electrochemistry my notes

what is the standard electrode potential table

Half equations written as reductions

What is the form of the equations in the electrode potential tables

Oxidised form + electron = reduced form

what is the RHS in the electrode potential table

Reduced form = species that can act as a reducing agent

In the electrode potential table what is the strongest reducing agent

Most negative electrode potential

In the electrode potential table what is the weakest reducing agent

Most positive electrode potential

Elechrochemical series definition

List of electrode potentials in order/ half cells in order of electrode potential

Conditions of SHE

100kPa, 289K, 1moldm³ ions H+ concentration

Redox half equation combination question: “construct the overall ionic equation by combining half equations”

Identify the reduction half equation, identify the oxidation half equation, balance the electons, add the equations together, write the final ionic equation

Redox feasibility steps

Identify the two half equations, the one with the more positive electrode potential is reduced, the one with the less positive electrode potential is oxidised, calculate Ecell, if Ecell is positive, the reaction happens

When do the redox feasibility steps work

For spontaneous reactions

Ecell equation and information

Ecell = electrode potential of reduction (so oxidising agent needed (LHS)) - electrode potential of oxidation (so reducing agent needed (RHS))

Writing half equations for lithium ion batteries

At the positive electrode (cathode) = reduction, at the negative electrode (anode) = oxidation

Half equation of lithium ion battery at the positive electrode

Li+ and electron and transition metal oxide = lithiated oxide

Half equation of lithium ion battery at the negative electrode

Lithium = Li+ and electron

When writing the half equations that happen at different electrodes (and information is given) what should you do

Balance the main atom, balance the oxygen with water then hydrogen with H+ then charge with electrons, and keep is all the same way round, don’t flip or reverse it

When they ask for “ the overall reaction that happens in fuel cells”

Combine the half equations as normal (check the electrons cancel, do not flip or reverse anything)

What does the EMF depend on

The concentrations/ partial pressures of the reacting series, as the cell runs, the reactants get used up and the products get used up, so must maintain concentrations of reactants, and constant partial pressures of the gases to maintain the EMF of a fuel cell

Writing a fuel cell

Anode (-)/ anode solution (-)//cathode solution (+)/cathode (+)

Rules for fuel cells

Group all of the aqueous ions together (including aqueous H+), separate state changes by a line, don’t include all species (only include it if it is part of the redox pair or an aqueous ion in the half cell)

What substances are always include in half cells

Gases, solids, and aqueous H+

What substances should you check before including in the fuel cell notation

H20, H+ (not aqueous), OH-, H2O2, any solvents

How to check if a substance (that should be checked) should be included in a fuel cell

Only include if changes oxidation state

When writing fuel cells do you include coefficients

No (only include if distinguishing)

Substances AND conditions needed for SHE

H2 (g) and 100kPa, 1moldm^-3 and H+, platinum electrode and temp of 298K

Electrochemistry cell practical template (6 marker)

Weigh out the solid and dissolve it in a small volume of the requires acid, make up to a known volume in a volumetric flask, place the electrode into the prepared solution to form the half cell, connect to a salt bridge, connect to a high resistance voltmeter, record cell EMF, use Ecell =E(cathode) - E(anode) (here must put RHS and LHS depending on the question), rearrange to get the standard electrode potential of the unknown half cell

In acidic conditions

With H+

When reversing a reaction in the electrode potential table

DO NOT reverse the sign of the value

Remember about Ecell equation

Isn’t always the most positive - the least positive, so must look at the half equations, Ecell = reduction - oxidation

What to do when asked to compare if reactants happen or not

Compare the electrode potentials

Purpose of a salt bridge

Complete the circuit, prevents charge build up, allows ions to flow between the two half cells

When to choose platinum as an electrode

Ion only half cell/ gas electrodes, non conducting solid (I2)

When to not choose platinum as an electrode

Half cell contains a solid metal/the solid present is a conductor (not I2), ask for a cheaper electrode (carbon in fuel cells)

When a system is being used to measure the standard electrode potential of the x system using the SHE

One side is the hydrogen electrode, and the other is the x half cell, the SHE needs H+ concentration of 1moldm^-3, usually produced by a strong acid (HCl), and H2 gas at 100kPa, the x solution (the electrode potential being measured) has standard conditions of each ion at 1moldm^-3, and if the other electrode solutiom is HCl, the mixture at the other will be XCl(charge of x)

Voltmeter

Very high resistance, so when it is in an electrode cell, only a tiny current can flow and the reaction essentially doesn’t proceed to completion, just enough to establish a potential

SHE equation, and about it

2H+ and 2electrons = H2 gas, when under standard conditions, the electrode potential is 0V

EMF equation

EMF = Eright - Eleft

Low resistance

Wire/bulb/lamp

If the concentration isn’t 1moldm^-3 then

The EMF won’t be the same as a solution under standard conditions

Anode vs cathode

Electrons flow from the LHS to the RHS so LHS is the anode = oxidation, but only when the LHS has a lower concentration as electrons flow from the lower concentration side to the higher concentration side

When does a concentration cell work

When there is a concentration difference, so the Ecell = 0, when the concentrations of the solutions are the same

Where the anode/cathode is depends on

Electrode potentials (normal cells), ion concentrations (concentration cells), how the cell is written

Normal Galvanic cell

Two different metals/ions

Concentration cell

Same metal, same ion, different concentrations

Electrolytic cell

Power supply present

What happens in a salt bridge

The ions in the ionic substance in the salt bridge move through the salt bridge and complete the circuit

Advantages of hydrogen fuel cells

High efficiency, no CO2 emissions (only water produced, unless hydrogen made from fossil fuels), quiet operation, continuous supply, lightweight

Disadvantages of hydrogen fuel cells

Storage issues (hydrogen gas is explosive), hydrogen productions may not be green, expensive catalyst (often platinum), infrastructure lacking

Advantages of lithium ion cells

High energy density, rechargeable, low self discharge (hold charge well), portable (ideal for phones)

Disadvantages of lithium ion cells

Can overheat/catch fire (when damaged), finite lifespan, mining issue (environmental and ethical concern), recycling is difficult

Advantages of lithium iodine cells

Very long life (very stable), reliable and safe (no leakage/explosion), high energy density (good for medical implants)

Disadvantages of lithium iodine cells

Low power output, not rechargeable

Use of lithium iodine cells

Used in pacemakers

Advantages of SHE

Universal reference (defined as 0.00V)

Disadvantages of SHE

Difficult to maintain (all the conditions), not practical for routine lab use (secondary standards (Cu2+/Cu) used instead)

Advantages of electrochemical cells (general)

Predict feasibility of reactions (if positive EMF then is feasible), can generate electricity (redox reactions drive current), used in batteries, sensors, and fuel cells

Disadvantages of electrochemical cells (general)

Conditions affect EMF (concentration, pressure, temperature), kinetics may prevent reaction (even if positive EMF, reaction may be slow (high activation energy) (need catalyst)

Internal combustion vs fuel cell

Fuel cells have a much higher efficiency so less energy wasted as heat, however fuel cells have hydrogen storage and production problem

Advantages of catalysis by transition metals

Speeds up redox reactions by providing an alternative pathway, cycle between oxidation states

Disadvantages of catalysis by transition metals

Some ions cannot catalyse (if their electrode potentials don’t allow both oxidation and reduction steps)