KA 2: reaction feasibility Flashcards
what is the standard enthalpy of formation (ΔH°f)
the enthalpy change when one mole of a substance is formed from its elements in their standard states
what is the standard state of a substance?
its most stable state at a pressure of 1 atmosphere and at a specified temperature (usually taken as 298K)
how can the standard enthalpy of a reaction be calculated and what’s the formula?
-the standard enthalpies of formation of the reactants and products
ΔH° = ΣΔH°f (products) - ΣΔH°f (reactants)
what is the entropy (S) of a system?
a measure of the degree of disorder of the system
the greater the degree of disorder….
the greater the entropy
what disorder do solids and gases have?
solids: low
gases: high
state the relationship between entropy and temperature
entropy increases as temp increases
what happens to entropy at the melting point of a substance?
rapid increase in entropy
what happens to entropy at boiling point?
even more rapid and larger change in entropy
state the second law of thermodynamics
the total entropy of a reaction system and its surroundings always increases for a spontaneous process
heat energy is released by the reaction system into the surroundings. what happens to the entropy of the surroundings?
increases
heat energy is absorbed by the reaction system from the surroundings. what happens to the entropy of the surroundings?
decreases
state the third law of thermodynamics
he entropy of a perfect crystal at 0 K is zero
what is the standard entropy of a substance?
the entropy value for the substance in its standard state
how can the change in standard entropy for a reaction system be calcuated? whats the formula?
-standard entropies of the reactants and products
ΔS° = ΣS°(products) - ΣS°(reactants)
what is the change in free energy for a reaction related to?
what’s the formula?
enthalpy and entropy changes
ΔG° = ΔH° - TΔS°
when would a feasible reaction occur?
change in free energy (ΔG°) between reactants and products is negative
what is a feasible reaction?
one that tends towards the products rather than the reactants (this doesn’t give an indication of the rate of the reaction)
how is the standard free energy change for a reaction calculated?
what’s the formula?
-using free energies of formation of the reactants and prodcuts
ΔG° = ΣΔG° (products) - ΣΔG° (reactants)
how can the feasibility of a chemical reaction under certain conditions be predicted?
from the calculated value of the change in standard free energy (ΔG°)
how can the temperatures at which a reaction may be considered feasible be estimated?
by considering the range of values of T for which ΔG° < 0
when is a reaction feasible under non-standard conditions?
if ΔG is negative
what does ΔG equal at equilibrium?
0
when will a reversible reaction proceed spontaneously until?
the composition is reached where ΔG=0
