Download Chemical Energetics

ENERGETICS
/THERMOCHEMISTRY
(AS)
• 1.Often chemical changes are
accompanied by changes in heat
content / enthalpy of the materials
reacting (H)
• 2. This change is shown by a change
in temperature.
• a. heat is lost to the surroundings
• Temperature of reaction mixture
rises / increases
• H is negative
• Exothermic reaction
• Enthalpy diagram / energy level
diagram
OR Reaction pathway/reaction
coordinate/extent of reaction
• b. heat is absorbed from the
surroundings , temperature of
reaction mixture decreases / falls
• H is positive
• Endothermic reaction
• 3. a. The value of H depends on
temp , pressure and concentrations
of reactants.
• b. H are measured under standard
conditions :
• Temperature = 298K ( 250 C )
• Pressure = 1 atm / 1.01 x 105 Pa
• Concentrations = 1 mol dm-3
• c. Any H measured under standard
conditions is described as standard
enthalpy change.
• Symbol : H
STANDARD
ENTHALPIES
Definitions
State symbols must be
included in equations
STANDARD
ENTHALPHY OF

FORMATION ( Hf )
Enthalpy change when 1 mole of a
compound is formed from its
elements in their standard states
, under standard conditions
temperature 298 K and pressure
of gases at 1 atm
• Steps in writing equations:
• 1. write formula of compound formed
• 2. identify elements required to form
the compound
• 3. balance equation to form 1 mole
of the compound
Examples
• Mg (s) + ½O2 (g) → 1MgO (s)
-602 kJ mol-1
602 kJ evolved for every 1 mole of
MgO formed
• ½ H2 (g) + ½ Cl2 (g) → 1HCl (g)
- 92.3 kJ mol-1
Examples
Notes :
1) Hf of elements is zero
Eg : Cu(s)  Cu(s) , Hf = 0
2) Hf are often theoretical only
• Exercise : Write the equation for
enthalpy of formation for : KMnO4 (s)
and H2O (l)
• K(s) + Mn(s) + 2O2(g) 1KMnO4(s)
• H2(g) + ½O2(g)  1H2O (l)
STANDARD
ENTHALPY CHANGE
OF COMBUSTION

( Hc )
Enthalpy evolved when 1 mole of
the element or compound is
completely burned in excess
oxygen , under standard
conditions
• Steps in writing equations :
• 1. identify products formed from
burning of compound in excess
oxygen
• 2. balance equation for 1 mole of
compound burnt
Examples
• 1 C (s) + O2 (g) → CO2 (g)
Hc = - 394 kJ mol-1
394 kJ evolved for every 1 mole of
carbon burnt
• 1 H2 (g) + ½O2 (g) → H2O (l)
•Exercise : Write an equation for the
enthalpy of combustion for C3H6 and
CH3OH
• 1C3H6 (g) + 9/2 O2 (g)  3CO2 (g)
+ 3H2O (l)
• 1CH3OH (l) + 3/2 O2 (g)  CO2 (g)
+ 2H2O (l)
STANDARD ENTHALPY OF
ATOMISATION ( Hat )
• Enthalpy required/absorbed
• 1 mole of gaseous atoms formed
• From the element in its standard
state under standard conditions
• Examples :
•
Fe (s)  1 Fe (g)
• Example :
• ½Cl2(g)  1 Cl (g) + 122 kJ mol-1
• molecules
atoms
• Note : from Data Booklet
• Bond energy Cl-Cl = + 244 kJ mol-1
•
Cl-Cl  2 Cl (g) 2 moles atoms
• Hat of Cl = ½ x bond energy Cl-Cl
STANDARD BOND
DISSOCIATION ENTHALPY
• Also called bond energy
• Energy absorbed
• Separate the 2 atoms in a covalent
bond in gaseous state under
standard conditions , per mole of
bond.
Examples
• 1) HCl (g) → H (g) + Cl (g) , +431 kJmol-1
• 2) CH4 contains 4 x C-H bond
• Total energy required to break all bonds in
CH4 = 1640 kJ
• ¼ CH4 = ¼ ( 4 C-H bond ) = one C-H
bond
• Average bond energy of one C-H bond
• = ¼ ( 1640 ) = +410 kJ
• ¼ CH4 (g)→¼ C (g) + H (g),+ 410 kJmol-1
STANDARD ENTHALPY OF
NEUTRALIZATION
• Energy evolved
• Acid reacts with base to form 1 mole of
water , under standard conditions
•
•
•
•
Examples :
NaOH + HCl → NaCl + 1 H2O
KOH + HCl → KCl + 1 H2O
Ionic equation(strong acid + strong base:
H+(aq) + OH-(aq)  1 H2O (l),
H neutralisation = -57 kJ mol-1
• Generally,
• 1) Strong acid / strong alkali :
•
H = - 57 kJ mol-1
• 2) Weak acid / base :
•
H = -(<57) kJ mol-1 (eg. -54 kJmol-1)
• Reason : Certain amount of energy
required to ionise the weak acid or base
first
STANDARD ENTHALPY OF
HYDRATION ( Hhyd )
• Energy evolved
• 1 mole of separate gaseous ions
dissolved in water under standard
conditions
• Exothermic as attraction/bond forms
between the ions and polar water
molecules
• Called ion dipole attraction
• Examples :
• 1 Na+ (g) → Na+ (aq)
• 1 Cl- (g) → Cl- (aq)
-406 kJmol-1
- 381 kJmol-1
• Hhyd for compounds =
• sum of Hhyd of constituent ions
Eg : Hhyd MgCl2 = Hhyd Mg2+ +
2 x Hhyd Cl= -1891 + 2(-381)
= - 2653 kJ
• H (hyd)  charge density of ions
• Charge density = charge/size
• Higher charge density , stronger ion
dipole attraction , more exothermic
H(hyd)
• Eg :
• H(hyd) Cl- > H(hyd) Br –
•
-381
-351
STANDARD ENTHALPY OF
SOLUTION ( Hsolution )
• Enthalpy change when 1 mole of a
substance dissolved in a stated
amount of solvent under standard
conditions
• Example :
• 1 KOH (s)  K+ (aq) + OH- (aq)
or KOH (aq)
- 57 kJ mol-1
STANDARD ENTHALPY CHANGE
OF REACTION ( Hr )
• Enthalpy change in a chemical
reaction , for the number of moles of
reactants as shown in a balanced
chemical equation under standard
conditions
• Example :
4 H2O + 3 Fe  Fe3O4 + 4 H2
 Hr = x kJ when 4 moles H2O
reacts with 3 moles Fe