Topic 9 & 19 - Oxidation and reduction Flashcards
Define oxidation and reduction
Oxidation is the loss of electrons
Recuction is th gain of electrons
OIL RIG
What is the oxidation number of atoms in the elemental state?
Zero
What is the oxidation number of a simple ion?
Same as the charge of the ion
Define redox reaction
A reaction in which changes in the oxidation numbers occur
What is the relationship between oxidation number and oxidation and reduction?
Oxidation → oxidation no. increases
Reduction → oxidation no. decreases
What is oxidation state?
The oxidation number of an element in a compound. (iron(II) oxide etc)
Write the half equations of
Zn(s) + Cu2+(aq) → Zn2+(aq) + Cu(s)
Zn(s) → Zn2+(aq) + 2e–
Cu2+(aq) + 2e– → Cu(s)
Define *oxidising agent *and reducing agent
Oxidising agent = oxidises the other reactant but is itself reduced
Reducing agent = reduces the other reactant but is itself oxidised
What can be said about metals’ redox abilities?
More reactive metals are stronger reducing agents than less reactive metals
What is the redox relationship between more reactive metals and less reactive metals?
A more reactive one is able to reduce the ions of a less reactive one
What can be said about non-metals’ redox abilities?
More reactive non-metals are stronger oxidising agents than less reactive non-metals.
What is the redox relationship between more reactive non-metals and less reactive non-metals?
A more reactive one is able to oxidise the ions of a less reactive one
Describe the reactivity series down Group 7 (halogens)
The tendency to be the oxidising agent and become reduced decreases down the group.
What happens in the half-cell containing a zinc strip in a voltaic cell?
- Zinc ions are formed
- Surface of the metal becomes negatively charged with respect to the solution
What happens in the half-cell containing a copper strip in a voltaic cell?
- Some copper ions in the solution accept electrons from the copper rod
- The rod becomes positive with respect to the solution
What happens in the anode of a voltaic cell and what charge does it have?
- Oxidation occurs
- The rod has a negative charge
What happens in the cathode of a voltaic cell and what charge does it have?
- Reduction occurs
- The rod has a positive charge
Which way do electrons flow in a cell?
From anode to cathode.
What is the voltage determined by in a voltaic cell?
The difference in the reducing strength of the two metals.
Draw a voltaic cell
Draw a diagram of an electrolytic cell
What happens in the anode of an electrolytic cell and what charge does it have?
- Oxidation occurs
- Positive charge
What happens in the cathode of an electrolytic cell and what charge does it have?
- Reduction occurs
- Negative charge
What is the energy conversion in an electrolytic cell
Electrical energy is converted into chemical energy
Show the reactions that occur in the electrolysis of molten NaCl
Anode: 2 Cl–(l) → Cl2(g) + 2e–
Cathode: 2 Na+(l) + 2e– → 2 Na(l)
Overall: 2 NaCl(l) → 2 Na(l) + Cl2(g)
What is the assigned electrode potential for the standard hydrogen electrode?
0 V
Describe the standard hydrogen electrode and the reaction that occurs in it
2H+(aq) + 2e– ⇔ H2(g)
Define standard electrode potential, Eº
When the standard hydrogen electrode is connected to another standard half-cell, the emf generated is known as the standard electrode potential
In what direction do electrons flow?
Towards the half-cell with the highest E° value
What is the cell potential?
The emf of a cell
E°cell = E°where reduction occurs – E°where oxidation occurs
How is the spontaneity of a reaction determined from the E° values?
E°cell is positive for all spontaneous reactions
What happens in the electrolysis of water?
- O2 evolve at anode and H2 evolve at cathode
- The pH at the anode decreases while the pH at the cathode increases
What happens in the electrolysis of NaCl(aq)?
- Cl2 is evolved at anode and H2 is evolved at cathode
- Cl2 has a strong smell and bleaching effect on blue litmus paper
- pH of the electrolyte increases
What happens in the electrolysis of CuSO4?
With graphite electrodes:
- Pink-brown colour develops as copper is deposited on the cathode
- O2 is evolved at the anode
- pH of the solution decreases
- The intensity of blue colour decreases
With copper electrodes:
- Pink-brown colour develops at the cathode
- Cu anode disintegrates
- No change in pH
- No change in the intensity of the blue colour
What is electroplating?
The process of using electrolysis to deposit a layer of a metal on top of other conductive substance
What are the features of an electrolytic cell used for electroplating?
- An electrolyte that contains the metal ions which are to be deposited
- The cathode made of the object to be plated
What are three purposes for electroplating?
- Decoration
- Corrosion control
- Improvement of function
