Electrode Potentials And Electrochemical Cells Flashcards
How do we get electricity?
By the movement of electrons
What type of cells make electricity
Electrochemical cells make electricity
How are electrochemical cells made?
• Made from two different metals dipped in salt solutions of their own ions and connected by a wire (the external circuit)
How many reactions are in an electrochemical cell?
• Oxidation
• Reduction
• Redox processes
How do electrons flow in an electrochemical cell?
Electrons flow through the wire from the more reactive metal to the less reactive one
What is the use of a voltmeter
• Found in the external circuit
• Used to measure the voltage between the two half-cells
• Finds the cell potential or electromotive force known as Ecell
What other variations of half-cells are possible?
• Half cells involving solutions of two aqueous ions of the same element.
Why is a platinum electrode used?
Platinum is an inert gas so it does not react with ions
What type of reaction happens at each electrode
Reversible reactions
What does the reversible arrow show?
That both reactions of each electrode can go in either directions (the flow of electrons)
How are half-equations for electrochemical cells written
• They’re always written with the reduction reaction going forward direction (with the electrons on the left-hand side)
What is the condition dependent on which direction the reaction will go in
• The reaction depends on on how easily each metal looses electrons ( how easily it’s oxidised)
What is the function of electrode potentials
• Measure how easily metals are oxidised
• A metals that’s easy to oxidise has a very negative electrode potential. One that’s harder to oxidise has a less negative (or positive) electrode potential
What are the guideline for wiring the overall equation for a cell
• Reduced means the half equation goes forward
• Oxidised means the half equation goes backwards
List the conditions that affect cell potentials
• Temperature
• Pressure
• Concentration
What is the standard electrode potential of a half cell?
The standard electrode potential of a half cell is the voltage measured under standard conditions when the half-cell is connected to a standard hydrogen electrode
What are the standard conditions for a half-cell?
• Any solutions must have a concentration of 1.00mol dm-3
• The temperature must be 298K (25C)
• The pressure must be 100kPa
Note
Is standard conditions are maintained, the reading in the voltmeter when a half-cell is connected to the standard hydrogen electrode will be the standard electrode potential of that half-cell
What is the function of the salt bridge
• Connects the solutions
• Made from filter paper soaked in KNO3(aq)
• Allows ions to flow through and balance out the charges
What is the function of the electrochemical series
It shows you what elements are reactive and which are not
What is an electrochemical series
An electrochemical series is basically a list of standard electrode potentials for different electrochemical half-cells
What can be calculated from the electrochemical series
Standard cell potentials
What is the function of a standard electrode potential
Standard electrode potential values can be used to calculate the standard cell potential Ecell or (EMF: electromotive force) when two half-cells are joined together.
Give the equation for the EMF of a cell
Ecell= Ereduced - Eoxidised
What is the first step in calculating the EMF
L First write the two half equations down as reduction reactions and write their E/V value from the electrochemical series
Describe the shorthand way of drawing electrochemical cells
• The half cell with a more negative potential goes on the left
• The oxidised forms go in the centre of the cell diagram
What do electrode potentials predict?
Whether a reaction will happen
What do the vertical lines on n a shorthand electrochemical cell show?
Double lines show a salt bridge
The more negative the electrode is, the more…..
Reactive the metal will be
What happens to metals when they react?
They loose electrons, they become oxidised
Give one advantage and one disadvantage of non-rechargeable batteries
• They are cheaper however,
• They cannot be recharge
Name a metal that is used in the electrodes in an alkaline hydrogen-oxygen fuel cell
Platinum containing electrodes
What electrolyte is used in an alkaline hydrogen-oxygen fuel cell?
Aqueous alkaline solution (OH-)
How is a lithium battery recharged vs a non-rechargeable battery
• To recharge these batteries, a current is supplied to force electrons to flow in the opposite direction around the circuit
• And reverse the reactions
• The reactions that take place in non-rechargeable batteries are difficult or impossible to reverse in this way
What happened to the half equations in a rechargeable batteries
The reactions are reversed
Give one type of electrochemical cells which provide electricity
Batteries
Give some types of batteries, their advantages and disadvantages
•Non-rechargeable batteries: can only be used until they run out. They are cheaper.
• Rechargeable batteries: Last longer and are cheaper in the long-run
What type of cells are used in mobile phones and laptops
Lithium cells
Describe how lithium cells provide electricity
• Lithium cell is made up of a lithium cobalt oxide (LiCoO2) and a graphite electrode
• The electrolyte is a lithium salt in an organic solvent
• Half equations: Check digital flashcards 🔴
• The Li+/ Li half cell has the more negative E value so goes in the direction of oxidation (backwards)
• The reaction that happens at the battery supplies, positive and negative electrode: Check did vital flashcards for answers🔴🔴
How do fuel cells generate electricity and give examples
• Chemicals that generate electricity are contained in the electrodes and the electrolyte that form the cell
• Chemicals are stored separately outside the cell and are fed in when electricity is needed
• alkaline hydrogen-oxygen fuel cell, used to power electric vehicles
• Hydrogen and oxygen gases are fed into two separate platinum-containing electrodes
• The electrodes are separated by an anion-exchange membrane that allows (OH-) and water to pass through it but not hydrogen and oxygen gases
• The electrolyte is an aqueous alkaline KOH solution
• Electrons flow from the negative circuit through an external circuit to the positive electrode
• The OH- ions pass through the anion-exchange membrane towards the negative electrode
Why do fuel cells have big advantages?
• using fuel cells in cars is more efficient
• able to convert more of their available energy into kinetic energy to get the car moving
• Internal combustion engines waste a lot of their energy producing heat
• Fir fuel cells the only waste product is water so no nasty toxic chemicals to dispose of and no carbon emissions
• Fuel cells don’t need to be recharged like batteries. As long as hydrogen and oxygen are supplied the cell will continue to produce electricity
What are some disadvantages of fuel cells
• Energy is needed to supply hydrogen and oxygen
• They can be produced from the electrolysis of water e.g.
• Re-using the waste product from the fuel cell but this requires electricity
• This electricity is generated by burning fossil fuels
• The whole process isn’t usually carbon neutral
• Hydrogen is also highly flammable, it needs to be handled carefully when it is stored or transported
What is the overall effect of alkaline hydrogen-oxygen fuel cell
Hydrogen and oxygen react to make water
Agents on the left hand side of the electrochemical series are more easily?
Reduced, so oxidising agents
