Energetics II Flashcards
Defn of spontaneous process
Spontaneous process is one that, once started, will continue without any external assistance
Endo/exo rctns are usually spontaneous and why
Exo are usually spontaneous as their prod are more stable than the rcts
Defn of entropy and symbol
Entropy (symb : S) of a system is a measure of the disorder of matter and energy in the system.
The more ways matter in the system can be arranged and the more ways energy in the system can be dispersed, the more disorganised the system is and the larger the entropy
Eqn of entropy and sig if < / >
ΔS = S (final/prod) - S (initial/rcts)
ΔS < 0 prod are less disordered than rcts
ΔS > 0 prod are more disordered than the rcts
Factors affecting entropy of a system (4)
- Change in temp
(increase temp, increase S)
As temp increases, the ave KE of the particles increase and there is a broadening of the energy distribution of the particles. Thus, there are more possible energy states in which particles can adopt at a high lvl. Hence, an increase in temp leads to an increase in entropy. - Change in phase
(Sgas»_space; Sl > Ssolid) - Change in amt of particles - esp gaseous particles
(No. Of particles increase, entropy increase)
A rctn where the prod have more aq/gaseous species than the rcts - increase in entropy - Mixing of particles
GASES: When mixing 2 or more gases with an increase in vol, entropy increases as there are more ways to arrange the particles
LIQUID/SOLID:
Entropy also increases when more solvent is added to a soln, as there are more ways to distribute the solute particles in a larger vol of soln
However, when dissolving solid or liquid solutes into a solvent,
(i) entropy increases as the solute particles are now free to move in soln, but
(Ii) entropy decreases as solvent molecules that were originally free to move become restricted by solvating the solute
The overall sign of ΔS depends on the balance b/w factors (i) & (ii)
- If the forces of attraction b/w solvent and solute are weak (eg id-id), usually (i) is more dominant and hence ΔS > 0
- If the forces of attraction b/w solvent and solute are strong (eg ion dipole interactions/h-bonding), (ii) may dominate which may result in ΔS < 0
Gibbs free Energy Change
- eqn
- units
- sig if it’s +ve / -ve / 0
ΔG = ΔH - T ΔS ——— greying hunter tracking son
Where ΔG = KJmol^-1
ΔH = kJ mol^-1
T = K
ΔS = J mol^-1 K^-1 ( so need to divide by 1000)
-ve - Forward rctn is spontaneous
+ve - Forward rctn is not spontaneous, ext energy needs to be supplied
0 - rctn has reached dynamic eqm
Limitations of the use of ΔG - indicates what and doesnt indicate what
Indicates thermodynamic feasibility (spontaneity) of a rctn
But NOT kinetic feasibility (where it proceeds at an observable rate). It depends on Ea
Comment on feasibility (sign of ΔG)
If ΔH > 0, ΔS > 0
ΔG = ΔH - TΔS
ΔH > 0, ΔS > 0 -> -TΔS < 0
At Low temp, ΔG > 0 becuz the + ve ΔH outweighs the -TΔS, hence the rctn is not spontaneous at low temp
At high temp, ΔG < 0 becuz the -ve TΔS outweighs the +ve ΔH, hence the rctn is spontaneous at high temp
OR (in another qn)
Since ΔH & -TΔS are both -ve, ΔG < 0 at all temp. This, the rctn is spontaneous at all temp
Graph of ΔG against temp T
How do u draw the graph of
2CO + O2 -> 2CO2
C + O2 -> CO2
2C + O2 -> 2CO
ΔG = (-ΔS)T + ΔH
2CO + O2 -> 2CO2
+ve grad ——— ΔS -ve so -ΔS is +ve
C + O2 -> CO2
Grad = 0
2C + O2 -> 2CO
-ve grad
How to draw Maxwell Boltzmann distribution diagram
Diff temp…. check notes
Describe and explain the entropy of the following system
1 mol of Cl (g) at 298K added to 1 mol of N2 (g) at 298K
Increase as mixing … increases the no. of ways of arranging the particles in the larger vol - increase in disorder and entropy of the system
Describe and explain the entropy of the following system
1 mol of Cl2 (g) at 298K is heated to 373K
increase as the increase in temp causes the broadening of the Maxwell-Boltzmann energy distribution of the particles - more possible energy states in which the molecules can adopt at a high temp - increase disorder in the system
Describe and explain the entropy of the following system
decrease in no. of moles of gaseous particles
Entropy decreases as the rctn results in a decrease in the no. of moles of gaseous particles in the system, causing a decrease in disorder and hence a decrease in entropy
Describe and explain the entropy of the following system
Increase in no. of moles of gaseous particles
Increase as with more particles - more ways to arrange the particles and more ways to distribute the energy in the system, and hence creating greater disorder and higher entropy in the system.
