Topic 9: Thermodynamics and Equilibrium Flashcards
When is a reaction spontaneous in terms of total change in entropy?
Total change in entropy has to be over 0.
How does entropy change in a system of heat flow from hot to cold area?
- Heat flow from hot to cold area so the heat amount, q, is negative for hot area.
- Entropy of hot area decreases and entropy of cold area increases.
Identify the equation for entropy
ΔS = q/T
Identify the relationship between entropy and temperature, density and pressure.
- Entropy increases in a material when the temperature increases.
- Entropy increases when molecules go from a more dense to a less dense state (solid → liquid → gas).
- Entropy increased when an ideal gas is expanded at constant temperature.
state the 3rd Law of Thermodynamics
At T=0K, entropy of a substance equals to 0.
How is the total change in entropy different between an isolated and open system?
ΔS total = ΔS system in an isolated system
ΔS total = ΔS system + ΔS surroundings in an open system
what is standard entropy?
Standard entropy is the entropy change in a substance when T increases from 0 to 298K
How can change in entropy of a system be calculated?
ΔS system = standard S of products - standard S of reactants
How is the heat or change in enthalpy of system related to heat of surroundings?
q surroundings = -q system = -ΔH system
When the change in entropy of surroundings happens only due to heat flow or change in enthalpy of the system, how is change in entropy of surroundings and total change in entropy calculated?
ΔS surr = -ΔH system/T
ΔS total = ΔS system - ΔH system/T
Where does the entropy increase come from in exothermic and endothermic reactions?
- Exothermic reactions: most (or all) of the entropy increase results from the change in entropy of the surroundings due to the heat flow out of the system into the surroundings
- Endothermic reactions: all of the entropy increase comes from the change in entropy of the system.
What happens to a system when it is at equilibrium?
Both reactants and products are constantly made and used up at the same rate, so the concentration does not change.
Differentiate equilibrium constant and reaction quotient
- Equilibrium constant (Keq) - the ratio between reactant and product concentration at which the reaction is at equilibrium
- Reaction quotient (Q) - similar to Keq but it is at any point of time
Predict the direction of reaction when Keq is small or large
- Small Keq: more reactants are favored
- Large Keq: more products are favored
Predict the direction of reaction in terms of reaction quotient
- Q < Keq: favor products formation - to the right
- Q > Keq: favor reactants formation - to the left
