Chem I Lesson 12 -Thermochemistry/Thermodynamics Basics Energy, Systems, State Functions and the First Law of Thermodynamics
A gas does 135J of work while expanding and at the same time absorbs 156J of heat. What is ΔU?
* Heat is absorbed: q is +156J * Work is done BY the system: -135J ΔU = -135 + 156 = +21J
Match these observable properties to the atomic and molecular behavior they correspond to: 1. temperature 2. pressure 3. volume
1. motion 2. force 3. bonding
Give formula definition of first law of thermodynamics
1. ΔU = q + w (with variation see below) ΔU = change in internal energy. Used for a closed system at rest with no magnetic or electric fields. Q is negative is heat is exiting system and positive if heat is entering system W is negative if work is done by system (energy is transferred away from system) W is positive if work is done on system (energy is transferred into system) 1. work done BY the system (-) 2. work done ON the system (+) *but work done by the system could be defined as (+) ** I don't get this, how do you know what sign to use? See notes
What does a negative vs. positive quantity of work mean?
A negative quantity of work means that the system loses energy, aka energy is passing from system to surroundings and system is doing work on surroundings A positive quantity of work means that the system gains energy, aka energy is passing from surrounding to system and surrounding is doing work on system
isovolumetric/isochoric process Give example
A thermodynamic process in which volume remains constant and no net pressure-volume work is done. No change in volume. Because the gas neither expands nor contracts, W=0 and so ΔU=Q The isovolumetric/isochoric process is a vertical line on a PV graph - so the area under the curve (the work done by the gas) is zero Ex: Heating water in a pressure cooker with fixed volume
isobaric process Give example
A thermodynamic process that occurs at a constant pressure Ex: simple example of an isobaric process is boiling water. You have a pot of water on the stove, it is at atmospheric pressure. As the water boils, the steam coming off expands There is nothing to contain the steam so it stays at atmospheric pressure. Energy is changed, pressure is constant.
Explain absolute zero in terms of molecular motion
Absolute zero is the temp at which there is no molecular motion/movement
Why does vapor pressure increase as temperature increases?
Because as temperature increases, more molecules have enough kinetic energy to escape into the gas phase
Why are isothermic and isobaric processes common?
Because it's easy to control temperature and pressure
Why do some molecules near the surface of a liquid evaporate?
Because they have enough kinetic energy to leave the liquid phase and escape into the gaseous phase
Bond dissociation is an ___________________ process
Bond dissociation is an endothermic (+) process Energy is absorbed when a bond is broken
Bond formation is an __________________ process
Bond formation is an exothermic (-) process Energy is released when forming a bond
What is Thermodynamics?
Broadly, the study of heat (thermo) through energy transfer and work (dynamics). Study of energy and its relationship to macroscopic properties of chemical systems
Condensation is facilitated by ___________ temperature or ________ pressure Give an example to explain this
Condensation is facilitated by LOWER temperature or HIGHER pressure Ex: In a covered or closed container, the escaping molecules are trapped above the solution. These molecules exert a countering pressure, which forces some of the gas molecules back into liquid phase (condensation) *Atmospheric pressure acts on a liquid in a manner similar to that of an actual physical lid
What's the critical point on phase diagram?
Critical point is point which there is no distinction between the liquid and gas phases
What is the transition from gas to solid called?
Deposition
What is the equation for energy of a particle with mass m that is at rest (rest mass energy)?
E = mc²
What does the "emissivity" of a material refer to in the Stefan Boltzman law?
Emissivity of the surface of a material is its effectiveness in emitting energy as thermal radiation. (All objects with a temp above 0K radiate heat) Emissivity is the ratio of energy radiated from a material's surface to that radiated from a perfect emitter, a 'blackbody' at the same temp/wavelength under the same conditions. Ratio of emissivity is between 0 - 1. 1 represents the blackbody radiator that absorbs 100% of incident radiation, so the appear black, but only exist in theory
What is the first law of thermodynamics?
Energy cannot be created or destroyed, only conserved
Energy entering a system is viewed by chemists as a _________ sign: If heat is absorbed by the system, what sign is q? If work is done on the system, what sign is w?
Energy entering a system is viewed by chemists as a positive (+) sign. If heat is absorbed by the system, q>0. If work is done on the system, w>0
Energy leaving a system is viewed by chemists as a ______ sign: If heat is given off by the system, what sign is q? If work is done by the system, what sign is w?
Energy leaving a system is viewed by chemists as a negative (-) sign: If heat is given off by the system, q<0. If work is done by the system, w<0.
Evaporation is an __________________ process
Evaporation is an endothermic process for which the heat source is the liquid water
Give an example of an isolated system and explain why
Ex: a thermos cannot exchange energy (heat and work) nor matter/mass with its surroundings *Note - no system can be isolated completely
Give an example of an open system and explain why
Ex: pot of boiling water can exchange energy (heat and work) and matter/mass with its surroundings
Give an example of a closed system and explain why
Ex: putting a lid on a saucepan and heating it. matter cannot escape but heat can can exchange energy (heat and work) but not matter/mass with its surroundings
True or False: Thermodynamics can be applied to microscopic phenomena like single collisions between two molecules Why?
False. Thermodynamic properties are only valid for systems composed of a large number of molecules because thermodynamic properties are based on probabilities - the rules of thermodynamics govern large systems containing many parts and usually cannot be applied to microscopic phenomena like single collisions between two molecules
Define heat
Heat (q) is the spontaneous transfer of energy between a system and its surroundings/another system from a region of higher temp to a region of lower temp (stopping only when thermal equilibrium has been reached)
Why are heat and work path/process functions?
Heat and work are not contained within a system, they are quantities of energy that we observe only when a system changes from one state to another This change from one state to another is indicative of a path function
Which are state functions? What are the others? I. Pressure II. Heat III. Density IV. Temp V. Volume VI. Work VII. Enthalpy
I, III, IV, V, VII = state functions II and VI = path functions state functions do not depend on the path taken path functions do depend on the path taken other state functions: internal energy, Gibbs free energy, entropy
Which is a type of potential energy? What are the others I. chemical II. vibrational III. translational IV. intermolecular V. rest mass energy VI. rotational
I, IV, V = potential energy II, III, VI = kinetic energy
Which types of energy does a monoatomic gas have? I. Vibrational II. Rotational III. Translational
III. Translational For monatomic gases, translational energy (straight line movement of the center of mass of a molecule) is the only contributor to KE Monatomic gases do not have vibrational energy because they have no covalent bonds around which to vibrate! Also have no rotational energy (rotation around center of mass)
If a system can exchange neither heat nor work with surroundings, q and w are both _________ and so ΔU for the system is ___________
If a system can exchange neither heat nor work with surroundings, q and w are both ZERO and so ΔU for the system is also ZERO
If energy transfer into surroundings causes ordered molecular collisions the energy transfer is defined a __________
If energy transfer into surroundings causes ordered molecular collisions the energy transfer is defined a WORK
If energy transfer into surroundings causes random molecular collisions the energy transfer is defined as _________
If energy transfer into surroundings causes random molecular collisions the energy transfer is defined as HEAT
What's an easy way to know if something is an intensive property vs. extensive property?
If the property is unchanged by altering the sample size, it's an intensive property. If the property is changed by altering the sample size, it's an extensive property
Relate intermolecular potential energy to intermolecular forces. Which phase states does intermolecular potential energy make up a substantial portion of internal energy?
Intermolecular potential energy is created by intermolecular forces (ionic bonding, hydrogen bonding, dipole-dipole interactions, Van Der Waals dispersion forces) between molecular dipoles. Liquids and solids
Give a conceptual definition of internal energy of a system
Internal energy of a system is the total energy - KE and PE - contained WITHIN a system, the collective energy of molecules measured on a microscopic scale Internal energy does not include macroscopic energies such as the KE of entire system moving as one unit or the PE of the entire system raised off the ground. Emphasis on INTERNAL
What is the unit of energy?
Joules (J) 1 Joule = 1 kgm²/s²
kinetic energy formula
KE=1/2mv² Units for KE are kgm²/s²
equation for kinetic energy for avg. translational KE per MOLE of molecules in any fluid What can we determine from the equation?
KE=3/2RT R=ideal gas constant= 8.314 J mol/K Increasing temperature increases translational KE! Remember that gases are fluids! *Note this is different from the equation for KE of a particle that contains Boltzman's constant
equation for kinetic energy for avg. translational KE per MOLECULE in any fluid
KE=3/2kT k = Botzman's constant = 1.38x10^-23 J/K
What are the three different forms of motion that together describe overall energy of molecular motion? What is their sum and how does it relate to temperature?
Motion of molecules can be divided into: translational, rotational, and vibrational. Together they describe thermal energy. Any increase in thermal energy increases temperature. Therefore, temp. is a measure of how fast molecules are moving or vibrating in a substance. When a substance gets hot, it's because its molecules move faster
Does a system contain heat? Explain
No! A system contains internal energy Heat is the transfer of energy from system to surroundings! Emphasis on TRANSFER
State functions are independent of the path taken, but are they independent of one another? Give an example
No, Gibbs free energy is dependent on enthalpy, entropy, and temp - all state functions
Can we say that something that is hot has greater thermal energy (in absolute terms) than something that is cold? Explain
No, because a large amount of lukewarm water can have a greater total heat content than a very small amount of hot water
What is the most important non-PV work?
Non expansion work aka Non PV work include: electrical work - think: firing neurons, muscle fibers contracting, batteries...
How does the zeroth law of thermodynamics discuss temperature?
Objects are in thermal equilibrium only when their temps are equal. Laws 1, 2, and 3 of thermodynamics are dependent on existence of temp.
What is PV work vs. non PV work?
PV work is work done with an associated change in volume. PV work assumes constant pressure. Non PV work refers to "non expansion" work
How is work defined in physics vs. in chemistry?
Physics is interested in work done by the system on the surroundings, and this work is defined as an energy transfer due to a force that usually changes the position or motion of an object. Since physics is interested in the surroundings, , work done by the system on surroundings is positive because change in energy (due to the work done) of the surroundings is positive Chemistry is interested in what work surroundings have done on a system, in order to get a chemical reaction for example
If you can't use Thermodynamics to understand individual molecules or collisions at the molecular level, what can you use?
Physics knowledge about force and motion
Give two examples of an intensive property, why?
Pressure and temperature are two intensive properties because they remain the same no matter how much of the substance is present. Intensive properties are independent of the size of the system or amount of substance present. Other intensive properties: boiling point, melting point, density, state of matter, ductility, malleability
What does a standard state of a substance in thermodynamics refer to?
Standard state of a substance is the most stable form (phase) of that substance Ex: H2 (gas), H20 (liquid), NaCl (solid), O2 (gas), C (solid, graphite) are the most stable forms of these substances under standard state conditions
What is the Stefan Boltzman law and what does it describe
Stefan Boltzman law describes the rate at which an object radiates magnetic waves (its radiated power P). This power depends on temperature and surface area
What is the fluid at the critical point called?
Supercritical fluid - two densities of the liquid and gas phase become equal and there's no distinction between the two phases
The capacity to do work arises from the __________________ of molecules.
The capacity to do work arises from the CONSTRAINT of molecules. An unconstrained system, like an ideal gar of infinite volume dispersed throughout the universe, can do no work.
What is vapor pressure?
The pressure that a gas exerts over the liquid at equilibrium (equilibrium between evaporation and condensation)
What is a state function?
The state of a system refers to its exact physical condition determined by the kinds/amounts of matter present, the structure of this matter at the molecular level, and the prevailing temp/pressure State functions are properties that describe the system in an equilibrium state but do not describe the process of how the system got to that equilibrium state.
What is boiling point, technically?
The temperature at which the vapor pressure of a liquid = the ambient (aka external, applied, incident) pressure
What is thermal conductivity, K, and what does it depend on?
Thermal conductivity, K, is an object's ability to conduct heat and it's based on the object's composition
What is radiation?
Thermal energy transfer via electromagnetic waves or light
What is convection?
Thermal energy transfer via fluid (liquid or gas) movements. Related- pressure or density drive warm fluid in the direction of cooler fluid (think circulation of convection oven)
Define conduction and give an example
Thermal energy transfer via molecular collisions where higher energy molecules of one system transfer energy to lower energy molecules of another system via collision Ex: a hot bowl of soup with a spoon
What is Thermochemistry and how does it relate to Thermodynamics?
Thermochemistry is the study of energy changes in the form of heat aka ENTHALPY CHANGES that occur during physical processes like phase changes (melting, boiling, etc.) and chemical reactions in which breaking bonds requires energy and forming bonds releases energy. Thermochemistry is the application of Thermodynamics, a physics concept, to chemistry/chemical reactions
adiabatic process
Thermodynamic process in which no heat is exchanged between system and its environment, so thermal energy of the system is constant throughout process. No energy exchange as heat. Because thermal energy is constant: Q=0 and so ΔU=-W Naturally occurring adiabatic processes are irreversible - entropy is produced Temp is not constant though! (Even though heat exchange = 0)!
isothermal process Give example
Thermodynamic process that takes place at constant temperature Constant temp implies constant internal energy because temp and internal energy are directly proportional Because internal energy is constant: ΔU=0 and so Q=W. Example: This isn't that complicated despite the name. Any reaction at constant temp counts - think any reaction occurring inside a refrigerator
Phase Diagram with solid-liquid, liquid-gas, and solid-gas interface What's the triple point on phase diagram?
Triple point is point at which the temperature and pressure at which the phases all exist at equilibrium
True or False: Spontaneous processes will not necessarily go to completion or happen fast
True
True or False: Thermodynamic state functions can be observed on an observable scale, they are macroscopic properties of a system
True
True of False; give an example Dividing one extensive property by another extensive property gives an intensive property
True density (intensive) = mass (extensive) / volume (extensive)
True or False: phase equilibria are analogous to the dynamic equilibria of reversible chemical reactions
True: The concentration of products and reactants are constant because the rates of the forward and reverse reactions are equal
Use thermodynamics/thermochemistry to understand the ___________________ of energy and how it translates to observable _________________ and _________________
Use thermodynamics/thermochemistry to understand the MOLECULAR BASIS of energy and how it translates to observable PHYSICAL PROPERTIES and CHEMICAL REACTIONS
Give two examples of an extensive property, why?
Volume and number of moles are two extensive properties because they depend on the amount of matter in the sample - they change with the amount of substance Extensive properties are dependent on the size of the system or amount of substance present. Other extensive properties: mass, size, weight, length
Work
Work is any energy transfer between a system and its surroundings (negative) or from a surrounding to a system (positive) W=-PΔV This work is "PV" work and has the units Joules (J) because work = energy. *MCAT could make work done by the system defined as positive, making the equation W=PΔV
What is the heat of vaporization at the critical point and for all temperatures and pressures above the critical point?
Zero
What are three types of thermal energy?
conduction, convection, radiation
What are the only two ways to transfer energy between systems?
heat (q) and work (w) Any energy transfer that is not heat is work
In general: solid phase found at _____ temp/_______ pressure liquid phase found at ________ temps and _________ pressure gas phase found at _______ temps/ ________ pressures
solid phase found at low temp/high pressure liquid phase found at moderate temps and moderate pressure gas phase found at high temps/low pressures
What are the standard state conditions in Thermodynamics vs. standard temp and pressure (STP) used in gas law calculations?
standard state conditions: 25C/298K, 1 atm, 1 M conc. STP: 0C/273K and 1 atm