Types of chemical reactions

Break down, swap one, swap two

1. Types of chemical reactions: from zero

2. Green to black

Heat green copper(II) carbonate. It turns into a black powder, and a gas comes off.

Pick the right answer

Predict: what happened to the green solid?

A. It broke down into two simpler substances
B. It just melted
C. It took in a gas from the air

Explanation

The green copper(II) carbonate broke down into two simpler substances: heat split it into a black solid and a gas. Melting would keep the same substance, and here a gas comes out of the solid instead of going in.

3. Thermal decomposition

Thermal decomposition

Breaking down a compound by heat into simpler substances.

CuCO3(s) \to[Δ] CuO(s) + CO2(g) CuCO X_{3} (s) Δ CuO (s) + CO X_{2} (g)

green solid → black solid + gas

Pattern: one compound in, two or more substances out. The

Δ Δ

over the arrow means heat.

4. Simple displacement

Simple displacement

A more active element takes the place of a less active one in its compound.

Zn(s) + CuSO4(aq) \to ZnSO4(aq) + Cu(s) Zn (s) + CuSO X_{4} (aq) ZnSO X_{4} (aq) + Cu (s)

Zinc is above copper in the activity series.

The blue colour fades, and reddish copper coats the zinc.

Metals above hydrogen also displace it from dilute acids:

Mg + 2HCl \to MgCl2 + H2 Mg + 2 HCl MgCl X_{2} + H X_{2}

5. Double displacement

Double displacement

Two compounds exchange their partners: AB + CD → AD + CB.

AgNO3(aq) + NaCl(aq) \to AgCl(s) v + NaNO3(aq) AgNO X_{3} (aq) + NaCl (aq) AgCl (s) ↓ + NaNO X_{3} (aq)

A white solid appears in the solution.

AgCl does not dissolve, so it separates as a solid. Such a solid is a precipitate, shown by ↓.

Another double displacement you know

NaOH + HCl \to NaCl + H2O NaOH + HCl NaCl + H X_{2} O

acid + base → salt + water

6. Spot the type

Sort into groups

Sort each reaction by its type.

$CaCO3 \to[Δ] CaO + CO2 CaCO X_{3} Δ CaO + CO X_{2}$ $Zn(OH)2 \to[Δ] ZnO + H2O Zn (OH) X_{2} Δ ZnO + H X_{2} O$ $Zn + H2SO4 \to ZnSO4 + H2 Zn + H X_{2} SO X_{4} ZnSO X_{4} + H X_{2}$ $Fe + CuSO4 \to FeSO4 + Cu Fe + CuSO X_{4} FeSO X_{4} + Cu$ $BaCl2 + Na2SO4 \to BaSO4 v + 2NaCl BaCl X_{2} + Na X_{2} SO X_{4} BaSO X_{4} ↓ + 2 NaCl$ $KOH + HNO3 \to KNO3 + H2O KOH + HNO X_{3} KNO X_{3} + H X_{2} O$

Pick the right answer

CuSO4 + 2NaOH \to Cu(OH)2 v + Na2SO4 CuSO X_{4} + 2 NaOH Cu (OH) X_{2} ↓ + Na X_{2} SO X_{4}

, what does ↓ after

Cu(OH)2 Cu (OH) X_{2}

tell you?

A. A gas leaves the reaction
B. An insoluble solid forms in the solution
C. The reaction needs heat

Explanation

The down arrow ↓ after

Cu(OH)2 Cu (OH) X_{2}

means it is a precipitate: an insoluble solid that forms in the solution. A gas leaving is shown by ↑, and heat by

Δ Δ

over the arrow.

7. Summary

Thermal decomposition: heat breaks one compound into simpler substances.
Simple displacement: a more active element replaces a less active one.
Double displacement: two compounds swap partners. A precipitate (↓) or salt + water may form.

Can you explain how to tell simple from double displacement at a glance?

Reveal

Type	What happens	Example
Simple displacement	A more active element takes the place of a less active one in its compound.	$Zn + CuSO4 \to ZnSO4 + Cu Zn + CuSO X_{4} ZnSO X_{4} + Cu$
Double displacement	Two compounds exchange their partners: AB + CD → AD + CB.	$AgNO3 + NaCl \to AgCl v + NaNO3 AgNO X_{3} + NaCl AgCl ↓ + NaNO X_{3}$

Next: roasting heats iron ores, and some of its changes are thermal decompositions.

Concepts in this lesson