Understanding Pure Substances and Thermodynamics Principles

Pure Substances



Heat Transfer Equations


Q1 = m * Cn * ΔT
m: mass of water
Cn: specific heat of ice

Q2 = m * Lf: heat of fusion (latent) for ice

Q3 = m * C * ΔTC: Heat sensible
C: 1 Kcal/Kg

Q4 = m * Lv: heat of vaporization (latent) for H2O
Lv: 540 Kcal/Kg
Lv: 970 BTU / LBM

Q5 = m * Cv * ΔT
Cv: 0.45 Kcal / Kg


Example Calculation


200 g of ice at -10 ºC to calculate the amount needed to transform 200 g of steam at 120 ºC. The external pressure is 760 mmHg. The specific heat of ice is 0.5 cal / g ºC and the value of 0.45 cal / g ºC.


Development Steps


  1. Heat the ice:
    Q1 = m * Cn * ΔT
    Q1 = 200 g * 0.5 cal / g ºC * 10 °C
    Q1 = 1000 cal
  2. Fuse the ice:
    Q2 = m * Lf
    Q2 = 200 g * 80 Kcal/Kg
    Q2 = 16000 cal
  3. Heat water:
    Q3 = m * C * ΔT
    Q3 = 200 g * 1 * 100
    Q3 = 20000 cal
  4. Vaporize water:
    Q4 = m * Lv
    Q4 = 200 g * 540
    Q4 = 108000 cal
  5. Heat steam:
    Q5 = m * Cv * ΔT
    Q5 = 200 g * 0.45 * 20
    Q5 = 1800 cal

Total Heat:
QTOTAL = Q1 + Q2 + Q3 + Q4 + Q5
QTOTAL = 146800 cal


Quality of Steam


x = mass of vapor / total mass


Moisture Vapor


y = liquid mass / total mass



Specific Volume


Vf = specific volume of saturated liquid (TAB)
Vg = specific volume of saturated steam (TAB)
Vfg = Vg – Vf


Thermodynamics Principles


Thermodynamics is the science of energy changes that occur in physical and chemical processes. A thermodynamic system is a part of the universe separated from the rest by arbitrarily defined boundaries, real or fictitious, to make the object of investigation. Systems are classified as open, closed, or isolated based on their ability to exchange energy with the environment.


Thermodynamic Properties


Thermodynamic properties can be extensive (dependent on the amount of matter) or intensive (independent of the amount of matter). The functions of state are thermodynamic variables whose value only depends on the current state of the system and not on the procedure by which the system reaches that state.


Transformation Processes


A transformation process is when a system exchanges energy with its surroundings, going from an initial state of equilibrium to another final state of equilibrium. Thermodynamic processes may be reversible or irreversible, depending on the ability to reverse the transformation by infinitesimal changes in the values of the variables.


First Principle of Thermodynamics


The variation of internal energy, ΔU, of a system equals the amount of heat exchanged between the system and its environment, plus the work done by the system.
ΔU = Q + W

Sign Convention: The flow of heat and work from the environment to the system are positive: Q > 0 and W > 0. The flow of heat and work from the system to the environment is considered negative: Q < 0 and W < 0.


Types of Processes


  • Isothermal: T = constant. The heat transfer between the system and the surroundings is equal to the work done by it. Q = -W
  • Adiabatic: Q = 0. The variation of internal energy equals the work done by the system.
  • Isochoric: V = constant. The heat exchanged for a system is equal to the change in internal energy, ΔU. ΔU = QV
  • Isobaric: P = constant. The heat exchanged for a system is equal to the change of enthalpy, ΔH.


Standard Enthalpy


The standard enthalpy of reaction, ΔH0, is the enthalpy in a reaction where standard reagents are transformed into products in standard states. The standard enthalpy of formation of a substance, ΔH0F, is the enthalpy for the formation of a mole of substance in its standard state from its elements in that state, which are assigned zero enthalpy.


Hess’s Law


If a reaction can be performed in several stages, links, or theoretical steps, its enthalpy change is equal to the sum of the enthalpies of reaction of these intermediate reactions.


Entropy


The entropy, S, is a state function that measures the degree of molecular disorder of systems. Entropy increases when the system becomes disordered and decreases with increasing molecular order.


Second Law of Thermodynamics


The entropy of the universe increases in a spontaneous process and remains constant in a process that is in equilibrium.


Standard Molar Entropy


The standard molar entropy, S0, of a substance is the entropy of a mole of this substance at a pressure of 1 atm and a temperature of 25 ºC.


Third Principle of Thermodynamics


The entropy of a pure crystalline substance, with perfect order, is zero at absolute zero.