CHEMISTRY As LEVEL(FORM FIVE) NOTES – PHYSICAL CHEMISTRY – Energetics

PHYSICAL CHEMISTRY – Energetics

THERMOCHEMISTRY

This is also termed as Energetics.

Definition

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

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

It is denoted by .

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 1 mole of a certain substance is completely burnt in a given moles of oxygen at standard state.
If ΔH 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 deals with the formation of compounds. It is denoted by ΔH°f.

Definition

Standard enthalpy of formation is the heat change which occurs when 1 mole of substance is formed from its elements 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.

Example

ΔH°f of CH3COOH

Example

ΔH°f of CH3–O–CH3

3. STANDARD ENTHALPY OF NEUTRALIZATION (EN)

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 takes 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 element is converted to gaseous atoms. Atomization energy applies for non-compounds and H is positive.
Atomization energy is denoted by “Eat”.

Example

Br2 (g) → 2Br (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 applies for metallic atoms 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 (EI)

Is the energy used to remove an electron from the outermost shell of an atom, gaseous atom or ions to form an ion(s).
It is denoted by “EI”.

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

H is positive.

Example 1

Ca+ (g) → Ca2+ + 2e⁻

Example 2

Al (g) → Al+ + e⁻

Al+ → Al2+ + e⁻

Al2+ → Al3+ + e⁻

7. ELECTRON AFFINITY (Eaff)

Definition

Is the energy required when one mole of non-metallic gaseous atom combines with one mole of electron.
It is denoted by Eaff.

Example

Cl (g) + e⁻ → Cl⁻

8. LATTICE ENERGY (EL)

Definition

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

Example

Na⁺ + Cl⁻ → NaCl

Al³⁺ + 3Cl⁻ → AlCl₃

Mg²⁺ + 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.

NaCl + Aq → NaCl (aq)

10. STANDARD ENTHALPY OF FORMATION

Is the heat change which occurs when one mole of a substance is formed from its elements at standard conditions.
eg. C + O2 → CO2 (g) ΔHf = -393 KJmol-1.

11. HEAT OF DILUTION

Is the heat change when one mole of a substance is dissolved in a given mole of water.

METHODS OF FINDING HEAT OF FORMATION OF A GIVEN COMPOUND

The calculations in thermochemistry are categorized in the following:

1. Calculation based on combustion data.
2. Calculation based on bond energies.
3. Calculation based on atomization data.
4. Calculation based on calorimetry information.
5. Determination of molecular formula by combustion data.
6. Calculation based on Born–Haber cycle.

CALCULATIONS BASED ON COMBUSTION DATA

Calculations involving combustion data have the following steps:

1. Identify the required equation.
2. Data presentation.
3. Data manipulation.
4. Conclusion.

Example 1

a) With one example in each briefly define the following terms:

1. Standard enthalpy of combustion.
2. Sublimation energy.
3. Standard enthalpy of formation.
4. Atomization energy.
5. Lattice energy.

Solution

(i) Standard enthalpy of combustion: is the heat energy given out when 1 mole of a certain substance is completely burnt in a given moles of oxygen at standard state.

(ii) Sublimation energy: Is the energy absorbed when one mole of solid atom is converted to gaseous atom.

(iii) Standard enthalpy of formation: is the heat change which occurs when 1 mole of substance is formed from its element.
eg. C + O2 → CO2 (g) ΔH°f = -393 KJmol-1

(iv) Atomization energy: is the energy absorbed when a given molecule or element is converted into gaseous atom.

(v) Lattice energy: Is the energy given out when 1 mole of ionic compound is formed from its ions.
eg. Na⁺ (g) + Cl⁻ (g) → NaCl

b) Calculate the enthalpy of formation of methane given that,

Enthalpy of combustion are:

• Carbon 394 KJmol-1.
• Hydrogen 286 KJmol-1.
• Methane 891 KJmol-1.

Solution

Required equation

C + 2H2 → CH4

Data presentation (KJ mol-1)

• C + O2 → CO2 -394
• H2 + O2 → H2O -286
• CH4 + 2O2 → CO2 + 2H2O 891

Data manipulation (KJ mol-1)

• C + O2 → CO2 -394
• H2 + O2 → H2O -286
• 2H2 + O2 → 2H2O -572
• 2H2 + O2 → CH4 + 2O2 891

C + 2H2 → CH4 -75

The enthalpy of formation of methane is -75 KJmol-1.

Example 2

Calculate the standard enthalpy of formation of ethane given that enthalpy of combustion:

• C = -394 KJmol-1
• Hydrogen = -216 KJmol-1
• Ethane = -1561 KJmol-1

Solution

Required equation

2C + 3H2 → C2H6

Given (KJmol-1)

• C + O2 → CO2 -394
• H2 + O2 → H2O -284
• Ethane -1561

Solution

Required equation

2C + 3H2 → C2H6

Data presentation

• 2C + 2O2 → 2CO2 -788
• 3H2 + 3/2 O2 → 3H2O -852
• C2H6 + 7/2 O2 → 2CO2 + 3H2O -1561

Data Manipulation

2C + 2O2 → 2CO2 -788 + 3H2 + 3/2 O2 → 3H2O -852 – 2C + 3H2 → C2H6 -1561

The standard enthalpy of formation of C2H6 is 125 KJmol-1.