Question

#1: A reaction in an electrolytic cell is as follows: 2NaCl (aq) + 2H2O (l) --> Cl2 (g) + H2 (g) + 2NaOH (aq). Which of the following reactions occurs at the cathode?

A. 2H2O (l) + 2e- --> H2 (g) + 2OH- (aq)

B. 2Cl- (aq) --> Cl2 (g) + 2e-

C. H2 (g) + 2OH- (aq) --> 2H2O (l) + 2e-

D. Cl2 (g) + 2e- --> 2Cl- (aq)

** no idea! :( @aaronq what do u think? :D

Answer 1

Try to identify oxidation and reduction reaction first.

Answer 2

the last one is anode right? (oxidation) ??

Answer 3

^ yep, use the reactions given to identify whats being reduced

Answer 4

No, it is reduction reaction.

Answer 5

Oxidation is loss of electrons , and reductions always occur at cathode...

Answer 6

oh :/ I'm confused hahaha :( idk how to tell which is cathode tho :(

Answer 7

In the last reaction, there is gain of \(e^{-}\) so it is , reduction reaction.

Answer 8

You can learn the concept of electroplating on net, but here is the trick :

Reduction always occur at cathode.
Oxidation always occur at anode.

Answer 9

ohh i see.. so the answer to my problem is the last equation?? D ? :O

Answer 10

Yes, I think . @aaronq do you favor me ? that it is D. ?

Answer 11

hmm D looks like reduction reaction, but it's not whats written in the equation above
2NaCl (aq) + 2H2O (l) --> Cl2 (g) + H2 (g) + 2NaOH

Answer 12

Cl is actually being oxidized from -1 to 0

Answer 13

Yes. Agreed.

Answer 14

Look at A)

Answer 15

im confused haha :/ wait so its asking for which is taking place at the cathode.. :/ so it's not D? :/

Answer 16

A. 2H2O (l) + 2e- --> H2 (g) + 2OH- (aq)

[Here is the removal of oxygen] - reudction reaction.

Answer 17

ohh okay so the answer to this is A then? :O

Answer 18

Wait iheartfood.

Answer 19

@aaronq , in the options given, only A and D are reduction reactions, right?

Answer 20

@Thoughts yeah, only A and D

Answer 21

kk :)

Answer 22

D - out, so left -- A

Answer 23

ohh okay!! :) awesome! thank you so much! :D

Answer 24

So it is A!

Answer 25

yep! @iheartfood do you know how to assign oxidation numbers?

Answer 26

awesome! :) ermm not really :/ how do i do that? :)

Answer 27

oh okay, because what would let you figure whats being oxidized and reduced easily (i.e. without guessing :P)

Answer 28

haha umm im gonna take a educated guess (its educated so its okay :P just for this tho :P ) and say that knowing how to assign oxidation numbers will help figure it out more easily? :D

Answer 29

hm there are basic rules, like everything in the first column is always +1 (e.g. Na) and everything in the second is +2 (Mg).. likewise everything in the 7th column is -1 (F), oxygen is always -2. the rest you have figure out by balancing the charges, wanna try some?

Answer 30

okay :) sure!

Answer 31

haha okay, do you know the periodic table (or have one handy)?

Answer 32

i can pull it out :)

Answer 33

okay, because you'll need it :)

Answer 34

kk its out :)

Answer 35

okay, easy one, what are the oxidation numbers of NaCl?

Answer 36

umm not too sure haha

+3 ?

Answer 37

no okay remember the ones on the first column are always +1 (because they always lose only one electron)

so Na = +1, right? now if the compound is neutral Cl must have a charge of?

Answer 38

-1 ?

Answer 39

yeah ! okay try MgO

Answer 40

+1 = Mg O=0 ?

Answer 41

hm no remember if the compound is neutral, the charges have to balance out. Now which group (column) is Mg in?

Answer 42

3?

Answer 43

oh oops i meant 2 :P

Answer 44

no thats row :P, groups are up and down

Answer 45

haha okay, so Mg is +2 because it's in group 2, right?

Answer 46

so it should be Mg= +2
O=-2 ?

Answer 47

mhmm :)

Answer 48

yes!

Answer 49

okay, what about H2O?

Answer 50

H= +1
O= -1 ?

Answer 51

oxygen is always -2.. and remember there are 2 hydrogens

Answer 52

ahh okay.. so it's

H= +2

O= -2 ?

Answer 53

hm H = +1 but there are 2 right, so +1 x 2 = +2 so charges are balanced

Answer 54

OH^-

Answer 55

ahh i see :)

Answer 56

do OH^- !! lol

Answer 57

haha okay! :)

erm O= -1

H=1 ?

Answer 58

not sure what to do with the - sign tho :(

Answer 59

nope! oxygen is always -2

Answer 60

oh right! my bad!

so O= -2

H= +2 ?

Answer 61

H is always +1 :P

Answer 62

oxygen + hydrogen =
-2 + 1 = -1 which is the charge on the ion

Answer 63

ohh okay :P

Answer 64

do NH3

Answer 65

kk N= - 9 ?? :/

H= +1

Answer 66

haha nooo why -9?

3 hydrogens = +1 x3 = +3
1 nitrogen = -3

Answer 67

ohh okay :P

idk why lol :P

Answer 68

haha okay, few more: F2 (2 is a subscript)

Answer 69

kk :P

F2=4 ?

Answer 70

after these i just have one more problem! :)

Answer 71

haha okay. what nooo, anything elemental like Al (s) or bound to the same element Cl2, is zero.

Answer 72

ohh okay.. so its just 0?

Answer 73

yes

Answer 74

same with all the noble gases, they're all zero

Answer 75

ahh okay :)

Answer 76

okay 2 more! Al(OH)3

Answer 77

OH is an ion that stays together, what is the charge on it?

Answer 78

kk :)


ermm idk :(

-2+1=-1 ?

Answer 79

Aaronq , nice teaching man. :)

Answer 80

agreed^^ :)

Answer 81

haha thanks guys !

Answer 82

:) i speak nothing but the truth :P

Answer 83

so the charge on OH is -1, is that what you said?

Answer 84

**typo :P

I agree with nothing but the truth :P

Answer 85

yeah :) is that right? :)

Answer 86

haha okay okay you philosophers

Answer 87

hehee :P

Answer 88

so if there are 2 OH ions, what is the charge on Al to make it balanced?

Answer 89

i mean 3 OH ions

Answer 90

+3?

Answer 91

yeah ! are you getting it, or no? lol

Answer 92

yes :)

Answer 93

haha okay good, because now you have to do an equation:

2NaCl (aq) + 2H2O (l) --> Cl2 (g) + H2 (g) + 2NaOH

Answer 94

aww :( wowzers!

kk lets see..

2NaCl= +2 , -2 ?

2H2O = +2, -2 ?

Answer 95

Cl2= -1 x 2 = -2?

H2= +1 x 2 = +2?

Answer 96

hm well, for NaCl, disregard the coefficient because you're assigning the oxidation number per species

Answer 97

ono remember anything attached to itself is zero? Cl2, H2, Br2, F2