Topic 7: Oxidation and reduction (redox reactions) (physical chem)

Question 1

define oxidation

Answer 1

loss of electrons

Question 2

define reduction

Answer 2

gain of electrons

Question 3

When does a redox reaction occur

Answer 3

when oxidation and reduction occur simultaneously (one species looses electrons which are then donated and gained by the other species)

Question 4

What is an oxidation number

Answer 4

Gives the oxidation state of an element or ionic substance
-e.g hydrogen = +1
-oxygen = -2
Group 7 = -1
Group 1 = +1

Question 5

Oxidising agent

Answer 5

-accepts electrons from the species that is being oxidised so it gains electrons and is reduced (oxidation no. becomes more negative)

Question 6

Reducing agent

Answer 6

-donates electrons to the species being reduced
-looses electrons and is oxidised (positive oxidation no.)

Question 7

half equations

Answer 7

-used to show separate oxidation and reduction reactions that occur

Question 8

state in terms of electrons the meaning of the term oxidising agent

Answer 8

electron accepter

Question 9

equation for oxidation of iron

Answer 9

Fe2+ –> Fe3+ + e-

Question 10

when is oxidation number the same as charge

Answer 10

only for monotomic elements

Question 11

Cl2 (aq) + 2Br (aq) –> 2Cl- (aq) + Br2 (aq) –> what is being oxidised and what is being reduced

Answer 11

Cl2 = reduced (neutral to negative)
Br = oxidised (negative to neutral)

Question 12

negative to neutral

Answer 12

oxidisation

Question 13

reduction vs oxidation

Answer 13

-Elements can either be oxidised or reduced (transfer of electrons between elements and the system)

-reduction = gain of electrons

-oxidation = loss of electrons

Question 14

Al3+ + 3e- –> Al what is happening in this equation

Answer 14

aluminum is reduced (gains electrons)

Question 15

2Cl- –> Cl2 + 2e- what is happening here

Answer 15

chlorine is oxidsed (looses electron)

Question 16

what side of the equation has oxidised and what side is reduction

Answer 16

oxidised = right hand side
reduction = left hand side

Question 17

steps to calculating half equations

Answer 17

1) molecule –> compound or compound –> molecule

2) Balance atoms

3) See charges to tell whether it is reduced or oxidised

4) Add electrons

5) Balance equation

Question 18

solid vs aqueous

Answer 18

Solid = no charge

Aqueous = charge

Question 19

Zn + CuSO4 –> ZnSO4 + Cu
write the two half equations

Answer 19

Zn –> Zn2+ + 2e- (oxidised)

Cu2+ + 2e- –> Cu (reduction)

Question 20

molecule to compound

Answer 20

reduction

Question 21

calculate the oxidation number in Fe2O3

Answer 21

Fe = +3
O = -2

Question 22

state in terms of electrons why iodine is classified as an oxidising agent in this reaction

Answer 22

-electron acceptor
-iodine gains electrons and becomes reduced

Question 23

what is oxidised and reduced in this equation using oxidation numbers

H2S + Cl2 –> S + 2HCl

Answer 23

S has been oxidised from -2 in H2S to 0 in S

Cl has been reduced from 0 in Cl2 to -1 in 2HCl

Question 24

define oxidation number

Answer 24

-Oxidation number shows us how many electrons are gained or lost by an element when atoms turn into ions

-Oxidation number –> the charge that an ion has or the charge it would have if the bonding electrons are completely transferred

Question 25

more positive vs negative oxidation number

Answer 25

The more electrons removed (positive oxidation number) = oxidation The more electrons added (negative oxidation number) = reduction

Question 26

rules for oxidation numbers

Answer 26

-uncombined element = zero -ions of just one atom = charge of the ion -neutral compound = zero -sum of oxidation numbers for an ion = charge of ion -Flourine in its compound = -1 -Group 1 metals = +1 -H = +1 except in metal hydrides which would be –1 -Oxygen = -2 expect when bonded to flourine or H2O2

Question 27

oxidising agent

Answer 27

oxidises another element and gets reduced itself by receiving electrons from the element becoming oxidised. (element reduced)

Question 28

reducing agent

Answer 28

Reducing agent --> reduces another element and gets oxidised by transferring its electrons to the elements being reduced (element oxidised)

Question 29

reducing and oxidation number in equation Mg + CuSO4 --> MgSO4 + Cu

Answer 29

-> Mg is oxidised (0 --> +2) --> Cu is reduced (+2 --> 0) --> Cu in CuSO4 is oxidising agent ---> Mg is the reducing agent

Question 30

equation for Cr3+ --> Cr2O7

Answer 30

2Cr3+ + 7H2O --> Cr2O7^2- + 14H+ + 6e-

Question 31

disproportionation

Answer 31

-Disproportionation is where an element is simultaneously oxidised and reduced in the same reaction -e.g Cl2 can be both oxidised and reduced in two separate compounds so has experienced disproportionation

Question 32

balancing redox equations

Answer 32

-chemical equation --> ionic equation --> half equation --> redox 1) Write half equations for reduction and oxidation 2) Add H2O to balance oxygen 3) Add H+ to balance hydrogen atoms 4) Add electrons to balance the charges (must be the same on both sides e.g -1 on both sides)

Question 33

combining half equations

Answer 33

1) Write both half equations balanced 2) Multiply so number of electrons on both equations are equal 3) Add half equations together and cancel out electrons, H+ ions and common species

Question 34

Explain in terms of oxidation states why magnesium is the reducing agent

Answer 34

Mg changes oxidation state from 0 to +2 so electrons are lost

Question 35

oxidation

Answer 35

ion --> molecule + electron

Question 36

A solution of sodium chlorate was added to a colourless solution of potassium iodidie. Suggest what happens and why

Answer 36

-solution goes brown -bc of iodine -iodine is oxidised

Question 37

oxidiation vs reduction

Answer 37

oxidation = increase in oxidation number reduction = decrease in oxidation number

Question 38

reducing agents are

Answer 38

electron donors (species that is oxidised)

Question 39

write a half equation for the reduction of chlorate ions to form chlorine in acidic conditions

Answer 39

2ClO- + 2e- + 4H+ ---> Cl2 + 2H2O

Question 40

true or false - oxidation state must equal charge of compound e.g SO4^2- --> charge of s is 6 instead of 8

Answer 40

true

Question 41

why is the oxidation state of chromium is zero in Cr(PF3)6

Answer 41

PF3 is neutral and the complex is neutral, so chromium must have an oxidation state of zero

Question 42

explain a low cost method for the extraction of copper from a dilute aqueous solution of copper sulfate

Answer 42

add scrap iron iron is more reactive than copper (better reducing agent) Fe + Cu2+ --> Fe2+ + Cu

Question 43

more reactive element =

Answer 43

better reducing agent

Question 44

equation for oxidation of iodine

Answer 44

2I- --> I2 + 2e-