Thermodynamics 3 Flashcards
(35 cards)
Describe a spontaneous reaction when it has a high activation energy.
Occurs slowly
What is entropy a measure of?
- Randomness or disorder of a system.
- increase in order = decrease in entropy
- increase in disorder = increase in entropy
How do we calculate change in entropy?
Final entropy - initial entropy
- If the change from initial to final results in an increase in randomness then Sfinal>Sinitial, thus, ΔS > 0
What is the Second Law of Thermodynamics?
The entropy of an isolated system tends to increase.
How to calculate a change in entropy of a substance?
The heart transferred reversibly to it divided by the temp. at which the transfer takes place.
What does it mean that entropy is a state function?
its value only depends upon the present state of the system.
Why reversible?
Equal temp. either side of a thermally conducting wall - no ‘hot spots’ and these would tend to spontaneously disperse and add to the entropy.
Why heat not work?
Heat - random molecular motion whereas work is ordered molecular motion
What does temperature got to do with entropy?
A given entropy change is more significant for a cold system than a hot one
Give the equation for entropy changes accompanying heating.
ΔS = C ln (Tf/Ti)
What does the equation ΔS = C ln (Tf/Ti) tell us?
That entropy changes are higher for materials with high heat capacities - more energy is required to cause a change in temp. for these materials.
What is entropy and second law?
The amount of heat passing to/from the surroundings is equal and opposite to the heat leaving/entering the system.
Therefore, ΔSsur = -q/T
At constant pressure wha does q equal?
q = ΔH therefore, ΔSsur = -ΔH/T
Describe entropy for an exothermic reaction?
The surrounding gains heat and its entropy increases
Describe entropy for an endothermic reaction?
The surrounding loses heat and entropy decreases.
Define first law of thermodynamics.
Energy can be converted
from one form to another, but energy cannot be created or destroyed.
Define the second law of thermodynamics.
The entropy of the
universe increases in a spontaneous process and remains unchanged in an equilibrium process.
State the equation for entropy and probability.
S = k ln W
K = 1.38 x 10^-23 J/K
W = the number of way that the state can be achieved
What is this third law of thermodynamics?
The entropy of a perfectly ordered crystalline substance at absolute zero
Describe entropy at higher temp.
- Greater molecular motion
- Broader distribution of individual molecular energies
- More randomness
- Higher entropy
Describe entropy at lower temperature.
- Less molecular motion
- Narrower distribution of individual molecular energies
- Less randomness
- Lower entropy
Define state function?
A function or property whose value depends only on the present state, or condition, of the system, not on the path used to arrive at that state.
How to calculate the standard entropy of a reaction?
ΔS^0rxn = ΣnS0(products) - ΣmS0(reactants)
Describe spontaneity and the entropy of the universe.
Processes occur spontaneously if the overall entropy of the universe is increased.
