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AQA A2 Electrochem
Electrochemistry
| Question | Answer |
|---|---|
| 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
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