Form 5 Chemistry – PHYSICAL CHEMISTRY – Energetics

THERMOCHEMISTRY

This is also termed as Energetics.

Definition

Is the branch of physical chemistry which deals with energy changes that occurs during the chemical reaction.

The energy change during the chemical reaction is either positive or negative.

It is denoted by H.

ENTHALPY OF REACTION/HEAT OF REACTION

Definition

Enthalpy is the energy change which takes place during chemical reaction.

THE COMMON ENTHALPIES OF REACTIONS

The following are the common Enthalpies of reactions:-

1. STANDARD ENTHALPY OF COMBUSTION

This is the heat energy given out when 1mole of a certain substance is completely burnt in a given moles of oxygen at standard state.
If H for combustion reaction is negative i.e. The reaction is exothermic.

Example of combustion reaction

HOW TO BALANCE THE COMBUSTION REACTIONS

In balancing the combustion reactions we normally balance the other materials apart from hydrogen and oxygen first.

Secondly balance hydrogen then finally oxygen.

                  

         

2. STANDARD ENTHALPY OF FORMATION
This deal with the formation of compounds. It is donated by  ΔH°f.

Definition
Standard enthalpy of formation is the heat change which occur when 1mole of substance is formed from its element at standard state. Standard enthalpy of formation can be positive or negative.
ΔH°f may be  +Ve  or  -Ve.

ΔH°f =  +Ve   Endothermic.
or
ΔH°f = -Ve     Exothermic.

Example
Standard enthalpy of formation of CO₂.
C+ O₂   →   CO₂      ΔH°f.
ΔH°f  of  CO₂  = ΔH comb of C

Example
ΔH°f  of  CH₃COOH

                   Example

                   ΔH°f  of  CH₃ – O – CH₃


3. STANDARD ENTHALPY OF NEUTRALIZATION (E_N)

Definition

This is the heat energy given out when one mole of water is formed from the reaction between acid and base at standard state. Or
This is the Enthalpy change which take place when one mole of water is formed from reaction between acid and base at standard condition.

     Example

    HCl + NaOH NaCl + H₂O H = – Ve

4. ATOMIZATION ENERGY

Definition

Atomization energy is the energy absorbed when a molecule or electron is converted to gaseous atoms. Atomization energy applied for non-compound and H is positive.
Atomization of energy is denoted by “Eat”.

           Example

                    Br_2
(g) 2Br_(g)

                     O_2(g) 2O_(g)

5. SUBLIMATION ENERGY (Es)

It is also known as dissociation energy.

Definition

Sublimation energy is the energy absorbed when one mole of solid atom is converted into gaseous atom.

Sublimation energy is applied for metallic atom and H is positive.

Sublimation energy is denoted by ‘Es’.

       Example

Al_(s) Al_(g)

Mg _(g) Mg_(g)

Ca _(s) Ca_(g)

6. IONIZATION ENERGY (E_I)

Is the energy used to remove an electron from the outer most shell of an atom,  gaseous atom or ions to form an ion(s).

It is denoted by “E_I “.

Ionization energy can be categorized as first 1^st, 2^nd and 3^rd depending on the nature of stable ions that can be formed from an atom.

H is positive.

Example1

     Ca ⁺_(g) Ca²⁺ +

 Example 2

Al _(g) Al ⁺ +

Al ⁺ Al²⁺ +

Al ⁺ Al ³⁺ +

7. ELECTRON AFFINITY ( E_aff )

Definition

Is the energy required when  one  mole  of non- metallic gaseous atom combine with one mole of electron.

It is denoted by E_aff.

Example

Cl _(g) + Cl^–

O _(g) + O^–

8. LATTICE ENERGY (E_L)

Definition

Is the energy given out when one mole of ionic compound is formed from its ions.

It is denoted by E_L.

Example

Na ⁺ + Cl^{– 1}
NaCl

Al ³⁺ + 3Cl^-1 AlCl3

Mg ²⁺ + 2Br^– MgBr2

           H = -ve

9. STANDARD ENTHALPY OF SOLUTION

Definition

Is the heat change when one mole of a compound is dissolved in a given moles of water at standard conditions.

Na Cl + Aq NaCl (aq)

      10. STANDARD ENTHALPY OF FORMATION
Is the heat change which occur when one mole of a substance is formed from its element at standard conditions.
eg.  C+ O₂→CO_2(g) ΔHf = -393KJmol^-1.