C2.2 Write balanced equations for common reaction types

Ontario Grade 11 Chemistry

Chemical Reactions

Recognize the reaction pattern, represent the products, and balance the atoms

A metal placed in a suitable solution may become coated with another metal, while a solid may form when two clear solutions are mixed. These visible changes can signal chemical reactions. At the particle level, atoms are rearranged into new substances. They are not created or destroyed. A chemical equation represents that rearrangement, and a balanced equation shows the same number of each kind of atom on both sides.

Before writing equations, recall two basics. A chemical formula uses symbols and subscripts to show which atoms are in a substance. A coefficient is a number placed in front of a formula; it counts whole particles or formula units. In this lesson, a reaction equation must first have correct formulas and then be balanced. Never change a formula’s subscripts to make an equation balance.

What you will learn

Identify common reaction types from their reactants and products.
Use course-level patterns and reference tables to write product formulas.
Balance equations by changing coefficients, not chemical formulas.
Check that atoms and, for ionic equations, net charge are conserved.

1. From an observation to a reaction pattern

A reaction may produce a gas, a new solid, or a substance with a different appearance. These observations suggest that substances have changed. The particle model explains this as atoms separating from their original partners and forming new arrangements. An equation records the starting substances, called reactants, on the left and the substances formed, called products, on the right.

The arrow means “forms” or “produces.” A plus sign separates substances that react or are produced together. A balanced equation has equal numbers of each type of atom on both sides. For an ionic equation, total charge must also be equal on both sides. State symbols show whether a substance is solid, liquid, gas, or dissolved in water:

(s)

,

(l)

,

(g)

, and

(aq)

. Add them when the information is known and useful.

\text{reactants} \rightarrow \text{products}

Reactants are written before the arrow; products are written after it.
Subscripts are part of a substance’s identity. Coefficients balance an equation.
Balance atoms, and preserve net charge when ions are shown.

2. Recognize common reaction types

A reaction type is a pattern that helps you predict or check the products. First identify the reactants and the pattern. Then use the correct formulas and any course-level reference table needed. The pattern does not replace checking whether a proposed reaction is possible.

In synthesis, two or more reactants form one product. In decomposition, one reactant breaks into simpler products. In single displacement, one element replaces another element in a compound. Use the activity series, a course reference list of elements ordered by their ability to displace other elements, to check whether a proposed replacement occurs.

In double displacement, two ionic compounds exchange ions. Use ion charges to write neutral product formulas. A solubility table can help determine whether a product is insoluble and forms a solid, or remains dissolved. In combustion, a substance reacts with oxygen. For complete combustion of a hydrocarbon, the products are carbon dioxide and water. Here, use that familiar pattern only to write and balance the equation; no further study of hydrocarbons is needed.

\begin{aligned}\text{synthesis: }&A+B\rightarrow AB\\\text{decomposition: }&AB\rightarrow A+B\\\text{single displacement: }&A+BC\rightarrow AC+B\\\text{double displacement: }&AB+CD\rightarrow AD+CB\end{aligned}

Synthesis: several reactants form one product.
Decomposition: one reactant forms several products.
Single displacement: an element replaces another element in a compound; check the activity series.
Double displacement: ions exchange partners; use charges and solubility information.
Combustion: a substance reacts with oxygen; complete hydrocarbon combustion forms carbon dioxide and water.

3. Write formulas, then balance

A correct equation begins with correct formulas. For ionic compounds, combine ions in ratios that give a neutral formula. For example, an ion with a

2+

charge and an ion with a

1-

charge combine in a one-to-two ratio. Do not carry the charges into the neutral compound’s formula. When an element appears by itself, use its correct elemental form from the course formula table; several elements occur naturally as diatomic molecules.

Once formulas are set, count each type of atom on both sides. Choose coefficients to make the counts equal. A coefficient multiplies every atom in the formula that follows it. If a polyatomic ion stays together on both sides, counting that group can make the balancing easier, but the final atom counts still need to agree.

Use the smallest whole-number coefficients. A missing coefficient means one. Do not write a coefficient of one. After balancing, recount the atoms. If the equation includes ionic species, add the charges on each side and confirm that they match. Balancing does not tell you by itself whether a reaction occurs; use the appropriate pattern or reference table for that decision.

aA+bB\rightarrow cC+dD

Write correct formulas before balancing.
Change coefficients only; never change subscripts.
Recount every element after balancing and check charge when relevant.
Use the activity series or solubility table when the reaction pattern calls for it.

4. A reliable equation-writing routine

Work in a fixed order. Identify the reaction type from the reactants. Predict products using the pattern and any relevant reference table. Write correct formulas, including elemental forms and ionic charges where needed. Add states when they are supported by the information given. Balance with coefficients, then verify atoms and charge.

For a double-displacement reaction, exchange the positive ions’ partners, then write each product formula so its total charge is zero. Check solubility to decide whether a product is aqueous or a solid. For single displacement, do not assume that every element can replace another; consult the activity series. For decomposition and synthesis, the reaction pattern narrows the likely products, but the formulas still must follow course conventions.

Equations in this topic do not require measurements or unit conversions. Coefficients are counts, not measured quantities, so significant figures do not apply to them. Keep state symbols attached to the formulas they describe.

Classify → predict → write formulas → add known states → balance → check.
Reference tables support product prediction; they do not change the balancing rules.
Coefficients are exact counting numbers in an equation.

Common reaction patterns

Type	General pattern	What to check
Synthesis	$\mathrm{A+B\rightarrow AB}$	One product forms; write its correct formula.
Decomposition	$\mathrm{AB\rightarrow A+B}$	One reactant forms simpler products.
Single displacement	$\mathrm{A+BC\rightarrow AC+B}$	Use the activity series to check replacement.
Double displacement	$\mathrm{AB+CD\rightarrow AD+CB}$	Write neutral formulas and check solubility.
Complete hydrocarbon combustion	$\mathrm{C_xH_y+O_2\rightarrow CO_2+H_2O}$	Balance carbon, hydrogen, then oxygen.

Worked example

Balance a double-displacement equation

Aqueous silver nitrate and aqueous sodium chloride form solid silver chloride and aqueous sodium nitrate. Write the balanced equation, including states.

Write the formulas
Silver nitrate, sodium chloride, silver chloride, and sodium nitrate have formulas based on their ions. The formulas are already neutral, so do not alter their subscripts while balancing.
$\mathrm{AgNO_3(aq)+NaCl(aq)\rightarrow AgCl(s)+NaNO_3(aq)}$
Count atoms
There is one silver, one sodium, one nitrogen, three oxygen, and one chlorine atom on each side. The silver chloride is shown as a solid, consistent with the stated product.
$\mathrm{Ag:1=1,\ Na:1=1,\ N:1=1,\ O:3=3,\ Cl:1=1}$
Check coefficients and charge
Every atom count already matches, so each coefficient is one. Each compound is neutral, and the total charge is zero on both sides. The equation is balanced as written.
$\mathrm{AgNO_3(aq)+NaCl(aq)\rightarrow AgCl(s)+NaNO_3(aq)}$

Answer:

\mathrm{AgNO_3(aq)+NaCl(aq)\rightarrow AgCl(s)+NaNO_3(aq)}

Check: Both sides contain one silver, one sodium, one nitrogen, three oxygen, and one chlorine atom. The net charge is zero on each side.

Common mistakes and how to avoid them

Changing a subscript to make the atom counts match.

Correction: Keep each formula fixed. Adjust only coefficients, because a changed subscript would describe a different substance.

Assuming a single-displacement reaction always occurs.

Correction: Check the activity series before accepting the predicted replacement.

Writing ionic product formulas without balancing their charges.

Correction: Use ion charges to form neutral compound formulas before balancing the equation.

Forgetting that a coefficient multiplies every atom in a formula.

Correction: Count all atoms in the formula, then multiply each count by its coefficient.

Lesson summary

A balanced equation represents the same atoms before and after a reaction.
Recognize synthesis, decomposition, single displacement, double displacement, and combustion patterns.
Write correct formulas first; balance by changing coefficients only.
Use the activity series and solubility table where needed, then check atoms and charge.

Check your understanding

Question 1

Which change is allowed when balancing a chemical equation?

Change a coefficient in front of a formula.
Change a subscript inside a formula.
Remove an element from one side.
Change an ionic charge to make totals match.

Show answer and explanation

Change a coefficient in front of a formula.

Coefficients change the number of particles without changing the substances. Subscripts and charges are part of the formulas and must not be altered to balance.

Question 2

Which reaction type has the pattern of one compound forming two or more products?

Synthesis
Decomposition
Double displacement
Combustion

Show answer and explanation

Decomposition

Decomposition begins with one reactant that separates into simpler products.

Question 3

In a double-displacement reaction, what should you check after predicting the products?

Whether the products have neutral formulas and whether solubility information supports a solid product.
Whether the products have different subscripts so the equation balances.
Whether every reactant is a gas.
Whether the coefficients include units.

Show answer and explanation

Whether the products have neutral formulas and whether solubility information supports a solid product.

Use ion charges to write neutral formulas, then consult solubility information to identify a possible solid product. Coefficients have no units.

Key terms

Reactant: A starting substance in a chemical reaction.
Product: A substance formed in a chemical reaction.
Coefficient: A number placed before a formula to show how many particles or formula units are represented.
State symbol: A label showing whether a substance is solid, liquid, gas, or dissolved in water.
Activity series: A course-level reference list used to judge whether one element can replace another in a compound.
Solubility: The ability of a substance to dissolve in water; a solubility table helps identify products that form solids.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C2.2. It is a study resource, not an official curriculum publication.

Before publication, the draft is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability, and then requires administrator approval. Errors can still occur, so corrections are welcomed.

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C2.2 · Write balanced equations for common reaction types

Ontario Grade 11 Chemistry

Recognize the reaction pattern, represent the products, and balance the atoms

What you will learn

1. From an observation to a reaction pattern

2. Recognize common reaction types

3. Write formulas, then balance

4. A reliable equation-writing routine

Common reaction patterns

Balance a double-displacement equation

Common mistakes and how to avoid them

Lesson summary

Check your understanding

Question 1

Question 2

Question 3

Key terms

Continue through SCH3U

About this lesson and its review