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Chemistry 3

Organic chemistry: Precipitaion, combustion, decomposition, corrosion, displacement reactions (and all the fun stuff from last year)

Table of Contents

Hey there! Please be advised that these notes are grouped in concepts, not textbook chapters. Feel free to use the textbook, or use that neat little search bar up the top there! Thanks!


Polyatomic ion valencies you need to remember

Valency Ion Formula
1+ Ammonium NH₄
1- Hydroxide OH
1- Nitrate NO₃
2- Carbonate CO₃
2- Sulfate SO₄
3- Phosphate PO₄

Remember! All transition metals have a valency of 2+ except for Gold, which has a valency of 1+

Endothermic and exothermic reactions

  • Endothermic reactions absorb heat
    • respiration
    • metal + acid
    • heat packs and warmers
  • Exothermic reactions release heat
    • photosynthesis
    • ice packs

TIP! To remember the difference between endothermic and exothermic reactions - remember that endo sounds like inhale (take in), and exo sounds like exhale (release) with respect to heat.


Hydrocarbons are compounds made up of only hydrogen and carbon.

**Molecular Formula ** Name
CH4 methane
C2H6 ethane
C4H8 buthane
C3H8 propane
C4H10 butane
C5H12 pentane
C6H14 hexane
C7H16 heptane
C8H18 octane
C9H20 nonane
C10H22 decane

Source: Table 16 at Introductory Chemistry, Chapter 16.1

Alkanes vs alkenes vs alkynes

  • Your standard hydrocarbons are known as alkanes (methane CH4, ethane C2H6, buthane C4H8, etc) - they are saturated and are joined by single bonds.
    • They are relatively unreactive apart from combustion reactions
  • Alkanels or *alcohols (methanol CH3OH, ethanol C2H5OH, butanol C4H7OH, etc) contain saturated hybocarbons bound to a hydroxide -OH .

Reactivity series

Reactivity series

Metals that are higher up the reactivity series are more reactive and more vulnerable to reactions such as corrosion.

Precipitation reactions

When solutions of different ionic compounds react with each other, they sometimes produce a precipitate - insoluble solid.

For example:

silver nitrate + sodium chloride solution -> sodium nitrate + silver chloride (precipitate)

Precipitation reactions are predicted using a table of solubilities.

Anion All soluble/insoluble except (slightly)
Chlorides Cl- all soluble except Ag+Hg₂2+ Pb2+
Bromides Br- all soluble except Ag+Hg₂2+Hg2+ Pb2+
Iodides I- all soluble except Ag+Hg₂2+Hg2+Pb2+  
Nitrates NO₃- all soluble, no exceptions    
Sulfates SO₄2- all soluble except Sr2+Ba2+Hg₂2+Hg2+ Ca2+Ag+
Phosphates PO₄3- all insoluble except Na+K+Rb+Cs+NH₄  
Carbonates CO₃2- all insoluble except Na+K+Rb+Cs+NH₄  
Hydroxides OH- all insoluble except Li+Na+K+Rb+Cs+Ba2+NH₄ Ca2+Sr2+

Cool tip! Given two ionic compounds (solutions) swap the metals OR swap the ions. For example, given:

sodium hydroxide + lead nitrate
# swap the metals (or gases) around!

-> lead hydroxide + sodium nitrate

Sometimes there will be no precipitate formed. Therefore, check your table of solubilities to see whether a precipitate can be formed.

Combustion reactions

Combustion reactions occur when oxygen reacts with another substance to produce energy in the form of heat, light and sound.

Alkanes and alkenes on BBC Bitesize (GCSE)

A combustion reaction is an exothermic reaction as it releases heat, and it often involves hydrocarbons.

Complete combustion

In complete combustion all reactants will combust provided sufficient oxygen is available. Carbon oxidises to carbon dioxide, and hydrogen oxides to water.

Remember! Oxidation is the process of gaining oxygen.

The general equation for complete combustion is:

hydrocarbon + oxygen -> carbon dioxide + water

For example, in methane:

methane + oxygen -> carbon dioxide + water
  CH4   +   O    ->      CO2       + H2O


Incomplete combustion

When there isn’t enough oxygen to allow for a complete combustion reaction, incomplete combustion occurs instead.

The general equation for incomplete combustion is:

hydrocarbon + oxygen -> carbon monoxide + carbon + water 

For example, in methane (balanced chemical eq.):

methane + oxygen -> carbon monoxide + carbon + water
   4CH4 +  5O2   ->       2CO       +   2C   + 8H2O

Note that carbon dioxide has been replaced with carbon monoxide + carbon

  • Carbon monoxide is poisonous!
  • Carbon is produced in the form of soot.

It was determined in an experiment that less energy is released with less carbon atoms.


i swear we never learnt this

Decomposition occurs when one substance breaks down into two substances. It is an example of an endothermic reaction

The general equation for a decomp. reaction is:

AB -> A + B

For example, with calcium carbonate1

CuCO3 -> CuO + CO2

It is usually observable by colour change.


Corrosion occurs when metal is exposed to air, water or other substances.

Corrosion on BBC Bitesize (National 4)

Some materials are more vulnerable to corrosion than others

  • Iron and steel rust
  • Tin doesn’t rust

In the real world (yea with all this coronavirus stuff I guess noone knows what that is anymore eh) corrosion is prevented through different measures:

  • Painting/greasing: Paint or grease protects the metal from air, water, and sunlight.
  • Electro-plating: tin-plated iron won’t rust as the iron is not exposed, and tin doesn’t rust. Electro-plating involves covering a metal in a thin layer of a less reactive metal.
  • Sacrificial Protection: Magnesium is more reactive in corrosion, therefore it can be sacrificed instead of the material under it. Sacrificial protection involves coating a metal in a more reactive metal.
  • Galvanising is similar to sacrificial protection, but it specifically refers to coating iron or steel with a zinc protective layer.

Displacement reaction

In a displacement reaction, a more reactive metal will displace a less reactive metal from its compounds2

Displacement reactions on BBC Bitesize (KS3)

Reactivity series

Some metals are more reactive than others.

Fe + Pb(NO₃)₂ -> Fe(NO₃)₂ + Pb  # displacement has occured
Mg + CuSO     -> MgSO₄ + Cu     # displacement has occured
Pb + Fe(NO₃)₂ -> NO REACTION    # no displacement
Au + Pb(NO₃)₂ -> NO REACTION    # no displacement

According to the Reactivity series, Magnesium is more reactive than zinc, and zinc is more reactive than iron, and iron is more reactive than copper, etc. This is best represented in this table displaying different reactions and whether an observable and visible reaction occurs.

  Magnesium Zinc Iron Copper
Magnesium sulfate
Zinc sulfate
Iron sulfate
Copper sulfate
Reactions seen 3 2 1 0

Source: BBC Bitesize ofc!

wow what’s that? they released the study guide?



  • Describe the indications that a chemical reaction has occurred
  • Group compounds into ionic and covalent depending on their properties
  • Explain the Law of Conservation of Mass
  • Describe and write equations for combustion, decomposition and precipitation reactions
  • Write word and balanced chemical equations for the following reactions
    • metal and acid reactions (also describe the test for hydrogen)
    • metal carbonate and acid reactions (also describe the test for carbon dioxide)
    • decomposition reactions involving metal carbonates and metal oxides
  • Use the solubility rules (provided – no need to memorise) to predict whether reactions will produce a precipitate


Describe the indications that a chemical reaction has occurred

To quote the Minnesota Science Teachers Education Project…

There are five signs of a chemical change:

  1. Color Change
  2. Production of an odor
  3. Change of Temperature
  4. Evolution of a gas (formation of bubbles)
  5. Precipitate (formation of a solid)

In a precipitation reaction, this might involve the formation of a solid (precipitate). In a combustion reaction, a change of temperature or production of a gas. In a corrosion reaction, this might involve a change of temperature or a precipitate (rust). In a displacement reaction, this might involve a colour change of sorts.

Group compounds into ionic and covalent depending on their properties3

Ionic compounds:

  • have higher melting points and electrical conductivity
  • electrostatic bonds between ions
  • tend to be hard and brittle
  • solid at room temp.
  • occurs between a metal and a non-metal

Covalent compounds:

  • have lower melting points and electrical conductivity
  • share electrons
  • tend to be soft and flexible
  • liquid/gaseous at room temp.
  • occurs between two non-metals

Explain the Law of Conservation of Mass

The law of conservation of mass or principle of mass conservation states that for any system closed to all transfers of matter and energy, the mass of the system must remain constant over time, as the system’s mass cannot change, so quantity can neither be added nor be removed. Therefore, the quantity of mass is conserved over time.4

In a nutshell:

mass cannot be created nor destroyed

Describe and write equations for combustion, decomposition and precipitation reactions



Complete combustion
hydrocarbon + oxygen -> carbon dioxide + water
Incomplete combustion
hydrocarbon + oxygen -> carbon monoxide + carbon + water 


A-B -> A + B


A-B + C-D -> A-D + B-C
where A and C are metals
and   B and D are non-metals

It might also help knowing your other general equations:


A + B -> A-B


metal + oxygen gas -> metal oxide

Acid + Base

acid + base -> salt + water

Acid + Carbonate

acid + carbonate -> salt + water + carbon dioxide

Acid + Metal

acid + metal -> salt + hydrogen

Test yourself on Quizlet!

Use the solubility rules (provided – no need to memorise) to predict whether reactions will produce a precipitate

Something is soluble if it can dissolve in water. In an exam, you’ll be given a solubility rules sheet, and you’ll need to use it (in conjunction with the periodic table)


  2. Displacement reactions on BBC Bitesize (KS3) - 

  3. Chemistry LibreTexts (UC Davis, CSU, US DoE) 

  4. Wikipedia, the free encyclopedia. 

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