Thermodynamics 7 Flashcards
can an extensive property become intensive? how? explain with examples
An extensive property can become intensive by specifying unit amount of the substance conerned.
For example, mass and volume are extensive properties but density and specific volume are intensive.
Density= mass/volume
specific volume= volume/mass
Heat capacity(C) is extensive property but specific heat capacity(c) and molar heat capacity(cₘ) is intensive.
an intensive property can be made by taking the ratio of 2 extensive properties.
differentiate reversible and irreversible reactions
->Reversible reactions:
- a process that is carried out infinitesimally slowly
-equilirbirum is not attained at any stage
- takes infinite time for completion
- imaginary and cannot be achieved in actual precise
-> NO reversible reaction
- a process that occurs rapidly
- equilbirium exists only after the completion of the process
- takes finite time for completion
- process actually occuring in nature.
can internal energy be calculated
the actual value of internal energy cannot be calculated as some of the properties involves in its calculation cannot be measured such as electronic energy, nuclear energy etc.
However, change in internal energy can be calculated when the state of system changes from one to another which isneeded.
why is internal energy a state function
Becaue its value depends only upon the final and initial states of the system and does not depedn on the path/method by whcih the state has been attained.
For example, consider a system which has water places in a thermos flask or insulated beaker. The internal energy can be changed in two ways
- doing mechanic work- using rotating paddle
- doing electronic work- using an electric immersion rod
the amount of work done is the same irrespective of the method through which it was done and produces the same change in state.
are q and w state functions? what abt q+w?
For a given change in state, q and w can
vary depending on how the change is carried
out. However, q +w = ∆U will depend only on
initial and final state. It will be independent of
the way the change is carried out. If there is
no transfer of energy as heat or as work
(isolated system) i.e., if w = 0 and q = 0, then
∆ U = 0.
what are the conditions where deltaH = deltaU
-if the ractions is carried out in a closed vessel and volume remains a contant
- if theere are no gaseous reactants or products, only solids and liquids
- if the moles of gaseuous reactnats and products are equal on both sides of the reaction
what are the properties of thermocehmical eqns
for exothermic reactions, the value of dH is -ve. for endothermic reactions, the valeu of dH is +ve.
-The stoichiometric coefficients denote the number of moles of products, reactants involved
-The physical states of ractnats and products must be mentioned because the heat absorbed or evolved depends on the phsyical states
If the stoichiometric coefficients are multiplied or divided by a number, then the valeu dH should also be multiplied or divided by the same number.
If a thermochemical reaction is reversed, then the value of dH remains same in magnitude but reversed in sign.
what does the enthalpy of transformation between phases depend upon
The magnitude of the enthalpy change
depends on the strength of the intermolecular
interactions in the substance undergoing the
phase transfomations. For example, the strong
hydrogen bonds between water molecules hold
them tightly in liquid phase. For an organic
liquid, such as acetone, the intermolecular
dipole-dipole interactions are significantly
weaker. Thus, it requires less heat to vaporise
1 mol of acetone than it does to vaporize 1 mol
of water
how is knowing reaction enthalpy used ?
Enthalpy change is a very useful quantity.
Knowledge of this quantity is required when
one needs to plan the heating or cooling
required to maintain an industrial chemical
reaction at constant temperature. It is also
required to calculate temperature dependence
of equilibrium constant.
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 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 does steel wall vessel ensure
The steel vessel is
immersed in water bath to ensure that no heat
is lost to the surroundings
explain how delta U is measures
A combustible
substance is burnt in pure dioxygen supplied
in the steel bomb. Heat evolved during the
reaction is transferred to the water around the
bomb and its temperature is monitored. Since
the bomb calorimeter is sealed, its volume does
not change i.e., the energy changes associated
with reactions are measured at constant
volume. Under these conditions, no work idone as the reaction is carried out at constant
volume in the bomb calorimeter. Even for
reactions involving gases, there is no work
done as ∆V = 0. Temperature change of the
calorimeter produced by the completed
reaction is then converted to qV
, by using the
known heat capacity of the calorimeter
2types of spontaneous processes
(i) process which requires no initiation- dissolution of sgar in water
(ii) process which requires some intiation: combustion of a fuel
is decrease in enthalpy a criterion for spontaneity
In most cases, a decrease in enthalpy of products compared to reactants , it is a spontaneous processs. (ie exothermic reacns)
However in
C(graphite, s) + 2 S(l) → CS2
(l)
this is endothermic, ie products have more energy, but its still spontaneous
The diffusion of 2 gases in a closed vessel isolated from its surroundings is also a spontaneous process but dH=0
It was discovered that decrease in enthalpy is not afactor for spontaneoity. so entropy was intorduced
