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C2.4 · Predict products of synthesis and decomposition reactions

Learn to predict products of synthesis and decomposition reactions through clear examples and targeted practice.

Ontario Grade 11 Chemistry

Chemical Reactions

Recognize reaction patterns, write the products, and check the equation.

A metal exposed to oxygen can form a new solid. In another familiar kind of change, heating a compound can produce more than one substance. These observations can be represented by chemical equations. A synthesis reaction combines two or more reactants to form one product. A decomposition reaction starts with one reactant and forms two or more products. To predict products, identify the reaction pattern, write the correct formulas, and then balance the equation. A pattern is a guide for a question that gives the reactants and relevant conditions; it does not show that every possible set of substances reacts.

What you will learn

Distinguish synthesis from decomposition by the number of reactants and products.
Use common Grade 11 reaction patterns to predict products.
Write correct chemical formulas before balancing an equation.
Balance a predicted equation using coefficients and check that atoms are conserved.

1. From observable changes to reaction patterns

When a reaction occurs, the starting substances may change in appearance. A new solid may form, or one substance may become several products. These observations can suggest a reaction, but the pattern is described most clearly by the number of reactants and products.

A reactant is a starting substance in a chemical reaction. A product is a substance formed by the reaction. In synthesis, two or more reactants form one product. At the particle level, atoms from the starting substances become part of the product. In decomposition, one reactant forms two or more products. Its atoms become arranged in separate substances.

The letters in the patterns below are placeholders. They do not stand for particular elements. Use the actual reactants in a question to predict actual products.

\mathrm{A+B\rightarrow AB\qquad AB\rightarrow A+B}

Synthesis has multiple reactants and one product.
Decomposition has one reactant and multiple products.
The reaction pattern describes the arrangement of substances, not their exact formulas.

2. Use common patterns to predict products

Begin with the reactants and any conditions stated in the question. Then select a common pattern that fits. One synthesis pattern is a metal combining with a non-metal to form an ionic compound. Another is a non-metal combining with oxygen to form a non-metal oxide. The elements named in the reactants determine which product is possible.

An ionic compound contains positive and negative ions. Its formula shows the smallest whole-number ratio of ions that gives no overall charge. For example, an ion with a charge of

2+

combines with two ions of charge

1-

to make a neutral compound. Write the resulting formula correctly before balancing.

A common decomposition pattern is a metal carbonate forming a metal oxide and carbon dioxide. A metal hydroxide commonly forms a metal oxide and water. The metal’s charge matters when writing the formula of its oxide. Do not copy a formula from a different metal without checking the ions.

The formula for carbon dioxide is determined by its two elements and their atom ratio. In a carbonate decomposition, the carbonate group’s carbon and oxygen appear in carbon dioxide, while the metal and remaining oxygen form the metal oxide. The product formulas must account for all elements in the reactant.

A pattern cannot add an element that is missing from the reactants. If a question does not give a useful pattern or enough information to identify products, do not invent products. Use only the reactants and conditions provided.

\mathrm{MCO_3\rightarrow MO+CO_2}

Use ion charges to write a neutral ionic formula.
A metal carbonate commonly forms a metal oxide and carbon dioxide.
A metal hydroxide commonly forms a metal oxide and water.
Check that the predicted products include every element in the reactants.

3. Write formulas before balancing

A chemical formula shows which elements are in a substance and how many atoms of each element are in one unit. A subscript is the small number after an element symbol. It is part of the formula, so changing it changes the substance. First decide what the products are and write their correct formulas.

A coefficient is a number placed in front of a formula. It tells how many units of that substance are represented. To balance an equation, adjust coefficients so each element has the same total number of atoms on both sides. A coefficient of one is understood and is not written.

For instance, hydrogen and oxygen can be represented as forming water. The formulas must stay unchanged while coefficients make the atom counts equal. This is an example of balancing, not a pattern for predicting every reaction.

Balancing checks that atoms are conserved in the equation. It does not determine which substances are products. If the formulas are correct, balance by counting each element on both sides and adjusting coefficients. Use the smallest whole-number coefficients that work.

State symbols show whether a substance is solid, liquid, gas, or dissolved in water. Include them when the question gives them or when they are part of the stated reaction information. Do not add unstated measurements or conditions.

\mathrm{2H_2+O_2\rightarrow 2H_2O}

Predict products and write correct formulas before balancing.
Change coefficients, not subscripts.
Count each element on both sides after balancing.
A balanced equation represents the proposed reaction; it does not prove that it occurred.

4. A checking routine

Use a consistent routine. First, identify whether the question describes synthesis or decomposition. Second, select a suitable common pattern from the reactants and stated conditions. Third, write the product formulas. Fourth, balance the equation with coefficients. Finally, count every type of atom on each side.

For synthesis, check that the single product contains the elements supplied by all reactants. For decomposition, check that the products together contain all elements in the one reactant. Then verify that the number of each kind of atom matches on both sides.

Keep product prediction separate from balancing. A correct atom count cannot fix an incorrect product formula. If the equation will not balance while the formulas stay fixed, revisit the product prediction and formula writing.

Identify the pattern before writing products.
Write formulas before using coefficients.
Check both the elements present and their atom counts.

Worked example

Predicting a carbonate decomposition

A question states that solid copper(II) carbonate decomposes when heated. Predict the products and write a balanced equation, including the stated solid state and the carbon dioxide gas product.

Identify the pattern
The reactant is a metal carbonate, and the question states that it decomposes. Use the common pattern: a metal carbonate forms the corresponding metal oxide and carbon dioxide.
Write the product formulas
Copper(II) indicates a copper ion with charge $2+$ . Oxide has charge $2-$ . One copper ion and one oxide ion make neutral copper(II) oxide. The other product is carbon dioxide.
$\mathrm{CuCO_3\rightarrow CuO+CO_2}$
Check and balance
Count atoms on each side. There is one copper, one carbon, and three oxygen atoms on each side, so every coefficient is already one. Add the states specified in the question and the gas state for carbon dioxide.
$\mathrm{CuCO_3(s)\rightarrow CuO(s)+CO_2(g)}$

Answer: The products are copper(II) oxide and carbon dioxide. The balanced equation is

\mathrm{CuCO_3(s)\rightarrow CuO(s)+CO_2(g)}

Check: Each side contains one copper atom, one carbon atom, and three oxygen atoms. The equation is balanced.

Common mistakes and how to avoid them

Changing a subscript to make an equation balance.

Correction: Keep each correct substance formula unchanged. Use coefficients in front of formulas to balance.

Assuming any two reactants must form a synthesis product.

Correction: Use the reactants and conditions given in the question. A pattern helps predict products, but does not mean every combination reacts.

Calling a reaction decomposition because it has multiple products, without checking the reactant side.

Correction: Decomposition starts with one reactant and forms two or more products.

Using the same metal oxide formula for every metal.

Correction: Check the metal ion’s charge and write the oxide formula that gives a neutral compound before balancing.

Lesson summary

Synthesis combines multiple reactants into one product. Decomposition starts with one reactant and forms multiple products.
Use the reactants, stated conditions, and common Grade 11 patterns to predict products.
Write correct product formulas first, then balance using coefficients.
A balanced equation has the same number of atoms of each element on both sides.

Check your understanding

Question 1

Which description matches a synthesis reaction?

One reactant forms two products.
Two or more reactants form one product.
One product forms two reactants.
Two reactants form two products.

Show answer and explanation

Two or more reactants form one product.

Synthesis combines multiple reactants to form one product.

Question 2

A metal carbonate follows the common decomposition pattern. Which products are predicted?

Metal oxide and carbon dioxide
Metal and oxygen
Metal hydroxide and hydrogen
Two metal oxides

Show answer and explanation

Metal oxide and carbon dioxide

The common pattern gives a metal oxide and carbon dioxide. The metal’s ion charge is needed to write the metal oxide formula.

Question 3

Which change is allowed when balancing a chemical equation?

Changing a subscript in a product formula
Changing the identity of a reactant
Adding a coefficient in front of a formula
Removing an element that appears on both sides

Show answer and explanation

Adding a coefficient in front of a formula

A coefficient changes the number of units represented while leaving each substance’s formula unchanged.

Key terms

Reactant: A starting substance in a chemical reaction.
Product: A substance formed in a chemical reaction.
Synthesis reaction: A reaction in which two or more reactants form one product.
Decomposition reaction: A reaction in which one reactant forms two or more products.
Ionic compound: A compound made of positive and negative ions in a ratio that gives no overall charge.
Subscript: A small number in a chemical formula that shows how many atoms of an element are present in one unit.
Coefficient: A number placed in front of a formula to show how many units are represented.
State symbol: A label showing whether a substance is solid, liquid, gas, or dissolved in water.

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Published by DoAssignment. This AI-assisted lesson follows Ontario Grade 11 Chemistry (SCH3U), expectation C2.4. 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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