Thermodynamics- 4 Flashcards
what is standard enthalpy of formation
The standard enthalpy change for the
formation of one mole of a compound from
its elements in their most stable states of
aggregation (also known as reference
states) is called Standard Molar Enthalpy
of Formation. Its symbol is ∆f H 0
,∆ = ∆ − ∆ H H H ∑ ∑ V
- applies only for 1 mole of ractant
- enthalpy of reacion= sum of enthalpies of products- sum of enthalies of reacns
what is standard enthalpy of combustion dcH
Standard
enthalpy of combustion is defined as the
enthalpy change per mole (or per unit amount)
of a substance, when it undergoes combustion
and all the reactants and products being in
their standard states at the specified
temperature.
what is enthalpy of atomisation daHo
The
enthalpy change in this process is known as
enthalpy of atomization, ∆aH
0
. It is the
enthalpy change on breaking one mole of
bonds completely to obtain atoms in the gas
phase.
In case of diatomic molecules, like
dihydrogen (given above), the enthalpy of
atomization is also the bond dissociation
enthalpy. In the case of metals, atomisation energy is also sublimation enthalpy
what is bond dissociation enthalpy
The bond dissociation enthalpy is the change
in enthalpy when one mole of covalent bonds
of a gaseous covalent compound is broken to
form products in the gas phase
what si mean bond enthalpy
In the case of polyatomic molecules, bond
dissociation enthalpy is different for different
bonds within the same molecule.
The Mean bond enthalpy is the enthalpy change
needed to break the covalent bond into gaseous atoms,
averaged over different molecules
We use values of mean bond enthalpy because every single bond
in a compound has a slightly different bond enthalpy. E.g. In CH4
there are 4 C-H bonds. Breaking each one will require a different
amount of energy. However, we use an average value for the C-H
bond for all hydrocarbons.
These values are positive because
energy is required to break a bond.
The definition only applies when the
substances start and end in the
gaseous state.
what is enthalpy of reaction in terms of bond enthalpies
The net enthalpy change of a
reaction is the amount of energy required to
break all the bonds in the reactant molecules
minus the amount of energy required to break
all the bonds in the product molecules
what is enthalpy of solution
Enthalpy of solution of a substance is the
enthalpy change when one mole of it dissolves in a specified amount of solvent. The enthalpy
of solution at infinite dilution is the enthalpy
change observed on dissolving the substance
in an infinite amount of solvent when the
interactions between the ions (or solute
molecules) are negligible.
dsolH= dhyd H + dlatticeH
For most of the ionic compounds, ∆sol H
0
is
positive and the dissociation process is
endothermic. Therefore the solubility of most
salts in water increases with rise of
temperature. If the lattice enthalpy is very
high, the dissolution of the compound may not
take place at all
MX + aq=M M+ + X-
Why do many fluorides tend
to be less soluble than the corresponding
chlorides
The fluorides of alkaline earth metals are relatively less soluble than chlorides due to the small size of fluoride it has high hydration as well as lattice enthalpy than chloride but in some cases, the difference is greater for fluoride than chloride, hence the fluoride of group 2 metals are less soluble.
what is hydration enthalpy
It is the the enthalpy change when 1 mole of a substance is dissolved to infinite dilution in an aqueous solution
state hess’s law
The amount of heat absorbed or evolved in a chemical reaction is the same whether the reaction occurs in 1 step or through a number of steps.
If a reaction takes place in several steps
then its standard reaction enthalpy is the
sum of the standard enthalpies of the
intermediate reactions into which the
overall reaction may be divided at the same
temperature.
what does hess law imply
-> The total amt o heat change depends only upon the nature pf reactants/ prodicts, and not the method through which the change is brough abt.
It follows from the factthat enthalpy is a state function.
what is 3rd law of thermodynamics
the entropy of a pure crystalline substance at absolute zero is zero.
at absolute 0, a perfectly crystalline substance has a perfect order of it s constituent particles.
this statement is confined to pure crystalline solids because theoritical arguments and practical evidences show that the entropy of super cooled liquids and solutiosn is not 0 at 0K
helps in calc. of absolute entropies of substances at room temp/ 0K
