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1

Redox Reactions

reduction- oxidation
- when on or more electrons are transferred from one species to the other

2

oxidation

loss of electrons, oxidation number increases

3

reduction

a gain of electrons, oxidation number decreases

4

half-reactions

they are like writing one element and the electrons it looses/gains

5

e- is product

oxidation

6

e- is reactant

reduction

7

oxidizing agent

the one that gets reduced

8

reducing agent

gets oxidized

9

oxidation number

elements start out with an oxidation number of zero

for ions composed of only one atom, the oxidation number is the charge on the ion

10

oxidation number of oxygen

is mostly -2 even if its covalent

Exception:

Perioxides (R2O2) has a charge of -1

11

oxidation number of hydrogen

is +1 whether its ionic or covalent

exceptions:

when Hydrogen is bound to a binary metal ex. NaH, it has an oxidation number of -1

12

fluorine oxidation number

has -1, as do most halogens

13

exceptions to oxidation numbers

halogens other than fluorine is combined with oxygen to form an oxygen, the oxidation number becomes POSITIVE.

ClO- becomes +1 oxidation for Cl

14

sum of oxidation number

must give charge of compound

ex. ClO2-

Cl is +3
O2 is -4

15

electrons are -ve so...

if oxidation number for a specific atom becomes more +
- it lost electrons and is oxidized

if oxidation number for an atom becomes more -ve
- it gained electrons and is reduced

16

how to balance redox eqns

- write out half reactions

17

how to do half reaction

1) write unbalanced reaction in ionic form
2) separate ion into two reactions (one compound and its ions, and the other)
3) balance atoms other than o and H in each reaction separately, then add water to balance the O and H+
4) add electrons to one side of each half reaction to balance the charges
5) if needed, multiply both balanced reactions by an integer to equalize the number of electrons transferred in reactions
6) add them together and cancel identical species

18

converting chemical energy into electrical energy

when reagents are mixed, the electron transfer occurs directly and no electrical work can be obtained

19

electrical work

can be obtained through a movement of electrons through a wire.

- a spontaneous redox reaction produces electrical energy in a cell

20

energy cells

are galvanic cells

21

galvanic cells

1) solutions (cu and zn 2+?) are kept in separate containers called half cells.

2) electrode is dipped in each half cell. electrodes provide a surface for each of the chemical half-reactions to take place. these electrodes are made of the two metals involving Solid zinc, in zinc solution and solid copper in copper solution) or from a chemically inert metal such as plutonium

3) the oxidation container and redox container are connected by a wire. and the transfer of electrons occurs indirectly through the external wire

22

galvanic cell reaction notes

as reaction continues on, the solid zinc will get smaller (oxidation, anode, +)

as reaction continues on, the solid copper will get larger (reduction, cathode, -)

voltmeter is attached to wire to measure stuff

23

galvanic cell reaction without salt bridge (??)

as reactions take place, the zinc oxidizes and their are more ions in solution. the copper gets reduced and copper ions go into the metal. which would oppose electron flow and stop reaction as the anode is getting positive and the cathode is getting -ve

24

galvanic cell design must contain

species in solution and concentration, electrode and what it is made of, direction of electrons, voltmeter, salt bridge, labels of cathode/anote and reduction/oxidation and half reaction enqs

25

cell potential

- is cell electrical energy,
- measure of the force behind redox run
- E (cell potential) describes the ability of each reagent to donate or withdraw electrons

26

cell potential depends on

chemical reaction and concentration of solutes and/or partial pressures of gases

27

voltmeter

- measure cell potential
- requires the flow of electrons which is slightly lower
- part of the cell potential is used to overcome the resistance of the wires in the voltmeter
- will measure a potential that is less than the maximum cell potential

28

shorthand notation

phase boundaries are represented by a: /

reactants are written before products

reduction is written on the right

ex.

Zn (s)/ ZN2+ (aq, 1 M) // Cu 2= ..

Reactants/Products//Reactants/Products

the double slash is the salt bridge.

29

oxidation number notation

write the charge BEFORE THE number

ex. -1 vs 1- (this is charge)

30

halogen-Ox ion charge (ex. ClO3-)

this will always have a charge of 1- overall