Electrochemistry

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AQA A Level Chemistry

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27 Terms

1
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salt bridge

filter paper soaked in/ inert tube filled with an ionic solution (usually KNO3) that has one end in cathode solution and one in anode solution

2
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function of salt bridge

to maintain electrical neutrality within the cell by providing ions to either side which completes the circuit by allowing ions to pass through it

3
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electrode potential

a measure of the ability of a metal to lose electrons

4
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Standard Hydrogen Electrode (SHE)

baseline used to compare electrode potential on half cells to when they are connected to it

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negative voltmeter value when connected to SHE

other metal is a better reducing agent (is being oxidised)

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positive voltmeter value when connected to SHE

SHE is a better reducing agent

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describe SHE

hydrogen gas is passed down to a platinum electrode in a solution of 1 mol dm-3 H+ at 298K and 100kPa

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role of voltmeter in a cell

measures the difference in electron build up either side of it

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conventional representation of cells

reduced species on the right, vertical line for phase boundary, double line for salt bridge, most oxidised species closest to salt bridge

10
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Evalues

show equilibria when a half cell is connected to SHE, if value is positive the ions are being reduced and if negative they’re being oxidised

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E cell

overall electromotive force of a whole cell, found by finding difference between Eof both half cells, if positive the reaction is feasible

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Ecell equation

Ecell = Eright(reduction) - Eleft(oxidation)

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why platinum is used as an electrode

it is inert and conducts electricity and acts as a source of electrons

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more positive E cell value

more reduced species, on right side

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more negative E cell value

more oxidised species, left side

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lithium cell positive electrode half equation

Li+ + CoO2 + e- → Li+[CoO2]-

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lithium cell negative electrode half equation

Li → Li+ + e-

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alkaline hydrogen cell positive electrode half equation

O2 + 2H2O + 4e- → 4OH-

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alkaline hydrogen cell negative electrode half equation

2H2 + 4OH- → 4H2O + 4e-

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hydrogen cell overall equation

O2 + 2H2 → 2H2O

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electrochemical series

the list of half cell reactions and their electrode potentials in numerical order

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half cell

one half of an electrochemical cell, constructed of a metal dipped in its ions or a platinum electrode with 2 aqueous ions

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advantages of hydrogen oxygen fuel cells

more efficient (less energy lost as heat), don’t need to be recharged, only waste product is water, no carbon dioxide produced

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disadvantages of hydrogen oxygen fuel cells

hydrogen is highly flammable, expensive to transport and store hydrogen, energy is required to make hydrogen and oxygen

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what happens when voltmeter in a cell is replaced by a bulb

EMF decreases to 0V because the concentration of ions in each solution becomes equal

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why an EMF value for a commercial cell is different to the theoretical value

non-standard conditions

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change made to a cell to allow the cell reaction to go to completion

replace the voltmeter with a lamp/ wire