Thermodynamics Flashcards
Topic 4, Lectures 19-22 - Tom Slater (69 cards)
Thermodynamics uses
Determines whether a reaction will take place
How much useful energy a reaction can generate
The ratio of reactants:products at equilibrium
Where it applies
Classical thermodynamics only deals with large systems and doesn’t apply at the level of individual molecules because it is dependent on the statistics of molecular distribution.
What can’t you determine with thermodynamics?
How fast reactions will occur/reach equilibrium
First law of thermodynamics
Energy cannot be created or destroyed, only converted between forms.
Zeroth law of thermodynamics
If two systems are in thermal equilibrium with a third system, they are also in equilibrium with each other. A=C, B=C, A=B. Temperature is a reliable way of measuring thermal balance
Thermal equilibrium
Where two thermal systems in contact stop exchanging heat, and have the same temperature.
Systems and surroundings
Chemical reactions represent the systems and anything external is the surroundings. Systems can be open,closed and isolated.
Open systems
Systems in which both matter and energy can be exchanged, e.g., a lidless pot of boiling water, the steam (matter) escapes and heat (energy) is released into the atmosphere.
Closed systems
Systems in which only energy can be exchanged, e.g., a reaction in a vessel with a lid, only heat (energy) is released but matter remains in the vessel.
Isolated systems
Systems in which neither matter nor energy are exchanged, e.g., a completely insulated vessel of boiling water. Though no system is perfectly isolated in real life.
Heat
Energy transfer that causes disorderly molecular motion. Heat transfer from system to surroundings causes random motion.
Work
Energy transfer that causes organised molecular motion. Work done causes motion of molecules in the same direction.
First law equation (closed system)
∆𝑈 = ∆𝑞 + ∆𝑤
The change in energy of a system is determined by the thermal energy added to the system plus the work done to a system.
Internal energy
The total kinetic and potential energy of the constituents of a system, rather than the whole system.
Internal energy of earth would account for the energy and motion of all things on earth but would not account for the energy associated with its motion around the sun.
Extensive variables
Depends on the amount of material (grams, moles, etc.)
Examples: heat, work, energy, mass, volume
Intensive variables
Independent of amount of material
Examples: pressure, temperature
Work done equation
∆𝑤 = − |𝐹 |∆𝑑
When distance is in the opposite direction to the force
The value of distance becomes negative
Expansion work
Ignoring the effects of friction and gravity, the expansion of gases can affect pressure and temperature. This has implications in areas such as pistons.
State functions
Properties of a system that depend only on the state of the system, as opposed to the path taken to reach that state.
Examples: internal energy, enthalpy, entropy, volume, pressure, gibbs free energy.
Process functions
Features of a system that depend on the path taken to transition between states. e.g. work done, heat. For example, the work done by expanding gas depends on whether pressure or temperature was constant during the expansion.
Maximising work
∆𝑤 = − 𝑉𝑖/𝑉𝑓 (𝑝𝑒𝑥𝑡𝑑𝑉)
Shows that most work is done when the external pressure is as close to the internal pressure as possible. Work done is also maximised when pressure is reduced gradually (infinitessimal changes.)
Work done—ideal gas at constant temperature
-𝑛𝑅𝑇(𝑙𝑛 𝑉𝑓/𝑉𝑖) calculates the maximum amount of work done
Expansion work equation
∆𝑤 = −𝑝𝑒𝑥𝑡∆𝑉
