Topic 13.2 - Energetics II: Entropy Flashcards
What is the equation for the energy change (J) of a reaction?
Q = mc(delta)T
Q - energy in Joules, J
m - mass of solution, g (equivalent to the volume of solution as we assume density = 1)
(delta)T - change in temperature, K
What is the equation for the enthalpy change of reaction when you know the energy change of the reaction?
(delta) H = Q/n
(delta)H - Enthalpy change of reaction, kJmol(-1)
Q - energy in Kilojoules, kJ
n - Number of moles of limiting reagant
What is the equation for the enthalpy change of reaction when you know the enthalpy change of formation for your products and reactants?
(delta) H reaction = (delta) H products - (delta) H reactants
All units are kJmol(-1)
What is the equation for the standard total entropy change of a reaction?
(delta) S total = (delta) S system + (delta) S surroundings
All units JK(-1)mol(-1)
Under standard conditions
What is the equation for the standard entropy of the system?
(delta) S system = (delta) S products - (delta) S reactants
All units JK(-1)mol(-1)
Under standard conditions
What is the equation for the standard entropy of the surroundings?
(delta) S surroundings = - (delta)H reaction/ T
(delta) S - Entropy change of surroundings, JK(-1)mol(-1)
(delta) H - Enthalpy change of reaction, convert to Jmol(-1)
T - Temperature, K
What is the standard equation for Gibbs Free energy?
(delta) G = (delta) H reaction - T(delta) S system
What is the equation for Gibbs Free energy that includes the equilibrium constant of a reaction?
(delta) G = -RTlnKc
(delta) G - Gibbs free energy of the reaction, KJmol(-1)
R - Gas constant, 8.31 Jmol(-1)K(-1)
T - Absolute temperature, K
Kc - Equilibrium constant, from this equation you cant know the units of Kc
Under standard conditions
What is the equation for Gibbs Free energy that includes the Ecell value of a reaction?
(delta) G = -nFEcell
(delta) G - Gibbs Free energy change, KJmol(-1)
n - the number of electrons involved in the redox reaction
F - Faraday constant, 9.65 x10^4 Cmol(-1)
Ecell - Standard cell potential, V
What is the equation for the standard total change in entropy that includes the Ecell value of the reaction?
(delta) S total =(nFEcell)/T
(delta) S total - Total entropy change of the reaction, JK(-1)mol(-1)
n - the number of electrons involved in the redox reaction
F - Faraday constant, 9.65 x10^4 Cmol(-1)
Ecell - Standard cell potential, V
What is the definition of bond enthalpy?
The energy required to break 1 mole of that bond
What is the equation for the standard total enthalpy change of a reaction when you are given the bond energies?
(delta) H total = (sum of) Bonds broken - (sum of ) Bonds made
What sign does entropy always have? Why?
+ve, as particles are always in constant motion so there is always a degree of disorder
What is the definition of Entropy?
Entropy, S, is a quantitative measure of the degree of disorder of a system
When is the only time that entropy of a system = 0?
At Absolute Zero
What is the definition of a Spontaneous reaction?
A reaction that does not require any external input of energy in order to occur
What is the definition of a Feasible reaction?
A reaction that naturally tends to happen, even if it is very slow as the activation is high
What is required, in terms of entropy, for a reaction to be feasible?
- For (delta) S total > 0
Are spontaneous reactions always feasible?
Yes
Are feasible reactions always spontaneous?
No - A reaction can be thermodynamically feasible but not occur due to a very high activation energy (kinetics)
As entropy increases…
- Disorder tends to increase (number of moles of substances)
- Energy tends to become dispersed
- A reaction is more likely to happen
What is the universe made up of?
The system and the surroundings
Describe the molar entropy in solids
- Relatively low as atoms are held in fixed positions and only vibrate slightly
Describe the molar entropy in liquids
- Higher entropy than solids as the atoms/particles are free to move
- There are many more ways of distributing the particles and energy (the complexity)
- Molecules with more atoms have higher molar entropies as they can vibrate, rotate and arrange themselves in more ways
Describe the molar entropy in gases
- Gases have the highest molar entropy as they are both free to move and are widely spaced (so the energy is dispersed)
- There are the most ways of distributing the particles and energy
- Molecules with more atoms have higher molar entropies as they can vibrate, rotate and arrange themselves in more ways
What is a fundamental principal of Entropy?
Random changes always tend to go in the way that increases the number of ways of distributing the molecules, energy and quanta (entropy)
If (delta) S system is > 0 then…
there has been an increase in disorder (entropy)
If (delta) S system is < 0 then…
there has been a decrease in disorder (entropy)
If a reaction is exothermic then the (delta) S of the surroundings will be…?
+ve
If a reaction is endothermic then the (delta) S of the surroundings will be…?
-ve
If you lower the temperature of the surroundings how will that affect the value of (delta) S of surroundings? Why?
It will increase the value of the change in entropy of the surroundings.
- Using the equation -(delta)H/T a decrease in temperature will cause and increase in (delta) S
- For a given quantity of energy, the increase in entropy is greater when the surroundings are cool than when they are hot
What is the relationship between (delta) S surroundings and T, temperature?
(delta) S is inversely proportional to T
In what situation can an endothermic reaction be feasible?
If the (delta) S of the system is more +ve than the -ve entropy of the surroundings
In what situation can a non-feasible reaction at room temperature become feasible?
An increase in temperature
For all spontaneous processes (delta) G must be…?
-ve
If (delta) H is -ve and (delta) S is +ve then (delta) G will be…?
-ve - ALWAYS FEASIBLE
If (delta) H is -ve and (delta) S is -ve then (delta) G will be…?
-ve - feasible at low temperatures for (delta) H > T(delta) S system
If (delta) H is +ve and (delta) S is +ve then (delta) G will be…?
-ve - feasible at high temperatures for T(delta) S system > (delta) H
If (delta) H is +ve and (delta) S is -ve then (delta) G will be…?
+ve - NEVER FEASIBLE
At the point that a reaction becomes feasible…
(delta) G/ (delta) S = 0
This means you can determine at what temperature a reaction becomes feasible
What are the units of (delta) G?
Kjmol(-1)
Can a feasible reaction still be called feasible even if it is slow?
Yes
In equilibria are both the forwards and backwards happening? What does this suggest about the feasibility of those reactions?
Yes, both are happening. This suggests that both the forward and back reactions are feasible (even though one will give a +ve (delta) G and the other -ve (delta) G)
Describe what a graph of total entropy change against extent of reaction would look like.
Endothermic: For an endothermic reaction the line would increase and reach a maximum quickly at a +ve entropy and then decrease to a -ve value
Exothermic: For an exothermic reaction the line would increase a lot first and meet a peak close to the end before decreasing slightly to a final +ve entropy value
Why are both the forwards and backwards reaction feasible in equilibria?
As both the forwards and backwards reaction go via an increase in entropy
Why is there a peak of total change in entropy during an equilibria reaction?
As a mixture of reactants and products gives a higher entropy value than one on their own
When does the maximum total entropy value occur?
At equilibrium - when (delta) S forwards = (delta) S backwards
At Kc = 1, (delta) G of the forwards reaction is…?
0 - The position of equilibrium is in the middle and neither the reactants nor products are favoured
At Kc > 1, (delta) G of the forwards reaction is…?
-ve - The position of equilibrium lies to the right, more products than reactants are being made
At Kc < 1 , (delta) G of the forwards reaction is…?
+ve - The position of equilibrium lies to the left, more reactants than products are in the equilibrium mixture
