Chemical Energetics Definitions Summary:
∆ < 0:
∆ > 0:
Exothermic/heat released
Endothermic/heat absorbed
Memorizing Standard Enthalpy Change and Energy: Released From CHANS’ LAB (Representation by Capital Letters)
No.
Standard
Symbol
Formula
Definition
Enthalpy
Heat
Note
When
Is
Change of
1
2
Reaction
Formation
∆⊝ /∆
∆⊝
Either
Molar
= ∆
⊝ quantities
of
reactants
in
− ∆
⊝ chemical
∆
⊝
reacting together
-Based on coefficients in balanced equation
formed from its constituent
-Theoretical Value
elements
-∆
⊝ of elements = 0 at standard state
equation
Either
1mol
of
substance
-∆
⊝ >0 => compound less stable than element =>
Possible decomposition. Vice versa
3
Combustion
∆⊝
∆⊝
Released
= ∆⊝ 1mol
of
completely
burnt
in
excess
substance
oxygen
1 mole of free
Dissolved in large amount of
gaseous ions
water
-Exothermic as heat is released
− ∆⊝ 4
Hydration
⊝
∆
⊝
∆
∝
Released
forming
solution
-Exothermic due to ion-dipole formation
at
infinite dilution
5
Atomization
∆⊝
Absorbed
1 mole of free
Formed from its element /
gaseous atoms
broken
/ 1 mole of
constituent elements in the
compound in
gaseous state
into
atoms
of
its
-Endothermic as energy needed to break bonds
-∆⊝ =0 for noble gases as they are already free gaseous
atoms in standard state
standard state
6
Neutralization
⊝
∆!"#
Released
1
mole
water
of
Formed
from
neutralization
between acid and base
+
-Exothermic due to formation of H2O from H and OH
-
⊝
-∆!"#
=Almost same for all strong acids & bases as all
ionize fully
⊝
-∆!"#
=More positive for weak acid & bases as energy
Compiled by Liu Ruoyang
required to ionize them
7
⊝
∆$%&
Solution
⊝
∆$%&
= −'( *+,
+
Either
1
⊝
∆
mole
of
solute
⊝
-∆$%&
>0 => Substance insoluble in H2O as energy
released in hydration cannot compensate energy
Dissolved in infinite volume of
water
absorbed in separating ions in the lattice. Vice versa.
Name
No.
Formula
Definition
Heat
8
9
10
Lattice Energy
'( *+, ∝
. × 0
. + 0
Electron Affinity
Released
Either
Absorbed
Note
When
Is
1 mole of solid ionic
Formed
compound
constituent free gaseous
of attraction between ions.
ions
-Measure of strength & stability of ionic compound
Added to 1 mole of
-Opposite of Ionization energy
gaseous atoms to form 1
-1 E.A always negative, 2
mole of gaseous singly
required to overcome electrostatic repulsion of the second
negatively charged ions
electron
broken
-Endothermic as energy required to break bonds
1 mole of electrons
1
mole
a
its
-Exothermic due to formation of strong electrostatic forces
st
nd
E.A. always positive as energy
Bond
Bond
Energy
dissociation
particular X-Y bond
-Same amount of energy is released when process is
enthalpy
in
reversed
a
of
from
particular
compound in the
-Measure of bond strength
gaseous state is
∆⊝
Average
bond
= 1* 2 23
enthalpy
− 1* 2 45
67+ 1 *ℎ9,
=
∑ 1 ((( +,
∑ 1 23
*All takes place under Standard Conditions of 298 K and 1 atm.
Compiled by Liu Ruoyang
Absorbed
1 mole of bonds
(average)
between atoms of X
and
Y
in
gaseous state is
the
broken