Grade by Grade
C. Chemistry: Chemical Reactions · Chemical Reactions

Word Equations, Chemical Equations and Molecular Models

C3.4 C2.2 C2.1 A1.12About 70 minutes
Learning goals
  • I can write word equations for chemical reactions.
  • I can write skeleton equations using correct formulas, diatomic elements and state symbols.
  • I can explain the difference between a coefficient and a subscript.
  • I can build and draw molecular models of simple reactions and use them to check whether atoms are conserved.

Words to know

word equation
An equation that names the reactants and products in words.
skeleton equation
An equation with correct formulas but not yet balanced.
balanced chemical equation
An equation with the same number of each kind of atom on both sides.
coefficient
A number in front of a formula that tells how many molecules or formula units there are.
diatomic element
An element whose molecules have two atoms: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂.
molecular model
A physical or drawn representation of a molecule, with balls for atoms and sticks for bonds.

The language of chemistry

A recipe tells you what goes in and what comes out: "flour + eggs + milk → pancakes". Chemists write reactions the same way, but they need more precision. They need to know exactly which substances react, in what numbers, and in what state. Over this lesson you will move from words, to formulas, to models you can build with your hands.

This lesson covers expectation C3.4 (write word equations and chemical equations), C2.2 (build molecular models of simple reactions and draw diagrams of them) and C2.1 (use the vocabulary of reactions).

Step 1: the word equation

A word equation names the reactants and products in words.

  • The reactants go on the left, the products on the right.
  • A plus sign (+) means "and" or "reacts with".
  • The arrow (→) means "produces" or "yields". It is not an equals sign: it shows a change.

Example: when natural gas (methane) burns in a furnace:

methane + oxygen → carbon dioxide + water

Read it aloud as: "Methane reacts with oxygen to produce carbon dioxide and water."

Step 2: the skeleton equation

Replace each name with its chemical formula, and add state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous.

CH₄(g) + O₂(g) → CO₂(g) + H₂O(g)

This is called a skeleton equation: it has the right substances, but it may not yet have the right numbers of each. (In a furnace, the water forms as a gas: water vapour. If the products cool down, it condenses to H₂O(l).)

Watch for diatomic elements. Seven elements exist as two-atom molecules when they are on their own: H₂, N₂, O₂, F₂, Cl₂, Br₂ and I₂ (remember the shape of a 7 on the periodic table, starting at nitrogen, plus hydrogen). So "oxygen" in a reaction is always O₂, and "chlorine gas" is Cl₂. Other elements are written as single symbols: Mg(s), Fe(s), C(s), Na(s).

Step 3: the balanced chemical equation

Atoms are never created or destroyed in a reaction (you will test this in the next lesson). So a correct equation has the same number of each kind of atom on both sides. To fix a skeleton equation, we put coefficients (big numbers) in front of formulas.

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g)

  • A coefficient tells you how many molecules (or formula units) there are. 2O₂ means two oxygen molecules.
  • A subscript is part of the formula and tells you how many atoms are in one molecule. You must never change a subscript to balance an equation, because that changes the substance.
  • A coefficient of 1 is not written.

You will learn a step-by-step balancing method two lessons from now. For now, practise reading and checking balanced equations.

2H₂(g)+O₂(g)→2H₂O(l)coefficient(2 molecules)subscript(2 atoms per molecule)state symbol(g) = gasarrow: "produces"state symbol(l) = liquidREACTANTSPRODUCTS
The parts of a balanced chemical equation. Coefficients count molecules; subscripts count atoms inside one molecule; state symbols give the state.

Molecular models: seeing the rearrangement

Equations are easier to understand when you can see them. Chemists use molecular models: balls stand for atoms and sticks stand for bonds.

When you model a reaction, you take apart the reactant molecules and rebuild the same atoms into the product molecules. If you have pieces left over, or not enough pieces, your equation is not balanced.

Look at the two reactions named in your curriculum:

  • C + O₂ → CO₂. One carbon atom and one oxygen molecule make one carbon dioxide molecule. This is what happens when charcoal burns completely.
  • 2H₂ + O₂ → 2H₂O. Two hydrogen molecules and one oxygen molecule make two water molecules. This reaction powers hydrogen fuel cells.
CC1 carbon atom+OOO₂1 oxygen moleculeOCOCO₂1 carbon dioxide moleculeAtom countleft: 1 C, 2 Oright: 1 C, 2 O
A model of C + O₂ → CO₂. The same three atoms appear on both sides; only the bonds between them have changed.
HHHH2H₂2 hydrogen molecules+OOO₂1 oxygen moleculeOHHOHH2H₂O2 water moleculesAtom countleft: 4 H, 2 Oright: 4 H, 2 O
A model of 2H₂ + O₂ → 2H₂O. Four hydrogen atoms and two oxygen atoms are rearranged into two water molecules. (The double lines are double bonds.)
Worked example

Example 1: From a sentence to an equation

Write a word equation and a skeleton equation: "Solid magnesium burns in oxygen gas to produce solid magnesium oxide."

  1. Reactants: magnesium and oxygen. Product: magnesium oxide.
  2. Word equation: magnesium + oxygen → magnesium oxide.
  3. Formulas: magnesium is an element, written Mg. Oxygen is diatomic, O₂. Magnesium oxide: Mg²⁺ and O²⁻ → MgO.
  4. Skeleton equation: Mg(s) + O₂(g) → MgO(s).
  5. Check: left has 2 O, right has 1 O, so it is not balanced yet. Coefficients fix this: 2Mg(s) + O₂(g) → 2MgO(s).

Word: magnesium + oxygen → magnesium oxide. Skeleton: Mg(s) + O₂(g) → MgO(s). Balanced: 2Mg(s) + O₂(g) → 2MgO(s).

Worked example

Example 2: Read a balanced equation

Read this equation in words and count the atoms on each side: N₂(g) + 3H₂(g) → 2NH₃(g).

  1. In words: "One molecule of nitrogen gas reacts with three molecules of hydrogen gas to produce two molecules of ammonia gas."
  2. Left: N₂ gives 2 N; 3H₂ gives 3 × 2 = 6 H.
  3. Right: 2NH₃ gives 2 × 1 = 2 N and 2 × 3 = 6 H.

Both sides have 2 N and 6 H, so the equation is balanced. (This reaction is how ammonia for fertilizer is made.)

Worked example

Example 3: Describe a model diagram

A diagram shows, on the left, one CH₄ molecule and two O₂ molecules, and on the right, one CO₂ molecule and two H₂O molecules. Write the equation and check it.

  1. Equation: CH₄ + 2O₂ → CO₂ + 2H₂O.
  2. Left: C = 1; H = 4; O = 2 × 2 = 4.
  3. Right: C = 1; H = 2 × 2 = 4; O = 2 (in CO₂) + 2 × 1 (in 2H₂O) = 4.

C 1 = 1, H 4 = 4, O 4 = 4: balanced. If you built it with model kits, every piece would be used and none would be left over.

Hands-on activity

Model kit reactions

You need: mini marshmallows or gumdrops in at least three colours (or three colours of modelling clay), toothpicks, a sheet of paper divided into a "reactants" side and a "products" side. Safety: toothpicks are sharp; work carefully and keep them away from young children. Do not eat the materials after handling them.

Choose a colour key and write it down, for example: one colour = C, one = O, one = H.

  1. C + O₂ → CO₂. Build one C atom and one O₂ molecule (two O joined by two toothpicks for the double bond) on the reactants side. Take them apart and rebuild them into CO₂ (O=C=O) on the products side. Any pieces left over?
  2. 2H₂ + O₂ → 2H₂O. Build two H₂ and one O₂. Rebuild into water molecules. How many water molecules can you make?
  3. CH₄ + 2O₂ → CO₂ + 2H₂O. Build the reactants, then rebuild. Did you need exactly two O₂?
  4. Draw a diagram of each model, before and after, using circles labelled with symbols. Write the balanced equation under each drawing.
  5. Test a wrong equation: try to build H₂ + O₂ → H₂O. What goes wrong?

What to look for: in steps 1 to 3 every piece is used. In step 5, one oxygen atom is left over, which shows that H₂ + O₂ → H₂O is not balanced; you need 2H₂ + O₂ → 2H₂O. Toothpicks (bonds) may be left over or needed, because bonds break and form, but atoms (marshmallows) must all be used.

Just for fun

Why do oxygen atoms always go out in pairs?

Because O₂ is better than one.

Your turn

1

What does the arrow mean in a chemical equation?

2

Write a word equation: "Hydrogen peroxide breaks down into water and oxygen."

3

Write the skeleton equation for the reaction in the previous question, with state symbols (hydrogen peroxide is aqueous, water is liquid, oxygen is a gas).

4

In 3CO₂, how many molecules are there, and how many oxygen atoms?

5

Which of these elements are written as diatomic molecules: sodium, chlorine, nitrogen, iron, iodine?

6

Count each type of atom on both sides: 2Na + Cl₂ → 2NaCl. Is it balanced?

7

Why can't you balance H₂ + O₂ → H₂O by changing it to H₂ + O₂ → H₂O₂?

More practice: warm-up

1

What do we call the substances on the left side of an equation?

2

What does (aq) mean?

3

Write the formula for oxygen gas.

4

In a molecular model, what do the balls and sticks represent?

5

What is the coefficient in 4Fe?

More practice: on your own

1

Write the word equation and skeleton equation: "Zinc metal reacts with hydrochloric acid to produce aqueous zinc chloride and hydrogen gas."

2

Count the atoms in 2C₂H₆.

3

Check whether this is balanced: CH₄ + O₂ → CO₂ + 2H₂O.

4

Write this equation as a sentence: 2KClO₃(s) → 2KCl(s) + 3O₂(g).

5

Describe how you would show with model kits that 2H₂ + O₂ → 2H₂O is balanced.

Challenge

1

Propane, C₃H₈, burns in a barbecue: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. Count every type of atom on each side.

2

A student's model of a reaction uses 2 N atoms and 6 H atoms as reactants and makes products with 2 N atoms and 6 H atoms. The products are all identical molecules, each with 1 N and 3 H. Write the balanced equation.

3

Iron reacts with oxygen to form iron(III) oxide. Write the word equation and the skeleton equation, then explain why the skeleton equation is not balanced.

Check your understanding

Quiz

1.

In the equation 2H₂ + O₂ → 2H₂O, what does the 2 in front of H₂O mean?

2.

Which is the correct skeleton equation for "hydrogen gas reacts with chlorine gas to make hydrogen chloride"?

3.

How many oxygen atoms are on the left side of CH₄ + 2O₂ → CO₂ + 2H₂O?

4.

Why must you never change a subscript to balance an equation?

5.

In a molecular model of a reaction, which must be the same before and after?

6.

What does the state symbol (g) mean?

For parents and teachers

Big idea: C3.4 asks students to write word equations and balanced chemical equations; C2.2 asks them to build molecular models of simple reactions (the curriculum's own examples are C + O₂ → CO₂ and 2H₂ + O₂ → 2H₂O) and draw diagrams of them. Building models makes it concrete that atoms are rearranged, not created or destroyed, which leads to the next lesson on conservation of mass.

Common difficulties:

  • Forgetting diatomic elements (writing O instead of O₂ for oxygen gas).
  • Mixing up coefficients and subscripts. Coefficients count molecules; subscripts are fixed parts of a formula.
  • Counting atoms with a coefficient: 3H₂O has 6 H and 3 O. Multiply the coefficient by every subscript in the formula.
  • Reading the arrow as "equals".

How to help: the marshmallow or clay models are worth the mess. Ask your teen to "prove" each equation by rebuilding the products from exactly the reactant pieces. Supervise around toothpicks with younger children nearby.