Unit 5 - redox equilibria

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Flashcards on Unit 5 - redox equilibria, created by jasmin.sahota on 06/15/2013.

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Question Answer
a reducing agent is a species that gives away electrons
an oxidising agent is a species that accepts electrons
oxidation states are sometimes called oxidation numbers
uncombined elements have oxidation states that = 0
the oxidation states of group 1 are +1
group 2 elements have an oxidation state of +2
aluminium +3
Fluorine have an oxidation state of -1
hydrogen has an oxidation state of +1 unless a metal hydride like NaH where it is -1
oxygen has an oxidation state of -2 except in peroxides such as h2o2 or fluorine where it is -1
the oxidation number of Cl is -1 except when with F and O, where it has positive values
the sum of all the oxidation states in a compound = 0, since all compounds are neutral
the sum of the oxidation states of a complex ion = the charge on the ion
complex ions are ions which have two or more atoms bonded convalently, but where the whole group of atoms has a charge
the symbol for sulphate (IV) ions is (SO3)-2
the symbol for sulphate(VI) ion is (SO4)2-
to check if something is balance, look 1. to see if there is the same number of atoms each side 2. the charges balance out
what are half cells? when a rod of metal is sipped into a solution of it's own ions e.g. zinc into zinc sulphate
the equation of zinc in sulfate solution born_haber_cycle_for_NaCl.png (image/png)
if the metal gains a negative charge we say it gains a negative electrical potential
if we measure the potential, it would tell us how readily electrons are released by the metal so basically how good a reducing agent the metal is
how potential difference (V) cannot be measured directly so what do we do? we connect together two different electrodes and measure the potential difference between them using a voltmeter.
what is a salt bridge? a piece of filter paper soaked in solution of a salt (usually saturated potassium nitrate)
why do we have a salt bridge? it is used rather than a wire to connect the circuit, to avoid further metal potentials in the cicuit
draw the setup of a zinc electrode and a copper electrode together born_haber_cycle_for_NaCl.png (image/png)
is one electrode is more negative than the other then it tells us that that metal loses its electrons more readily than the other metal. it is a better reducing agent
electrons flow from negative to positive
the left electrode is always the negative one
we can do half equations at each electrode, to show the redox equations
if we want to compare the tendency of different metals to release electrons we must have a standard electrode, called the standard hydrogen electrode
describe the standard hydrogen electrode 1.bubble hydrogen gas into a solution of H+(aq) ions 2. as hydrogen doesn't conduct we use platinum 3.
platinum is used because it is very unreactive. it is coated with finely divided platinum to increase the surface area
the potential of the standard hydrogen electrode is defined as zero
is the standard hydrogen electrode is connected to another electrode, the measured voltage, called the electromotive force (emf) is... the electrode potential of that cell
draw the diagram for the standard hydrogen electrode born_haber_cycle_for_NaCl.png (image/png)
electrodes with a negative emf are better at releasing electrons and therefore better reducing agents than hydrogen
changing the conditions, such as concentrations of ions, or temperature of an electrode... changes the electrical potential
a list of emf values for metal/metal ion standard electrodes is given in a table called the electrochemical series
to get the overall emf between two metal you do the calculation the largest negative value - the smaller value
cell representation = shorthand for writing down the cell formed by connecting two electrodes
a vertical line in cell representation indicates a phase boundary e.g. between a solid and a solution
a double vertical line in cell representation is the salt bridge
this shows what happens at the two electrodes when they're not connected Zn2+(aq) + 2e- <--> Zn(s) emf = -0.76v Cu2+(aq) + 2e- <--> Cu(s) emf = +0.34v what happens to the half equations when they are connected? what is the overall equation and state whether it's feasible. then show reverse of the equation and state if it's feasible electrons will tend to flow from the negative(zinc) to the positive(copper) so half equations become Zn(s) --> Zn2+(aq) +2e- Cu2+(aq)v+ 2e- --> Cu(s) therefore the overall equation is Cu2+(aq) + Zn(s) --> Zn+2(aq) + Cu(s) this is feasible the reverse of this equation Cu(s) + Zn+2(aq) --> Zn(s) + Cu2+(aq) is not feasible
what does NO P.R stand for? the more Negative the electrode potential will go in the Oxidation direction, and the more Positive the electrode potential will go in the Reduction direction
the half-equations in the electrochemical series are always written as reduction equations
when two half equations are put together, which ones goes in the direction of oxidation the one with the more negative emf goes in the oxidation direction, so the half equation is backwards from the one shown in the electrochemical series
when two half equation are put together which ones goes in the direction of reduction? the one with the more positive emf, and therefore goes in the same direction as the equation from the electrochemical series
the emf of a cell = the emf(RHS) - the emf(LHS)
the emf of the standard hydrogen electrode = 0.00V
when measuring the electrode potentials using a standard hydrogen electrode, it is important to do it under it's standard states which are: 1. any solutions of ions have a conc of 1.00moldm-3 2. the temperature must be 298K (25C) 3. the pressure must be 100kPa
in a standard hydrogen cell, hydrogen gas is bubbled into a solution of aqueous H+ions
what type of electrode is used in a standard hydrogen electrode platinum
electrons flow through the wire in a cell, from the most reactive metal to the least
platinum is used because it conducts electricity, it is inert
how easily a metal oxidises depends on it's electrode potential a metal that is easily oxidised has a very negative electrode potential, while one that's harder to oxidise has a less negative electrode potential
in a cell representation, the reduced species goes on the edge of the diagram, followed by the oxidised species in the middle
the more reactive metal is the one that loses it's electrons more easily, and therefore has the more negative electrode potential
the more reactive the non-metal is the one that wants to gain electrons to form a negative ion, and therefore has a more positive electrode potential.
when predicting the outcome of reactions, what are the steps? 1. write two half equations as reduction reactions 2. put the one with the more negative electrode potential above the other on 3. draw two anticlockwise arrows 4. one arrow from the products of top equation to the reactant of the top equation 5. one arrow going from the reactants of the bottom equation to the products of the bottom equation 6. the substances at the end of the arrows (non-pointy end) are what you mix together 7. the substances at the pointy ends are what you end up with
two types of non-rechargeable batteries are zinc/copper cells zinc/carbon cells
Non -rechargeable batteries use what type of reaction irreversible reactions
Non -rechargeable batteries are used in gadgets that don't use a lot of power, and are only used for short periods of time
the negative electrode in a zinc-carbon dry cell is the zinc casing
the positive electrode in the zinc-carbon cell is carbon, which acts like the inert platinum electrode in the hydrogen electode this is surrounded by a mixture of maganese oxide and powdered carbon
the electrolyte in the zinc-carbon cell is a paste of ammonium chloride, not a liquid
the half-equations for the zinc-carbon cell are Untitled.png (image/png)
draw the diagram of the zinc-carbon cell Untitled.png (image/png)
half-equations have non-reversible arrows because it is not practical to reverse them in a battery
half equations in non-rechargeable batteries can be made reversible under the right conditions but... trying to do this is a battery can make it leak or explode. this is because the zinc electrode forms the casing of the battery, so becomes thinner as the zinc is oxidised.
another reason why the zinc-carbon electrode cannot be recharged is because the ammonium ions would produce hydrogen gas, which would escape from the battery. without the hydrogen, the ammonium ions couldn't be reformed by reversing the reactions
rechargeable batteries are recharged by reversing the cell reactions
A type of rechargeable battery is a lead-acid battery
rechargeable batteries are done by applying an external voltage to drive the electrons in the opposite direction
lead acid batteries are used in car batteries
lead acid batteries consist of six 2v cells connected in series to give 12v
each lead acid cells consists of two plates dipped into a solution of sulfuric acid
the positive plate in a lead acid batter is made up of lead coated with lead(IV) oxide, PbO2
the negative plate in a lead acid battery is made up of lead
when discharging in a lead acid battery, which direction does the electrons move from the lead plate to the lead(IV) oxide coated plate
show the equation for the reaction that occurs at the lead plate born_haber_cycle_for_NaCl.png (image/png)
show the equation that occurs at the lead-dioxide coated plate born_haber_cycle_for_NaCl.png (image/png)
portable batteries are also rechargeable batteries
there are two types of disposable batteries nickel and cadmium and lithium ion
which battery is more expensive, zinc-carbon one, or nickel/cadium one? the nickel cadium one is more expensive, however it charges up to 500 times, reducing the effective cost significantly
to recharge the nickel/cadium and the lithium ion batteries, what do we need to do? a current is supplied to force the electrons to flow in the opposite direction around the circuit and reverse the reactions.
why is it possible to recharge these batteries because none of the substances can escape or are used up
the pros of a non-chargeable battery are.. 1. cheaper than rechargeable batters 2. work for longer than rechargeable batteries 3. can be recycled 4. less likely to contain toxic metals lead and cadium, so less hazardous in landfill sites, if the contents leaks out
the cons of non-rechargeable batteries is... 1. have to be replaced every time they run out, therefore more expensive in the long run 2. cannot supply as much energy as rechargeable ones, so not devices that use a lot of power
what is an electrolyte is a substance that contains free ions and can conduct electricity
the alternative to burning hydrocarbons for fuel is.. the fuel cell, which uses oxygen and hydrogen as fuels to produce electricity to drive an electric motor
a fuel cell consists of two platinum-based electrodes separated by a special polymer electrolyte which allows ions to pass through it.
the half equations for a fuel cell are born_haber_cycle_for_NaCl.png (image/png)
the diagram for a fuel cell is born_haber_cycle_for_NaCl.png (image/png)
the only emission from a fuel cell is water
although hydrogen seems green it isn't as most hydrogen comes from crude oil, or it can be produced by electrolysis where the electricity for that is made by burning fossil fuels