (aq) aqueous, meaning dissolved in water.

Molecular models show why we balance

Fig 1Ball-and-stick models of two reactions. In C + O₂ → CO₂, one carbon atom and two oxygen atoms appear on each side. In 2H₂ + O₂ → 2H₂O, both sides have 4 hydrogen atoms and 2 oxygen atoms.

When carbon burns, one carbon atom joins one oxygen molecule to make one carbon dioxide molecule. Count the atoms: 1 C and 2 O on the left, 1 C and 2 O on the right. That equation is already balanced. Hydrogen burning is different. The skeleton $H_{2} + O_{2} \to H_{2} O$ has 2 oxygen atoms on the left but only 1 on the right, as if an oxygen atom vanished. We cannot rewrite water as $H_{2} O_{2}$ , because that is hydrogen peroxide, a different substance. Instead we make two water molecules, which uses both oxygen atoms. Now there are 4 hydrogen atoms on the right, so we need two hydrogen molecules on the left.

2 H_{2} (g) + O_{2} (g) \to 2 H_{2} O(l)

Balanced: 4 H and 2 O on each side.

AChange coefficients

Changes how many molecules or formula units react
$2 H_{2} O$ is still water, just two molecules of it
This is the only thing you adjust when balancing

BNever change subscripts

Changes what the substance is
$H_{2} O_{2}$ is hydrogen peroxide, not water
Subscripts come from ion charges or known molecular formulas

The atom inventory method

Write the word equation, then the skeleton equation with correct formulas (use the naming rules from the first two lessons, and remember the seven diatomic elements).
Make an atom inventory: list each element and count its atoms on the left and on the right.
Balance one element at a time by changing coefficients. A good order is metals first, then non-metals other than H and O, then hydrogen, and oxygen last.
If a polyatomic ion such as $SO_{4}$ or $NO_{3}$ appears unchanged on both sides, count it as a single unit.
Update the whole inventory after every change, because one coefficient affects every atom in that formula.
When every row matches, check that the coefficients are the lowest whole-number set (2, 4, 2 should be 1, 2, 1).

Step	Equation	Al (left \| right)	O (left \| right)
Skeleton	Al + O₂ → Al₂O₃	1 \| 2	2 \| 3
Make the O count on the right even	Al + O₂ → 2Al₂O₃	1 \| 4	2 \| 6
Balance O	Al + 3O₂ → 2Al₂O₃	1 \| 4	6 \| 6
Balance Al	4Al + 3O₂ → 2Al₂O₃	4 \| 4	6 \| 6

Atom inventory for aluminum reacting with oxygen. Oxygen comes in pairs (O₂), so an odd count of O on the other side is doubled first.

Fig 2Balancing methane combustion with an atom inventory. The skeleton CH₄ + O₂ → CO₂ + H₂O has carbon balanced but hydrogen (4 and 2) and oxygen (2 and 3) unbalanced; CH₄ + 2O₂ → CO₂ + 2H₂O has 1 C, 4 H and 4 O on each side.

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Change the coefficients in front of each formula and watch the atom counts on both sides update until every element balances.

Seven diatomic elements

Hydrogen, nitrogen, oxygen, fluorine, chlorine, bromine and iodine exist as two-atom molecules when they are on their own:

H_{2}

N_{2}

O_{2}

F_{2}

Cl_{2}

Br_{2}

I_{2}

. When one of them is a reactant or product by itself, write it with the 2. Inside a compound, the ordinary formula rules apply, so sodium chloride is still NaCl.

The final check

Write a quick tally under your answer, for example Na 2 | 2, H 4 | 4, O 2 | 2, and make sure the coefficients cannot all be divided by the same number. A balanced equation tells you the ratio of particles in the reaction: in

2 H_{2} + O_{2} \to 2 H_{2} O

, hydrogen and oxygen molecules always react in a 2 to 1 ratio.

Word Equations and Balanced Chemical Equations

Key things to remember

The parts of a chemical equation

Molecular models show why we balance

The atom inventory method

Worked examples

Where marks get lost

Flashcards 8

Practice