9.0 THERMOCHEMISTRY

9.1 Concept of Enthalpy

9.2 Calorimetry

9.3 Hesss Law

9.4 Born-Haber Cycle

THERMOCHEMISTRY

8.4 Born-Haber CycleTHERMOCHEMISTRY 9.1 : Concept of enthalpy

Concept of Enthalpy Definition of Enthalpies

Endothermic & Exothermic Reaction

FormationCombustionAtomizationNeutralizationHydrationSolution/dissolutionThermochemical Equation

Standard Condition of ReactionEnthalpyStandard Enthalpy

Energy Profile Diagram

Concept of Enthalpy

Thermochemistry

ExothermicEndothermic

A study of heat change in chemical reactions.

Two types of chemical reactions:

Important TermsHeat is energy transferred between twobodies of different temperatures.

5

Surroundings is everything that lies outside the system.

System is any specific part of the universe whose change we are going to observe.

6Types of SystemOpen system is a system that can exchange mass and energy with its surroundings

Closed system is a system that allows the exchange of energy with its surroundings

Isolated system is a system that does not allow the exchange of either mass or energy with its surroundings

open

mass & energy

Exchange:

closed

energy

isolated

nothing

EXAMPLE: water in a flask

Any process that release energy from the system to surrounding.

H = negative value (H < 0)Total energy of products is less than total energy of reactants.

Results in a decrease in the enthalpy of the system.

Exothermic Reaction

2H2(g) + O2(g)

2H2O(l)

Ener

gy /

Enth

alpy (

kJ)

Progress of reaction

H = 571.6 kJ

Exothermic reaction

Energy Profile Diagram2H2(g) + O2(g) 2H2O(l) H = 571.6 kJ

2H2(g) + O2(g)

2H2O(l)

Ener

gy /

Enth

alpy (

kJ)

Progress of reaction

H = 571.6 kJ

(Ea),

2H2(g) + O2(g) 2H2O(l) H = 571.6 kJ

Any process that absorb energy from the surrounding to the system

H = positive value (H > 0)

Total energy of products is more than total energy of reactants

Endothermic Reaction

Results in an increase in the enthalpy of a system.

Al2O3(s)Ener

gy /

Enth

alpy (

kJ)

Progress of reaction

H = + 1676 kJ

Endothermic reaction

Energy Profile DiagramAl2O3(s) 2Al(s) + 3/2 O2 (g) H = + 1676 kJ

Al2O3(s)H = + 1676 kJ

Al2O3(s) 2Al(s) + 3/2 O2 (g) H = + 1676 kJ

Enthalpy is an extensive property, its magnitude depends on the amount of the substance present

Used to quantify the heat flow into or out of a system in a constant-pressure process

Enthalpy (H)

H = Hproducts Hreactants

The difference between the enthalpies of the products and the enthalpies of the reactants

Enthalpy Change (H)

Enthalpy change measured at standard conditions (298 K, 1 atm etc)

STANDARD STATES : Gas: 1 atm and it behave ideally Aqueous solution: 1 M Pure substance (element or compound): Most stable form of substance at 1 atm and temperature (usually 25oC)

Standard Enthalpy (Ho)

EXAMPLE: (standard states of pure substance)

oxygen O2(g)

hydrogen H2(g)

sodium Na(s)

bromine Br2(l)

water H2O(l)

Specify whether the substance exists as atom/molecule andits states of matter!

The chemical equations that shows the enthalpy changes

2H2O (l) 2H2 (g) + O2 (g) H = + 572 kJ (Endothermic)

2H2(g) + O2(g) 2H2O(l)

Note: A forward reaction has opposite sign of the reverse reaction

Thermochemical Equation

H = 572 kJ (Exothermic)

Two important aspects of H:

Sign positive or negative

Magnitude stoichiometrical relationship

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (l) H = 890.4 kJ

STOICHIOMETRY OF THERMOCHEMICAL EQUATION

Sign positive or negative

Magnitude stoichiometrical relationship

2H2O(l) 2H2(g) + O2(g) H = 572 kJ endothermic2H2(g) + O2(g) 2H2O(l) H = 572 kJ exothermic

2H2(g) + O2(g) 2H2O(l) H = 572 kJ

572 2

H2(g) + O2(g) H2O(l) H = 286 kJ21

Two important aspects ofH

There are many kind of enthalpies such as: Enthalpy of formation Enthalpy of combustion Enthalpy of atomisation Enthalpy neutralisation Enthalpy hydration Enthalpy solution (dissolution)

Type Of Enthalpies

Heat change when 1 mole of a compound is formed from its elements in their stable states.

Enthalpy of Formation, Hf

Example:

H2 (g) + O2(g) H2O (l) H = 286 kJ

The standard enthalpy of formation, Hfo is the heat change when 1 mole of a compound is formed from its elements in their stable states at standard conditions

(1 atm and 298 K).

Standard Enthalpy of Formation, Hfo

The standard enthalpy of formation of any element in its most stable state form is ZERO.

Hf (O2 ) = 0 Hf (Cl2) = 0o o

EXAMPLE: (standard states of pure substance)

oxygen O2(g)

hydrogen H2(g)

sodium Na(s)

bromine Br2(l)

water H2O(l)

Specify whether the substance exists as atom/molecule andits states of matter!

Heat released when 1 mole of a substance completely combusted in oxygen gas under stated condition.

C(s) + O2(g) CO2(g) H= 393 kJ

Enthalpy of Combustion, Hcomb

The heat absorbed when 1 mole of gaseous atoms is formed from its element under stated condition.

Na(s) Na(g) H= +109 kJ

Enthalpy of Atomization, Hatom

Example:

Cl2(g) Cl(g) H= +121 kJ

Heat released when 1 mole of water formed when acid reacts with base under stated condition.

HCl(aq) + NaOH(aq) NaCl(aq) + H2O(l) H= -58 kJ

HNO3(aq) + Ba(OH)2(aq) Ba(NO3)2(aq) + H2O(l) H = Hneut

Enthalpy of Neutralization, Hneut

Heat released when 1 mole of gaseous ion is hydrated in aqueous solution under stated condition

Na+(g) Na+(aq) H= -406 kJH2O

Enthalpy of Hydration, Hhyd

The heat change when 1 mole of a substance dissolves in solvent(water) to form an infinite dilute solution. KCl(s) K+(aq) + Cl(aq) H= +690 kJ

Enthalpy of Solution, Hsoln

Write a balanced equation and draw an appropriate enthalphy diagram for each of the following:a) The combustion of 1 mol of methane

in O2b) The formation of 1 mol of NO2 from its

elements (heat is absorbed)

Exercise 1 9.1

30

Exercise 2 9.1Name type of enthalpy change for the Following reactions. Predict whether the reactions endothermic or exothermic.(a)C(graphite) + 2H2(g) CH4(g)(b)CH2CH2(g) + 3O2(g) 2CO2(g) + 2H2O(l)(c) HNO3(aq) + NaOH(aq) NaNO3(aq) + H2O(l)(d) I2 (s) I (g)

(e) KCl (s) K+ (aq) + Cl- (aq) (f) Cl(g) Cl(aq)

H2O

H2O31

END OF CHAPTER 9.1END OF CHAPTER 9.1