Electrode potentials and cells PMT

redox reaction?

an oxidation reaction paired with a reduction reaction

reducing agent?

electron donor

oxidising agent?

electron acceptor

displacement reaction?

redox reaction where one element (stronger reducing) takes the place of another (weaker oxidising)

why can zinc displace copper but copper cant displace zinc?

zinc is a stronger reducing agent than copper, so displaces copper;

copper is a stronger oxidising agent

single displacement?

one element is displaced

double displacement?

two elements are displaced

are single displacements redox?

all single displacements are redox

are double displacements redox?

never redox

where are the strongest reducing agents in the electrochemical series?

at the top

where are the strongest oxidising agents in the electrochemcial series?

at the bottom

what will each metal act as a reducing agent for?

anything below it in the electrochemical series, elements with weaker reducing power

ions of each metal will act as an oxidising agent for?

elements above it in the series/weaker oxidising power

what makes redox reactions stronger?

elements that are further apart in the series = release more energy

what causes release of energy in redox?

transfer of electrons

what is a half cell?

a single electrode/metal dipped in a solution of its ions

what is an electrochemical cell?

Two half cells connected together using a salt bridge, wires and a voltmeter

what is a salt bridge?

liquid junction of unreactive ions that can moce between the solutions to carry flow of ions but will not interfere with the reactions

function of salt bridge?

completes the circuit, allows transfer of ions

battery?

two or more cells connected together

how can half cells reach equilibrium in a closed system?

reversible reactions

compare reducing agents equilibrium to oxidising agents equilibrium?

left of the oxidising agents, stronger = more to left

in which half cell are there more delocalised electrons?

more in the metal that acts as the reducing agent, because equilibrium lies to the left

which way do electrons flow?

away from reducing agent (more -vely charged), towards oxidising agent (more +vely charged)

what reaction occurs at the anode (+ve)?

oxidation

charge of anode?

Positive (attract anions)

reaction at cathode (-ve)?

reduction

charge of cathode?

Negative (attract cations)

which way do cations (+ve) flow in a cell?

towards the cathode/negative electrode

which way do anions (-ve) flow in a cell?

towards anode

how is the equilibrium in the reducing agent half cell disrupted?

• conc. of e- decreases as it goes to oxidising

• shifts to left to oppose change

• more electrons flow through the wire, to oxidising

how is the equilibrium in the oxidising agent half cell disrupted?

• conc. of e- increases

• equilibrium shifts to right to oppose change

how to measure reducing strength of a cell?

add a voltmeter -> potential difference shows reducing strength

how does resistance affect voltage/p.d?

more resistance = lower voltage

electromotive force, E?

maximum voltage possible between two electrodes, not affected by resistance

potential difference?

actual votage measured between two electrodes, is affected by resistance

conventional cell representation rules?

• most negative half cell goes on the left

• most oxidised species from each half cell goes next to salt bridge

• salt bridge = double line ||

• includes state symbols

• reduced, oxidised || oxidised, reduced

draw standard hydrogen electrode/half cell

what is the SHE used for?

measuring standard for hald-cell potentials, has a cell potential of 0V, measured under standard conditions

standard conditions used for electrochemical cells?

• solutions of 1.0moldm-3 concentration

• 298K

• 100kPa

if the hydrogen electrode is connected to a stronger reducing agent, which reaction happens?

hydrogen ions pick up e- from Pt and are reduced to H2 (g)

2H+ (aq) + 2e- ⇌ H2 (g)

if the hydrogen electrode is connected to a weaker reducing agent (stronger oxidising agent), which reaction happens?

H2(g) oxidised to hydrogen ions and e- flow to oxidising electrode

hydrogen electrode consists of?

strip of platinum dipped in a solution of hydrogen ions (H+, from HCl) bubbled through hydrogen gas

pros of using platinum in SHE?

metallic, so will conduct electricity, but are also inert so will not interfere with the reaction

how to calculate a half cell potential?

measure under standard conditions and compare to the SHE, which is 0V

what does a negative potential mean in terms of oxidation and reduction?

substances are more easily oxidised and will lose electrons

what does a positive potential mean in terms of oxidation and reduction?

substances are more easily reduced and will gain electrons to become more stable

cell EMF calculation?

what EMF value would you get for a favourable, spontaneous reaction?

positive value, more positive = more favourable

anticlockwise rule to combine half cell reactions?

• write the most negative EMF on top

• draw anticlockwise arrows around reactions

• balance electrons on both sides

• write out the cell reaction

are positive electrode potential species better oxidisng or reducing agents?

oxidising agents, so will oxidise species more negative than it

are negative electrode potential species better oxidisng or reducing agents?

reducing, so will reduce those less negative than it

effect of increasing the concentration of solutions used in the electrochemical cell on cell EMF? why?

makes the cell EMF more positive as fewer electrons are produced in the reaction

effect of increasing pressure of the cell on EMF?

more negative, as more electrons are produced (equilibrium determined from half equations x)

3 types of uses for electrochemical cells?

produce non-rechargeable, rechargeable or fuel cells (commercial cells for energy)

reactions that take place within a rechargeble cell?

reversible reaction, meaning reactants can reform

common cells used as rechargeable batteries in phones, laptops and cars?

lithium ion cells

what do lithium ion cells/batteries consist of?

lithium cobalt anode and graphite (carbon) cathode, an electrolyte of a lithium salt in an organic solvent to carry the flow of charge

half equation for the negative electrode in a lithium ion cell?

half equation for the positive electrode in a lithium ion cell?

full half equation for a lithium ion cell?

Li + Li+ + CoO2

in order to be recharged, what needs to be applied?

a current over the cell, which forces the electrons to move in the opposite direction, causing reaction to reverse = recharging

why cant non-rechargeable cells recharge?

reactions used arent reversible