Chemistry Redox and Cells and Electrolysis notes and mistakes

when we have charge and know the equation at the fuel cell, what is the percent efficiency (can be charge or elec) what do we use

Using Q = n(e-) x F, from redox

when balancing the half equation and there is like some random ion in there, like Pb (s) → PbS04(s)

Add the ion to the side which does not have it.

You must add the SO4 2- (aq) ion to the side which does not have it.

Important derivation/shortcut formula for n(e-)

n(e-) = I t / F ,

I - current (A)

t - time (s)

F- Faradays constant, 96500 C

when it says there is a semipermeable membrane

it allows only some certain substances to pass through

what is green hydrogen

A form of hydrogen produced through electrolysis, using renewable energy sources, resulting in zero net contribution to greenhouse gases

Method to produce green hydrogen

• electrolysis of water, through
  —alkaline electrolysis circuit

purposes of membranes

-acts as a selective barrier preventing ions from contaminating another solution.
e.g “in membrane cells, stopping Cl'- from contaminating NaOH soln”

-to prevent a spontaneous reaction

-to prevent the excessive release of thermal energy

-to prevent direct contact between oxidant and reductant.

what is molten

when there is liquid without water and very hot

the energy/voltage required to make a recharge reaction

GREATER than E (reduction) - E (oxidation).

OR E( higher/more pos value) - E (more/neg value)

MUST SAY GREATER than this voltage required

if an electrolytic cell with Br- (aq) and Sn 2+ (aq) what will the reductant and oxidant be?

Sn 2+ will be both the oxidant and reductant, as there are two equations

Do the polarities switch during recharge?

NO, it does NOT, the electrode polarity does not change from recharge to discharge.

All that changes is the direction of the electron flow,
HOWEVER, what once was the cathode (site of reduction) is now the anode (site of oxidation)

IT is STILL positive!!!

What do you assume if there is concentrated NaCl and Brine or 5M NaCl

This means that chloride oxidation is preferred over water oxidation,

chloride undergoes oxidation in this situation

EVEN THOUGH water is a stronger reductant in the electrochemical series and still stronger,
BECAUSE of the non SLC concentrated conditions we must use 2Cl- (aq)

Cls (g) + 2 e- => 2Cl- (aq),

O2 (g) + 4H+ (aq) + 4e- => 2 H20 (l)

despite this, you must choose 2Cl- (aq) as a preferred reductant for the anode.

ONLY SPECIAL CONCENTRATION CASE YOU MUST MUST MUST KNOW.

Disadvantages of fuel cells in motor vehicles

expensive,

flammable as it is in hydrogen

Advantages of fuel cells

more energy efficient as directly chemical to electrical energy.

When balancing with an oxygen, and there’s multiple which do you use

The one which gives a stronger reaction/gradient

This is identified as a reducing agent, which version should you use to balance half equation ‘ 2Fe (s) or Fe(s)

Fe(s)

The electrolyte in the equation is

The ion in the question, and it needs to be bonded to something to be used as an electrolyte.

K + for OH-

If something is molten what is the state?

Liquid

What is the name of the fuel cell correspond to

The electrolyte

where does the co2 go in molten carbonate fuel cell? Out of cell?

No it gets recycled from the anode to the cathode

Why are the two half cells separated in fuel cell?

• to ensure electrons flow across the circuit and harness electrical energy

• To avoid reduction and oxidation to happen spontaneously and stopping the flow of electrons

Compare question layout

Define both things in context!

Then difference, then similarity.

Then link to context o

Green chemistry principles linking to fuel cells

Catalysis- electrodes contain catalysis which improve the rate of reaction and improve efficiency.

Design for energy efficiency- direct conversion of chemical to electrical energy.

Prevention of waste- no production of CO2 use of renewable feedstocks

02 produced via photosynthesis, to renewable

H2 electrolysis of H20. Energy provided by renewables

Green hydrogen

Green electricity and water resulting in green hydrogen

Issues with hydrogen

explosive and flammable

Colourless odorless

Storage and transport difficult due to the great volume it takes up, it requires high compression,

Differences between fuel cells and galvanic cells

Fuel cells are a continuous supply,

Porous electrodes

Secondary cells

rechargeable cells,

Electrical to chemical

Non spontaneous

Reluctant above oxidant

Discharge (like a galvanic)

Generates electricity, and is spontaneous

Chemical to electrical

Ph as H+ products would result in what?

H+ consumed means that the acidic nature of the cell would reduce and it would become more neutral hence increasing its pH

If substance solid can you break into ion for oxidising agent or reducing agent?

No, you must include the whole thing if not aqueous

Redox flow battery, how is this a hybrid of fuel and secondary cell

Because of a continuous supply of reactants, it is a fuel cell

Because it can be recharged,

Function of membrane

Transport electrons between electrodes

To prevent a spontaneous redox reaction

What would happen if the barrier was removed between two half cells

Current stops

Spontaneously react, (Release thermal E, confirm first so don’t say this till confirmed)

Stops thinning reductant, thing undergoing oxidation

Stops thickening oxidant, thing undergoing reduction

Why is the hydrogen half cell assigned a 0.00 value, what is its role in other voltages

It is a benchmark and reference, to compare other voltages

For example, Ni half cell E is lesser than the hydrogen half cell at -0.28, hence it is a stronger reducing agent than hydrogen and Ni 2+ is a weaker oxidising agent than 2H+.

Advantages of lead acid cell

Rechargeable, long life, reliable, high currency

Disadvantages of lead acid cell

Lead is poisonous, heavy, acid is corrosive, low energy density.

Why are fuel cells more efficient than thermal power stations?

Fuel cells - directly from Chem to electrical

Thermal power from - Chem to thermal to mechanical to electrical,

in each step there is a energy loss, so the more steps, the more energy loss and lesser efficiency

Sources of hydrogen gas

Mostly from fossil fuels.

Steam reforming of fossil fuels

Steam reforming of biogas

Decomposing of water,electrolysis

GCP’s pertinent to fuel cells

Design for energy efficiency-

Designing safer chemicals-

Catalysis- incorporates catalysts into the porous electrode to speed up rate of reaction by reducing the activation energy required for a successful collision/

Use of renewable feedstocks- (for microbial fuel cells)

How is design for energy efficiency pertinent to fuel cells?

because the direct conversion of chemical to electrical energy, MUST LINK TO ALL PARTS OF QUESTION, SO THE ECONOMIC AND/OR ENVIORNMENTAL

allows a higher energy efficiency for the same fuel used. This is compared to the comparatively less efficient coal powered stations with multiple steps and lower energy efficiencies.

→releasing lesser GHG’s like CO2 in the atmosphere, reducing GHG effect,( MINIMISES NEGATIVE ENVIRONMENTAL IMPACTS)

→More efficienct means less fuel used, hence lesser economic cost

How is designing safer chemicals used in regards to fuel cells

The fuel cells achieve their function and minimise toxicity and enhance safety by producing H20 a relatively harmless chemical as a product.

Contrarily, you can talk about how hydrogen fuel cells, are sourced from flammable and combustible hydrogen and can be unsafe as a result.

How does use of renewable feedstocks relate to MICROBIAL fuel cells

MICROBIAL Fuel cells can be derived from renewable feedstocks such as soils, wastewater,

Why is a cell not at 100% efficiency

Because of side reactions, forming products, giving resistance to the cell, reducing energy efficiency as a result

Features of fuel cells which are good

Porous electrode- the porousity increases the surface area

Catalyst- the catalyst increases rate of reaction by providing an alternative reaction pathway which reduces the required activation energy and increases the proportion of successful collisions

Heat siphoning - capture and reuse excess heat, enhancing overall efficiency