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AQA A2 Electrochem

Electrochemistry

QuestionAnswer
When E0 are given in a data table, how do you answer the question "predict the colour of the species at the end of the reaction" Order the E0; "SEP of couple1 is more positive than SEP couple2"; so (reactant) can get oxidised/reduced/gain/lose e-; and becomes (product); NOW CHECK IF THAT PRODUCT can get FURTHER oxidised/reduced
What is the Oxidation number of Sulfur in Sulfate ions? (SO4)-2; sulfur is +6
What is the oxidation number of Chromium in (Cr2O7)-2 ions? +7
How do you work out a whole equation, given the two half equations and the E0? Order the E0; the TOP/less positive/more negative one is flipped; multiply so that e- cancel each other; add the species on left; add species on right
How do you work out Ecell? Order the Eo; Ecell=E0(bottom)-E0(top)
Write the equation for the conversion of (SO4)-2 into SO2 apply the OHe principle: (SO4)-2  SO2 + 2H2O; (SO4)-2 + 4H+ --> SO2 + 2H2O; (SO4)-2 + 4H+ + 2e- SO2 + 2H2O
Define standard electrode potential teh electromotive force of a half-cell, compared with a standard hydrogen half cell, measured under standard conditions
When drawing a cell diagram, what should you remember? the salt bridge dips INSIDE the solutions; write the standard conditions on the diagram; write “ALL” solutions at 1mol.dm-3;check that the electrode is either the metal itself OR PLATINUM
When would you use a platinum electrode? in all cases: non-metal ion/non-metal element or two metals ions; all cases except where the couple contains a metallic element
When asked to draw the cell diagram to measure the standard electrode potential for Zn+2/Zn, what is the trick/trap? you have been given only ONE half-cell; the second half-cell must be the standard hydrogen half-cell
E0(Ag+/Ag)=+0.80V and E0(Cu+2/Cu)=+0.34V; which chemical is the reducing agent? order the couples: the reducing agent is the top right so here Cu/copper metal (actually a very poor reducing agent: a good one would have E0 very negative)
E0(Ag+/Ag)=+0.80V and E0(Cu+2/Cu)=+0.34V; which chemical is the oxidising agent? order the couples: the oxidising agent is the bottom left so here Ag+/silver ion
E0(Ag+/Ag)=+0.80V and E0(Cu+2/Cu)=+0.34V; what is the cell potential? bottom-top=(+0.80)-(+0.34)=+0.56V
E0(Ag+/Ag)=+0.80V and E0(Cu+2/Cu)=+0.34V; what is the half-cell that produces electrons? order couples; the half-cell that produces the e- is the “top” one/the one with the least positive/the one with the most negative E0
E0(Ag+/Ag)=+0.80V and E0(Cu+2/Cu)=+0.34V; write the equation for the feasible reaction order the couples; reverse the top equation; multiply the equation so that the e- balance; add all species on left; add on the right; simplify; here Cu + 2Ag+  Cu+2 + 2 Ag
Describe how to write the equation for a reaction given E0 of two couples order the couples; reverse the top equation; multiply the equation so that the e- balance; add all species on left; add on the right; simplify; here Cu + 2Ag+  Cu+2 + 2 Ag
When provided with more than one E0 for compounds of the same element (Vanadium), what is the trick/trap? remember that the product of the first reaction can react further!
An excess of Iron(II) was added to a solution containing V2+(aq) ions. Predict which vanadium species at the end of the reaction. V3+/V2+=-0.26V; VO^2+/V3+=+0.34V; Fe3+/Fe2+=+0.77V;VO2^+/VO^2+=+1.00V VO2^+
CHALLENGE: why would a reaction NOT take place, even if the E0 suggest it should? high activation energy; conditions are not standard
CHALLENGE: Predict the effect on the cell potential when increasing the concentration on Cu+2 + 2e-  Cu=+0.34V and Ag+/Ag=+0.80V to minimise the change, ie increasing [Cu+2], the system will try to decrease [Cu+2], the Cu+2/Cu equilibrium shifts right, less e- are produced so electrode potential will becomes more positive and cell potential is decreased
CHALLENGE: Predict the effect on the cell potential when increasing the concentration on Cu+2 + 2e-  Cu=+0.34V and Fe+2/Fe=-0.44V to minimise the change, ie increasing [Cu+2], the system will try to decrease [Cu+2], the Cu+2/Cu equilibrium shifts right, more e- are reacting so electrode potential will becomes more positive and cell potential will increase
CHALLENGE: explain why Copper does not react with diluted nitric acid but DOES react in concentrated (E0 Cu+2/Cu=+0.34; E0 H+/H2=0.00; E0 NO3-/NO2=+0.80) The Eo predict Cu can react with NO3-; but not feasible under standard cond (high Ea?); increased [NO3-] makes equilibrium NO3-/NO2 shift right; more e- are reacting; electrode pot more positive; E cell increases: reaction becomes more feasible
E0(Ag+/Ag)=+0.80V and E0(Cu+2/Cu)=+0.34V; what is the positive terminal? order couples; the negative terminal is the half-cell that produces the e- is the “top” one/the one with the least positive/the one with the most negative E0; the positive terminal is the other one: here Ag+/Ag
Name three rechargeable cells Ni-Cd or Lithium ions or a lead-acid battery
Define a fuel cell a cell that produces electrical energy but where the fuel and the oxidant need to be provided continuously
Write the (overall) equation in the hydrogen fuel cell H2 + 0.5 O2 --> H2O
Write the half-equation taking place at the negative terminal (hydrogen half cell) in the alkaline hydrogen fuel cell 2H2 + 4OH- --> 4H2O + 4e- (better learn by heart)
Write the half-equation taking place at the positive terminal (oxygen half cell) in the alkaline hydrogen fuel cell O2 + 2H2O + 4e- --> 4OH- (better learn by heart)
Describe how Hydrogen gas can be stored in cars Hydrogen gas will be liquefied AND stored under pressure. In an insulated tank, Hydrogen can then be adsorbed or absorbed.
What are the problems linked to Hydrogen fuel cells? Production of H2(g) still uses fossil fuels; risk of explosions; Storage and transport of H2(g) to pumps; storage of H2(g) in cars; ad-or ab-sorbers have a limited lifetime; high production costs of fuel cells; limited lifetime of fuel cells
What are the advantages of Hydrogen fuel cells? more efficient than internal combustion engine; no CO2/only water produced = reduction of greenhouse effect; no CO/toxic produced
What is the fuel in a hydrogen-rich fuel cell? Methanol or Ethanol
Describe how Methanol can replace Hydrogen in Hydrogen-rich fuel cells As a fuel or to produce Hydrogen
What is the salt bridge soaked in? saturated solution of Potassium Nitrate
In a lead acid battery, the following reactions take place: Pb + (SO4)-2  PbSO4 + 2e- and PbO2 + 4H+ + 2(SO4)-2 + 2e-  PbSO4 + 2H2O. Write the equation for the discharge reaction Pb + PbO2 + 4H+ + 2(SO4)-2 -->2 PbSO4 + 2 H2O
In a lead acid battery, the following reactions take place: Pb + (SO4)-2  PbSO4 + 2e- and PbO2 + 4H+ + 2(SO4)-2 + 2e-  PbSO4 + 2H2O. Write the equation for the reaction taking place during recharging 2 PbSO4 + 2 H2O --> Pb + PbO2 + 4H+ + 2(SO4)-2
In a Lithium ion battery, E0(Li+/Li)=-3V and E0(Li+ + CoO2 + e-  LiCoO2)=+1V. Write the equation for the reaction taking place during recharging LiCoO2--> Li + CoO2
Created by: UrsulineChem