First-Law Mechanics & Enthalpy Flashcards
(14 cards)
Write the first-law equation that links internal energy, heat, and work.
ΔE=q+w.
Clarify the sign convention for w.
w<0 when the system performs work on the surroundings; w>0 when the surroundings do work on the system.
Why is internal energy a state function even though q and w are path-dependent?
Because any difference in path-specific heat and work values still sums to the same ΔE for identical initial and final states.
In an isolated system, what is ΔE and why?
ΔE=0 because neither heat nor work crosses the system boundary.
Express enthalpy in terms of internal energy, pressure, and volume.
H=E+PV.
Why is enthalpy especially convenient for constant-pressure chemistry?
Because ΔH equals the heat exchanged at P=const, eliminating the need to treat PΔV expansion work explicitly.
Define extensive versus intensive properties and classify enthalpy.
Extensive properties depend on system size; enthalpy is extensive.
Describe how standard molar enthalpies of formation enable calculation of reaction enthalpy changes.
ΔH_rxn°=∑nΔH_f°(products)−∑nΔH_f°(reactants).
How does the system-centric sign convention ensure consistency across thermodynamic quantities?
By defining positive values for energy entering the system (heat absorbed, work done on the system) and negative values for energy leaving, calculations align directly with measured energy balances.
Why must both magnitude and sign be reported for thermodynamic data?
Magnitude conveys extent, while sign conveys direction of energy or entropy flow, both of which are essential to interpret spontaneity.
Provide an example where work is positive yet ΔE is negative.
If a gas is compressed by the surroundings (w>0) but simultaneously releases more heat than work absorbed (q<0 with |q|>|w|), the net ΔE becomes negative.
Why is PV-work the primary form of pressure–volume work in chemistry?
Chemical processes often occur in closed containers where volume changes against an external pressure represent mechanical work exchange with surroundings.
Under what circumstances can electrical work be significant in thermodynamics?
In electrochemical cells, where charge transfer through an external circuit does electrical work equal to −nFE.
Differentiate reversible and irreversible PV-work in terms of magnitude.
For identical volume changes, reversible compression/expansion performs the maximum (for expansion) or requires the minimum (for compression) amount of work because the external pressure is matched differentially to internal pressure throughout the process.
