Exam Four Flashcards
A large K value means a […] value of G.
large
Equation of delta G:
DG = -(RTlnK)
When K > Q, the reaction proceeds […] and the reaction is […].
towards the products; spontaneous
When K < Q then the reactions proceeds […] and is […].
towards the reactants; nonspontaneous
S and T relationship:
entropy is directly related to the temperature (as T increases so does S)
What causes an increase in entropy?
- increase in temperature
2. s < l < aq
Calculating the entropy of the universe steps:
- CALCULATE DH: [Heats of formation (P-R)]
- CALCULATE SURROUNDINGS: (-DH/T)
- CALCULATE UNIVERSE: (Ssys + Ssurr)
At equilibrium DSuniverse =
0; so DSsys = -DSsurr
Can a reaction occur spontaneously even though DSsys is negative?
yes; if the DH is negative (DSsurr will be positive)
Can a reaction occur spontaneously when the reaction is endothermic?
yes; only if DSsys > DSsurr
Predict:
DH = - DS = +
spontaneous at all T DG is (-)
Predict:
DH = +
DS = -
nonspontaneous at all T
DG = (+)
Predict:
DH = +
DS = +
spontaneous at higher T; nonspontaneous at lower T
Predict:
DH = -
DS = -
spontaneous at lower T; nonspontaneous at higher T
What is the equation to solve the temperature at which a reaction becomes spontaneous:
DH = T(DS) and T = (DH)/(DS)
How do you balance a redox reaction in an acidic medium?
- separate into half-reactions and determine which is being oxidized/reduced
- first balance all atoms other than O and H
- balance O molecules by adding H2O
- balance H by adding H+ ions
- balance charge by adding electrons (multiply if needed so electrons equal)
- add the half reactions
How do you balance a redox reaction in a basic medium?
- separate into half-reactions and determine which is being oxidized/reduced
- first balance all atoms other than O and H
- balance O molecules by adding H2O
- balance H by adding H+ ions
- balance charge by adding electrons (multiply if needed so electrons equal)
- balance H+ ions by adding OH- ions to both sides of the reaction (formation of H2O)
- add the half-reactions
If the salt is pure, predicting the products of its electrolysis is straightforward…
the cation will be reduced and the anion oxidized
The more easily reduced agent species is the stronger […] agent.
oxidizing
The more easily oxidized species is the stronger […] agent.
reducing
How do you determine the relative strengths of electrode species molten salts?
NOT cell potential; use periodic trends
ELECTROLYSIS:
If the metal holds its electrons more tightly it has a […]. Thus, as a […] , it gains electrons more easily, so it is the stronger […] agent and is […] at the cathode.
higher ionization energy; cation; oxidizing agent; reduced
ELECTROLYSIS: If the nonmetal holds its electrons less tightly than another, it has a lower […]. Thus, as […]. it loses electrons more easily, so it is the stronger […] agent and it is […] at the anode.
electronegativity (EN); an anion; reducing; oxidized
ELECTROLYSIS: How do you predict the electrode products of aqueous salt solutions?
- reduction with the less negative (more positive) electrode
- oxidation with the less positive (more negative) electrode potential occurs
Cations of less active metals are […] to the metal.
reduced
List less active metals:
Au, Ag, Cu, Cr, Pt, Cd
Cations of more active metals are […] to the metal.
not reduced
List active metals:
Group 1A, 2A, and Al
Anions that are […] because of overvoltage from O2 formation include […] except […].
oxidized; halides; F-
Anions that are not oxidized include…
F-
common oxoanions (SO4, CO3, NO3, PO4)
Voltaic cells are also known as…
galvanic cells
A voltaic cell uses a […] redox reactions to generate electrical energy.
spontaneous
List the characteristics of voltaic cells:
- spontaneous
- electrical energy generated from the difference in free energy between higher energy reactants and lower energy products
- system does work on the surroundings (load)
List the characteristics of electrolytic cells:
- uses electrical energy to drive a nonspontaneous redox reactions
- external power source supplies free energy to convert lower energy reactants into higher energy products
- surroundings (power supply) do work on the system (electrolytic cell)
- electroplating and the recovery of metals from ores utilize this
What is the notation for a volatic cell?
- anode on the left (X—X+)
- cathode on the right (Y+—Y)
- anode/cathode separated by physical boundary
- comma separates the half cell components that are in the same phase
- concentrations are placed in parathesis
The more positive the E value…
the more readily the reactions (as written) occurs
The species that goes under oxidation, it is the stronger…
reducing agent
The species that undergoes reduction is the stronger…
oxidizing agent
How do you write a spontaneous reaction without a guide?
- reverse one of the HR into an oxidation step so the different of the electrode potentials gains a positive Ecell
- cathode minus anode = +Ecell
- reactants should be the stronger oxidizing/reducing agent
How can metal displace other metals from solution? What is an example of this?
any metal that is lower in the list can reduce the ion of a metal that is higher up and thus displace that metal from solution; and E cell is overall positive
zinc can displace iron
What metals can displacer H2 from water?
metals that are active enough to reduce H2O lie below that half-reaction
and if E cell is overall positive
Faraday’s Constant:
9.65*10^4
Equation of E cell given K
E cell = (0.0592/n) x logK
Find K given the E cell:
log K = (n x Ecell) / 0.0592
Describe the electron flow in a galvanic/voltaic cell:
the anode is negative because electrons are generated and the cathode is positive because electrons are removed
Describe the electrons flow in the electrolytic cell:
external power source generates electrons for the cathode so it’s negative, and removes from the anode so it’s positive
ELECTROLYSIS: Lysing of an aqueous solution means there’s a mixture of…
We compare the electrode potentials of […] to predict the electrode products.
ions and water; ions and water
Overvoltage of water reduction and oxidtion:
0.4-0.6
