22. Enthalpy and Entropy
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What does entropy (S) represent?
the dispersal of energy and disorder within a chemical making up a chemical system
What does a higher amount of disorder mean for enthalpy
higher ( more +ve) enthalpy
When is there no entropy?
0K / -273C
How do you calculate delta S?
S (products) - S (reactants)
When does entropy increase?
when: solids melt liquids boil ionic solids dissolve in water the number of gaseous molecules increases temperature increases
What is standard entropy?
the entropy of one mole of a substance at standard conditions
What are the units for standard entropy?
J K-1 mol-1
Why are standard entropy values always +ve
because absolute zero is the most ordered possible state, so at any temperature greater than that, the entropy will be greater, therefore more +ve
What is the Gibbs Free Energy equation?
delta G = delta H - Tdelta S
At what point does a reaction become feasible?
when delta G < 0
Why may some reactions, although theoretically feasible, never take place?
activation energy is too high, catalyst needed
What does delta G being equal to zero mean for the reaction?
it is at equilibrium and the reaction is just becoming feasible
When will a reaction never be feasible?
when delta H is positive and delta S is negative
If delta H is -ve and delta S is +ve what will the sign of delta G be, and what effect will temperature have on feasibility?
-ve always feasible
If delta H is -ve and delta S is -ve what will the sign of delta G be, and what effect will temperature have on feasibility?
probably -ve at low temperatures the reaction will be feasible as temperature increases, feasibility decreases
If delta H is +ve and delta S is +ve what will the sign of delta G be, and what effect will temperature have on feasibility?
probably +ve will not be feasible at low temperatures as temperature increases, feasibility increases
If delta H is +ve and delta S is -ve what will the sign of delta G be, and what effect will temperature have on feasibility?
+ve never feasible
Rearrange Gibbs Free Energy equation for delta S
delta S = (delta H - delta G)/T
When does delta G = 0
at changes of state
What is the equation for delta S at changes of state?
delta S = (delta H)/T
What is the limitation of delta G?
indicates thermodynamic feasibility but does not take into account kinetics or the rate of reaction (i.e. whether the activation energy is too high, or whether a catalyst is needed)
Define lattice formation enthalpy
the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions
Write an equation for the lattice formation enthalpy of NaCl
Na+(g) + Cl- (g) => NaCl (s)
Define enthalpy of sublimation
the standard enthalpy change when 1 mole of a solid is changed into gaseous atoms under standard conditions