Vocabulary (and not vocabulary)
Define spontaneous and nonspontaneous processes
spontaneous: occur naturally under certain conditions; no continual input of energy from an external source
non spontaneous: will not occur unless driven by the continual input of energy from an external source
If you are given 3 compounds and told to label them from increasing to decreasing entropy, what are the 3 steps you would use?
1. solid<liquid<<gas
2. molar mass
3. molecular complexity
write the equation relating Gibbs free energy, enthalpy, entropy, and temperature
∆
Calculate ∆
Label as spontaneous or nonspontaneous:
a. ice melting at room temperature
b. a ball rolling uphill
c. water freezing at room temperature
a. spontaneous
b. nonspontaneous
c. nonspontaneous
Define entropy. What is it used to study?
measure of randomness or disorder of a system. It is used to study matter and/or energy dispersal
Label the following in order of increasing entropy (1 mole of each and equal T)
(a) Cl2(g), Br2(g), and Br2(l)
(b) H2O(l), H2O(g), and H2O(s)
(c) Ne(g), Cl2(g), and S8(g)
b. solid, liquid, gas
c. molar mass
Relate ∆ to 0 when:
a. the reaction is spontaneous in the forward direction
b.the reaction is nonspontaneous in the forward
c. direction the system is at equilibrium
∆G < 0,
∆G > 0,
∆G = 0
When do you use standard free energy values? what equation can you use when not all parts of a chemical equations are in their standard states? What is this equation at equilibrium?
A standard free energy change (∆G°) has all chemicals in their standard states (i.e. solids, liquids, gases must be pure and unmixed!)
∆G = ∆G° + RT(lnQ)
∆G° + RT(lnK)
T/F: A spontaneous reaction always occurs very quickly
false
Calculate the change in entropy for the following reaction: CH3CH2CH3(g) + 5O2(g) → 3CO2(g) + 4H2O(g)
Given:
S˚ (J mol–1 K–1):
CH3CH2CH3 (g)=269.9
O2 (g)=205.138
CO2 (g)=213.74
H2O (g)=188.825
100.9 J/K mol
When a substance is heated, what happens to entropy? Why?
increases- energy distribution widens, particles expand (matter is dispersed)
Relate entropy and enthalpy to zero
Process WILL BE Spontaneous at ALL Temperatures
ΔG < 0 at any temperature
ΔS > 0
ΔH < 0
Calculate the entropy of the surroundings for the following two reactions.
a.) C2H8(g) + 5 O2(g) → 3 CO2(g) + 4H2O(g)
ΔH = -2045 kJ
b.) H2O(l) → H2O(g)
ΔH = +44 kJ
6860 J/K
-150 J/K
Predict the sign for ∆Srxn (+, –, or ≈ 0)
(a) MgO(s) + CO2(g) → MgCO3(s)
(b) 2C8H18(g) + 25O2(g) → 16CO2(g) + 18H2O(l) (c) CH4(g) + H2O(g) → CO(g) + 3H2(g)
*look at change in moles of gas*
Calculate ΔSuniv for the combustion of octane in air.
2 C8H18(g) + 25 O2(g) ⟶ 16CO2(g) + 18H2O(l)
For this reaction at 298.15 K: ΔH° = –11,024 kJ and ΔS° = -1383.9 J K–1.
+
What happens to entropy when:
a. volume compressed
b. a substance is sublimed
c. a substance is mixed with something
a. decrease
b. increased
c. increased
Relate entropy and enthalpy to zero
Process Spontaneous at High Temperatures
ΔG < 0 at HIGH temperatures
ΔS > 0
ΔH > 0
relate entropy and enthalpy to zero
Process Spontaneous at Low Temperatures
ΔG < 0 at LOW temperatures
ΔS < 0
ΔH < 0
what is the 3rd law of thermodynamics?
“The entropy of a pure perfect crystalline substance at 0 K is zero”
Consider the combustion of propane gas:
C3H8(g) + 5O2(g) ⟶ 3CO2(g) + 4H2O(g) ∆Hrxn= –2044 kJ
Calculate the entropy change to the surroundings at 25 ˚C.
6.86 kJ K–1
what happens to entropy when a gas is dissolved?Why?
decreases; motion of gas particles/ability to disperse decreases
A reaction has ∆H = 182.2 kJ mol–1, ∆S = 213 J K–1 mol –1. Determine the T (in °C) when the reaction switches between spontaneous and non-spontaneous.
855.4K
relate entropy and enthalpy to zero
Process is NOT Spontaneous at ANY Temperature
ΔG > 0 at high temperatures
ΔS < 0
ΔH < 0
Spontaneity stops/starts when
∆G= 0