Topic 14 - Redox II Flashcards
What happens to an element’s oxidation number when it is oxidised?
It increases.
Do s-block elements react by gaining or losing electrons?
Losing them
Do p-block elements react by gaining or losing electrons?
- Metals -> Losing electrons
* Non-metals -> Gaining electrons
Do d-block elements react by gaining or losing electrons?
It varies, by but they tend to form positive ions with positive oxidation numbers.
Describe the structure of a basic electrochemical cell.
- Anode and cathode
- Each electrode is dipped in a solution of its own ions
- Salt bridge between solutions
- External circuit connecting electrodes, with voltmeter
In an electrochemical cell, what is the salt bridge made of?
Filter paper soaked in a salt solution
What are the two processes in an electrochemical cell?
- Oxidation
* Reduction
In an electrochemical cell, which electrode does oxidation always happen at?
Anode
Remember: A + O are vowels!
In an electrochemical cell, explain which metal becomes the anode and which becomes the cathode? Where do oxidation and reduction occur?
MORE REACTIVE METAL:
• Forms ions more readily, so it loses electrons electrons more easily
• This is oxidation
• So this is the anode (relatively positive)
LESS REACTIVE METAL:
• Forms ions less readily, so it instead accepts electrons and ions from the solution, reforming the metal
• This is reduction
• So this is the cathode (relatively negative)
In an electrochemical cell, which electrode does reduction always happen at?
Cathode
Do reactive or unreactive metals form ions more readily?
Reactive
In an electrochemical cell, which direction do electrons flow in?
From the more reactive metal to the less reactive metal.
What is the symbol for cell potential?
E(cell)
What is cell potential?
The difference between the two half-cells in an electrochemical cell.
Do half-cells have to involve a metal and solution of metal ions?
No, they can also involve:
• A solution of two different aqueous ions of the same element (in the same half-cell) with an inert electrode
• Gas with solution of its own ions and an inert electrode
Give an example of a half-cell with a solution of two aqueous ions of the same element. Describe the structure.
• Solution of Fe³⁺ and Fe²⁺ • Platinum (or graphite) electrode • Oxidation or reduction (depending on whether the half-cell has the anode or cathode) occurs on the electrode surface Possible reactions: • Fe²⁺(aq) -> Fe³⁺(aq) + e⁻ • Fe³⁺(aq) + e⁻ -> Fe²⁺(aq)
When an inert electrode is required in an electrochemical cell, what elements tend to be used?
- Platinum
* Graphite
Given that zinc is more reactive than copper, explain what happens in the zinc/copper electrochemical cell. Include half-equations.
- Zinc loses electrons more easily, so it is oxidised to the ion, releasing electrons into the circuit
- Zn(s) -> Zn²⁺(aq) + 2e⁻
- Copper ions in the solution are reduced using electrons from the circuit, to form copper solid
- Cu²⁺(aq) + 2e⁻ -> Cu(s)
Describe how a half-cell can be set up with a gas.
- The gas is bubbles over a platinum/graphite catalyst sitting in a solution of its aqueous ions
- Oxidation or reduction occurs on the surface of the electrode
When drawing electrochemical cells, which half-cell is on the left?
The one where oxidation occurs (anode).
Are the reactions at each electrode reversible?
- Yes
* The direction each reaction goes in depends on how easily the metal loses electrons
Which way are half-reactions at each electrode in an electrochemical cell written?
The reduction reaction is the forward direction.
e.g. Zn²⁺ + 2e⁻ -> Zn
Describe how you can set up an electrochemical cell involving two metals.
1) Get a strip of each of the metals you’re investigating. Clean the surfaces using emery paper (or sandpaper).
2) Clean any grease or oil from the electrodes using some propanone.
3) Place each electrode into a beaker with a solution containing ions of that metal. If any solution contains an oxidising agent that contains oxygen (e.g. MnO₄⁻), add acid too.
4) Create a salt bridge to link the two beaters together. Do this by soaking a piece of filter paper in salt solution.
5) Connect the electrodes to a voltmeter using crocodile clips and wires. There should be a voltmeter reading.
What things are used to clean the electrodes when preparing an electrochemical cell?
- Emery paper (or sandpaper) -> Cleans surface
* Propanone -> Cleans off any grease or oil