Thermodynamics

Question 1

What is thermodynamics? And what variables there are?

Answer 1

Thermodynamics is the study of energy.
The system is what we investigate, while the environment is everything else. Thermodynamics describe the exchange of energy between the system and the environment.

Question 2

What are state variables? How can we differentiate them?

Answer 2

State variables are variables that describe the state of the system.

1. Extensive state variables: when two systems are combined, the variables adds up (volume, # molecules, energy … )
2. Intensive state variables: when two system are combined, the variables don’t change or average out (temperature, pressure concentration …)

Question 3

What does the First Law of thermodynamics tell us?

Answer 3

The first law tells us that the energy is conserved:
ΔU=q-w
where U is the internal energy of the system; q is the flux of heat; w is the work.
- if U decrease: heat is produced; work is done.
- if q is positive: energy is added to the system.
- if q is negative: energy is produced and U is lost.

Question 4

What is Enthalpy?

Answer 4

Enthalpy change (ΔH)  is the heat added to or produced by the system at constant pressure.
	ΔH=qp=ΔU+pΔV

Question 5

What is work?

Answer 5

Work (w) is the energy transferred to or from an object via the application of force along a displacement.
w=pΔV

Question 6

What is multiplicity?

Answer 6

Multiplicity (W) is the number of microscopic arrangements that have the same macroscopic appearance.
The higher is W, the higher is the probability that the molecules will arrange in that state. This is called “probability drive”.
The more probable is the state, thus the higher is W, the more likely is that the particles will arrange equally in space, and less likely that they will arrange all on one side. The most probable state is called “equilibrium”.

When two systems are combined:
Wtot=W1*W2

Question 7

What does the Second Law of thermodynamics tell us?

Answer 7

The second law tell us that in a spontaneous reaction, the total Entropy needs to increase.

Question 8

What is Entropy?

Answer 8

Entropy represents the unavailability of a system’s thermal energy for conversion into mechanical work, often interpreted as the degree of disorder or randomness in the system.
S=kb*ln(W)
S=q/T
Entropy leads to an even distribution of particles or energy over the system.

Question 9

How are systems discriminated?

Answer 9

1. Isolated system: a system that can’t exchange molecules or heat with its environment.
   ΔS(sys)>0
2. Open system: a system that can exchange molecules or heat with its environment.
   ΔS(tot)=ΔS(sys)+ΔS(env)>0
   There is an increase in S(tot), which will occur spontaneously.

Question 10

What is the entropy of the environment?

Answer 10

ΔS(env)= -q(sys)/T

–> ΔS(tot)=ΔS(sys) - ΔH(sys)/T

Question 11

What is Gibbs Free Energy?

Answer 11

Gibbs free energy is used to predict if a process will occur spontaneously at constant pressure, and it expresses how much work a system can be made to do.
G=H-T*ΔS

Question 12

What is the relation between enthalpy and entropy in reactions?

Answer 12

1. ΔH<0 & ΔS>0:
   
   • enthalpically favoured (exothermic)
   • entropically favoured
   • spontaneous
2. ΔH<0 & ΔS<0
   
   • enthalpically favoured (exothermic)
   • entropically opposed
   • spontaneous only at T0 & ΔS>0
   • enthalpically opposed (endothermic)
   • entropically favoured
   • spontaneous only at T>ΔH/ΔS
3. ΔH>0 & ΔS<0
   
   • enthalpically opposed (endothermic)
   • entropically opposed
   • non-spontaneous