5.2.2 - Enthalpy and Entropy Flashcards
What is entropy ?
A measure of disorder in a system.
- The more disorder, the higher level of entropy.
- An increase in disorder and entropy will lead to a positive entropy change ∆S˚ = +ve
- If number of ways of arranging the energy
(W) is high, then system is disordered
and entropy (S) is high.
What degrees are reactions ethalpalpicly favorable ? (What values do we want ∆H to be)
ΔH<0(exothermic reaction)
What happens at 0K
At 0K substances have zero
entropy. There is no disorder as particles are stationary
(absolute zero)
- It is not possible for a substance to have a standard entropy of less than zero.
- Only perfect
crystals at absolute zero
(T = 0 K) will have zero.
How can entropy change be calculated ?
What values do we want entropy to be ?
Positive value as the reaction is entropically feasible
Units for entropy
JK-1 mol-1
What is gibbs free energy
- It tell us if the reaction is feasible or not.
- ∆G = ∆H - T∆S
- Always ensure that units are Jmols, JK mol and K
What value does ∆G have to be to consider it a feasible reaction ?
∆G = 0 or a -value (negative)
How does Temperature affect feasibility
y= mx+c to the ∆G = ∆H - T∆S equation.
c = ∆H
The gradient(m) of this graph is equal to -∆S
∆H, ∆H and ∆H value feasibility table.
What formula shows temperature
T = ∆H/∆S
Explain the limitations of predictions made by ∆G about
feasibility, in terms of kinetics
No Information on Reaction Rate: ΔG tells us if a reaction can occur but not if it will occur at a practical rate. (might occur at a slow rate of reaction).
- Activation Energy Barrier: Kinetics are controlled by the activation energy (Ea), the energy required to reach the transition state. Even if ΔG is negative (indicating a favorable reaction), a high activation energy can make the reaction so slow that it appears not to proceed at all.
Rearrange gibbs equation to make ΔS the subject
