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Thermodynamics Lecture 4: (Thermochemical Properties of fuels (Combustion…
Thermodynamics Lecture 4:
Physical & chemical changes
A phase is a specific state of matter that is uniform throughout in composition and physical state
The liquid and vapor states of water are two of its phases
The term phase is more specific than state because a substance may exist in more than one solid form each one of which is a solid phase
Enthalpies of vaporization
The vaporisation of a liquid, such as the conversion of liquid water to water vapor when a kettle boils at 100 degrees, is an endothermic process (DELTA H > 0 ) because heating is required to bring around the change
At a molecular level, molecules are being driven apart from each other and this requires energy
One of the body's strategies for maintaining its temperature at about 37 degrees is to use the endothermic character of vaporization of water because the evaporation of perspiration requires energy and withdraws it from the skin
the energy that must be supplied as heat at a constant pressure per mole of molecules that are vaporized under standard conditions (1 bar) is called the standard enthalpy of vaporization of liquid and is denoted DELTA vap H
For example, 44kJ is required to vapoize 1 mole of H2O at 1 bar and 25 degrees celcius so DELTA vap H = +44kJ per mole
A thermochemical equation shows the standard enthalpy change (including the sign ) that accompanies the conversion of an amount of reactant equal to its stoichiometric coefficient in the accompany chemical equation (in this case 1 mole of H20).
If the stoichiometric coefficients in the chemical equation are multiplied through by 2, then the thermochemical equation would be written: 2 H20 - DELTA H = +88kJ
This equation means that 88kJ of heat is required to vaporize 2 moles of H2O at 1 bar and at 298.15 K
Enthalpies of fusion
another common phase transition is fusion or melting as when ice melts to water
The change in molar enthalpy that accompanies fusion under standard conditions ( pure solid at 1 bar) is called the standard enthalpy of fusion DELTA fus H
Its value for water st 0 degrees celcius is +6.01kJ per mole
Notice that the entahalpy of fusion is much less than the enthalpy of vaporization: in vaporization the molecules become completetly separated from each other, whereas in melting the molecules are loosened without separating
Entalpies of condensing and freezing
The reverse of vaporization is condensation, the reverse of fusion (melting) is freezing
The molar enthalpy changes are respectively, the negative of the enthalpies of vaporization and fusion. This is because the energy that is supplied (during heating ) to vaporize or melt the substance is released when it condenses or freezes
Bond Enthalpy
For example, the dissociation of the hydroxyl radical OH(g), we have HO(g) - H(g) + O(g) DELTA H = +428kJ
The corresponding standard molar enthalpy change is called the bond enthalpy
A complication when dealing with bond enthalpies is that their values depend on the molecule in which the two linked atoms occur
For instance the total standard enthalpy change for the atomization ( the complete dissociation into atoms ) of water
It is not twice the O-H bond enthalpy in H2O even though two O-H are dissociated
There are two different dissociation steps: 1) First an O-H bond is broken in a H2O molecule: H2O(g) - HO(g) + H(g), DELTA H = + 492kJ, 2) second is the O-H bond is broken HO(g) - H(g) + O(g)
Thermochemical Properties of fuels
Combustion of methane in a natural gas flame
CH4(g) + 2 O(g) - CO2 + 2 H2O (l)
As discussed in lectures in 2 & 3 H m = U m + pV m
For condensed phases (solid and liquids), pV m is so small that it may be ignored
For gasses treated as perfect, pV m may be replaced by RT
The combination of reaction Enthalpies, Hess's law
this fundamental law arises from the fact that the enthalpy is a state function, this implies that the enthalpy change for a process is the same no matter what pathway we take in going from the initial to the final state
Standard Enthalpies of formation
We need to simplify even further the process of predicting reaction enthalpies of biochemical reactions
Kirchhoff's equation
WE need to know how to predict the reaction enthalpy of a biochemical reaction at one temp from its value at other temps
Suppose we want to know the enthalpy of a particular reaction at body temperature. 37 degrees celcius but we have data available for 25 degrees celcius, or suppose we want to know if the oxidation of glucose is more exothermic if it takes place in the body of an artic fish at 0 degrees than when it takes place at mammalian body temperatures
initially we assume that the heat capacity does not vary with temperature in the range T1 to T2 in which case we have already shown that H(T2) - H(T1) = Cp (T2-T1)
Entropy and the second law of thermodynamics
reversible and irreversible processes
A reversible change in thermodynamics is one that can be reversed by an infinitesimal modification of a variable
a system is said to be in equilibrium with its surroundings if such an infinitesimal change in the conditions in opposite directions results in opposite changes in its state
When the internal and external pressures are equal the system is in equilibrium
if the external pressure is decreased an infinitesimal amount then the gas inside the piston expands slightly which can be reversed by an infinitesimally small increase in the external pressure, this is therefore a reversible thermodynamic change
REVERSIBLE PROCESSES: are infinitely slow, are at equilibrium, do maximum work
IRREVERSIBLE PROCESSES: go at finite rate, are not at equilibrium, do less than the maximum work
The first law tells us which changes are permissible(only those changes where the internal energy of an isolated system remains constant are allowed).
The second law, through another state function(the entropy) tells us which of these are spontaneous
The thermodynamic definition of entropy is motivated by the idea that a change (energy is dispersed in a disordered fashion) depends on how much energy is transferred as heat which stimulates disordered motion in the surroundings, in contrast work stimulates a uniform motion of the atoms in the surroundings and hence does not change the entropy of the system)
We can define ENTROPY as dS= dq rev / T
in which q rev is the heat change for a reversible path between two states of the system
For our initial encounter with the concept we can identify entropy with disorder, - if the matter and energy are distributed in a disordered way (as in gas for example ) then entropy is HIGH, whilst - if matter and energy are distributed in an ordered way( as in a crystal) the entropy is low
The statistical definition of entropy we have already noted that the thermodynamic definition of entropy its related to disorder or randomness, the greater the disorder the greater the entropy, although as we have already seen it is not necessary to use this directly to develop a thermodynamic relationship : dS= dq rev / T
However it is useful to look at the molecular basis for Entropy, a large and complex subject know as statistical thermodynamics which we will only touch on this course,
Boltzmann postulated that the Entropy was related to the number of ways that a distribution can be achieved
S= k ln W
W= states/configurations/arrangements
Spontaneous Processes
one of the most important questions that we can answer using thermodynamics is what criterion decides the direction of a spontaneous process
It is attractive to think that all spontaneous processes lead to either a lowering in the internal energy or to an increase in the disorder of the system, however practical experience shows that there are many processes which appear to contradict this; there are processes where the entropy decreases (burning a magnesium ribbon where the products have low entropy but the gas consumed has a high entropy). A more sophisticated approach considers the entropy change of the system and the surroundings together (the Universe ) and the second law of thermodynamics states that "IN A SPONTANEOUS PROCESS THE ENTROPY OF THE UNIVERSE INCREASES"