C. Chemistry: Chemical Reactions · Chemical Reactions

Synthesis and Decomposition Reactions

C2.3 C3.5 C3.4 C2.1 A1.5 A1.8About 75 minutes

Learning goals

I can recognize synthesis (A + B → AB) and decomposition (AB → A + B) reactions.
I can predict the products of simple synthesis and decomposition reactions and write balanced equations for them.
I can describe the reactants and products of reactions such as burning magnesium and decomposing hydrogen peroxide from observations.
I can describe the tests for oxygen, hydrogen and carbon dioxide gas, and explain what a catalyst does.

Words to know

synthesis reaction
A reaction in which two or more reactants combine to form one product: A + B → AB.

decomposition reaction
A reaction in which one compound breaks down into two or more simpler substances: AB → A + B.

catalyst
A substance that speeds up a reaction without being used up.

enzyme
A protein made by living things that acts as a catalyst, such as catalase.

electrolysis
Using electricity to decompose a compound, such as splitting water into hydrogen and oxygen.

limewater
A clear solution of calcium hydroxide that turns milky when carbon dioxide passes through it.

Sorting reactions

There are millions of chemical reactions, but many of them follow a few simple patterns. If you can recognize the pattern, you can often predict the products of a reaction you have never seen before. Expectation C2.3 asks you to investigate synthesis, decomposition and displacement reactions and represent them with models, word equations and balanced equations; C3.5 asks you to describe their reactants and products from observations.

This lesson covers the two simplest patterns: building up (synthesis) and breaking down (decomposition). The next two lessons cover displacement reactions.

The two patterns. In synthesis, simpler substances combine into one product. In decomposition, one compound breaks apart into simpler substances.

Synthesis reactions: A + B → AB

In a synthesis reaction, two or more reactants combine to form one product. (Synthesis means "putting together".)

Magnesium burns in air: 2Mg(s) + O₂(g) → 2MgO(s). The silvery metal ribbon burns with a dazzling white light and leaves a white powder, magnesium oxide. Old camera flashes and some fireworks use this.
Sodium and chlorine: 2Na(s) + Cl₂(g) → 2NaCl(s).
Making ammonia for fertilizer: N₂(g) + 3H₂(g) → 2NH₃(g).
Iron rusting (simplified): 4Fe(s) + 3O₂(g) → 2Fe₂O₃(s).
Carbonic acid in sparkling water: CO₂(g) + H₂O(l) → H₂CO₃(aq).

Predicting the product of a metal + nonmetal synthesis: the product is a binary ionic compound. Use the ion charges to write its formula, then balance. Example: aluminium + chlorine → AlCl₃, so 2Al + 3Cl₂ → 2AlCl₃.

Decomposition reactions: AB → A + B

In a decomposition reaction, one compound breaks down into two or more simpler substances. Decomposition usually needs energy: heat, electricity or light.

Hydrogen peroxide releases oxygen: 2H₂O₂(aq) → 2H₂O(l) + O₂(g). This happens slowly in the bottle (which is why it comes in dark bottles that block light) and very fast with a catalyst, such as manganese(IV) oxide in the lab or the enzyme catalase found in yeast, potatoes and liver. A catalyst speeds up a reaction without being used up.
Electrolysis of water: 2H₂O(l) → 2H₂(g) + O₂(g). Electricity splits water into hydrogen and oxygen. (Pure water conducts electricity poorly, so a little of an ionic compound is dissolved in it first.)
Heating limestone: CaCO₃(s) → CaO(s) + CO₂(g). This is the first step in making lime and cement, and one reason cement making releases a lot of carbon dioxide.
Potassium chlorate: 2KClO₃(s) → 2KCl(s) + 3O₂(g).

Predicting products: a binary compound usually decomposes into its two elements (remember diatomic gases). Example: sodium chloride, melted and split by electricity, gives sodium and chlorine: 2NaCl → 2Na + Cl₂.

Electrolysis of water. Hydrogen collects at the negative electrode and oxygen at the positive one. The volume of hydrogen is twice the volume of oxygen.

Identifying the gases: three classic tests

Many synthesis and decomposition reactions produce gases. In a school lab, your teacher can identify them with simple tests (C3.5 asks you to describe products on the basis of observation):

Oxygen: a glowing wooden splint (blown out, still red at the tip) bursts back into flame.
Hydrogen: a burning splint held at the mouth of the tube makes a "pop" sound, as the hydrogen burns: 2H₂ + O₂ → 2H₂O.
Carbon dioxide: a burning splint goes out, and bubbling the gas through clear limewater (calcium hydroxide solution) turns it milky, because a calcium carbonate precipitate forms.

These tests involve flames, so they are done only in a lab under a teacher's supervision.

Three gas tests. Oxygen relights a glowing splint; hydrogen pops with a burning splint; carbon dioxide turns limewater milky. (Lab demonstrations only.)

Worked example

Example 1: Classify, predict and balance

Classify each reaction as synthesis or decomposition, predict the products and write a balanced equation: (a) calcium + oxygen → ?, (b) water + electricity → ?, (c) potassium + bromine → ?

(a) Two elements combine: synthesis. Ca²⁺ and O²⁻ → CaO. Skeleton: Ca + O₂ → CaO. Balance O with 2CaO, then Ca with 2Ca: 2Ca + O₂ → 2CaO.
(b) One compound breaks down: decomposition. Water → hydrogen + oxygen (both diatomic): 2H₂O → 2H₂ + O₂.
(c) Two elements combine: synthesis. K⁺ and Br⁻ → KBr. Bromine is diatomic: 2K + Br₂ → 2KBr.

(a) synthesis, 2Ca + O₂ → 2CaO; (b) decomposition, 2H₂O → 2H₂ + O₂; (c) synthesis, 2K + Br₂ → 2KBr.

Worked example

Example 2: From observations to an equation

A teacher heats a strip of magnesium. It burns with a blinding white light and leaves a white powder. Describe the reactants and products and write the word and balanced equations.

Reactants: magnesium (shiny grey metal) and oxygen from the air (colourless gas).
Product: magnesium oxide, a white powder. The bright light and heat are evidence of a chemical change.
Word equation: magnesium + oxygen → magnesium oxide.
Balanced: 2Mg(s) + O₂(g) → 2MgO(s). It is a synthesis reaction.

Synthesis: 2Mg(s) + O₂(g) → 2MgO(s). Never look straight at burning magnesium: the light is bright enough to hurt your eyes.

Worked example

Example 3: Interpreting a gas test

A student adds a pinch of manganese(IV) oxide to hydrogen peroxide. Bubbles form quickly, and a glowing splint held in the tube bursts into flame. At the end, the black powder is still there. Explain.

The glowing splint relit, so the gas is oxygen.
The reaction is the decomposition of hydrogen peroxide: 2H₂O₂ → 2H₂O + O₂.
The manganese(IV) oxide is still there unchanged, so it is a catalyst: it sped up the reaction without being used up. That is why it is not written as a reactant.

Decomposition: 2H₂O₂(aq) → 2H₂O(l) + O₂(g), sped up by the catalyst MnO₂.

Hands-on activity

Inquiry: catalase in a potato

Question: Does a raw potato break down hydrogen peroxide faster than a cooked potato?

You need: a grown-up, eye protection, dish gloves, 3% hydrogen peroxide from a pharmacy, a raw potato and a piece of potato that has been boiled for 5 minutes and cooled, a knife and cutting board (grown-up cuts), 3 small clear glasses, a ruler, a measuring spoon.

Safety: wear eye protection and gloves; the grown-up pours the hydrogen peroxide and handles the knife. Do not eat the potato pieces afterwards.

Predict: which glass will foam most? Explain using the idea of a catalyst.
The grown-up cuts two cubes about 1 cm on each side: one from the raw potato and one from the cooked potato.
Pour 2 tablespoons of hydrogen peroxide into each glass. Label them raw, cooked and control (no potato).
Drop the potato cubes into their glasses at the same moment. After 2 minutes, measure the height of the foam with the ruler.
Optional extension: try a cube of raw potato that is cut into smaller pieces (same total size). Does surface area change the rate?

What to look for: the raw potato is covered in bubbles of oxygen and makes the most foam; the cooked potato makes few or no bubbles; the control has almost none. Raw potato contains the enzyme catalase, a natural catalyst. Boiling changes the shape of the enzyme so it stops working. The cut-up potato foams faster because more catalase is exposed. Clean-up: pour the liquid down the sink with lots of water; put the potato pieces in the compost or garbage.

Just for fun

Why is a magnesium ribbon always invited to Canada Day parties?

Because once it gets fired up, it really lights up the room. (Please enjoy from a distance: burning magnesium is too bright to look at directly.)

Your turn

Write the general pattern for a synthesis reaction and for a decomposition reaction.

Classify: 2H₂ + O₂ → 2H₂O

Classify: 2HgO → 2Hg + O₂

Predict the product and balance: Li + O₂ → ?

Predict the products and balance: the decomposition of magnesium chloride, MgCl₂.

What is a catalyst?

What observation shows that a gas is oxygen?

More practice: warm-up

How many products does a synthesis reaction have?

How many reactants does a decomposition reaction have?

What white powder forms when magnesium burns?

Which gas makes a "pop" with a burning splint?

What happens to limewater when carbon dioxide bubbles through it?

More practice: on your own

Write the balanced equation for the synthesis of aluminium oxide from its elements.

Write the balanced equation for the decomposition of hydrogen peroxide, and name the gas test for one product.

Calcium oxide reacts with water to make calcium hydroxide. Write the balanced equation and classify it.

In electrolysis of water, 40 mL of hydrogen is collected. About how much oxygen is collected? Explain.

Why does hydrogen peroxide come in brown or opaque bottles?

Challenge

Sulfur trioxide (from burning sulfur-containing fuels) reacts with water in the air to make sulfuric acid, which falls as acid rain. Write the balanced equation and classify the reaction.

Older car airbags inflated when sodium azide, NaN₃, decomposed into sodium and nitrogen gas. Write and balance the equation.

A student says, "Cooking the potato killed the catalyst, so the reaction can't happen at all." Is she right? Explain using the results of the potato inquiry.

Check your understanding

Quiz

Which equation shows a synthesis reaction?

What is the product when magnesium burns in oxygen?

A glowing splint bursts into flame in a test tube of gas. The gas is...

Which is the balanced equation for the electrolysis of water?

A catalyst...

What usually decomposes into its two elements when enough energy is added?

For parents and teachers

Big idea: C2.3 and C3.5 ask students to investigate synthesis and decomposition reactions, describe their reactants and products from observations (the curriculum's examples include magnesium burning and producing oxygen from hydrogen peroxide), and represent them with word equations and balanced equations. Gas tests are a classic way to identify products.

Common difficulties:

Forgetting diatomic gases when predicting products (writing O instead of O₂).
Writing ionic product formulas wrongly. Use charges: Al and Cl make AlCl₃.
Writing the catalyst as a reactant. It is often written above the arrow instead.
Confusing the gas tests: glowing splint relights (oxygen), burning splint pops (hydrogen), limewater turns milky (carbon dioxide).

Safety at home: the potato inquiry uses 3% hydrogen peroxide from a pharmacy; please pour it yourself, supervise the knife, and make sure eye protection and gloves are worn. Flame tests and burning magnesium are for the school lab only: do not try them at home.