Chapter 9-Redox

Question 1

An element lower in the reactivity series can do what to an element higher in the reactivity series?

Answer 1

Oxidise

Question 2

An element higher in the reactivity series can do what to an element lower in the reactivity series?

Answer 2

Reduce

Question 3

Metal higher in the series?

Answer 3

Displace metals lower in the series from its salt

Question 4

Writing half equations (4 steps)

Answer 4

1) Balance the # of atoms of the element being oxidised/reduced on both sides
2) Add water molecules to balance the # of oxygen atoms on the 2 sides
3) Add hydrogen ions to balance the # of hydrogen atoms on the 2 sides
4) Add the e- so that the electrical charges on both sides are balanced

Question 5

BOD

Answer 5

Biochemical Oxygen Demand
→ The measure of dissolved oxygen (in ppm) required to decompose the organic matter in water = biologically over a set time period (usually 5 days)

Question 6

Winkler Method

Answer 6

1) Manganese (II) sulfate = added to water to create a white precipitate of manganese hydroxide (Mn(OH)2)

Question 7

The amt of product that results from electrolysis depends on? (3)

Answer 7

1) amt of e- required to produce one mole of product
2) The magnitude of the current (rate of flow of e-)
3) The time taken for which the current is passed

Question 8

Charge (coulomb) = ?

Answer 8

Current (Ampere) x Time (seconds)

Question 9

Amt of e- (moles) = ?

Answer 9

Charge passed (c) / 96500 cmol^-1 (Faraday’s constant)

Question 10

In Voltaic cell, the more reactive metal will be at which electrode?

Answer 10

Negative electrode = anode

Question 11

Cell diagram is written which way?

Answer 11

+ Oxidation is on LHS

+ Reduction is on RHS

Question 12

Standard electrode potential

Answer 12

The potential (voltage) of the REDUCTION half-eq under standard conditions measured relative to the SHE

Question 13

SHE conditions (4)

Answer 13

+ Inert Platinum electrode → coated w/ ‘Platinum Black’ = v. finely divided platinum that catalyses the electrode equilibrium
+ 1 mole of H ions
+ H gas at 298K
+ Pressure of 1 atm (100kPa)

Question 14

Predictions made using the electrode potentials don’t indicate what?

Answer 14

Ea (Activation Energy)

** If it has a high Ea, even the most favourable reactions may NOT occur in practice!

Question 15

If the electrode potential is quite close (0.2V diff.) what happens?

Answer 15

Equilibrium = rather than completion will occur

** Can use Le Chatelier’s Principles

Question 16

What is the equilibrium that is set up?

Answer 16

Ox + ne- ⇌ Red

Question 17

↑ [Ox] / ↓ [Red] what happens to equilibrium, number of electrons and E° ?

Answer 17

1) Equi shifts to the RHS
2) ↓ in # of electrons
3) E° more +

Question 18

↓ [Ox] / ↑ [Red] what happens to equilibrium, number of electrons and E° ?

Answer 18

1) Equi shifts LHS
2) ↑ in # of electrons
3) E° more -

Question 19

Metal/ Metal ion Electrode Equilibrium

Answer 19

(M^n+) + ne- ⇌ M

Question 20

1) ↑ conc. of metal ions = ?

2) ↓ conc. of metal ions = ?

Answer 20

1) E° more +

2) E° more -

Question 21

Typical sort of half equations → seq. usually go in what steps? (4)

Answer 21

1) Balance the atoms apart from oxygen and hydrogen
2) Balance the Oxygen’s by adding water molecules
3) Balance the Hydrogen’s by adding Hydrogen ions
4) Balance charges by adding e-‘s