Thermodynamics Flashcards
What is meant by a thermodynamic system?
-A thermodynamic system is one that is governed by conservation of energy, separated from its surrounding by real or imaginary boundaries and contains a great number of interacting particles which are composed of a great number of subsystems. -Heat (Q) or matter (# of moles n) may penetrate the boundaries. Work (W) can be performed by or on the system.
There are 3 types of thermodynamic systems, what are they?
(i) Isolated: the system cannot exchange energy or matter with its surroundings Q = 0 , W = 0 , ∑n = 0 (energy remains constant whenever any chemical or physical processes occur)
(ii) Closed: the system can exchange energy but not matter Q ≠ 0 , W = 0 , ∑ n = 0
(iii) Open: the system can exchange both energy and matter (ex: an alive system) Q ≠ 0 , W ≠ 0 , ∑n ≠ 0
What is the law of energy conservation
The amount of energy in a closed and isolated system remains constant whenever any physical or chemical processes occur in the system.
There are two types of systems parameters, what are they?
(i) Global (extensive): describes the system as a whole and possesses additive properties; total value of a global parameter for the whole system is the sum of the values of its individual parts (mass, total charge, total number of particles, etc.)
(ii) Local (intensive): non-additive depends on time and spatial coordinates (temperature, pressure, chemical potential, etc)
The state of a system is described by its state functions. What are state functions? Literally what are they, and how are they related to one another by the state equation? What are the state variables, and which equation relates them to one another?
State functions: properties of a thermodynamic system whose magnitude depends only on the initial and final states of a system and not on the path of change.
The state functions are • enthalpy (H) • entropy (S) • free energy (G) • internal energy (E or U) related by the state equation ∆G = ∆H -T∆S
The state variables are: • number of moles (n) • Pressure P • Temperature T • Volume V related by the state equations pV=nRT
What is the First Law of Thermodynamics?
-The change in internal energy of a system is equal to the heat added to a system minus the work done by the system. -This relationship stems from conservation of energy ∆U = Q - W = Uf - Ui
(Ui and Uf = initial and final values of U)
W+ when the system performs work
W- when work is done on the system by its surroundings
Q+ when heat enters the system
Q- when heat flows out of the system
What is the Second law of thermodynamics? What is the second law of thermodynamics stated in terms of entropy?
Heat flows spontaneously from a substance at a higher temperature to a substance at a lower temperature and does not flow in the reverse direction.
-Entropy of the universe is always increasing.
Entropy: a measure of the disorder of a system (JK-1)
-Entropy is also known as the thermodynamic function that describes the degradation of energy
For reversible isothermal processes:
-The entropy of the system and its surroundings does not change
∆ S = 0
For irreversible isothermal processes:
-The entropy of the system + surroundings (the universe) increases
∆S > 0
- Each spontaneous irreversible adiabatic process leads to an increase in the entropy of the universe.
- Spontaneous irreversible processes tend toward equilibrium. At equilibrium, entropy is at it’s maximum and disorder is at its greatest → this is the most probable arrangement of the system
List definitions fr each of the Thermodynamic Functions. (U,H,S,F,G)
Internal Energy (E or U) (joules):
- The change in internal energy of a system is equal to the heat added to a system minus the work done by the system.= sum of all energies in the system.
- This relationship stems from conservation of energy:
∆U = Q - W
W+ when the system performs work
W- when work is done on the system by its surroundings
Q+ when heat enters the system
Q- when heat flows out of the system
- Thermal energy: kinetic + potential energy associated with the random motion of particles
- In an isolated system the internal energy (U) is constant
Enthalpy (H) (j)
H = U + p∆V
- This state function expresses heat changes at constant pressure (of isobaric processes)
- The change in enthalpy is equal to the heat absorbed or given off by a system at constant pressure.
Exothermic reaction: (∆H -) heat is given off by the system
Endothermic reaction: (∆H+) heat is added to the system
*For spontaneous exothermic chemical processes occuring at constant pressure, heat is given off, the enthalpy of the system decreases and the system reaches equilibrium.
Entropy (S) (j.k-1)
- Entropy:* a measure of the disorder of a system (JK-1)
- Entropy is also known as the thermodynamic function that describes the degradation of energy
Free Energy (F) (Joules):
-also known as the Helmholtz function, it is defined by:
F = U - TS
- the unit of free energy is Joule
- its decrease equals max work done in an isothermal reversible process
Interpretation:
-The total energy is composed of the free (available) E that can be used for work done at isothermal ireversible process and of the [TS] portion of energy which is not useful. F=U-TS lower=more work
*A spontaneous irreversible process occuring at constant temperature is accompanied by a decrease in free energy which reaches a minimum at equilibrium
Free Enthalpy (G) (Joules) – Gibbs function
Represents the maximum amount of energy released by a process occuring at constant temperature and pressure that is available to perform useful work.
∆G = ∆H – T∆S
*the unit of free enthalpy is Joule
-A spontaneous process occuring at constant temperature and pressure is accompanied by a decrease in free enthalpy which reaches a minimum at equilibrium
-∆G = negative for sponateous processes
+∆G = requires energy
What is Chemical Potential of a thermodynamic system? Law of Hess?
Chemical potential (µi) is the measure of the affinity of a given substance - the gibbs free energy of one mole of a given substance. E.g one one of petrol would have a -ve gibbs free energy.
Formula (3.22)
- This formula indicates that at a constant temperature and pressure, a partial change in the free enthalpy corresponds to a partial change in the # of moles. i.e. a change in composition is related to a change in energy
- Which reactions will take place in system and their rate depend on their chemical potential and on the amount of substance.
Chemical reaction change composition of system, therefore state changes as well; ? states ? ?Energy, and each trngle E can be expressed as product of extensive and inextensive factor unit is (J.MOL-1)
delta G= partial change enthalpy
delta n= partial change in no of moles
During a chemical reaction:
- Law of Hess: the reaction heat does not depend on the reaction path, not on intermediate but on final and initial states i.e the change of enthalpy in a chemical reaction (i.e. the heat of reaction at constant pressure) is independent of the pathway between the initial and final states.
Thermoregulation in homiothermic organisms, Stephan-Boltzmann law states what?
- Temperature effects the rate of chemical processes in the body as proper enzymatic action is dependent on an optimal temperature of 37 ºC.
- In order to maintain a constant temperature and proper functioning of organs, heat production as a result of chemical processes and muscle activity must be offset by heat loss to the surroundings.
- A constant body temperature is maintained by 4 different types of thermal losses.
Radiation:
Each body of a certain temperature emits energy in the form of electromagnetic waves. Radiation in the infrared region of the spectrum is given off at temperatures corresponding to the surroundings.
- -*According to the Stephan-Boltzmann Law, the energy irradiated by a black body is proprtional to the 4th power of temperature.
- -*Thus the net quantity of heat lost by the organism is equal to the difference between the 4th powers of the temperature of the body and its surroundings
- -*The quantity of heat that irradiates is dependent on blood flow to the skin and is decreased by clothes and other insulating materials.
- -*contributes to 40 -60 % of total thermal losses
- Flow of heat:*
Blood distributes heat to various parts of the body. To increase heat loss, blood flow to the skin is increased. To decrease heat loss, skin is diverted from the surfaces of the body by constriction of skin capillaries
Conduction of heat:
Through direct contact with objects at temperatures different from that of the body.
*flow of heat and conduction are collectively responsible for 15-30% of losses.
Evaporation:
Method of heat loss during perspiration and respiration
The organism secretes a water and salt mixture through sweat glands in the skin. The water droplets saturate the air in the skin’s immediate vicinity (relative humidity at this point is 100% and movement of air carries the saturated air away from the body producing a cooling effect
- Dew point- temperature at which the partial pressure of the vapor equals the equilibrium vapor pressure -100%)
- production of sweat increases during physical exercise, higher body temperatures (fever), and elevated environmental temperatures
This process contributes to 20 – 25% of total thermal losses
~Below 19ºC, thermal losses are minimal
~29-31ºC, production of heat and thermal losses are at equilibrium
~Above 31ºC, thermal losses are not sufficient to regulate body temperature and evaporation takes over
* Thermal comfort of an organism is dependent on temperature of the air, relative humidity, and movement of air.
pV Diagram
- a Pv- diagram is used to describe corresponding changes in volume and pressure in a system
- in thermodynamics it can be used to estimate the net work performed by a thermodynamic cycle (net work is the area enclosed by the lines of the diagram)
Measurement of Temperature
Temperature is a measure of the average kinetic energy of molecules in a substance.
The SI unit of temperature is Kelvin.
- The lower limit of temperature on the Kelvin scale is 0 and is called absolute zero
- Kelvin temperature is related to the Celsius scale by
Tk = Tc +273
The relationship between the Celsius and Fahrenheit scale is
Tf = (9/5)Tc +32
The Fahrenheit scale has a different zero point. Water freezes at 32 F.
Various methods exist for measuring temperature. All of these are based on changes in the thermometric properties of matter. (expansion, changes in electrical resistance, emmision of radiation, voltage changes)
- Liquid thermometers –used in the laboratory setting. The thermometric property that is exploited here is volumetric expansion. When a liquid at Vi is heated by a temperature T, its volume increases by ∆V. Thus the column of mercury expands in proportion to the temperature increase.
Formula for volumetric expansion is ∆V = βVi∆T where β is the coefficient of volumetric expansion
- Medical thermometers – it is a mercury thermometer used to measure body temperature. The scale is from 35-42ºC.
- Calorimetric thermometer- used for the measurement of small temp differences
- Thermocouple –Measures the voltage difference induced by changes in temperature. Consists of thin wires of two diff metals welded together. One of the junctions (the hot junction“) is placed into thermal contact with the object. The other junction is kept at constant temp. A thermocouple generates a voltage difference that is measured by a voltmeter.
- Electrical resistance thermometer –based on the fact that the electrical resistance of a metal wire changes with temperature. A platinum wire is placed in contact with the object to be measured and electrical resistance is measured as a function of temperature increase
- Thermistor- based on the fact that the electrical resistance of a semiconductor decreases with increased temp. i.e. the density of free e- increases. Resistance is measured as a function of temperature. Produces very reliable results.
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Thermography –method of measuring the intensity of emitted radiation due to a temp source
- the intensity of radiation emmitted increases with increasing temp
- produces a thermogram which displays the various infrared intensities in color
- used in medicine, to detect malignant cancers, since cancer tissue is associated with increased metabolic activity and elevated temp. - Bimetallic strip – two metals with different expansion coefficients are welded together, when temp is increased, the strip bends toward the side with the lower expansion coefficient –it is used to control temp in devices that need to maintain a constant temp (thermostats)
Calorimetric measurements
A calorimetric thermometer is applied for the measurement of small temperature differences within a limited range of scale. Its capillary is enlarged at the lower end, which results in a shorter length of thermometer when special scaling around both of the base points is conserved.
Calorimetry is the method for measurement of the amount of thermal energy. Calorimeters measure heat. The most common is the mixing calorimeter. Dewar’s vessel with coupled walls and vacuum between them is filled with water. The heat supplied Q (J) is calculated from the calorimetric equation:
Q = (M + K) cΔT … where, M = mass of heated water (kg), K = water value is the amount of water (kg), which requires the same amount of heat to increase temp. by 1∘c as that consumed by the device,
c = specific heat capacity of water and ΔT = temperature change of water.
Calorimetry is applied for the measurement of energy requirements of organisms. 17 MJ/kg for proteins and 38 MJ/kg for fats. For adults energy turnover = about 11 MJ/day because roughly 5MJ/m2/day.
Energy exchange per unit mass is 20-25 x higher in kidneys than rest of body.
Specific heat capacity
Specific heat: the amount of heat energy needed to raise the temperature of 1 kg of a substance by 1 ºC or 1 K. [J.kg-1.K-1]
The specific heat of a substance varies with temperature. Eg. liquid water has a higher specific heat than ice.
Specific heat is a property of the given substance.
-The quantity of heat ∆Q required to change the temperature ∆T of a substance is proportional to the object’s mass and the specific heat of a substance.
∆Q = mc∆T
*Where m is the mass of the object and c is the specific heat
Latent heat
L is the latent heat (units J/Kg): the amount of heat required to change the phase of 1kg of a particular substance
- heat of fusion:* latent heat for a phase change between solid and liquid
- heat of vaporization:* latent heat for a phase change between liquid and gas
Phase Change
Temperature remains constant when a substance undergoes a phase change.
- The amount of heat gained or lost by a system undergoing a phase change or
- The amount of heat required to change the phase of a particular substance is given by
∆Q =mL
*Water has a relatively high latent heat of fusion and evaporation. This means that a great amount of heat energy must be supplied to change the phase of water.
-For water, the specific latent heat of fusion at melting temperature 0ºC is 334kJ/kg and the specific latent heat of evaporation at boiling temperature 100ºC is 2.26 MJ/kg.
