Electrodes Flashcards
what is the standard electrode potential of a half cell
- this is the e.m.f of a half cell compared with a standard hydrogen half cell, measured at 298K with solution conconcentrations of 1moldm-3 and a gas pressure of 100kpa
describe half cells
- a half cell comprises of an element in two oxdiation states, the simplest half cell has a metal placed in an aqueous solution of its ions
For example
a copper half cell consists of a solution containing Cu2+ ions (oxidation state +2) into which a strip or rod of copper metal (oxdation state 0) is placed
an equilibrium exsits at the surface of the copper between these oxidation states of copper
Cu2+ + 2e- = Cu
- the forward reaction involves electron gain
- the reverse reaction involves electron loss
- the solid metal acts as an electrode when the half cell is connected to another half cell to form a cell
describe bromine half cell
- half cells can also be made from non metals in equilibrium with non metal ions
- Br2 + 2e- = 2Br-
describe the hydrogen half cell
- 2H+ + 2e- = H2
- if hydrogen half cell were to be connected to another half cell to form a cell there is no solid piece of metal that can act as the electrode this is overcome by the use of platinum electrode
- platinum is inert and does not react at all - purpose is to be in contact with both the H2 and H+ ions and to allow the transfer of electrons into and out of the half cell via a connecting wire
- surface of the platinum electrode is coated with platinum back which is a spongy surface in which the electrons can be transferred between the non metal and its ions
- HCl can be used as a source of H+
describe metal ion/metal ion half cells
- this type of half cell contains ions of the same element in different oxidation states for example a half cell can contain Fe3+ and Fe2+ ions
Fe3+ + e- = Fe2+ - this is a type of half cell that would need to involve a platinum electrode as there is no solid piece of metal that could act as an electrode
a standard Fe3+/Fe2+ half cell is made up of
- a solution containing Fe2+ and Fe3+ ions with teh same concentration
- an inert platinum electrode to allow electrons to pass into or out of the half cell via a connecting wire
different half cells have….
different electrode potential, when two half cells are connected together to form a cell they have an overall cell potential
what is an overall cell potential
- this is a measure of how well electrons can be pushed around the cell, the larger the overall cell potential the more electrons are pushed around
- actual value of the overall cell potential will depend on the electrode potential of the half cells involved
how can we determine the standard electrode potential of a half cell
- determine the standard electrode potential by connecting it to a hydrogen half cell
- ## the tendency for different half cells to accept or release electrons is measured as an electromotive force e.m.f or voltage, the hydrogen half cell has an e.m.f value of 0V so can be used as a reference
whats the wire used for in measuring the standard electrode potential
- it allows the electrons carrying charge to flow through it
what is the purpose of a salt bridge
- it connects the two solutions and allows ions carrying the charge to be transferred between the half cells, made from a piece of filter paper soaked in aqueous solution of an ionic substances usually KNO3
what is the electrochemical series
- this is when we can list half cells in order of their standard electrode,
if a electrode potential has a negative value then ..
the backward reaction will occur when compared with the standard hydrogen electrode
if an electrode potential has a positive value then…
the forward reaction will occur when compared to the standard hydrogen electrode
the more negative the E value
- the greater tendency toward the half cell undergoing oxidation when connected in the cell
the more positive the E value
- the greater tendency towards the half cell undergoing reduction when connected in a cell
what are the E values used for
- they can be used to predict what will happen when half cells are connected together to form a cell
what does a standard hydrogen half cell consist off
- 1 moldm-3 of Hcl as the source of H+ ions
- hydrogen gas at 100 Kpa
- an inert platinum electrode to allow electrons to pass into or out of the half cell via a connecting wire
how do you make cells from half cells
- made by connecting together two half cells with different electrode potentials:
- one half cell releases electrons
- other half cell gains electrons
- difference in electrode potential is measured with a voltmeter
- joined by a wire and salt bridge this allows the charge to be carried between each cell via the electrons and ions
describe the copper-zinc cell
- Zn2+(aq) + 2e- = Zn(s) E= -0.76V
- Cu2+ + 2e- = Cu E= +0.34V
- E is more negative for Zn therefore Zn is more likely to undergo oxidation and lose electrons as the half cell has the more negative value therefore it has a greater tendency towards the equilibrium shifting left
- Zn2+/Zn = equilibrium releases electrons into the wire making zinc the negative electrode, looses electrons and shift left
- electrons flow along the wire to the Cu electrode of the Cu2+/Cu half cell, gains electrons and moves right
what does the reading on the voltmeter tell us
- the reading on the voltmeter measures the potential difference of the cell, this is the difference between the electrode potentials of the half cell
- the bigger the value the further away from the equilibrium position the reaction moves
- ## the reading on the voltmeter can be taken as the cell potential as long as any ions of the same element have a concentration of 1 moldm-3 or are equimolar
how to calculate the standard cell potential
E cell = positive terminal - negative terminal
what is the cell reaction
- it is the sum of the overall chemical reaction taking place in the cell - the sum of the reduction and oxidation half reactions taking place in each of the half cell
how do we use standard cell potentials to predict the feasibility of reactions
- by calculating the cell potential for a reaction, using the standard electrode potential for each half cell we can determine whether electrons are likely to flow and hence the feasibility of any reaction
- in the electrochemcial series the species will react with teh species below it and to the left of the equation
describe storage cells
- non rechargeable cells - provide electrical energy until the chemicals have reacted to such an extent that the voltage falls, the cell is then flat and discarded
- rechargeable cells - the chemicals in the cell react providing electrical energy, the cell reaction can be reversed during recharging, the chemicals in the cell are regenerated and the cell can be used again - common examples include nickel and cadmium batteries, lithium ion and lithium polymer batteries used in laptops
- fuel cells - the cell reaction uses external supplies of a fuel and an oxidant which are consumed and need to be continuously supplied, the cell will continue to provide electrical energy so long as there is a supply of fuel and oxidant
risk and benefits of fuel cells
- provide high levels of battery life
- less pollution
- more efficient at producing energy than conventional combustion as less energy is wasted as heat
- toxic when eaten
- rapid discharge of current which can cause fires and explosions
- have some controls on batteries including restrictions on the transport of lithium based batteries and limited sales of batteries to individual consumers in some countries
describe fuel cells
- modern fuel cells are based on hydrogen or hydrogen rich fuels such as methanol
- fuel cell uses energy from the reaction of a fuel with oxygen to create a voltage
how they work - reactant flow in and products flow out while the electrolyte remains in the cell
- fuel cells can operate virtually continuously so long as the fuel and oxygen continue to flow into the cell, fuel cells do not have to be recharged
name some fuel cells
- hydrogen oxygen
- methanol
- natural gas
- petrol
what are the limitations of predictions of feasibility from cell potentials
electrode potentials and concentration
- non standard conditions alter the value of an electrode potential,
- Cu2+ + 2e- = Cu
- the equilibrium opposes the change by moving to the right
- electrons are removed from the equilibrium
- the electrode potential becomes less negative or more positive
- a change in electrode potential results from concentration changes means that predictions are made on the basis of the standard value may not be valid
will a reaction actually take place
- predictions can be made about the equilibrium position but not about the reaction rate which may be extremely slow because of a high activation energy
- actual conditions used for a reaction may be different from the standard conditions used to measure E values, this will affect the value of the electrode potential
- standard electrode potentials apply to aqueous equilibria - many reactions take place under very different conditions
general rule
- the larger the difference between E values the more likely that a reaction will take place
- if the difference between E values is less than 0.4V then a reaction is unlikely to take place
The more reactive a metal is ….
the more it wants to loose electrons to form a positive ion
- more reactive metals have a more negative standard electrode potentials
the more reactive a non metal is …
the more it wants to gain electrons to form a negative ion, they have more positive standard electrode potential
what happens when the conditions are not standard
- changing the concentration or temperature of a solution can cause the electrode potential to change
- for example if you increase the concentration of ZN2+ the equilibrium will move to the left reducing the ease of electron loss and the electrode potential of Zn/Zn2+ becomes less negative and the whole cell potential will be lower
- if you increase the concentration of Cu2+ the equilibrium will move right increasing ease of electron gain, the electrode potential of Cu2+/Cu becomes more positive and the whole cell potential will be higher
how does a fuel cell work
- at the anode the platinum catalyst splits the H2 into protons and electrons
2. the polymer electrolyte membrane only allows H+ across and this forces the e- to travel around the circuit to get to the cathode
3. an electric current is created in the circuit which is used to power soemthing like a car
4. at the cathode oxygen combines with the H+ from the anode and the e- from the circuit to make H20 which is the only waste product
