Topic 9 & 19 - Oxidation and reduction

Question 1

Define oxidation and reduction

Answer 1

Oxidation is the loss of electrons

Recuction is th gain of electrons

OIL RIG

Question 2

What is the oxidation number of atoms in the elemental state?

Answer 2

Zero

Question 3

What is the oxidation number of a simple ion?

Answer 3

Same as the charge of the ion

Question 4

Define redox reaction

Answer 4

A reaction in which changes in the oxidation numbers occur

Question 5

What is the relationship between oxidation number and oxidation and reduction?

Answer 5

Oxidation → oxidation no. increases

Reduction → oxidation no. decreases

Question 6

What is oxidation state?

Answer 6

The oxidation number of an element in a compound. (iron(II) oxide etc)

Question 7

Write the half equations of

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

Answer 7

Zn(s) → Zn²⁺(aq) + 2e^–

Cu²⁺(aq) + 2e^– → Cu(s)

Question 8

Define *oxidising agent *and reducing agent

Answer 8

Oxidising agent = oxidises the other reactant but is itself reduced

Reducing agent = reduces the other reactant but is itself oxidised

Question 9

What can be said about metals’ redox abilities?

Answer 9

More reactive metals are stronger reducing agents than less reactive metals

Question 10

What is the redox relationship between more reactive metals and less reactive metals?

Answer 10

A more reactive one is able to reduce the ions of a less reactive one

Question 11

What can be said about non-metals’ redox abilities?

Answer 11

More reactive non-metals are stronger oxidising agents than less reactive non-metals.

Question 12

What is the redox relationship between more reactive non-metals and less reactive non-metals?

Answer 12

A more reactive one is able to oxidise the ions of a less reactive one

Question 13

Describe the reactivity series down Group 7 (halogens)

Answer 13

The tendency to be the oxidising agent and become reduced decreases down the group.

Question 14

What happens in the half-cell containing a zinc strip in a voltaic cell?

Answer 14

• Zinc ions are formed
• Surface of the metal becomes negatively charged with respect to the solution

Question 15

What happens in the half-cell containing a copper strip in a voltaic cell?

Answer 15

• Some copper ions in the solution accept electrons from the copper rod
• The rod becomes positive with respect to the solution

Question 16

What happens in the anode of a voltaic cell and what charge does it have?

Answer 16

• Oxidation occurs
• The rod has a negative charge

Question 17

What happens in the cathode of a voltaic cell and what charge does it have?

Answer 17

• Reduction occurs
• The rod has a positive charge

Question 18

Which way do electrons flow in a cell?

Answer 18

From anode to cathode.

Question 19

What is the voltage determined by in a voltaic cell?

Answer 19

The difference in the reducing strength of the two metals.

Question 20

Draw a voltaic cell

Answer 20

Question 21

Draw a diagram of an electrolytic cell

Answer 21

Question 22

What happens in the anode of an electrolytic cell and what charge does it have?

Answer 22

• Oxidation occurs
• Positive charge

Question 23

What happens in the cathode of an electrolytic cell and what charge does it have?

Answer 23

• Reduction occurs
• Negative charge

Question 24

What is the energy conversion in an electrolytic cell

Answer 24

Electrical energy is converted into chemical energy

Question 25

Show the reactions that occur in the electrolysis of molten NaCl

Answer 25

Anode: 2 Cl^–(l) → Cl₂(g) + 2e^–

Cathode: 2 Na⁺(l) + 2e^– → 2 Na(l)

Overall: 2 NaCl(l) → 2 Na(l) + Cl₂(g)

Question 26

What is the assigned electrode potential for the standard hydrogen electrode?

Answer 26

0 V

Question 27

Describe the standard hydrogen electrode and the reaction that occurs in it

Answer 27

2H⁺(aq) + 2e^– ⇔ H₂(g)

Question 28

Define standard electrode potential, Eº

Answer 28

When the standard hydrogen electrode is connected to another standard half-cell, the emf generated is known as the standard electrode potential

Question 29

In what direction do electrons flow?

Answer 29

Towards the half-cell with the highest E° value

Question 30

What is the cell potential?

Answer 30

The emf of a cell

E°_cell = E°_{where reduction occurs} – E°_{where oxidation occurs}

Question 31

How is the spontaneity of a reaction determined from the E° values?

Answer 31

E°_cell is positive for all spontaneous reactions

Question 32

What happens in the electrolysis of water?

Answer 32

• O₂ evolve at anode and H₂ evolve at cathode
• The pH at the anode decreases while the pH at the cathode increases

Question 33

What happens in the electrolysis of NaCl(aq)?

Answer 33

• 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

Question 34

What happens in the electrolysis of CuSO₄?

Answer 34

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

Question 35

What is electroplating?

Answer 35

The process of using electrolysis to deposit a layer of a metal on top of other conductive substance

Question 36

What are the features of an electrolytic cell used for electroplating?

Answer 36

• An electrolyte that contains the metal ions which are to be deposited
• The cathode made of the object to be plated

Question 37

What are three purposes for electroplating?

Answer 37

• Decoration
• Corrosion control
• Improvement of function