ELECTRODE AND POTENTIAL Flashcards
how many half cells does 1 cell contain
2 half cells connected by a salt bridge
what does a half cell contain
a metal and a salvation of the compound that contains the metal
what does the half cell do
produce a small voltage if connected to a circuit
how is a voltage produced
it will form when it is connected together and the zinc half cell will have the tendency to oxidise to the zinc ion and release electrons.
how is the potential difference produced
more electrons will build up in zinc compared to copper, the potential difference will be created between them.
how is potential difference measured
the zinc strip is negative and copper strip is positive, the potential difference will be measured by a high resistance voltmeter
why do we need a high resistance voltmeter
to stop the current from flowing so reaction can not occur
why is a salt bridge used
to connect the circuit so free moving ions can conduct the charge
how is a salt bridge made
made from filter paper soaked in a salt solution - potassium nitrate the salt needs to be un reactive with the electrode and electrode solution
what happens if the current mistakenly flows
the voltmeter should be removed and replaced by a bulb, the reaction will occur separately at each electrode and voltage will fall to 0 as the reactants will be used up
what doe the solid vertical line in a convectional cell represent
represents a boundary between the phases of the solution
what does the double line represent
the salt bridge between the two half cells
what is written on the right side of the representation
the more positive half cell
what happens is we doe not have a metal
we use a potassium electrode because it has a conducting surface for electron transfer
why do we use platinum
it is un reactive and can conduct energy
define standard hydrogen electrode
comparison electrode which all other potentials are measured against, it is assigned the potential of 0
what are the conditions of the standard hydrogen electrode
hydrogen gas has pressure of 100kpa, solution has hydrogen ion at 1.0 mol dm, temperature at 298 K, no current flowing, platinum electrode
which e cell will oxidise and which will reduce
the more positive will have an increasing tendency for species to reduce and act as oxidising agent, the more negative has an increasing tendency for species to oxidise and act as a reducing agent
how to work out e cell
e cell= e reduction - e oxidation
what is e cell a measure of
how far the from the equilibrium does the cell reaction lie, the more positive the e c ell the more likely the reaction will occur because the current can flow for reaction to occur and e cell will fall to 0 while reaction occurs
le chateliers principle on concentration
increasing the concentration of reactants will increase the e cell therefore decreasing the concentration if the reactants will decrease the e cell
le chateliers principle on temperature
most cells reactions will be exothermic in the spontaneous direction so a temperature rise will lead to a decrease in the e cell because the equilibrium will shift back to oppose the increase
what happens if the e cell is positive
it indicates that the reaction will occur, this may be slow or not as effective. this is dependent on the activation energy, a high activation energy means a reaction will not occur.
why can electrochemical cells be used for
as commercial source of electrical energy
what are non rechargeable cells
not reversible e.g. lithium manganese dioxide
what are fuel cells
use energy from reactions of a fuel with oxygen to create a voltage
how can fuel cells maintain a constant voltage over time
they are constantly fed with fresh oxygen and hydrogen so maintain. constant acceleration of reactants, this is different to ordinary cells where the voltage drops over time as the reactant concentration will drop
why do we use a higher temperature is cells
at standard conditions the rate is too low to produce an appreciable current so the higher temperature is therefore used to increase the rate, however the reaction will be exothermic so using le chateliears principle e cell would fall so higher pressure would counteract this
what are advantages of fuel cell
less pollution and less CO2 because pure hydrogen will melt only water whilst hydrogen rich fuels produce only small amount of air pollutants and co2, it also has greater efficiency.
what are some limitations to fuel cells
it is expensive to produce, storing the hydrogen is hard to keep safe and feasible, it is a pressured liquid and has limited life cycle of a solid absorber, due to limited lifetime it will require regular placements and disposal to have a Hugh production cost, uses toxic chemicals in its productions
how is hydrogen stored in fuel cells
as a liquid under pressure, adsorbed on the surface of solid materials, absorbed with a solid material
how is hydrogen accessed
it is available by the electrolysis of water but this is expensive process to be a green fuel the electricity it will need to be produced by renewable resources
what are some advantages of ethanol
ethanol fuel cells are made by renewable sources in a carbon neutral way, renewable materials to produce ethanol by fermentation are abundant, ethanol is less explosive and easier to store than hydrogen.
