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Prepared by Lawrence Kok

Video Tutorial on Hess’s Law, Born Haber Cycle and Lattice Enthalpy of ionic compounds.

Video on Energetics and Born Haber Cycle

Born Haber Cycle on NaCI Born Haber Cycle on LiF

Born Haber Cycle on CaF2 Hess’s Law calculation

Hess’s Law

H = Enthalpy ( Heat Content )ΔH = Enthalpy change ( Change in heat content )

Hess’s Law • Overall heat change of a chemical rxn is independent of its pathway• Energy change in changing A + B → C + D is the same regardless of theroute (independent of route) by which the chemical changes occurs.

Energy Level or Energy Cycle Diagram can be used to determine ΔH

Hess’s Law – Used to calculate ΔH for rxn which cannot be determined experimentally

Energy Level or Energy Cycle Diagram

Cal ΔH for C(s) + 1/2O2(s) → CO(g) which cannot be determined experimentallyEx 1

Hess’s Law – Used to calculate ΔH for rxn which cannot be determined experimentally

Answer = -111kJmol-1

Energy Level or Energy Cycle Diagram

Cal ΔH for C(s) + 1/2O2(s) → CO(g) which cannot be determined experimentallyEx 1

Cal ΔH for 2NO2(g) → N2O4(g) ΔH cannot be determined experimentally

Hess’s Law – Used to calculate ΔH for rxn which cannot be determined experimentally

Energy Level or Energy Cycle Diagram

Ex 2

Cal ΔH for 2NO2(g) → N2O4(g) ΔH cannot be determined experimentally

Hess’s Law – Used to calculate ΔH for rxn which cannot be determined experimentally

Energy Level or Energy Cycle Diagram

Answer = -24 kJmol-1

Ex 2

Born Haber Cycle is a Hess’s Cycle • Used to calculate Lattice enthalpy for IONIC COMPOUNDS • Used to calculate ΔH that cannot be determined experimentally

Lattice Enthalpy – Heat absorbed to convert 1 MOL of IONIC compounds into GASEOUS ions

Lattice Enthalpy – Heat released when 1 MOL of IONIC compound formed from its GASEOUS ions

Atomization Enthalpy – Enthalpy change when 1 MOL of GASEOUS atoms formed from its elements under STD condition

Electron Affinity Enthalpy – Enthalpy change when 1 MOL of GASEOUS ATOMS gain electrons to form 1 MOL of negatively charged ions

Born Haber Cycle is a Hess’s Cycle • Used to calculate Lattice enthalpy for IONIC COMPOUNDS • Used to calculate ΔH that cannot be determined experimentally

1st Ionization Enthalpy – Enthalpy change when 1 MOL of ELECTRONS are removed 1 MOL of atoms of elements in gaseous state.

2nd Ionization Enthalpy – Enthalpy change when 1 MOL of ELECTRONS are removed 1 MOL of UNIPOSITIVE ions in gaseous state.

Formation Enthalpy – Enthalpy change when 1 MOL of substance are formed from its elements

Born Haber Cycle to determine Lattice Enthalpy for NaCI

ΔH obtained experimentally1

Born Haber Cycle to determine Lattice Enthalpy for NaCI

ΔH obtained experimentally1

2

Born Haber Cycle to determine Lattice Enthalpy for NaCI

ΔH obtained experimentally

Lattice enthalpy

1

2

Born Haber Cycle to determine Lattice Enthalpy for MgO

ΔH obtained experimentally1

Born Haber Cycle to determine Lattice Enthalpy for MgO

ΔH obtained experimentally1

2

Born Haber Cycle to determine Lattice Enthalpy for MgO

ΔH obtained experimentally

Lattice enthalpy

1

2

Born Haber Cycle to determine Lattice Enthalpy for CaCI2

ΔH obtained experimentally1

Born Haber Cycle to determine Lattice Enthalpy for CaCI2

ΔH obtained experimentally1

2

Lattice enthalpy

Born Haber Cycle to determine Lattice Enthalpy for CaCI2

ΔH obtained experimentally1

2

Lattice Enthalpy • Measure of electrostatic forces of attraction between ions in its crystal structure• Lattice enthalpy depends on SIZE of ions and CHARGE on ions

Lattice Enthalpy • Measure of electrostatic forces of attraction between ions in its crystal structure• Lattice enthalpy depends on SIZE of ions and CHARGE on ions

Size increase ↑ - Lattice Enthalpy decrease ↓ - Separation bet ions increase ↑ - Electrostatic forces between ions decrease ↓ - Lattice enthalpy decrease ↓

Lattice Enthalpy • Measure of electrostatic forces of attraction between ions in its crystal structure• Lattice enthalpy depends on SIZE of ions and CHARGE on ions

Size increase ↑ - Lattice Enthalpy decrease ↓ - Separation bet ions increase ↑ - Electrostatic forces between ions decrease ↓ - Lattice enthalpy decrease ↓

Charge ↑ – Lattice enthalpy increase ↑ - Stronger electrostatic forces between ions increases ↑ - Lattice enthalpy increase ↑

To determine the degree of ionic/covalent character

To determine the EXPT lattice enthalpy Na+

(g) + CI- (g) → NaCI(s)

Uses of Born Haber Cycle

To determine the degree of ionic/covalent character

To determine the EXPT lattice enthalpy Na+

(g) + CI- (g) → NaCI(s)

Uses of Born Haber Cycle

Theoretical Lattice enthalpy (Calculated) can be calculated by make assumptions

1

To determine the degree of ionic/covalent character

To determine the EXPT lattice enthalpy Na+

(g) + CI- (g) → NaCI(s)

Uses of Born Haber Cycle

Lattice Enthalpy can be obtained by TWO Methods• 1st Method: Experimental Lattice Enthalpy (Actual) determined from BHC• 2nd Method: Theoretical Lattice Enthalpy (Calculated) using formula

1

2

Theoretical Lattice enthalpy (Calculated) can be calculated by make assumptions

Uses of Born Haber Cycle – to determine the degree of ionic /covalent character

High Difference in EN value Small Difference in EN value

Uses of Born Haber Cycle – to determine the degree of ionic /covalent character

High Difference in EN value Small Difference in EN value

High Degree of IONIC character

100% Ionic

EXPT (Actual ) = Theoretical (Calculated) Lattice enthalpy Lattice enthalpy

Uses of Born Haber Cycle – to determine the degree of ionic /covalent character

High Difference in EN value Small Difference in EN value

High Degree of IONIC character Has IONIC and COVALENT character ( NOT 100% Ionic)

100% Ionic

EXPT (Actual ) = Theoretical (Calculated) Lattice enthalpy Lattice enthalpy

EXPT(Actual) Lattice enthalpy is MORE (Ionic + Covalent character)

EXPT (Actual) > Theoretical (Calculated) Lattice enthalpy Lattice enthalpy

Uses of Born Haber Cycle – to determine the degree of ionic /covalent character

Video on Energetics and Born Haber Cycle

Born Haber Cycle on NaCI Born Haber Cycle on LiF

Born Haber Cycle on CaF2 Hess’s Law calculation

Acknowledgements

Thanks to source of pictures and video used in this presentation

Thanks to Creative Commons for excellent contribution on licenseshttp://creativecommons.org/licenses/

Prepared by Lawrence Kok

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