O defg: Entropy Flashcards
Define entropy.
A measure of the number of ways of arranging molecules and their associated energy quanta.
When may endothermic reactions be spontaneous?
When entropy increases during the reaction.
- Endothermic interactions happen spontaneously since energy isn’t evenly distributed
- All reactions are somewhat reversible
- But if products are messier and more spread apart than reactants, they won’t react - so it’s one-way
How are the following represented symbolically?
- General entropy change
- The entropy change for a given reaction
- ΔS
- ΔSsys
Entropy values are quoted at standard conditions, these being…
298 K, 100 kPa
What is the unit of entropy?
J K-1 mol-1
Not kJ
What is the sign of ΔSsys when:
- products are more entropic than reactants?
- products are less entropic than reactants?
- Entropy increases: positive
- Entropy decreases: negative
What factors increase the entropy of a collection of molecules?
- Greater volume (molecules more spread out)
- Greater total energy
- More molecules sharing the energy
- More complex molecules (more atoms = more energy levels)
What state of matter has the lowest entropy?
Solid
For each of the following changes, state whether the entropy of the system would increase, decrease or remain the same.
- Oil mixes with petrol
- A suspension of oil in water separates into 2 layers
- Car exhaust gases are adsorbed onto the surface of a catalyst in a catalytic converter
- Increase
- Decrease
- Decrease
State and explain which substance in the following pairs would have the higher standard molar entropy.
- Separate samples of copper and zinc / sample of brass
- Liquid pentane / liquid octane
- Brass; mixtures have higher entropies than the pure substances
- Octane; more complex molecules have higher entropies
C
- Hexane and pentane are liquids, so have lower entropy
- But pentane is less complex than hexane, so has lower entropy
- Propane and butane are gases, so have higher entropy
- But propane is less complex than butane, so has lower entropy
- Entropies increase for the first four alkanes as the molecules contain increasing numbers of atoms. The number of energy levels increases so there are more ways of distributing energy
- Pentane is a liquid and butane a gas so butane has the higher entropy
Write the equation used to calculate the total entropy change of a process.
ΔStot = ΔSsys + ΔSsurr
ΔStot = ΔSsys - ΔH/T
Total entropy change = entropy change of system + entropy change of surroundings
In mathematical terms, what is required for an endothermic reaction to be feasible?
ΔStot is positive
ΔSsys + ΔSsurr > 0
How do you work out ΔSsys?
Sproducts - Sreactants
How do you work out ΔSsurr?
ΔSsurr = -ΔH / T
ΔH is the enthalpy change of the system. Therefore, the enthalpy change of the surroundings is -ΔH.
Prove that water will not freeze at 10oC.
ΔSsys = -22.0 J K-1 mol-1
ΔH = -6.010 kJ mol-1
ΔStot = ΔSsys + ΔSsurr
ΔH = -6010 J
T = 273 + 10 = 283 K
ΔSsurr = -ΔH/T = - (-6010) / 283 = +21.2 J K-1 mol-1
ΔStot = -22.0 + 21.2 = -0.80 J K-1 mol-1
ΔStot < 0 so entropy decrease; the water will not freeze
Will calcium carbonate decompose to produce carbon dioxide at 1000 oC?
ΔSsys = +159 J K-1 mol-1
ΔH = +179 kJ mol-1
ΔStot = ΔSsys + ΔSsurr
ΔH = +179,000 J
T = 273 + 1000 = 1273 K
ΔSsurr = -ΔH/T = -(+179,000) / 1273 = -140.6 J K-1 mol-1
ΔStot = 159 - 140.6 = +18 J K-1 mol-1
ΔStot > 0 so entropy increases; it decomposes
What is the requirement of entropy at equilibrium?
ΔStot = 0
Should be equally easy to go forwards + backwards
What must the temperature be for water and ice to exist together in equilibrium? Give your answer in oC.
ΔSsys = +22.0 J K-1 mol-1
ΔH = +6.01 kJ mol-1
ΔStot = 0 (equilibrium)
ΔSsys + ΔHsurr = 0
ΔSsys - ΔH/T = 0
ΔH = 6010 J
22.0 - 6010/T = 0
T = 6010/22.0 = 273 K = 0 oC
Why may a reaction be feasible but not spontaneous?
- Sponteneity depends on enthalpy as well as entropy
- Kinetic factors affect rate: a reaction may not be spontaneous if Ea is large (i.e. so slow it is said not to occur)
Fill in the table by stating:
- Whether a reaction in each scanario would be feasible
- The conditions required for each reaction to occur spontaneously
1) Entropy is a measure of the number of ways of arranging molecules and their associated energy quanta.
2) ΔSsys is often positive because ions move from a regular lattice into a solution with more ways of arranging molecules and their associated energy quanta.
Ca2+ has high charge density, so ion-dipole interactions are stronger (solvation shells more regular) than bonding in ionic lattice. Lattice has higher entropy than solution, so ΔSsys is negative.
2NO(g) + O2(g) → 2NO2(g) ΔrH = -114 kJ mol-1
Give the sign of ΔSsys for this reaction, with a reason. Use this to explain whether the reaction becomes more or less feasible at higher temperatures. (3)
- ΔSsys must be positive for reaction to be feasible
- ΔStot = ΔSsys -ΔH/T. ΔH is negative so -ΔH/T is positive. As T increases, -ΔH/T decreases, so ΔHtot decreases. Becomes less feasible at higher temperatures
An experiment was carried out to determine the standard enthalpy change of solution for a compound which readily dissolves at room temperature. During the dissolving process a decrease in temperature was recorded.
What can be assumed from these results?
- The compound dissolves more readily at lower temperatures.
- The dissolving process has a positive ΔsurrS.
- The dissolving process has a positive ΔsysS.
- The process has a high activation energy.
3
- 1 incorrect since it is endothermic, so should say “higher temperatures”*
- 2 incorrect since endothermic → Δ is H positive → ΔsurrS = -ΔH/T is negative*
- 3 correct since endothermic, so entropy must increase for it to be feasible*
- 4 incorrect since readily dissolves at room temperature*
