Unit 9 Flashcards
What is Enthalpy (AH)?
Energy change in a reaction.
What is Activation Energy (E)?
Energy required by a reaction so it can start (lighting the candle).
What is Gibbs Free Energy (AG)?
Energy released by a reaction that can be used to help it continue without external energy being added.
The candle keeps burning on its own after you light it.
What is Entropy (AS)?
The dispersion of energy through a system. Gases have a higher S than liquids, liquids more than solids, and 2 moles of substance have more entropy than one mole of substance.
What characterizes thermodynamically favorable reactions?
They have a (-AG).
What types of reactions are ALWAYS thermodynamically favorable?
Reactions with (-)AH and (+)AS.
What does a negative AG indicate about K values?
If AG is (-), then Ko > 1.
What should be noted about the units of AH and AS?
AH and AS are usually NOT given in the same units. When using AG° = AH° - TAS°, make sure they match units.
What is the value of AG at equilibrium?
AG = 0 at equilibrium. Neither direction of the reaction is favored.
What is the value of R in the equation AG° = -RT ln K?
The value for R is 8.314 J/mol K.
What does it mean if a reaction with a (-)AG does not proceed at a measurable rate?
They are said to be under ‘kinetic control.’ High activation energy is a common reason for this.
What are coupled reactions?
Must have an intermediate in common, one reaction has a -AG that provides energy for the other reaction to move forward.
What are the three steps to dissolving?
(1) Bonds must be broken between the particles of solute. This is +AH and +AS.
(2) Solvent must move apart to make room for the solute. This is +AH and +AS.
(3) New attractions between the solute and solvent particles form. This is -AH and -AS.
What is oxidation?
Losing electrons, happens at the anode. OIL, An Ox.
What is reduction?
Gaining electrons, happens in the cathode. RIG, Red Cat.
In a battery, where do electrons flow?
Electrons flow from anode to cathode.
What is the function of a salt bridge?
Cations flow to the cathode, and the anions flow to the anode.
What happens to the mass of the cathode and anode while a battery is discharged?
The cathode gains mass and the anode loses mass.
What happens to E°cell if you reverse a reaction?
The sign of E°cell changes.
What happens to E°cell if you double a reaction?
E°cell DOES NOT change.
How do you calculate E°cell?
E°cell = E°Reduction + E°Oxidation.
What does a more (+) E° indicate?
The half-reaction with a more (+) E° is the reaction that takes place at the cathode.
What must be balanced when adding two half reactions?
The electrons must be balanced so they cancel out.
What is the equation relating AG° and E°cell?
AG° = —nFE°
If AG° is (-), then E°cell is (+). Reminder: n = # of electrons transferred.
What is the formula for Electroplating/Electrolysis Calculation?
I = nF/time
What happens to the voltage (E°cell) of the battery if Q increases?
The voltage (E°cell) drops.
Remember to calculate Nernst from end to the beginning, in order to get the correct order of operations.
What is Gibbs free energy?
Energy that is ‘free’ and available to do work.
What is the equation for cell potential?
E_cell = E_cathode - E°_anode
What does ΔG° represent?
ΔG° = [ΔG’ products - ΔG°f Reactants]
What is the relationship between entropy and energy?
Entropy increases when energy spreads out.
What is the equation for free energy of dissolution?
ΔG° = ΔH° - TΔS°
What does ΔG° > 0 imply?
Thermodynamically unfavored - Slightly soluble insoluble salts.
What does ΔG° < 0 imply?
Thermodynamically favorable - Soluble salts.
What is the significance of standard reduction potentials?
Used to compare reduction potentials.
What is the difference between galvanic and electrolytic cells?
Galvanic cells are thermodynamically favorable; electrolytic cells are thermodynamically unfavorable.
What is Faraday’s Law?
a = amount of charge, t = time in seconds.
What are standard conditions?
1 M solution, 1 atm gas, 25°C.
What is the Nernst equation at non-standard states?
ΔG = ΔG° + RT ln Q
What occurs at equilibrium?
Q = K, ΔG = 0.
What does K represent in thermodynamics?
K = e^(-ΔG°/RT)
What does a negative ΔG° indicate about E°?
If ΔG° < 0, E° > 0, K > 1 - Thermodynamically favorable.
What does a positive ΔG° indicate about E°?
If ΔG° > 0, E° < 0, K < 1 - Thermodynamically unfavorable.
As volume increase what else increases?
Entropy
As molescule move faster what increases?
Entropy
In electrolytic cell representations, what is one key factor to make the reaction favorable?
The volumes of the solutes should not be equal
Which equation should you use when trying to account for an increase in mass?
I= q/t
