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THE MCAT-CHEM 4
ELECTROCHEMISTRY
| Question | Answer |
|---|---|
| What oxidation state does oxygen have in peroxides? | -1 |
| What oxidation state does oxygen have normally? | -2 |
| What oxidation state does an elemental state have? | 0 |
| What oxidation state does hydrogen have when bond to non-metals? | +1 |
| What oxidation state does hydrogen have when bond to metals? | -1 |
| Another name for galvanic cell | voltaic cell |
| Galvanic / voltaic cell | electrochem cx that derives electrical energy from spontaneous redox rxn taking place within the cx |
| Electrolytic cx | electrochem cx in which energy from an applied voltage is used to drive an otherwise nonspontaneous rxn |
| Electrons ALWAYS flow from? | anode --> cathode |
| What happens at anode? Cathode? | oxidation / reduction |
| Naturally, what species will be the anode and what species will be the cathode? | -anode = highest oxidation potential -cathode = highest reduction potential |
| What is the salt bridge for? | maintains charge neutrality of each half cell |
| If the cell potential is positive, the rxn is spontaneous / nonspontaneous | spontaneous |
| If the cell potential is negative, the rxn is spontaneous / nonspontaneous | nonspontaneous |
| Which species are the most easily reduced? | electronegative species |
| Is the cell potential for a galvanic cell positive or negative? Electrolytic cell | -positive -negative |
| Calculate cell potential. | Reduction potential + oxidation potential --> don't multiply potentials by anything |
| What is the reduction potential for hydrogen? | 0 |
| What species has positive reduction potentials? | oxygen, halogens, nonreactive metals |
| What species has negative reduction potential? | reactive metals that want to get rid of outer electron |
| Current for an electrochem cell. | I = q / t (coulombs of charge per second) |
| Faraday's constant | F = q / n (total charge / total moles of electrons) |
| Faraday's law | I * t = n * F (current * time ) = (moles of e * Faradays) |
| Why can electrolytes conduct electricity? | motion of ions |
| Without electrolytes, can there be a circuit? | NO --> electricity can't travel |
| Which type of cell requires potential / voltage input? | electrolytic |
| Percent yield | experimental yield / theoretical yield * 100 |
| What does a coulomb equal to? | Amps * second |
| Cu2+ + 2e --> Cu How many moles of electrons are required to reduce one mole of Cu2+ | 2 moles of electrons |
| If one electron is required to reduce Ce4+ and 2 electrons are required to oxidize Sn2+, how many Ce4+ are reduced for every Sn2+ oxidized? | 2 |
| Where is the strongest oxidizing agent? Reducing agent? | upper left / lower right |
| A negative reduction potential means it is easier / harder to reduce? | harder --> better reducing agent |
| A positive reduction potential means it is easier / harder to reduce? | easier --> worse reducing agent |
| What is the reduction equation for hydrogen? | 2H+ + 2e --> H2 --> E = 0 |
| What does galvanic cell convert? | converts chemical energy to electrical energy |
| What is a half cell? | strip of metal and solution together |
| Where does current flow in relation to electrons? | current flows in opposite direction |
| Why do electrons in galvanic cell naturally flow to cathode? | electrons in anode have higher potential energy than cathode |
| Galvanic cell in relation to work. | Galvanic cell always has chemical energy that can be converted to work |
| What does a positive cell potential indicate with G? | negative G and spontaneous reaction |
| Equilibrium constant, K, and Go vary with? | temperature |
| K = 1, then delta G = ? K <1, then delta G =? K >1, then delta G =? | -0 ->0 -<0 |
| What are standard state conditions? | 298 K, conc. of all solutions is 1M, 1 atm |
| What does electrochemical cell imply? | either galvanic or electrolytic cell |
| What are electrolytic cells used for? | metal plating, purifying metals, electrophoresis |
| In a galvanic cell, the anode is positive/negative and the cathode is? | -negative / positive |
| In an electrolytic cell, the anode is positive/negative and the cathode is? | -positive / negative |
| What are galvanic cells used for? | batteries |
| If oxidation takes place, does reduction always have to take place? | yes |
| T/ F: An Atom in the reducing agent gets oxidized | TRUE |
| T/F: If an atom of the reductant loses two electrons, an atom of the oxidant must gain 2 electrons | FALSE |
| A galvanic cell with a positive potential can____ | perform work |
| If Eo =0, can the rxn still be spontaneous? | YES --> depends on chemical concentration |
| Forward and reverse rates are ONLY equal during ______. | equilibrium |
| Does enthalpy and Gibbs free energy value reverse when rxn is reversed? | YES |
| Galvanic cells are spontaneous so delta G =? | < 0 |
| Electrolytic cells are non-spontaneous so delta G=? | > 0 |
| Does an electrolytic cell require a salt bridge? | NO |
| What type of cell is a concentration cell and is it spontaneous / non-spontaneous? | galvanic / spontaneous (delta G < 0) |
| What does deltaG =? | -n*F*E(cell) --> energy is proportional to both cell voltage and number of electrons involved |
| Galvanic cells _____ energy while electrolytic cells ______ energy. | release / store |
| Voltages add in series / parallel? | series |
| The more a reaction proceeds in the forward direction, the greater / smaller its voltage? | greater in a galvanic cell |
| To be a sacrificial metal, the metal must have a higher/lower oxidation potential than the protected metal? | higher oxidation potential |
| What defines an unfavorable reaction? | negative reduction potential |
| Electroplating results from the reduction of a _____ into a metal. | cation |
| What is a desirable property of an electroplating material? | cation has large positive reduction potential (not easily oxidized) |
| What is the oxidation number of pure elements? | 0 |
| What is the second more eN element? | oxygen |
| What is the oxidation number of oxygen in H2O2? | -1 |
| What are the money metals and how are they special? | Au, Pt, Ag, Hg, Cu, Ni --> negative oxidation potentials so don't rust or tarnish |
| Does a positive oxidation potential mean it is easier or harder to oxidize? | easier |
| Does a positive reduction potential mean it is easier or harder to reduce? | easier |
| More easily reduced = better _____ agents. | oxidizing |
| More easily oxidized = better _____ agents. | reduced |
| equation that relates free energy and emf? | delta G = -nFE |
| What does a change in Gibbs free energy represent in terms of work? | max. amount of nonPV work available from a reaction at constant temperature and pressure |
| What does a negative delta G indicate in terms of work? | works is done by system |
| Under standard conditions, the free energy of a spontaneous reaction is? | negative |
| At equilibrium, the free energy of any reaction is? | 0 |
| Positive potentials correspond to ____ reactions. Negative potentials correspond to _____ reactions. | spontaneous nonspontaneous |
| What are conditions fro standard potential? | concentrations are at 1M and voltage is 1.1V |
| At equilibrium, what is the value of the free energy and cell potential? | zero |
| At equilibrium, what is the relationship between Q and K? | Q = K |
| relate delta G to delta Go | deltaG = Go + RT ln(Q) |
| The log of a number <1 is positive /negative? | negative |
| In a concentration cell, where will the cation concentration be greatest? | cathode |
| Are electrolytic cells used to decompose a salt? Power an electrical device? | yes / no |
| What are galvanic cells used for? | voltage source and pH meter |
| What determines the emf? | temperature, concentrations, and reactions |
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