Chapter 23 - Redox and electrode potentials Flashcards
What is reduction ?
- Gain of electrons
- Decrease in oxidation number
What is oxidation ?
- Loss of electrons
- Increase in oxidation number
What is the oxidation number on an uncombined element ?
0
What is the oxidation number on a combined hydrogen ?
+1
What is the oxidation number on a combined oxygen ?
-2
What is the oxidation number on an ion of an element ?
Ionic charge
What is an oxidising agent ?
- Takes electrons from the species being oxidised
- The oxidising agent contains the species that is reduced
What is a reducing agent ?
- Adds electrons on to the species being reduced
- The reducing agent contains the species that is oxidised
What is a voltaic cell ?
An electrochemical cell that converts chemical energy into electrical energy
What do you need to make a voltaic cell ?
- You need chemical reactions that transfer electrons from one species to another
- This is because electrical energy results from the movement of electrons
What is a half cell ?
A half cell contains the chemical species present in a redox reaction
What does two half cells make ?
A voltaic cell, connecting two half cells allows for electrons to flow
Why must the chemicals be kept apart in the two half cells ?
This is because if they were allowed to mix, electrons would flow in an uncontrolled way and heat energy would be released rather than electrical energy
What is a metal/metal ion half cell ?
- Simplest half cell
- Consists of a metal rod dipped into a solution of its aqueous metal ion
What is electron transfer like in an isolated half cell ?
There is no net transfer of electrons either into or out of the metal
What is electron transfer like when two half cells are connected ?
The direction of electron flow depends upon the relative tendency of each electrode to release electrons
What is an ion/ion half cell ?
Contains ions of the same element in different oxidation states
Give an example of an ion/ion half cell
- Mixture of aqueous iron(ii) and iron(iii) ions
- Inert metal electrode
What metal is used in an ion/ion half cell ?
- Inert metal electrode
- Typically platinum
In an operating cell, which metal loses electrons ?
- The electrode with the more reactive metal
- It is oxidised
- This is the negative electrode (anode)
In an operating cell, which metal gains electrons ?
- The electrode with the less reactive metal
- It is reduced
- This is the positive electrode (cathode)
What does standard electrode potential measure ?
The tendency to be reduced and gain electrons
What is the standard chosen half cell ?
- Hydrogen gas
- H+ ions
- Inert platinum electrode
What is the standard electrode potential of a hydrogen electrode ?
0V
What are the standard conditions for electrode potentials ?
- 298K (25C)
- 100 kPa (1 bar)
- 1 moldm-3
Define standard electrode potential
The e.m.f of a half cell connected to a standard hydrogen half cell under standard conditions
How can you measure a standard electrode potential ?
Connect the half cell to a standard hydrogen electrode
What does the wire connecting the electrodes allow for ?
A controlled flow of electrons
What connects both solutions ?
A salt bridge
What does the salt bridge allow for ?
Allows ions to flow
What does the salt bridge typically contain ?
A concentrated solution of an electrolyte that does not react with either solution
What happens as the Ev value gets more negative ?
The greater the tendency to lose electrons and undergo oxidation
What happens as the Ev value gets more positive ?
The greater the tendency to gain electrons and undergo reduction
What Ev do metals tend to have ?
Negative Ev values and lose electrons
What Ev do non metals tend to have ?
Positive Ev values and gain electrons
How do you calculate the standard cell potential ?
E° cell = E° (positive electrode) - E° (negative electrode)
What are the three main types of cells ?
- Primary cells
- Secondary cells
- Fuel cells
How often are primary cells recommended to be used ?
Single use only
How is electrical energy produced by primary cells ?
Oxidation and reduction at the electrodes
Can reactions be reversed in primary cells ?
No they can not
What happens when the chemicals are used up in a primary cell ?
- The voltage falls
- The battery goes flat
- Cell will need to be discarded or recycled
What are modern primary cells like ?
Alkaline based on zinc and manganese dioxide and a potassium hydroxide alkaline electrolyte
Are secondary cells rechargeable ?
Yes they are
Can reactions be reversed in secondary cells ?
- Yes they can be
- This is done via recharging
What happens as a result of reversible reactions in secondary cells ?
Chemicals in the cell are then regenerated and the cell can be used again
Give some examples of secondary cells
- Lead acid batteries in car batteries
- Lithium ion cells in laptops, phones and cameras
What does a fuel cell do ?
It uses the energy from the reaction of a fuel with oxygen to create a voltage
How does a fuel cell work ?
- The fuel and oxygen flow into the fuel cell and the products flow out
- The electrolyte remains in the cell
How often do fuel cells work ?
They operate continuously provided that the fuel and oxygen are supplied into the cell
Do fuel cells have to be recharged ?
They do not have to be recharged
What are hydrogen fuel cells said to be ?
Green
Why are hydrogen fuel cells said to be green ?
- This is because they do not produce carbon dioxide
- Water is the only combustion product
What other fuel cells are being made ?
Methanol fuelled cells
What type of electrolyte can a hydrogen fuel cell have ?
- Alkali
- Acid
