AOS 1 - Primary galvanic cells & fuel cells

How to solve a complex redox reaction in alkaline/basic conditions?

1. Balance equation like normal.

2. For the amount of H+ ions in the equation add OH- ions to either side.

3. Combine the OH- and H+ to form water and balance equation.

How does MnO4- change in a reaction?

The purple MnO4- ion is reduced to produce the near colourless Mn2+.

How does (Cr2O7)2- change in a reaction?

The orange (Cr2O7)2- ion is reduced to green Cr3+ ions.

Where do reactions occur in a galvanic cell?

Anode - Oxidation

Cathode - Reduction

Which way do electrons flow in a galvanic cell?

Electrons flow from the anode to the cathode (negative to positive).

How to determine standard electrode potential (E) in a galvanic cell?

Expressed in volts (V)

What is a galvanic cell?

Electrochemical cell in which chemical energy from spontaneous redox reactions is converted into electrical energy.

What is the purpose of a salt bridge?

Contain ions that free to move so that they balance charges formed in the half cells. W/o a salt bridge the cathode half cell will accumulate negative ions while the anode will accumulate positive ions which will stop the movement of electrons around the external circuit.

What are properties of an inert electrode?

-Not reactive

-Be solid

-Be conductive

e.g. platinum or carbon

What is a primary cell?

a cell that cannot be recharged. Once the reaction reaches equilibrium the cell will be 'flat' & must be discarded.

e.g. Zinc-carbon cell.

What is a fuel cell?

an electrochemical device that converts chemical energy into usable electrical energy & heat w/o combustion

Similarities between fuel cells & galvanic cells?

-Convert chemical energy to electrical energy through redox.

-Site of oxidation & reduction are physically separate.

-Oxidation occurs at negative electrode, anode & reduction at the positive electrode, cathode.

-Electrolytes must occur b/w electrode to allow flow of ions that make up the internal circuit.

Differences between fuel cells and galvanic cells?

-FC do not store reactants, they must be continuously supplied from an external source.

-PGC produce electricity until the chemical reaction finished, at which point the electrical energy is exhausted.

-FC do not lose charge or need to be recharged provided reactants are supplied continuously.

-FC electrodes are porous- this provides a high surface area & the opportunity to introduce catalysts.

-Energy Efficiency: FC 35-70% < PGC 45-90%

List advantages of fuel cells?

-FC convert chemical energy directly to electrical.

-FC will generate electricity provided fuel is supplied.

-Hydrogen FC produce water as a byproduct.

List disadvantages of fuel cells?

-FC require a constant supply of reactants.

-Often expensive catalysts & electrolytes.

-Some FC are no effective.

-Issues surrounding storage & safety of hydrogen.

-Hydrogen mainly sourced from fossil fuels.

What is the purpose of a porous electrode?

-To allow gases to diffuse through & come int contact w/ the electrolyte seeping out.

-Porous nature provides a high surface area & an opportunity to introduce a catalyst improving overall efficiency.

What is the purpose of an electrolyte in fuel cells?

-Transporting ions b/w electrodes.

-Separates the oxidant and reductant preventing spontaneous reactions.

-Membrane are a type of electrolyte that conducts H+ ions.

What are renewable feedstocks?

an organic material derived from recently living organisms or natural processes that can be replenished over a short timescale.

e.g. biomass, wastewater or CO2.