Unit 3, Topic 2: Redox Reactions

Question 1

Redox reactions

Answer 1

• involve simultaneous reduction and oxidation reactions
• They involve the transfer of electrons from one chemical species to another

Question 2

Oxidation

Answer 2

Losing electrons

Question 3

Reduction

Answer 3

Gaining electrons

Question 4

Oil Rig

Answer 4

Oxidation Is Loss Reduction Is Gain

Question 5

Metals

Answer 5

Tend to oxidise - This is due to their differing electronegativities

Question 6

Non-metals

Answer 6

Tend to reduce - This is due to their differing electronegativities

Question 7

The number of electrons lost

Answer 7

determined by the electron configuration – tries to reach most stable electron configuration

Question 8

Oxidising agent

Answer 8

An oxidising agent or oxidant enables another chemical to be oxidised – they themselves are reduced

Question 9

Reducing agent

Answer 9

A reducing agent or reductant enables another chemical to be reduced – they themselves are oxidised

Question 10

Oxidation number

Answer 10

• Oxidation numbers and oxidation states are interchangeable terms
• The +- sign is before the number i.e. +2 and not 2+
• Based off the assumption that all bond in the compound are ionic

Question 11

Oxidation numbers in use

Answer 11

• Oxidation number can be used to determine whether the species will oxidise or reduce – an element in its highest oxidation state can only be reduced, vice versa is true – element in lowest oxidation state can only be oxidised

Question 12

Oxidisation (oxidation state)

Answer 12

increase in oxidation state

Question 13

Reduction (oxidation state)

Answer 13

decrease in oxidation state

Question 14

Oxidation number of free elements (e.g. O2)

Answer 14

zero

Question 15

Oxidation number of a simple ion

Answer 15

equal to charge e.g. oxidation number of Na+ is +1

Question 16

Oxidation number of main group metals

Answer 16

Main group metals have oxidation number equal to charge

Question 17

Hydrogen oxidation state

Answer 17

Hydrogen is normally +1 except in metal hydrides

Question 18

Oxygen oxidation state

Answer 18

Oxygen is normally -2 except in fluorine where it has a positive number and in peroxides where oxidation number is -1

Question 19

Fluorine oxidation state

Answer 19

Fluorine is always -1

Question 20

Sum of all oxidation numbers in a neutral compound

Answer 20

zero

Question 21

The sum of all oxidation numbers in a polyatomic ion

Answer 21

equal to the charge

Question 22

The most electronegative element

Answer 22

assigned the lowest oxidation number e.g. OF2 where O is +2 and F is -1+2 − (2 × −1) = 0

Question 23

Balancing redox reactions

Answer 23

1. Identify what has been oxidised and what has been reduced – half equations
2. A) Balance equations for everything but oxygen and hydrogen
   B) Balance oxygen using H2O(l)
   C) Balance hydrogen using H+(aq)
3. If necessary, multiply both equations by an integer so that the number of electrons is equal on both sides
4. Add two half equations cancelling out spectator ions etc.
5. Check for balancing
6. (If in basic solution) cancel out H+ ions by adding OH- ions to both sides of the equation

Question 24

Galvanic cells

Answer 24

• Galvanic cells (voltaic) are a type of electrochemical cell where chemical energy is converted to electrical energy
• Designed so that the half cells (oxidation and reduction half equations) are kept separately and are connected by an external circuit – facilitates transfer of electrons

Question 25

anode - Galvanic

Answer 25

negative

Question 26

cathode - galvanic

Answer 26

positive

Question 27

Salt bridge

Answer 27

Helps balance charges

Question 28

Half-Cells

Answer 28

* Each half cell is an electrode in contact with a solution * The chemical species in the half cells form a conjugate redox pair * If one member of the pair is metal, it is likely to be the electrode * If no metal is present, an inert electrode is used like platinum or graphite

Question 29

Salt Bridge

Answer 29

* The salt bridge houses ions that are free to move * The cations and anions in the bridge are able to move to either half-cell to balance electrical charge * Known as an internal circuit

Question 30

From an oxidised species to reduction species:

Answer 30

* The more positive the E° value is, the stronger it is as an oxidising agent * The more negative the E° value is, the stronger it is as a reduction agent

Question 31

E˚

Answer 31

determined off a standard half-cell with a hydrogen cell

Question 32

E˚ of hydrogen cell

Answer 32

0 V

Question 33

E𝐶𝑒𝑙𝑙° formula

Answer 33

E𝐶𝑒𝑙𝑙° = E𝑅𝑒𝑑𝑢𝑐𝑡𝑖𝑜𝑛° − E𝑂𝑥𝑖𝑑𝑎𝑡𝑖𝑜𝑛°

Question 34

Cell voltage greater than 0

Answer 34

spontaneous reaction

Question 35

cell voltage less than 0

Answer 35

nonspontaneous reaction

Question 36

Electrolytic Cell

Answer 36

Type of electrochemical cell where electrical energy is converted to chemical energy (opposite of Galvanic Cells) - Electrolytic cells have two electrodes in contact with the electrolyte connected to an external power supply

Question 37

Electrolysis

Answer 37

involves passing electrical energy through a conducting liquid o Causes non-spontaneous redox reactions to occur

Question 38

Cathode - electrolytic

Answer 38

reduction

Question 39

anode - electrolytic

Answer 39

oxidation

Question 40

power supply - electrolytic cells

Answer 40

The power supply draws electrons from the positive electrode to the negative electrode

Question 41

anions - electrolytic cells

Answer 41

Anions move towards anode

Question 42

cations - electrolytic cells

Answer 42

cations move toward the cathode

Question 43

Electrolysis of a Molten Salt

Answer 43

* Inert electrodes (platinum or graphite) are used – doesn’t interfere with the reaction * Electrons are supplied to the negative electrode and cycles through to the positive electrode

Question 44

Electrolysis of an aqueous solution

Answer 44

* In aqueous solutions, the water itself may be oxidised/reduced * Therefore, all four electrode potentials must be considered * The combination of oxidation and reduction with the more positive potential will occur

Question 45

Concentrated vs dilute

Answer 45

* In a concentrated NaCl solution, the secondary oxidation reaction (2𝐶𝑙(−𝑎𝑞) → 𝐶𝑙2(𝑔) + 2𝑒−) will occur * This is because the original electrochemical series is determined off standard conditions with 1 mol solutions * Also, in a concentrated solution, there are more chloride ions vs hydroxide ions – therefore their production will dominate

Question 46

Electroplating

Answer 46

- industrial use of electrolysis - Involve depositing a layer of metal on the surface of another metal - object to be plated is connected to the negative terminal - it becomes the cathode - the positive electrode (anode) is made of the metal to be plated onto the cathode and the solution contains ions of said metal - these ions will be reduced into the cathode

Question 47

Electrorefining

Answer 47

* Involves extracting pure metals from ores * When copper is extracted from ores, it is 98% pure and is further purified by electrorefining * Impure copper is at the anode and a pure copper sheet is the cathode * Oxidation of the copper occurs forming Cu2+ ions o Attracted to the cathode – essentially plating copper plates * Impurities left (zinc, nickel) are more reactive and become oxidised – stays as ions * Less reactive (gold, silver) aren’t oxidised – collects at the bottom of the solution