Synthesis Reaction Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Synthesis Reaction.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A chemical reaction in which two or more simpler substances (elements or compounds) combine to form a single, more complex product.

Building something bigger from smaller parts — like assembling LEGO bricks into a structure.

Read the full concept explanation →

How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Synthesis Reaction starts by comparing the reactant-product pattern, charges, states, and conserved atoms.

Common stuck point: Students often know a formula related to synthesis reaction but skip the recognition step: Does the balanced equation match a recognizable pattern of reactants, products, ions, oxygen, or electron transfer? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Does the balanced equation match a recognizable pattern of reactants, products, ions, oxygen, or electron transfer?

Worked Examples

Example 1

easy

Write the balanced equation for the synthesis of magnesium oxide from magnesium and oxygen.

Answer

2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}

First step

Synthesis reactions have the general form:

\text{A} + \text{B} \rightarrow \text{AB}

Full solution

2
Unbalanced: $\text{Mg} + \text{O}_2 \rightarrow \text{MgO}$ .
3
Balance: $2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}$ . Check: Mg 2=2, O 2=2. ✓

Synthesis (combination) reactions join two or more reactants to form a single, more complex product. Metal + oxygen → metal oxide is a common pattern.

Example 2

medium

Predict the product and write the balanced equation for the synthesis reaction between sodium and chlorine gas.

Example 3

easy

Write the balanced synthesis of calcium chloride from its elements.

Example 4

medium

Predict and balance the synthesis between potassium and oxygen to form

\text{K}_2\text{O}

Example 5

medium

Balance and classify:

\text{NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl}

Example 6

hard

A 2.4 g sample of Mg burns and gains mass. Predict the mass of MgO formed. (Mg=24, O=16)

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

Identify this reaction type:

\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3

Example 2

hard

When magnesium ribbon burns in air, it reacts with oxygen to form a white powder. Write the balanced equation and classify the reaction type. Predict the product if sodium reacted with chlorine gas under similar conditions.

Example 3

easy

Classify:

2Mg + O_2 \rightarrow 2MgO

. What reaction type is this?

Example 4

easy

N_2 + 3H_2 \rightarrow 2NH_3

a synthesis reaction?

Example 5

easy

Complete:

CaO + H_2O \rightarrow

? (synthesis)

Example 6

easy

Which equation is a synthesis reaction? (a)

2H_2O \rightarrow 2H_2 + O_2

(b)

S + O_2 \rightarrow SO_2

Example 7

easy

Balance the synthesis reaction:

H_2 + Cl_2 \rightarrow HCl

Example 8

easy

Two elements combine to form one compound. Name the reaction type.

Example 9

easy

Complete:

2Na + Cl_2 \rightarrow

Example 10

easy

Is forming rust,

4Fe + 3O_2 \rightarrow 2Fe_2O_3

, a synthesis?

Example 11

medium

A metal M reacts with oxygen to form

M_2O_3

. Write the balanced synthesis equation.

Example 12

medium

Predict the product and balance:

Mg + N_2 \rightarrow

? (Mg is +2, N is -3)

Example 13

medium

Which is a synthesis reaction, and balance it:

SO_3 + H_2O \rightarrow

Example 14

medium

Classify and balance:

P_4 + O_2 \rightarrow P_4O_{10}

Example 15

medium

CO_2 + C \rightarrow 2CO

a synthesis reaction? Explain via product count.

Example 16

medium

Lime water forms from

CaO

. Write and balance the synthesis:

CaO + CO_2 \rightarrow

Example 17

medium

Balance the synthesis:

Fe + S \rightarrow FeS

, then state if any coefficient is needed.

Example 18

medium

A student says

2H_2 + O_2 \rightarrow 2H_2O

is decomposition. Correct them.

Example 19

medium

Predict and balance:

Li + N_2 \rightarrow

? (Li is +1, N is -3)

Example 20

challenge

A 0.48 g sample of Mg burns in oxygen forming MgO. Mass of MgO formed? (Mg=24, O=16)

Example 21

challenge

Equal moles of

H_2

and

N_2

react:

N_2 + 3H_2 \rightarrow 2NH_3

. Which is limiting if 3 mol of each?

Example 22

challenge

Metal X forms oxide

X_2O_5

. If 0.20 mol X reacts fully, how many mol

O_2

are consumed?

Example 23

easy

Classify the reaction

2\text{Cu} + \text{O}_2 \rightarrow 2\text{CuO}

Example 24

easy

Balance the synthesis:

\text{K} + \text{Cl}_2 \rightarrow \text{KCl}

Example 25

easy

Complete the synthesis product:

\text{Ba} + \text{O}_2 \rightarrow

? (balanced)

Example 26

easy

\text{C} + \text{O}_2 \rightarrow \text{CO}_2

a synthesis reaction?

Example 27

medium

Balance the synthesis:

\text{Al} + \text{O}_2 \rightarrow \text{Al}_2\text{O}_3

Example 28

medium

Identify whether

\text{Na}_2\text{O} + \text{H}_2\text{O} \rightarrow 2\text{NaOH}

is synthesis. Justify in one sentence.

Example 29

medium

Balance the synthesis:

\text{Fe} + \text{O}_2 \rightarrow \text{Fe}_2\text{O}_3

Example 30

medium

Sulfur dioxide reacts with oxygen to form sulfur trioxide. Write the balanced synthesis equation.

Example 31

medium

Balance:

\text{Ca} + \text{N}_2 \rightarrow \text{Ca}_3\text{N}_2

Example 32

medium

Which is the synthesis reaction? (a)

\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2

(b)

\text{CaO} + \text{CO}_2 \rightarrow \text{CaCO}_3

Example 33

medium

Balance:

\text{H}_2 + \text{Br}_2 \rightarrow \text{HBr}

Example 34

hard

0.96 g of sulfur (

M=32

) burns completely in oxygen to form

\text{SO}_2

(

M=64

). What mass of

\text{SO}_2

is produced?

Example 35

hard

How many grams of

\text{Al}_2\text{O}_3

(

M=102

) form when 5.4 g of Al (

M=27

) reacts with excess

\text{O}_2

Example 36

hard

\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3

. With 2 mol

N_2

and 3 mol

H_2

, find the limiting reactant and the moles of

\text{NH}_3

formed.

Example 37

hard

Predict the formula of the product when lithium reacts with oxygen, then write the balanced synthesis equation.

Example 38

hard

Identify the reaction type and balance:

\text{P}_4 + \text{Cl}_2 \rightarrow \text{PCl}_3

Example 39

challenge

A 5.85 g sample of Na (

M=23

) reacts with

\text{Cl}_2

(

M=71

) to form NaCl. If 11.7 g of NaCl (

M=58.5

) is collected, find the percent yield.

Example 40

challenge

When

\text{P}_4

(

M=124

) reacts with excess

\text{O}_2

to form

\text{P}_4\text{O}_{10}

(

M=284

), what mass of product forms from 6.20 g of

\text{P}_4

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Related Concepts

Chemical Reaction Chemical Equation Decomposition Reaction

Background Knowledge

These ideas may be useful before you work through the harder examples.

chemical reactionchemical equation