Elements from the Sea - Electrolysis

Question 1

What is the rule for the oxidation number of an element?

Answer 1

oxidation number = 0

Question 2

What is the rule for the oxidation number of a simple ion?

Answer 2

oxidation number equal to the charge of the ion

Question 3

What is the oxidation number of oxygen in compounds (normally)?

Answer 3

-2

Question 4

What is the oxidation number of Hydrogen in compounds (normally) ?

Answer 4

+ 1

Question 5

What is the rule for the oxidation number of a compound?

Answer 5

The sum of the oxidation numbers of the elements present = 0

Question 6

What is the rule for the oxidation number of complex ions?

Answer 6

sum of oxidates = charge of the ion

Question 7

If the oxidation state goes up, has it been oxidised or reduced?

Answer 7

Oxidised

Question 8

If the oxidation state goes down, has it been oxidised or reduced?

Answer 8

Reduced

Question 9

What is electrolysis?

Answer 9

Using electricity to make a chemical reaction occur

Question 10

What are the charges of the anode and cathode?

Answer 10

Anode - positive

Cathode - negative

Question 11

What are the rules for what is produced at the anode if electrolysis is done in aqueous conditions?

Answer 11

If the metal is group 1 or 2 -> hydrogen

Any other metal -> the metal present

If an acid is present -> hydrogen

Question 12

What are the rules for what is produced at the cathode if electrolysis is done in aqueous conditions?

Answer 12

If a halide is present -> halogen produced

If a sulfate or nitrate is present -> O₂

If a hydroxide is present -> O₂

Question 13

What is produced if a reactive anode is used?

Answer 13

copper

Question 14

What is the balanced half equation for the production of hydrogen from a metal salt in electrolysis?

Answer 14

2H₂O_(l) + 2e^- -> 2OH^-_(aq) + H_2(g)

Question 15

What is the balanced half equation for when oxygen is produced at the cathode when a sulfate/nitrate is present?

Answer 15

2H₂O_(l) -> O_2(g) + 4H⁺_(aq) + 4e^-

Question 16

What is the balanced half equation for the production of oxygen at the cathode when a hydroxide is present?

Answer 16

4OH^-_(aq) -> O_2(g) + 2H₂O_(l) + 4e^-

Question 17

What is disproportionation?

Answer 17

When two (or more) molecules start with the same oxidation state and end with 2 different states

Question 18

What is an example of disproportionation?

Answer 18

production of bleach:

starts as Cl₂

ends as OCl^- and Cl^-

Question 19

What is a dynamic equilibrium?

Answer 19

when the rate of forward reaction = rate of backward reaction

Question 20

What is Kc?

Answer 20

the equilibrium constant at a given temperature

Question 21

What does it mean if the Kc value is very high?

Answer 21

more product than reactant is being formed

Question 22

What does it mean if the Kc value is close to 1?

Answer 22

the equilibrium is balanced

Question 23

What does it mean if the Kc value is very small?

Answer 23

More reactants are being formed than products

Question 24

How can Kc be calculated?

Answer 24

[product] / [reactant]

Question 25

What would the Kc value of

Cl₂ + 2OH^- ⇌ Cl^- + OCl^- + H₂O

be?

Answer 25

Kc = [Cl^-] [OCl^-] [H₂O] / [Cl₂] [OH^-]²

Question 26

What is the effect on the equilibrium if the pressure is increased?

Answer 26

if reactants has more moles than products:

forward reaction favoured as there will be a reduction in moles of gas present - original pressure restored

if products has more moles than reactants:

back reaction favoured as there will be a reduction in moles of gas present - original pressure restored

Question 27

What will the effect on increasing the temperature on the equilibrium if the back reaction is endothermic?

Answer 27

back reaction favoured - endothermic will reduce the temperature back to original conditions

equilibrium will shift to the left

Question 28

What concentration is being calculated in an iodine-sodium thiosulphate titration?

Answer 28

the concentration of the oxidising agent

Question 29

What are the stages of a thiosulphate titration?

Answer 29

1. measure 25cm³ of the oxidising agent (potassium iodate)
2. add to excess potassium iodide (this will oxidise iodide ions)
3. find out moles of iodine produced by titrating against sodium thiosulphate

Question 30

How can the accuracy of a iodine-sodium thiosulphate titration be improved?

Answer 30

towards the end of the titration, add starch solution to see the colour change easier

will go from blue-black to colourless

Question 31

What is the balanced equation for the redox reaction between iodine and thiosulphate ions?

Answer 31

I_2(aq) + 2S₂O₃^2-_(aq) -> 2I^-_(aq) + S₄O₆^2-_(aq)

Question 32

What is a by-product?

Answer 32

a product produced by another reaction that we don’t want or need

Question 33

What is a co-product?

Answer 33

Another product produced from the same reaction that isn’t necessarily useful

Question 34

What is atom economy?

Answer 34

the % of useful products produced in a reaction

Question 35

What is the equation for atom economy?

Answer 35

mass of useful products / mass of useful reactants

Question 36

What is a waste product?

Answer 36

a co-product that isn’t useful

Question 37

What type of reaction has the best atom economy?

Answer 37

addition reactions

Question 38

What type of reactions have the worst atom economy?

Answer 38

elimination reactions