Electrode potential and electrochemical cells

What is an oxidising agent

• Electron acceptor

What is a reducing agent

• Electron donor

What are electrode potentials

• A value that indicates how readily a substance gains electrons

What is an electrochemical series

• List of electrode potentials for half equations in numerical order

What is the role of the voltmeter in a cell

• Measures potential difference (EMF)

• I.e the difference in electrode potentials for the 2 half cells

Role of the salt bridge

• Maintain electrical neutrality/complete the circuit

• Does this because it has mobile ions

What can be used for the salt bridge and why

• Potassium nitrate solution

• Allows the movement of ions

• And will not react with chemicals in the half cells

What conditions are standard electrode potentials measured under?

• 298k

• 1moldm^-3 (conc for solutions)

• 100kPa (for gases)

When is a platinum electrode used and why?

• When you have a cell that doesn’t contain solid metals (as electrodes)

• Platinum provides a solid surface for electron transfer

• And is inert (unreactive)

Equation to workout Ecell/EMF

• Larger electrode potential - Smaller electrode potential

General formula for conventional representation for a cell

Why is a standard hydrogen electrode used ?

• Used to measure unknown electrode potentials

What side of the cell representation is the standard hydrogen electrode always on?

• LEFT

Equation for standard hydrogen electrode

What can be used for H+ source in SHE

• 1 moldm-3 solution of

• HCL or HNO3

The potential of other half cells is compared to the potential of

• SHE which has a potential of 0 V

What can cause the value of electrode potentials to change ?

• Changing the conditions

• Any change that causes the eqm to shift right (e.g increasing conc of reactants) will increase the electrode potential

• Any change causing eqm to shift left will decrease electrode potential

Applying a change to what conditions changes electrode potential

• concentration

• Pressure (if gas is involved)

In an electrochemical cell when EMF is measured with a voltmeter…

• no current is flowing

What happens when you replace a voltmeter with a lamp in a cell?

• Electric current will flow through the circuit

• EMF will decrease

3 types of cells

• Non rechargeable

• Rechargeable

• Fuel

What happens in non rechargeable cells?

• 1 or both cell reactions cannot be reversed

Pros and cons of non-rechargeable cells

Pros

• Cheaper to produce and buy

Cons

• Single use - Thus use up finite resources

• Can end up in landfill and pollute the environment by releasing toxic substances

What are rechargeable cells

• Cells where electrode reactions can be reversed by applying a reverse potential

Pros and cons of rechargeable cells

Pros

• Minimise waste going to landfills, therefore reduce pollution

Cons

• More expensive

• Applying a reverse potential requires electricity→ CO₂ released from power stations → which causes global warming

Example of a rechargeable cell

• Lithium ion cell

What reaction occur at the positive electrode of a lithium ion cell

What reaction occurs at the negative electrode of a lithium ion cell

Overall reaction occurring in lithium ion cell?

• Li + CoO₂ → LiCoO₂

How do fuel cells work?

• Fuel cells are used to generate an electric current

• Reactants for the fuel cell can be continuously supplied

• Therefore fuel cells do not need to be electrically recharged

• Meaning the concentration for the reactants remain constant

• (EMF will not change with use)

Why are liquid fuels preferred to gas fuels in fuel cells?

• Liquid fuels can be stored and transported more easily than gases

What happens in hydrogen-oxygen fuel cell

• Converts hydrogen(fuel) and oxygen(air) into water

• Which releases electrical energy in the process

What are the 2 main conditions for hydrogen oxygen fuel cells

• Acid

• Alkaline

Depends on the electrolyte used

What is the overall reaction in hydrogen-oxygen fuel cell

2H₂ + O₂→ 2H₂O

Explain how a hydrogen oxygen fuel cell works (alkaline)

• 2 electrodes separated by an electrolyte (KOH)

• Each reactant is introduced from either side of the cell

• Water is removed

• Hydrogen is oxidised at the negative electrode - by reacting with OH from electrolyte, releasing electrons

• H2+2OH^- →2H2O + 2e-

• These electrons travel through the external circuit and arrive at the positive electrode

• Where oxygen gains the electrons at the positive electrode and reacts with water (from the electrolyte)

• O2 +2H2O +4e- → 4OH-

• Hydroxide ions produced move through the electrolyte to complete the circuit

Benefits and risks of a hydrogen-oxygen fuel cell?

(Essentially the benefits of producing electricity this way?)

• Water is the only product /no greenhouse gases

• Efficient compared to combustion

Risks

• Generating hydrogen requires reacting methane with steam which produces CO2

• Fossil fuels burnt to generate electricity to get make hydrogen