Chem 175 - Chapter 18 Chemical Thermodynamics (Exam 4)
Isothermal Processes
(no temp change in surroundings) - entropy change of the surr is related to the thermal energy change of the sys ∆S surr = -q sys / T = -∆H sys / T
Arrange the compounds from highest to lowest standard molar entropy, Sº - O2 - N2 - CH3OH (g)
- CH3OH - O2 - N2 more atoms per molecule and higher molar mass increases the entropy
Non-spontaneous Processes
- don't happen under indicated conditions
3rd Law of Thermodynamics
- entropy of a pure, perfectly ordered, crystalline substance at absolute zero is zero - at ab. zero there is no thermal energy to cause particles in solid to move S = k*ln(l)
Standard Free Energies of Formation, ∆Gf°
- free energy change in the formation of a substance from its elements in their standard states
Spontaneous Processes
- happen under indicated conditions (quickly or slowly) but the process occurs ∆S univ > 0
Entropy (S)
- measure of degree of disorder or randomness in a system - describes how spread out a systems energy is
Microstates
- number of energetically equivalent arrangements possible for the system
Macrostate
- set of energetically equivalent microstates
2nd Law of Thermodynamics
- spontaneous processes always result in an overall increase of enthalpy S = k*ln(w) w = microstates k = 1.38 x 10⁻²³ J/K
Gibb's Free Energy, G
- state function defined in terms of the enthalpy, entropy, and temp of the system, and used to determine if a process is spontaneous T∆S univ = ∆H sys + T∆S sys G = H - TS ∆G° = ∆H° - T∆S°
Chemical Thermodynamics
- study of how heat and work are involved in chemical reactions and physical changes (like state changes)
1st Law of Thermodynamics
- total energy of the universe is constant; energy is neither created not destroyed, but it can be transferred
For a particular reaction, Δ𝐻 = 139.99 kJ/mol and ΔS = 298.7 J/(mol·K). 1. Calculate ΔG for this reaction at 298 K. 2. What can be said about the spontaneity of the reaction at 298 K? a. The system is spontaneous in the reverse direction. b. The system is at equilibrium. c. The system is spontaneous as written.
1. ∆G = 50.98 kJ/mol Δ𝐺 = Δ𝐻 − 𝑇Δ𝑆 2. a
Which law? The energy of the universe is constant and can neither be created not destroyed.
1st
Which law? ∆U = q + w
1st
Which law? For any reaction that occurs, then entropy of the universe must increase.
2nd
Which law? ∆S univ > or = to 0
2nd
The ΔHvap of dichloromethane is 28.06 kJ*mol−1 and its ΔSvap is 89.65 J*mol−1 *K−1. What it the boiling point of dichloromethane?
39.84°C Δ𝐺 = Δ𝐻 − 𝑇Δ𝑆 T = Δ𝐻 / Δ𝑆 *at equil, Δ𝐺 = 0
Which law? S = kb x ln(W)
3rd
Which law? The entropy of a perfect crystal at absolute zero is 0.
3rd
The Δ𝐻fus of silver is 11.30 kJ⋅mol−1, and its Δ𝑆fus is 9.150 J⋅mol−1⋅K−1 . What is the melting point of silver (in °C)?
961.82°C ∆G = 0 since it is at equil Δ𝐺 = Δ𝐻−𝑇Δ𝑆
If ΔG° > 0, then K < or > 1.
< 1
If ΔG° = 0, then K ____ 1.
=
Only ΔG ___ 0 is true at equilibrium.
=
In an exothermic rxn, the q sys ___ 0. < or >
>
If ΔG° < 0, then K < or > 1.
>1
A reaction has a standard free‑energy change of −18.70 kJ mol−1. Calculate the equilibrium constant for the reaction at 25 °C.
Keq = 1888.35 Δ𝐺° = −RTln𝐾
When ∆H > 0, ∆S < 0, and -T∆S > 0, the rxn is _______________.
Nonspontaneous at all temps
When ∆H > 0, ∆S > 0, and -T∆S < 0, the rxn is _______________.
Spont at high temps Nonspont at low temps
When ∆H < 0, ∆S < 0, and -T∆S > 0, the rxn is _______________.
Spont at low temps Nonspont at high temps
When ∆H < 0, ∆S > 0, and -T∆S < 0, the rxn is _______________.
Spontaneous at all temps
Complete the statement. ΔG = ΔG° when a. 𝑄 = 1 b. 𝐾 = 0 c. 𝑄 = 𝐾 d. 𝐾 = 1 e. 𝑄 = 0
a
When Δ𝐺 = 0,... a. the reaction is at equilibrium. b. the reaction mixture will shift toward the reactants. c. only the reactants A and B are present. d. all of A and B have reacted to give pure AB.
a
Which process is necessarily driven by an increase in the entropy of the surroundings? a. the freezing of water b. the melting of ice c. the sublimation of dry ice d. the vaporization of water
a
Which of the reactions are spontaneous (favorable)? a. 2Mg(s)+O2(g)⟶2MgO(s) Δ𝐺 = −1137 kJ/mol b. C(s)+H2O(l)⟶CO(g)+H2(g) Δ𝐺 = 90.8 c. AgCl(s)⟶Ag+(aq)+Cl−(aq) Δ𝐺 = 55.6 kJ/mol
a ∆G is neg when the rxn is spon
What statement about the spontaneity of a reaction and the rate of a reaction is true? a. A spontaneous reaction is always a slow reaction. b. The spontaneity of a reaction is not necessarily related to the rate of a reaction. c. A spontaneous reaction is always a fast reaction.
b
Which process is necessarily driven by an increase in the entropy of the surroundings? a. the sublimation of dry ice b. the vaporization of water c. the freezing of water d. the melting of ice
c when water freezes, the solid is more ordered, so the entropy of the sys is neg causing the surr to be pos
Predict how the entropy of the substance is affected in each process. Fe (s, 250K) → Fe (s, 25K)
decreases
∆n gas = -1 the entropy ________.
decreases
∆n gas = 0 the entropy ________.
has little or no change
At __________ temps surr have higher KE and the release of heat energy by the sys will increase that KE by a lower % than at _______ temps.
higher lower
The effect of entropy change of sys on entropy of surr is smaller at _________ temps than _______ temps.
higher lower
Predict how the entropy of the substance is affected in each process. Ar (l) → Ar (g)
increases
Predict how the entropy of the substance is affected in each process. Cu (s, 300K) → Cu(s, 800K)
increases
∆n gas = 1 the entropy ________.
increases
An increase in temperature increases the KE, which _________ the entropy.
increases the particles move faster and energy is dispersed faster and throughout the system
Reactant-favored, product-favored, or insufficient information? 2A(g) + B(g) ⇌ 4C(g) ∆H° = 322 kJ
insufficient info
Reactant-favored, product-favored, or insufficient information? A(g) + 2B(g) ⇌ 2C(g) ∆H° = -89
insufficient info
Spontaneous or nonspontaneous? Iron metal is extracted from iron ore
nonspon
Spontaneous or nonspontaneous? Water is separated into H2 and O2 gas at 25°C
nonspon
Reactant-favored, product-favored, or insufficient information? A(s) + B(g) ⇌ 2C(g) ∆H° = -109 kJ
product-favored
Reactant-favored, product-favored, or insufficient information? 2A(g) + 2B(g) ⇌ 3C(g) ∆H° = 254 kJ
reactant-favored
Spontaneous or nonspontaneous? A cup of hot coffee cools to room temperature
spon
Spontaneous or nonspontaneous? Iron rusts in moist air
spon
Spontaneous or nonspontaneous? Perfume molecules spread out to fill the entire room
spon
When ΔG < 0, the reaction is ________ in the fwd direction. spon or nonspon
spontaneous
∆S is positive when the distribution becomes more or less equally distributed.
true
Spontaneous as written above a certain temperature ΔH = ΔS =
ΔH = + ΔS = +
Spontaneous in reverse at all temperatures ΔH = ΔS =
ΔH = + ΔS = -
Spontaneous as written at all temperatures ΔH = ΔS =
ΔH = - ΔS = +
Spontaneous as written below a certain temperature ΔH = ΔS =
ΔH = - ΔS = -
Classify the phase changes by the signs of the system's ΔH and ΔS. gas to liquid
Δ𝐻 = − & Δ𝑆 = −
Classify the phase changes by the signs of the system's ΔH and ΔS. gas to solid
Δ𝐻 = − & Δ𝑆 = −
Classify the phase changes by the signs of the system's ΔH and ΔS. liquid to solid
Δ𝐻 = − & Δ𝑆 = −
Classify the phase changes by the signs of the system's ΔH and ΔS. liquid to gas
Δ𝐻= + & Δ𝑆= +
Classify the phase changes by the signs of the system's ΔH and ΔS. solid to gas
Δ𝐻= + & Δ𝑆= +
Classify the phase changes by the signs of the system's ΔH and ΔS. solid to liquid
Δ𝐻= + & Δ𝑆= +
For a gaseous reaction, standard conditions are 298 K and a partial pressure of 1 atm for all species. For the reaction C2H6(g) + H2(g) ⇌ 2CH4(g) the standard change in Gibbs free energy is Δ𝐺°=−69.0 kJ/mol. What is ΔG for this reaction at 298 K when the partial pressures are PC2H6 = 0.200 atm, PH2 = 0.400 atm, and PCH4 = 0.800 atm?
∆G = -63.85 kJ/mol Δ𝐺 = Δ𝐺 + RTln(Q)
Equation for ∆S univ =
∆S univ = ∆S sys + ∆S surr
Equation for ∆S° rxn =
∆S° rxn = Σm[S°(products)] - Σn[S°(reactants)]
