Classifying
Chemical Reactions

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Decomposition Reactions

Decomposition Reaction Video

When a single reactant breaks apart to form  two or more new substances
General form

 

XY--> X+ Y

Example
H₂CO₃(aq) → H₂O(l) + CO₂(g)

Thermal Decomposition

Some substances will only decompose when heated to extremely high temperatures.

Example

2Na2N3 (s) --> 2Na (s)+ 3N2 (g)

Just 100g of NaN₃ can produce 56litres of nitrogen gas in 0.03 seconds.  This is used in car air bags

Combination (Composition) Reactions

Combination Reactions Video

When two or more reactants combine to form one new substance
General Form
X + Y --> XY

Example
H₂(g) + Cl₂(g) → 2HCl(g)

 

 

Displacement Reactions

Replacement Reactions Video Part A Replacment Reactions Video Part B

Ionic Compounds

Substances made up of a lattice (or crystal) structure. The substances have a positive ion called a cation and a negative ion called an anion.
There are normally hard, brittle and brightly coloured.

The cations are atoms (or compounds) that have lost an electron and the anions are atoms (or compounds) that have gained an electron.

• Some common cations that have lost 1 electron are
• H+, Li+, Na+, K+ and NH₄+

 

• Some common cations that have lost 2 electrons Ca2+, Mg2+, Ba2+, Cu2+ and Fe2+

• Some common cations that have lost 3 electrons are Fe3+ and Al3+.

 

• Some Common anions that have gained 1 electron are F-, Cl-, Br- and Br-

• Some Common anions that have gained 2 electron are O2-, S2-, SO42-and CO₃^2-
• Some Common anions that have gained 3 electron are N3- and PO43-.

 

Predicting Precipitates

Scientists use a list of solubility rules to help them predict if something will dissolve or not

From the table predict which of the products will be a precipitate
KCl(aq) + AgNO₃(aq) → KNO₃( (aq)) + AgCl( (s))

Oxidation and Reduction Reactions

Not always evident when an oxidation reaction takes place

Oxidation reactions

Usually occurring when oxygen is combined (joined) with another chemical. This can result in the reaction called combustion
An example of combustion is

methane + oxygen → carbon dioxide + water
CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

Combustion Video

Reduction reactions

Usually occur when oxygen is removed from another substance.  It is the opposite of oxidation.

copper(II) oxide + carbon → copper + carbon dioxide
2CuO (s) + C (s) → 2Cu (s) + CO₂ (g)
Here oxygen is removed from Copper and added to Carbon.
Notice there is both a reduction reaction and an oxidation reaction occurring. This is called a redox reaction.

Redox Reactions and electrons

Redox reactions do not always involve oxygen. They are many reaction that involve the transfer of electrons from one substance to another.

OILRIG – Oxidation Is L oss, Reduction Is G ain

Metal Displacement Reactions

These are replacement reactions which are simple redox reactions where the electrons move from one metal to another
Example
Cu²⁺(aq) + Zn(s) → Cu(s) + Zn²⁺(aq)

Activity Series

Replacement reactions do not work all the time.  This is because some substances are more reactive than others.
The activity series (see table ) summarizes this.

Extraction of Metals

Metals are extracted from their ore.  They bond to these substances because of their activity.  Very non-reactive substances are usually found in pure form like silver and gold.  Other very reactive metals are never found in their pure form and must be extracted.

Three groups are

• Native metals
• carbon reduction
• Electrolysis

The ones needing electrolysis are the most reactive.

Native metals

Metals are not very reactive like silver and gold. We find them in nature and they are separated from the surrounding sand and rocks by such things as crushing and froth floatation.

Carbon Reduction

Naturally occurring metals found in ores. Some common ores are found in the table on page 156.

Extraction is done by carbon reduction where the ore is heated with a pure form of carbon (possibly coke) and oxygen is forced in.

The reduction of iron ore works like this in a 2 step process

Step 1
carbon + oxygen gas → carbon monoxide gas
2C(s) + O₂(g) → 2CO(g)
Step 2
Iron(III) oxide + carbon monoxide gas à iron + carbon dioxide gas
Fe₂O₃(s) + 3CO(g) → 2Fe(s) + 3CO₂(g)
This whole process is known as smelting

Electrolysis

Carbon reduction does not work with the very reactive metals like Aluminium, Potassium or Calcium
The process whereby an electrical current is passed through the molten ore is called electrolysis.

The electricity pushes the electrons onto the metal ions and thus neutralises then.
The process uses large amounts of electricity and is very expensive. Some common metals extracted this way are Aluminium, Potassium and Sodium.

Sodium can be extracted from molten salt by electrolysis.

The result is two half reactions

sodium ion+ an electron → sodium
Na⁺(aq) + e^– → Na(s)
And
chloride ion → chlorine gas + 2 electrons
2Cl^–(aq) → Cl (g) + 2e^–

Forward To Part C