Entropy & Enthalpy
A chemical reaction has ΔH = –75 kJ/mol. Is this reaction endothermic or exothermic?
Exothermic
What are the signs for ΔH and ΔS in a reaction that is always spontaneous, regardless of temperature?
ΔH is negative, ΔS is positive
In a redox reaction, what substance is the oxidizing agent?
The species that is reduced.
Write the Nernst Equation:
E = Eo − (0.0591/n)logQ
What does a positive ΔS value indicate about the disorder or randomness of a system?
Entropy Increases or there is more disorder
Calculate ΔG and determine whether the reaction is spontaneous at 298 K:
ΔH = +60 kJ/mol
ΔS = +200 J/mol·K
ΔG = +0.4 kJ/mol, so the reaction is not spontaneous at 298 K.
What happens to the mass of the anode and cathode during a redox reaction in a voltaic cell?
Anode loses mass ; Cathode gains mass
Given the half-cell potentials:
Zn²⁺/Zn = –0.76 V Cu²⁺/Cu = +0.34 V
Calculate the standard cell potential (E°cell) for Zn + Cu²⁺ → Zn²⁺ + Cu
+1.10 v
When a gas condenses into a liquid, what happens to the entropy?
Entropy Decreases because the molecules become more orderly.
A reaction has ΔH = –120 kJ and ΔS = –200 J/K. At what temperature (in K) will the reaction become non-spontaneous?
600K
What is the function of the salt bridge in a galvanic (voltaic) cell?
Allows for the flow of ions to prevent charge build-up.
For the redox reaction:
Ag+ (0.01 M) + Cu (s) → Ag (s) + Cu2+ (1.0 M)
Given that Eocell = 0.46 V, n = 2, calculate E at 25oC.
0.41 v
Using Hess’s Law, calculate the enthalpy change for:
A → C
Given: A → B = +50 kJ
B → C = –30 kJ
ΔH = +20 kJ
A reaction has ΔG = –45 kJ/mol and ΔS = –120 J/mol·K. Find ΔH at 298 K.
-80.76 kJ/mol
In an electrochemical cell, how can you identify the anode and cathode
Anode - site of oxidation ; Cathode - site of reduction
If E°cell = +1.25 V, what is the Gibbs free energy change for a reaction involving 2 moles of electrons?
(Use ΔG = –nFE°cell, with F = 96,485 C/mol)
ΔG = –241.2 kJ
Calculate ΔH for the combustion of methane:
CH₄ + 2O₂ → CO₂ + 2H₂O
Standard enthalpies of formation (kJ/mol):
CH₄ = –75 kJ/mol O₂ = 0 kJ/mol
CO₂ = –394 kJ/mol H₂O = –286 kJ/mol
-891 kJ/mol
Determine if the following reaction is spontaneous at 298 K:
ΔH = –60 kJ/mol, ΔS = –100 J/mol·K
–30.2 kJ/mol ; Spontaneous
Give the proper line notation for this balanced battery equation:
H2 (g) + Fe3+ (aq) → 2H+ (aq) + Fe2+(aq)
Pt(s) | H2 (g), H+ (aq) || Fe3+ (aq) , Fe2+ (aq) | Pt (s)
Calculate the equilibrium constant K for the following reaction at 25°C:
Fe3+ + e- → Fe2+ , Eo = +0.77 V
1.07 x 1013