CH301 Notes Chapter 9 part 1 Thermodynamics - study of the changes or transfers in energy accompanying chemical and physical processes. Will two (or more) substances react when mixed under specified conditions? What energy changes and transfers are associated with this? How far does the reaction occur? Things to think about: What happens when iron rusts? Why doesn’t iron ‘unrust?” A soft metal and a green gas react violently to from a stable (relatively) harmless white crystalline solid. Why does the reverse not occur? Why do two gases (separated by a valve) mix when the valve is opened? Why don’t they ‘unmix?’ What happens if you just put hydrogen and oxygen in a balloon? Thermodynamics shows that SOME processes only occur in ONE direction by themselves. It does NOT show how fast they happen!!! Thermodynamic Terms system: region of interest, or, substances involved in the chemical and physical changes. In lab, its the chemicals inside the beaker. surroundings: environment around the system - where observations are made. system + surroundings = universe. Types of Systems: depends on how energy and/or matter is exchanged with surroundings: OPEN: exchange energy & matter (Example: human body) CLOSED: exchange energy only. (Example: sealed ice pack) ISOLATED: NO exchange (energy or matter) (Example: Thermos flask with rigid lid) ALL MATH IS DONE FROM THE POINT OF VIEW OF THE SYSTEM!!!! See below for details. Energy: capacity to do work or produce heat three kinds: electromagnetic (E = hν) kinetic (E = ½ mv2) potential (many types e.g., chemical) 2 -2 units: Joules (J) 1 J = 1 kg m s Calorie (cal) 1 calorie = 4.184 J 1 human heartbeat uses about 1J. The First Law of Thermodynamics: “The internal energy in an isolated system is constant.” “The total energy in the universe is constant.” “Energy is neither created nor destroyed in chemical reactions and physical changes”. NOTE: We can change from one form of energy to another!! WORK: w: Motion against an opposing force. WORK = FORCE x DISTANCE UNITS: Joule, J (see energy above) Types: (there are others but these are important for CH301) Expansion (expansion against an external force) Nonexpansion (work is done but there is no expansion against an external force) positive w: Work done ON system BY surroundings negative w: Work done BY system ON surroundings HEAT: q : Energy transferred due to a temperature difference. UNITS: Joule, J. (see energy above) positive q: Heat flows INTO system: ENDOTHERMIC process negative q: Heat flows OUT of system: EXOTHERMIC process Work vs. Heat HEAT is random motion of particles
WORK is organized motion of particles.
TEMPERATURE: Related to the average energy of the particles in a system. Two systems may be at the same temperature, but possess different amounts of energy, due to their size.
State Functions ONLY depend on current state of system; INDEPENDENT of how system got there! Always written in CAPITAL letters: Pressure P ,Volume V , Absolute Temperature T, Internal Energy E, Enthalpy H, Entropy S, Free Energy G.. e.g., Temperature: A glass of water at 298K. What about beforehand? Change in a STATE FUNCTION is INDEPENDENT of route taken by system! Back to the glass of water: It starts at 25°C, so T = 298K. I take it away and bring it back. It is now at 23°C, so T = 296K. Does it matter HOW that change occurred? All we can say is ΔT = - 2.00 K. This can be a useful tool… if the route was complex! Non-State Functions DEPENDENT on how system got between initial and final thermodynamic state! NOT written in capital letters: heat, q work, w. e.g., Consider two systems: A: starts at 3.5°C and cooled to 1.2°C B: starts at 3.5°C heated to 10°C, cooled to 1.2°C For BOTH samples ΔT is 1.2 – 3.5°C = -2.3°C = -2.3 K but q and w are different for A and B. INTERNAL ENERGY, E: Total energy (all forms) inside system We NEVER know the absolute value of E – dont know exact energy of all component particles We CAN find the CHANGE in E: ΔE = E (final) - E (initial) ANY CHANGE in a quantity X is always given by:
ΔX = X (final) - X (initial)
IMPORTANT NOTE FOR HW SERVER - QUEST Some other books use U rather than E for Internal Energy. You may see this in questions. Changes in Internal Energy, ΔE E may only change via heat or work: ΔE = q + w q>0 heat absorbed by system q<0 heat emitted by system w>0 surroundings did work on system w<0 system did work on surroundings Example: If 1200 joules of heat are added to a system and the system does 800 joules of work on the surroundings, what is the energy change for the system, ΔE?
Expansion Work against a Constant External Pressure System: Piston, area A: pushes against external (surrounding) pressure Pext ; experiences opposing force f = Pex A (remember pressure Pext is a force per unit area, we are stating the total force f) Move piston out a distance d : Work done by system on surroundings: system loses energy: Work = d Pex A But d A = ΔV so w = - Pex ΔV Pex MUST BE CONSTANT! Expansion Work Against a Vaccum Pex = zero so w = zero! FREE EXPANSION- No expansion work done by system- no energy lost. -1 -2
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Getting Units to Work Out: If P is in Pa (1Pa = 1kg m s ) and ΔV is in m , we get w in J. But if ΔV is in liters, and P is in atm always use the conversion factor (will be given on exams) -3 3 1 L. atm = 10 m x 101325 Pa = 101.325 Pa.m3 = 101.325 J Example: Ideal gas is allowed to expand from a volume of 10L to a volume of 40L against a constant opposing pressure of 0.75atm. Find the work done.
