Types of Chemical Reactions Chemistry Tutorial

Key Concepts

• A chemical change, or chemical reaction, involves the formation of at least one new substance.
• In a chemical reaction, atoms from reactants are rearranged to form products.
• There are many ways to classify chemical reactions.⁽¹⁾

  In this tutorial we will use the following classifications:

  (a) synthesis reactions (or direct combination reactions)

  (b) decomposition reactions

  (c) single replacement reactions (or single displacement reactions)

  (d) double replacement reactions (or double displacement reactions)

• In a synthesis reaction, elements or compounds react to produce a new compound.
• In a decomposition reaction a compound is broken up into smaller compounds or elements.

  Decomposition reactions can be further classified as:

  (i) thermal decomposition (decomposition when heated)

  (ii) electrolytic decomposition (decomposition when an electric current is passed through)

• In a single replacement reaction one element replaces (or displaces) another element in a compound.
• In a double replacement reaction, atoms or ions in both reacting compounds are exchanged to form products.

  Double replacement reactions may be further classified as:

  (i) precipitation reactions

  (ii) neutralisation reactions

  (iii) double decomposition reactions

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Synthesis Reactions (direct combination reactions)

Synthesis reactions are also known as direct combination reactions.

In a synthesis reaction, two or more reactants combine to form a product:

The reactants may be elements or compounds, but the product must be a compound.

(i) Both reactants are elements:

(ii) Both reactants are compounds:

Decomposition Reactions

In a decomposition reaction a compound is broken down into smaller chemical species (elements or compounds).⁽⁴⁾

Heat or electricity can be used to decompose a compound.
If heat is used to decompose a compound the reaction is known as a thermal decomposition reaction.
If electricity is used to decompose a compound the reaction is known as an electrolytic decomposition reaction.

(i) Decomposition using heat (thermal decomposition)

(ii) Decomposition using electricity (electrolyic decomposition)

Single Replacement Reactions

Single replacement reactions are also known as displacement reactions or substitution reactions.

In a single replacement reaction, one element in a compound is replaced by a different element to form two products.
One of the products is a compound and the other is an element.
The element in the compound that is to be replaced is known as the less active element.
The element that becomes part of the new product compound is known as the more active element.

The element to be replaced can be either a metal or a non-metal.

(i) Replacement of a metal in a compound by a more active metal.⁽⁵⁾

(ii) Replacement of a less active metal in a compound by a more active non-metal.

(iii) Replacement of a non-metal in a compound by a more active non-metal.

(iv) Replacement of hydrogen in water by an active metal.⁽⁶⁾

(v) Replacement of hydrogen in acid by an active metal.⁽⁷⁾

Double Replacement Reactions

Double replacement reactions are also known as double displacement reactions or metathesis reactions.

In a double replacement reaction, atoms or ions in both reacting compounds are exchanged to form products.

(i) Precipitation reactions

(ii) Neutralisation reactions

(iii) Double Decomposition reactions

The most common example of this type of reaction is the reaction between an acid and a carbonate or hydrogencarbonate (bicarbonate) ⁽⁹⁾
Initially, the acid reacts with the carbonate (or hydrogencarbonate) to produce a salt and carbonic acid, H₂CO₃.
Then the carbonic acid spontaneously decomposes to produce carbon dioxide and water.
So, the overall reaction is represented as:

acid + carbonate (or hydrogencarbonate) → salt + carbon dioxide + water

(a) acid + carbonate

(b) acid + hydrogencarbonate (acid + bicarbonate)

Classification Key for Chemical Reactions

It is possible to use the following key to decide how to classify a particular reaction:

Worked Examples Using the Reaction Classification Key

Question 1. Nitrogen triiodide is a very unstable compound, touching it causes it to explode and form nitrogen gas and iodine vapour.

Would you classify this nitrogen triiodide reaction as a synthesis, decomposition, single replacement or double replacement reaction?

Solution 1:

1. Write a balanced chemical equation for the reaction:

   word equation:	nitrogen triiodide	→	nitrogen gas	+	iodine gas
   chemical equation:	2NI3	→	N2	+	3I2

2. How many reactants are there?

   1 reactant

3. 1 reactant that breaks apart to form simpler pure substances (elements or compounds) is a decomposition reaction.

Question 2. When magnesium metal is left lying out on the laboratory bench it reacts with oxygen in the atmosphere to form magnesium oxide.

Would you classify this reaction as a synthesis, decomposition, single replacement or double replacement reaction?

Solution 2:

1. Write a balanced chemical equation for the reaction:

   word equation:	magnesium	+	oxygen gas	→	magnesium oxide
   chemical equation:	2Mg	→	O2	+	2MgO

2. How many reactants are there?

   2 reactants (therefore not a decompositon reaction)

3. How many products are formed?

   1 product is formed.

4. When two elements, magnesium and oxygen, combine to form a single product, magnesium oxide, the reaction is classified as a synthesis reaction.

Question 3. When an aqueous solution of colourless lead(2+) nitrate (lead(II) nitrate) is added to an aqueous solution of colourless potassium iodide solution, a bright yellow precipitate of lead(2+) iodide (lead(II) iodide) is formed.

Would you classify this reaction as a synthesis, decomposition, single replacement or double replacement reaction?

Solution 3:

1. Write a balanced chemical equation for the reaction:

   word equation:	lead(2+) nitrate (lead(II) nitrate)	+	potassium iodide	→	lead(2+) iodide (lead(II) iodide)	+	potassium nitrate
   chemical equation:	Pb(NO3)2	+	2KI	→	PbI2	+	2KNO3

2. How many reactants are there?

   2 reactants (therefore not a decompositon reaction)

3. How many products are formed?

   2 products are formed (therefore not a synthesis reaction)

4. Is one of the products an element?

   No. Both products are compounds. (therefore not a single replacement reaction)

5. This is a double displacement reaction.

4. When carbon and lead(2+) oxide (lead(II) oxide) are heated together a reaction takes place which results in the formation of metallic lead and carbon dioxide.

Would you classify this reaction as a synthesis, decomposition, single replacement or double replacement reaction?

Solution 4:

1. Write a balanced chemical equation for the reaction:

   word equation:	lead(2+) oxide lead(II) oxide	+	carbon	→	lead	+	carbon dioxide
   chemical equation:	2PbO	+	C	→	2Pb	+	CO2

2. How many reactants are there?

   2 reactants (therefore not a decompositon reaction)

3. How many products are formed?

   2 products are formed (therefore not a synthesis reaction)

4. Is one of the products an element?

   Yes.

5. This is a single replacement reaction in which the more active element, carbon, replaces the less active element, lead, from the compound, lead(2+) oxide (lead(II) oxide).