3.1 Redox And Standard Electrode Potential Flashcards
Oxidation in terms of electron transfer
Loss of electrons
Reduction in terms of electron transfer
Gain of electrons
Steps for separating a redox equation into two half equations
- Separate parts associated with each element
- If applicable - if oxygen is in a half equation then all water and hydrogen ions mist be associated with that half equation
- If applicable- simplify
- Balance the charges using electrons
How can a redox half equation be replicated as an experiment in the lab
In an electrochemical half cell
Metal/metal ion half cell
Electrode made of the metal dipped into a 1 moldm-3 solution of the metal ions
Solution half cell
• an element in an aqueous ion is oxidised or reduced to form different aqueous ions
• consists of a solution made up of both forms of concentration 1 moldm-3 and an electrode made of platinum and coated with fine platinum powder (platinum black)
Gas half cell
• reduced or oxidised species is a gas and the other species is an aqueous ion
• made up on 1 moldm-3 solution of the aqueous ions associated with the gas, platinum black electrode dipped into solution and a glass cover over the platinum electrode that surrounds the electrode in a steady stream of the gas at 1 atm
What is a cell diagram
A way of representing the electrochemical cell by showing the different species involved in the redox reactions in both half cells
Rules for setting up cell diagrams
- Metal electrodes are placed far left and far right of the cell and shod include any platinum electrodes used
- A double vertical line separates the 2 half cells - shows the salt bridge
- If there is a change in state between the 2 species in a half cell then a single vertical line is shown as a phase boundary
- If there is no change between states it is shown as a comma
- The reduced form of each half cell is either the conducting metal or the species next to the platinum electrode
- The oxidised form of each half cell is closest to the salt bridge
What does a cell diagram mirror
How the cell should be set up
What is standard electrode potential (E•)
The potential difference when any half cell is connected to the standard hydrogen electrode under standard conditions
How can a cell potential of an electrochemical cell be measured
With a high-resistance voltmeter
Can the individual potential of a half-cell be accurately measured alone
No
What is needed to obtain the individual reduction potential of a half-cell
Standard electrode potential
What is the electrode potential of the standard hydrogen electrode
0 volts
How can standard electrode potential be measured
By pairing it with the standard hydrogen electrode and measuring the cell potential
What is the oxidation potential of an electrode
The negative of its reduction potential
The more positive the standard electrode potential the more likely the system is to…
Gain electrons (be reduced)
If 2 half cells are connected the electrons flow from..
The more negative to more positive half cell
Equation that shows how the reading on the high-resistance voltmeter is given
Standard electrode potential = E•(more positive) - E•(less positive)
Standard hydrogen electrode- how is it set up
Hydrogen gas at a pressure of 1 atm bubbles over an inert platinum electrode dipped in 1 moldm-3 H+(aq) at a temperature of 298K
Which terminal and side of the circuit is the standard hydrogen electrode connected to
Positive terminal
Left hand side of the circuit
If the voltmeter gives a positive reading during standard hydrogen electrode it means
The hydrogen gas is oxidised to H+ ions
Electrons flow from left to right
If a voltmeter gives a negative reading during standard hydrogen electrode it means
The H+ ions are reduced to hydrogen gas
Electrons flow from right to left
What is EMF
The maximum potential difference which is present between two electrodes
Rules to follow to calculate cell EMF using the equation E•cell=E•(more positive)-E•(more negative)
• most negative half-cell is on the left hand side of the cell and connected to the positive terminal of the voltmeter
• the left hand side electrode always undergoes oxidation and so is the negative electrode as the electrons are released at this electrode
State the function of the salt bridge
To complete the circuit by allowing ions to move whilst also preventing the solutions from mixing
What must the EMF be in order for the redox reaction to be feasible
Positive after using the equation
E•(cell) = E•(reduction)-E•(oxidation)
How are fuel cells similar to electrochemical cells
They convert chemical energy into electrical energy through a redox reaction
How are fuel cells different to electrochemical cells
They require a continuously flowing source of fuel as well as a continuously flowing source of an oxidising agent
Fuel cells can produce electricity continuously as long as what 2 things are supplied?
Fuel
Oxygen
Are fuel cells efficient?
Yes, very
Convert between 50-70% of the chemical energy in the fuel into electrical energy
The remaining energy is lost as heat
Why is hydrogen fuel cells being trialed as an alternative for powering vehicles?
It releases energy more efficiently and is much lighter than traditional batteries
What happens during a hydrogen fuel cell
The fuel (Hydrogen gas) is passed over the platinum electrode and so the platinum acts as a catalyst
At the anode hydrogen gas is oxidised to H+ ions
The H+ ions then pass through a semi-permeable membrane and reach the cathode where they accept electrons and react with oxygen to form water
Overall reaction of hydrogen fuel cell
O2 + 2H2 –> 2H2O
Reaction that occurs at the anode of hydrogen fuel cell
H2 –> 2H+ + 2e-
Reaction that occurs at cathode of hydrogen fuel cell
O2 + 4H+ + 4e- –> 2H2O
Advantages of hydrogen fuel cell
• water is the only waste product - no carbon dioxide or greenhouse gases produced
• far less energy is lost as heat - more efficient
• hydrogen gas can be produced by the electrolysis of water - a renewable energy source
Disadvantages of hydrogen fuel cells
• hydrogen gas is highly flammable - difficult and expensive fuel to store
• most common source of hydrogen is reformation of methane from fossil fuels - crude oil is still used and so carbon dioxide still formed when hydrogen is used just it is produced at an earlier stage
• electrolysis to produce hydrogen requires a lot of energy - this will cause the production of carbon dioxide unless electricity is produced from a renewable source
