Reaction Feasibility Flashcards
why can’t we determine ΔH directly?
because the actual enthalpies of reactants and products cannot be measured directly
what is standard enthalpy of formation?
the enthalpy change for forming one mole of a substance from its elements in their standard states
what is the standard state of a substance?
its most stable state at 1 atm pressure and 298 K (25oC)
what is the standard enthalpy of formation for an element in its most stable form?
zero
what must be done before summing enthalpy of formation?
each value must be multiplied by its soichiometric coefficient
what are the units for enthalpy change (ΔH)?
kJ mol-1
what is entropy? (s)
a measure of the degree of disorder in a system
how does entropy (s) change with temperature?
entropy increases as temperature increases
what are the entropy levels for solids vs gases?
solids - low disorder, low entropy
gases - high disorder, high entropy
what happens to entropy at phase changes? (liquid to gas etc)
mp - rapid entropy increase
bp - even larger and more rapid entropy increase
what is standard entropy?
the entropy content of 1 mole of a substance at 1 atm pressure and 298 K
what is a feasible reaction?
a reaction that tends toward the products rather than the reactants
does feasibility indicate reaction rate?
no, a feasible reaction may still occur very slowly
what does the third law of thermodynamics state?
the entropy of a perfect crystal at 0 K is zero
what does the second law of thermodynamic state?
the total entropy change (ΔS) of a system and its surrounding always increases in a spontaneous process
what happens when a system releases heat?
entropy of surroundings increases
what happens when heat is absorbed by a system?
entropy of surroundings decreases
when is a reaction feasible in terms of entropy?
when the overall entropy change is positive
what are the units for entropy?
J K-1 mol-1
when is a reaction feasible based on ΔG?
if ΔG
what happens when ΔG = 0?
the reaction is at equilibrium (just feasible)
what happens when ΔG is positive?
the reaction is not feasible
can a reaction be feasible under non-standard conditions?
yes, as long as ΔG is negative
when does a reversible reaction stop?
when ΔG = 0, meaning the reaction has reached equilibrium
