5.2.3 Redox and electrode potential

what is redox reaction

reaction where both reduction and oxidation happens

what is the oxidation number of Hydrogen

+1 ( unless when bonded to a metal)

what is the oxidation number of Oxygen

-2 ( unless H₂O₂)

what is oxidation

losing e⁻

what is reduction

gaining e⁻

what does OILRIG stand for

what is it when a ion becomes more positive

Oxidation ( losing electrons )

what is it when a ion becomes less positive & negative

reduction ( gains electrons)

when balancing half equations where do e⁻ go when element is reduced

the + e⁻ goes to the REACTANTS side when oxidation number gains e⁻

( Reduced Reactants )

when balancing half equations where do e⁻ go when element/ion is oxidised

the + e⁻ goes to the PRODUCTS when oxidation number loses electrons

method to balancing half equations

Work out the oxidation number /state for element/ion

balance the elements

work out if electrons were lost or gained

add to reactants side ( gained) or product side ( lost)

balance O₂ with H₂O

balance hydrogens with H⁺

method to combine half equations

work out the half equation

balance the e⁻ ( same on each equation) / whatever you do to e⁻ do to equation

cancel out the same species on both equations ( e⁻ / H₂O)

add the reactants then add the products

what are the conditions for metal ion cells

25°C

101 kpa

1 mol dm⁻³

what factors does electrode potential depend on

Temperature

Pressure

concentration

in the metal/metal ion cells what is the function of the salt-bridge 

regulates neutrality 

metal/metal ion cells how do e⁻ and ions carry charge ?

e⁻ → charge carriers through the wire

ions → charge carriers through the solution/ salt bridge

when working out P.D which cell is always oxidised 

the more negative E⁰ is always oxidised

when working out P.D which cell is always reduced

the more positive E⁰ is always reduced

why is the redox system oxidised 

 in a more negative reaction the equilibria moves to the left 

why is the redox system reduced

the more positive reaction = equilibria moves to the right 

In a P.D reversible reaction what direction does the more E⁰ negative go

goes left ( oxidation)

how to work out the E⁰ cell

E⁰ ( Positive) - E⁰ ( Negative)

Standard electrode potential defintion

the difference in potential difference between a standard electrode potential and a standard hydrogen half-cell.

what is the best oxidising agent in half-cell equations 

the half-cell equation with the most positive E⁰/V

Metal/metal ion cell ( Image)

Why may results be different in a E° reaction

Not standard conditions

Rate of reaction too slow

Ea too high

Disadvantages and advantages of electrochemical cells

Disadvantage: Use toxic chemicals /Chemicals are flammable

Advantages: cheap to make

what is the hydrogen half cell equation

2H⁺ + 2e⁻ ⇌ H₂

what is the E° for the hydrogen half cell

0v

In a redox system what is effect of increasing concentration of reactant 

equilibria moves to the right → more products are made

E° → becomes more positive 

in redox system what is the effect of increasing concentration of products

equilibria moves to the left/ more reactants are made 

E° → becomes more negative 

what is the salt bridge made out of ?

KNO₃ ( inert ions)

how to measure standard electrode potential of a half-cell

standard hydrogen electrode ( left) connected standard electrode potential of metal ( right)

how to work out equation when energy storage is recharging

full equation ( combine half equations ) in reverse

reactants →products / products → reactants 

what is a fuel cell

cell that produces electricity by reacting a fuel with oxygen

how to work out half-equation at other electrode from overall equation + half equation ( from one electrode)

add anything that appears in overall equation but not half equation

to get rid of H⁺ and e⁻ →add to other side 

2 differences between fuel cell and modern storage cell

fuel cell → fuel reacts with O₂ to give electrical energy

Refuel → constant supply of O₂ and fuel

one advantage of using ethanol over hydrogen fuel in a vehicle 

hydrogen is flammable 

ethanol is liquid its easier to store

method to working out formula of hydrated salt

use conc and volume ( mean titre) of given salt to work out mols of salt ions

if in 25cm³ x 10 → 250cm³ ( moles )

mass / mols —> mr of hydrated salt

mr ( hydrated salt) - mr of ions = mr of H₂O

then divide by 18 = •H₂O

volume to reach endpoint = mean titre

Thiosulfate titration what are the colour changes

tiitration solution brown → straw colour

when starch is added straw to blue

Thiosulfate titration what is the endpoint and isnt starch added initally

endpoint = blue colour to colourless

no sharp endpoint but gradual colour change if starch added initally

redox titrations how to go from 250cm⁻³ to 25cm⁻³ when diluting only

moles in 250cm⁻³ x 10 ( mention diluting with water only)

how to work out mass in (g dm⁻³)

conc x Mr

how to convert g → mg 

x 1000

Advantages of fuel cells compared to combustion of fossil fuels

produce less CO₂

Fuel cells are more efficient

redox system when does cell potential change ?

Concentrations are changing

Ways of storing hydrogen as fuel in cars

absorption

Liquid under High pressure

How to know which redox system to use for each electrode 

oxygen electrode is always more positive E°V ( reduction)

Hydrogen is more negative