Types of Chemical Reactions

Reactions in chemistry occur in numerous ways, and when reactions occur from chemical substances called reactants, they often led to the formation of new substances called products.
The most common of these reactions can be grouped into the following
i. combination reactions
ii. catalytic reactions
iii. decomposition reactions
iv. displacement reactions
v. double decomposition reactions
vi. thermal dissociation reactions
vii. reversible reactions
viii. oxidation and reduction reactions
COMBINATION REACTIONS
In this type of reaction, more than one substance combine to form a single compound termed the product.
e.g hydrogen + oxygen = water

H₂ + ¹/₂O₂  →  H₂O

PbO₂ + SO₂ → PbO₄

Decomposition reaction

When substances decompose, they will produce new substances. That is a sample compound may decompose to produce new ones. Other substances may require heat to decompose. This process of applying heat to decompose a substanc is termed thermal decomposition.

2Pb(NO₃)_{2  (S)} ^heat → 2PbO_(S)  +  NO_4(g)  + O_2(g)

Displacement reaction

In displacement reactions, one of the reactants will replace another in a compound to form a new product

e.g.  Cl_2(g) +  2KBr_(aq) → 2KCl_(aq) + Br_2(l)

Double decomposition reaction

This type of reaction occurs when the reactants decompose and exchange their constituents to form new substances

e.g. NaCl_(aq) + H₂SO₄(aq) → NaHSO₄(aq) + HCl(aq)

Double decomposition reactiion is useful in the preparation of volatile or insoluble substances

e.g. KNO_3(aq) + H₂SO_4(aq) → HNO_{3 (g)} +  KHSO_4(aq)

Catalytic reaction

 Chemical reactions that occur at a slow are usaully subjected to other substance that alters the rate of the reaction. The chemical substance that alters the reaction rate is a catalyst.

Reversible reaction

Most chemical reactions are reversible to some extent. In a reversible reaction, the products formed dissociates back to the reactants. A double arrow indicates a reversible reaction.

e.g. 3Fe(s) +4H₂O(l) → Fe₃O₄(g) + 4H₂(g)

Thermal dissociation

This reaction uses heat to dissociate molecules of substances into two or more substances. This rection is reversible upon cooling the products.

e.g NH₄Cl (S) ^heat→ NH₃(g) + HCl (g)^cold→ NH₄Cl (S)

Oxidation and Reduction Reaction(redox)

In redox reaction electrons are always transferred, from one atom to another. One atom donates the electron while the other atom receives the electron. Modern expanation of redox reaction includes the tranfer of hydrogen, electronegative and electropositive elements.

In redox reaction, the oxidizing agent donates electron while the reducing agent receives electron.

Oxidation: it occurs when electron, hydrogen and electropositive elements is removed from their atoms, elements or compounds. The atom, element or compound, which removes the electron is the reducing agent.

Reduction: in reduction reactions, atoms, electrons, hydrogen is added to their atoms or elements.

OXIDATION is a process involving loss of electrons, while reduction is a process involving gain of electrons

Ionic equation

Electron transfer

in a redox reaction, electrons are transferred as shown below.

2kl(aq) + Fe₂(SO₄)₃(aq) → I₂(s) + K₂SO₄(aq) + 2FeSO₄(aq)

In the above equation, iodine donates two electrons to form iodine molecule while iron accepts electron to become iron III from iron II state.

Ironic equation: 2I^-(aq) + 2Fe³⁺(aq) → I₂(s) + 2Fe²⁺(aq)

The two half equations

2I^-(aq) → I₂(s) + 2e^- ……………………………………….oxidation

2Fe³⁺(aq) + 2e^-→ 2Fe²⁺(aq) ................................................reduction

N/B. A half reaction cannot occur unless there is a complimentary half reaction. This is because an atom cannot donate an electron if there is no other atom to receive the electron.

Oxidation number: this is the electrical charge carried by an atom, ion or molecule as determined by a set of arbitrary rules. The oxidation number expresses the oxidation state of an element.

How to determine oxidation number

1.     The oxidation number of an element when not combined with other elements is zero, often wriiten as Na^o.

2.     The oxidation number of simple elements is the same as the charge on the simple ekement. E.g. Na⁺= +1, Mg²⁺= +2, Al³⁺=+3, Cl^-= -1, S^2-= -2

3.     When the ion is made-up of more than one element, the oxidation number is the sum of the charges. E.g. OH^- = -1. This is so becaouse the net charge is -1. That is [ (H=+1, plus O= -2) = -1]

4.     The total sum of all charges in a compound is zero. E.g. MgCl₂ has an oxidation number of zero (MgCl₂)

N/B. Oxygen has an oxidation state of -2, but in peroxides its oxidation state is -1. Likewise hydrogen has oxidation state of +1 but in hydrides its oxidation state is -1.

Example

What is the oxidation number of manganese atom in potassium tetraoxomanganate (VII) KMnO₄.

We know that the total sum of all charges in a compound is zero, then;

KMnO₄= 0.

That is K + Mn + O₄ =0

Let y represent Mn in the equation so that

K + y + O₄ = 0

But K= +1, and O = -2

Hence,  K + y + O₄ = 0 will be

 (+1) + y + 4(-2) = 0

(+1) + y + (-8) = 0

(+1) + (-8) + y = 0

(+1-8) + y =0

-7 + y = 0

Therefore y= +7. The oxidation number of Mn is +7.