electrochemical exam questions bank

Li(s) | Li+(aq) || Li+(aq) | MnO2(s) , LiMnO2(s) | Pt(s) Li+(aq) + e- → Li(s) -3.04 write a half-equation for the reaction that occurs at the positive electrode of this cell. Calculate the standard electrode potential of this positive electrode emf= 2.91 - answer-Li+ + Mn02 + e- --> LiMn02(s) 2.91 = (x) - (-3.04) even though oxidation, don't change sign explain the function of salt bridge? - answer-ions in ionic substance in salt bridge move through salt bridge to maintain charge + balance use data to justify why S042- ions should not be capable of oxidising Br- ions? - answer-E* for S042- < what are the 2 ways to use hydrogen as fuel for cars. what is the major advantage of using fuel cell? - answer-one way in fuel cell to power an electric motor or as fuel in an internal combustion engine internal combustion engine wastes > energy aluminium used as electrode is rubbed with sand paper before use, explain why? - answer-To remove oxide layer on aluminium what would you draw for diagram of a right hand standard hydrogen electrode with no salt bridge? - answer-H2 gas given below the electrode Pt electrode how can a simple salt bridge be prepared? In aluminium and hydrogen fuel cell why would this be unsuitable? - answer-dipping filter paper into KC032 C032- ions react with acid (H+) in standard hydrogen electrode or with Al3+ conc of H+ / Al3+ change / EMF identify the weakest reducing agent from species in table? explain how you deduced your answer? - answer-(least oxidising --> so look on RHS) (reducing = LFH) Species on RHS has largest electrode potential + E* can normal cell reactions be recharged and why? - answer-no cell reactions not reversed (Zn oxidised to Zn0) Zn0 not be reduced to Zn by passing current through explain how salt bridge D provides an electrical connection between the 2 electrodes? suggest why KCl not suitable salt for salt bridge in this cell? - answer-ions able to move through Cl- ions react with Cu2+ ions In cell, 2 Cu electrodes, same solution of CuS042- . left hand side is more concentrated with H2S04. in external circuit, of cells, e- flow through ammeter from right to left suggest why e- move in this direction? - answer-CuS04 in LFH electrode , > concentrated reduction at LFH electrode oxidation at RHS electrode Explain why the current in the external circuit of this cell (*2 = Cu electrode with CuS04 diff concentration) falls to zero after cell operated for some time? - answer-eventually CuS04 in each electrode will be at same concentration suggest why recharging of lithium cell may lead to release of C02 into atmosphere? - answer-electricity for recharging cell may come from burning fossil fuels Deduce half-equations for electrode reactions in hydrogen-oxygen fuel cell? overall equation? - answer-positive electrode: H2 + 2OH- → 2H2O + 2e- negative electrode: O2 + 4e- + 2H2O → 4OH- 2H2 + 02 ---> 2H20 Use half-equations to explain how electric current can be generated? - answer-hydrogen electrode produces e- e- flow to oxygen electrode oxygen electrode accepts e- Explain why fuel cell not need to be recharged? - answer-hydrogen / reactants supplied continuously provide energy for vehicle, hydrogen used either in fuel cell or in internal combustion engine. Suggest main advantage of using hydrogen in fuel cell rather than in internal combustion engine? - answer-in fuel cell > energy from hydrogen-oxygen reaction converted to useful energy less energy wasted Identify one major hazard associated with the use of a hydrogen-oxygen fuel cell in a vehicle? - answer-hydrogen is flammable / explosive Solar cells generate an electric current from sunlight. cells are often use to provide electrical energy for illuminated road signs. Explain why rechargeable cells are connected to these solar cells? - answer-solar cells not supply electrical energy all time need sunlight rechargeable cells store electrical energy for use when cells not working Suggest one reason why many waste disposal centres contain a separate section for cells