1.2 Enthalpy, Entropy and Free Energy Flashcards
Define feasible/spontaneous changes, with an example
changes that have a natural tendency to occur without any external help, eg. Aqueous silver ions and aqueous iodide ions spontaneously form a yellow precipitate of silver iodide.
how do feasible changes relate to stability of a system?
A feasible change occurs due to stability - feasible changes move a system from a less stable state to a more stable state.
What is the temperature change in most feasible reactions, and when is this not the case?
- Most feasible reactions are exothermic
- Some can be endothermic, but there must be an additional factor that determines whether or not the reaction will take place.
Give an example of a feasible endothermic reaction
Ammonium carbonate + ethanoic acid
This reaction shows an increase in disorder as a solid and solution produces a gas and solution - hence there is an increase in entropy. The reaction is therefore feasible despite being endothermic as it results in a change in entropy.
How does feasibility relate to entropy?
feasible reactions increase the disorder/entropy of a system (eg. changes of state, increased number of gas molecules etc)
How do the states of matter relate to entropy?
In terms of randomness/disorder:
solid < liquid < gas
define entropy (S)
a measure of disorder or randomness of a system
Why is entropy always a positive value?
all substances possess some degree of disorder, as particles are always in constant motion
state the second law of thermodynamics
The sum of the entropy changes for a chemical system and its surroundings must be positive for a process to be feasible.
define standard entropy (S^θ)
The absolute value of a substance’s entropy under standard conditions
Describe the relative standard entropy values of different states of matter
Gases generally have larger standard entropies than liquids, which have larger values than solids
Define standard entropy change (∆S^θ)
the entropy change per mole for conversion of reactants in their standard states into products in their standard states, at a stated temperature
state the equation to calculate the standard entropy change
∆S^θ = S^θproducts - S^θreactants
state the equation used to calculate free energy change
∆G= ∆H – T∆S
define Gibbs’ Free Energy
The energy associated with a chemical reaction that can be used to do work
What is the criterion for a feasible reaction?
∆G^θ < 0
what can be assumed if ∆G is positive?
the reaction is not feasible at that temperature, although it may be feasible at another temperature
what can be assumed if ∆G=0?
the system is in a state of equilibrium, ie. there is no thermodynamic tendency for the reaction to proceed in either direction.
What units should ∆S be if used in the free energy equation?
kJ/K/mol
What can be assumed about the feasibility of a system if ∆H is negative and ∆S is positive?
∆G is always negative
therefore feasible at any temperature
What can be assumed about the feasibility of a system if ∆H is positive and ∆S is negative?
∆G is always positive
therefore not feasible at any temperature
What can be assumed about the feasibility of a system if ∆H is negative and ∆S is negative?
∆G can be positive or negative
feasible below certain temperatures
What can be assumed about the feasibility of a system if ∆H is positive and ∆S is positive?
∆G can be positive or negative
feasible above certain temperatures
What features must a reaction system have to proceed satisfactorily?
- Thermodynamically stable (ie. negative ∆G value)
- Kinetically stable (ie the reaction must have a low activation energy)
why might a system with a negative ∆G value still react very slowly, and how can this be amended?
Although it is thermally feasible, it might not be kinetically stable. A catalyst can be used to increase the rate of reaction by lowering the activation energy.
