1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Electrolysis, charge of ions.

  1. Jul 30, 2010 #1
    Hi, my names Adam, this is my first post im in year 10 UK (10th grade i believe)

    This is not a homework question, its advice towards the analysis of my chemistry coursework.
    Im predicted an A* in chemistry so this is why my experiment is more complicated than others in my class.I based my experiment on electrolysis, I changed the charge of the ions, referring to the formula I=nAve, by increasing charge of ions (e), current (I) will increase, which is directly proportional to moles deposited. I weighed the cathode before and after using my 3 electrolytes, which had a charge of +1,+2 and +3. I noticed the trend i would expect, higher charge = higher mass deposited thus higher moles deposited. This may sound stupid, but why is the weight higher, do higher charge ions have a higher mass? are more attracted to the cathode? can you explain the weight difference.

    Many Thanks, Adam
     
  2. jcsd
  3. Jul 31, 2010 #2

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    "Not homework?" Schoolwork is "homework." This needs to be moved.

    That said, you have omitted ALL the information pertaining to the actual conduct of the experiment. You've given us a statement of the object of the experiment, and of your conclusion, but nothing about experimental details. Fill in the blanks and we'll give you a few hints through the trickier conceptual parts (which you've completely misunderstood to this point).
     
  4. Jul 31, 2010 #3
    ok, sorry, shall i run through my method?
     
  5. Jul 31, 2010 #4

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Yes. You might want to read the homework guidelines --- it'll save a lot of time.
     
  6. Jul 31, 2010 #5
    ok, i badly feel as if i have got off to the wrong start :frown:

    Our objective was to see how changing the four major factors in the formula I=nAve affects the overall current (Amps). My group of three was made up of the smarter people in the class, so instead of changing A (surface area of electrodes) or v (velocity - temperature or voltage) we changed e (charge of ion).

    Because I is directionally proportional to moles deposited we will weigh the cathode ( because we are changing the postive ions) before and after we electolyse the electrolyte. By finding the mass change we can then divide it by the atomic mass of the element and find the moles deposted.

    We used three electrolytes :
    Silver Nitrate AgNO3 +1 ion charge
    Copper Chloride CuCl +2 ion charge
    Iron Chloride FeCl + 3 ion charge

    As i expected, we saw the trend we expected, the higher the charge of ion, the bigger mass change, more moles deposited.

    Because the higher charge ions have a higher charge, are they attracted to the cathode more vigerosly, do they have a higher mass. Essentially, can you help explain why the higher charged electrolytes deposited more metal on the cathode. This may be a stupid question.

    Hope this helps Adam
     
  7. Jul 31, 2010 #6

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Much better, thank you.

    Now, we still need to know some things: currents you observed for each cell; times you left the cells running between weighings of the cathodes; cathode materials; anode materials --- and, if just the list of missing information is hint enough for you, drop in again sometime.
     
  8. Jul 31, 2010 #7
    ok, i first used copper electrodes, but after my preliminary tests, i found in the CuCl solution, copper was transfering off the anode into the electrolyte, then onto the cathode, this makes the test unfair, so i changed to graphite electrodes which are very unreactive.

    I electrolysed the solutions for 2minutes at 12V each time.

    Do you need the info on the currents? as i have already found out the moles deposited from the mass change. But i would like to know more about the characteristics of +1,+2, and +3 charged ions.
     
  9. Jul 31, 2010 #8

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Currents AND mass changes.
     
  10. Aug 1, 2010 #9
    okay, this is going to get really complicated now.

    I tested each electrolyte 3 times to find an average, i eliminated any outliers.

    Element # Test No. # Mass Change # Atomic Mass # Moles Deposited

    Ag+1-------I]1) 0.082 /108 = 0.000759 ***OUTLIER***[/I]
    --------------2) 0.033 /108 = 0.000305
    --------------3) 0.032 /108 = 0.000296
    AVERAGE MOLES DEPOSITED FOR +1 = 0.0003008

    Cu+2--------1) 0.070 /63.5 = 0.00110 ***OUTLIER***
    --------------2) 0.040 /63.5 = 0.00063
    --------------3) 0.030 /63.5 = 0.00047
    AVERAGE MOLES DEPOSITED FOR +2 = 0.00055

    Fe+3----------1) 0.110 /56 = 0.00196
    ---------------2) 0.110 /56 = 0.00196
    ---------------3) 0.106 /56 = 0.00189
    AVERAGE MOLES DEPOSITED FOR +3 = 0.00194
    Sorr its hard to read, this format wont let me set it out clearly

    Now if i simplify my averages, i get 30, 55, and 190. This shows the trend i would have expected, although i believe this is wrong. The ion charge went up by 1 each time (+1,+2,+3) so i would expect the averahes to be more linear, say 50, 100 and 150. Agian, hers the complicated bit, on Cu+2, we see i have taken Test No.1 as the outlier, well suppose that is the correct value, and Test No 2 and 3 are actually outliers, then my averages would look like this. 30, 110, 190 ---- this is almost a perfectly linear incline, going up by 80 each time.
    Does this make sense?
     
  11. Aug 1, 2010 #10

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    CURRENTS! Those numbers from that "thing" called an ammeter; they're what the experiment is all about.

    The mass changes are what? Grams, tons, kilograms?

    You used the same electrodes throughout the experiment?

    What was your power supply?
     
  12. Aug 1, 2010 #11
    I didnt put the currents in because they dont seem to be included in my results. That "thing" called an ammeter was connected to a variable resistor in my circuit.
    The initial amp reading at the start of each test was 0.65 amps.
    Readings after ...
    Silver+1 1)0.71 2)2.62 3)2.92
    Copper+2 1)0.75 2)0.81 3)0.78
    I dont have the results for Iron+3.

    Sorry about the mass, yes they are grams. After preliminary findings, i changed to graphite electrodes, yes i used them for all tests to keep it fair.

    Input volts i have already said ...

    You seem to have ignored my moles results, do you agree? do you understand my theory about
    the wrong outliers, and that it should be linear?

    and i just need to know about the charged ions; my original question.
     
    Last edited: Aug 1, 2010
  13. Aug 1, 2010 #12

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Enough with the "20 Questions" --- you're probably trying to get this finished up to turn in Monday. You've been cooperative, so we'll get to the bottom line --- your experiment is a disaster both in design and data, and none of that is your fault; you haven't picked up too well on hints steering you in more productive directions with your analysis, and that could be my fault, yours, your school's, or some combination.

    Graphite anodes? Probably not a problem. Graphite cathodes? Kiss of death for weighing purposes, kiss of death for "constant area." Constant voltage? Pointless --- a perfect electrolysis setup is going to present a zero voltage drop. Compare your electrolytes at constant current to see the effect of ionic charge.

    "Relationship between mass and charge?" No silver platter for you on this one: 1) recall the definition of an ion; 2) compare the mass of that ion to that of the neutral parent atom; 3) plate that ion out on your cathode and think about what is involved in the interactions of ions, electrons, and neutral atoms in the plating process.

    Recall the definition of electric current; compare that to the mass required per unit time to carry an equivalent amount of charge for mono-, di-, and tri-valent cations.

    I'll duck in every hour or so for questions --- you got handed something totally inappropriate to any reasonable expectations for your level in the educational process, and it has confused you far more than enlightened you.
     
  14. Aug 1, 2010 #13
    First let me thank you for your time and help:smile:

    My design is ok, and my data could be better lets say, but it does show a trend. In england were currently on summer holidays, so this isnt due for at least another 6 weeks.

    I dont understand what your saying about the electrodes, i can assure you the weighing was fair and we kept everything constant. The depth the electrodes were in the solution was measured, and the distance apart was kept constant.

    I wanted to know more about ions, i already understand a bit, and a bi about ionic bonding. (not trying to sound big headed) but im top of my GCSE class, but a lot of what your saying is going over me. We havnt learned about mono, di or trivalent cations, could you explain easily? if not, then its not a problem. I wanted to know how to justify the different mass changes, is because +3 ions are attracted to the cathode more vigerously then the +1 ions. Or is it because +3 ions already have a bigger mass than +1 ions. Sorry if im sounding stupid here, but im trying.

    I have attached a copy of my coursework so far, its not much and im sure its not perfect, but maybe it can explain my experimant better. I have a lot of results, explanation, analysis and conclusion material in note form in my book which i have yet to write up. I will knuckle down over the next few days to get a better version for you to see, if you still care.

    Once again, thanks for your help :) please take a look at the attachment.

    Adam
     

    Attached Files:

  15. Aug 2, 2010 #14

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Six weeks --- that's plenty of time to work this out for you. Haven't been able to open the attachment yet --- takes a little time for it to clear hurdles set up to reduce spam. Once it's clear, I'll look that over and get back to you.
     
  16. Aug 4, 2010 #15
    I can view it, and theres 3 views? is it visible to you yet?
     
  17. Aug 5, 2010 #16

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    "There was an error opening this file." Eleven views. Can you give me a short summary with "spoilers," and we'll get on with this?
     
  18. Aug 5, 2010 #17

    Gokul43201

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Opened fine for me. Here are the important parts:

    27xnof8.png oge040.png

    vgr9tf.png wukoyg.png

    2yv2gc9.png wtvecw.png
     
  19. Aug 5, 2010 #18

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Thankee, Gokul // Give me a couple hours with this, AdamCFC, and I'll get back to you with a couple more questions/hints.
     
  20. Aug 5, 2010 #19
    PM me your email address and ill send it in either Word or PDF format :),

    EDIT: sorry for some reason Gokuls post didnt come up at first, cheers for that :) Like i say, thats all i have written up, i have a results table, a graph, although thats on paper but i could re-create on word, and some analysis, ill upload more during the weekend :)
     
    Last edited: Aug 5, 2010
  21. Aug 5, 2010 #20

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    "By increasing any of the above, overall, ... ;" and, you are controlling n, A, v constant to test your hypothesis.

    The way PF is supposed to work is that you ask for help, and people give you hints rather than doing the work for you --- my preference is to ask you questions that lead you to the answer to your question. I ain't messing with you --- I want you to understand the principles behind the things you're trying to learn.

    "A" we won't worry about for now, you've taken appropriate steps to keep that constant; there are refinements we can discuss later.

    Now for the first couple questions: 1) do you understand the distinction between voltage and velocity for "v?" 2) how are you keeping "n" constant for your three solutions? State the concentrations you used for each, and remember the definition you've given for "n."
     
  22. Aug 5, 2010 #21
    Here goes; Velocity is the speed and direction the ions have, does this relate to kinetic energy of the ions? By increasing the voltage or temperature you can give the ions more energy, through either thermal energy or kinetic energy and this can increase the velocity. So to keep the "v" constant, i must keep the voltage and temperature the same. I can keep the voltage the same through the power-pack, and eliminate errors by using the same power-pack throughout my experiment, as its very hard to keep the temperature the same in a classroom enviroment, i have/will state that im doing all 3 tests during a 3 day period, at room temperature. This is as accurate as i can make it.

    I am keeping "n" constant by using the same amount of electrolyte at the same molarity? I used 0.1mol concentrations for each electrolyte. "n" is number of ions.

    Is this right though :(, i am assuming all three solutions have the same proportion of ions? <--- does that make sense?

    Thanks, Adam
     
  23. Aug 5, 2010 #22

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    "Yes --- and --- no." The expression you've been given to work with, "I=nAve," is missing a couple details: 1) n is the number of ions per unit volume (concentration); 2) it is necessary to sum over ALL ion species. The actual "drift" velocity of ions in aqueous solution at room temperature is so low as to be insignificant compared to the velocities due to random thermal motion (fingers crossed, you've been introduced to Brownian motion, or Boltzman, or some discussion of molecular/atomic vibration at temperatures above absolute zero. If not, holler.

    You're aware that you have to control these factors, and did what you could with the resources available. Cool.

    Okay, much as I suspected. I drop silver nitrate, AgNO3, into water and it dissolves to give me a silver cation, Ag+,and a nitrate anion, NO3- in solution, TWO charge (current) carrying species.

    Given that information, what happens when I drop cupric chloride, CuCl2[/ (one copper atom combined with two chlorine atoms), into water?

    And, it's your bedtime --- catch you in the morning.
     
  24. Aug 6, 2010 #23
    No,we havnt been introduced to any of those laws :(, i think thats more A level stuff, do you have the same grade system in america, GCSE, A Level?

    Would Copper Chloride issolve into Cu+ anion, and 2Cl- cations.
    Would Iron Chloride dissolve into Fe+ anion and Cl- cation.

    Finally, because the Anode is positive, because the metals become postively charged, dont they become an-ions, instead of cations?
     
  25. Aug 6, 2010 #24

    Bystander

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    The energy a charged particle (anion or cation) would acquire "falling" freely through an electrical potential difference is just the potential difference in volts times the charge number times 100kJ/mol; this is forty times greater than the thermal energy of particles of equivalent mass at room T. However, when "freefall" is obstructed as in ions in aqueous solution, tens to hundreds of collisions with neutral particles occur for every nanometer of travel in the potential gradient, robbing the ion of the energy it would have picked up in freefall.

    The educational fads in the U.S.? Gibberish, political correctness, sensitivity, and who knows what other nonsense translated into seat time equals diplomas for nincompoops, and actual learning and demonstrated skills are penalized --- guess that'd be "Lysenko's New Soviet Man."
    "Conservation of charge," one of seven conservation laws chiselled literally in stone; ions produced from a neutral substance HAVE TO OBEY.

    One mole of silver salt equals two charge carrying species, one mole of copper? And iron?
    Cation, a positively charged species formed when a neutral species gives up an electron; Cathode, an electrode that gives electrons to other species, or vacuum (in a CRT), and "cat-" can be regarded as a prefix meaning "source of electrons."

    Anode, an electrode that "sinks" electrons from an electrical circuit or from species in solution; Anion, formed when a neutral species accepts, takes, picks up, acquires an electron from some electron source.

    "Cathode" and "anode" can be a little tricky to interpret on occasion --- stick with the definitions, and you'll usually be okay.
     
  26. Aug 6, 2010 #25
    Ok, so:
    -Anode is the positive electrode
    -Cathode is the negative electrode
    -An-ions are attracted to the An-ode because they are negatively charged
    -Ca-tions are attracted to the Ca-thode because they are positively charged

    Also, do i need to include information about the ion masses? if so, would Cu+2 be, the mass of a copper atom, minus the mass of 2 electrons?

    I will upload a improved version soon, if you cant view it, shall i email it to you, because im not sure to show it how gokul did.

    Regards, Adam
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook