Simple Electrolysis - Kinetics ?

[Merlin66]

Hello,
I'm trying to look at 2Cl-(aq)) =Cl2(aq) + 2e- for a basic aqueous solution.
I've made with (n(Cl-) = n( e-) = 0,32 moles production of Cl- for 1h at 8.5 Amperes)

RATES
A1
-start
10 moles = mol("Cl2") - 0.32
20 save moles*time
-end
A2
-start
10 moles = mol("Cl-") + 2* 0.32
20 save moles*time
-end
KINETICS 1
A1; -formula Cl2 1
A2; -formula Cl- 1
-time 8

is it good ?

[dlparkhurst]

Did you look at the output?

I would split Cl2 as a separate element to allow disequilibrium between Cl- and Cl2. Here is a first-order decay simulation converting Cl- to Cl2. I used O2(g) as a means to add/remove electrons; you could also make the formula HCl. You can adjust the script for the direction of the reaction, and the rate.

SOLUTION_MASTER_SPECIES
    [Cl2]         [Cl2]            0     Cl2             71
SOLUTION_SPECIES
    [Cl2]    =    [Cl2]
    log_k 0.0
RATES
Cl2_production
10 k = 0.1 / (3600)  # 0.1 per hour
20 rate = k * TOT("Cl")
30 moles = rate * TIME
40 save moles
END
SOLUTION
Cl  1
END
USE solution 1
EQUILIBRIUM_PHASES
O2(g) -0.7 10
KINETICS
Cl2_production
-formula [Cl2] 1 Cl -2
-step 86400 in 24 steps
USER_GRAPH 1
    -headings               Time Cl Cl2
    -axis_titles            "Days" "Moles per kilogram water" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x total_time/3600
20 graph_y TOT("Cl"), TOT("[Cl2]")
  -end
    -active                 true

[Merlin66]

Thank you dlparkhurst !
It work well, but it's the redox way.
I'm trying to made the inverted way, ie using Cl2 to make Cl- with 2e
Unfortunatly, the solution 1 doesnt give any Cl2, then I cant produce nothing.
Btw I like the way to use O2 as equilibrium for electron

I'm searching.

[dlparkhurst]

SOLUTION_MASTER_SPECIES
    [Cl2]         [Cl2]            0     Cl2             71
SOLUTION_SPECIES
    [Cl2]    =    [Cl2]
    log_k 0.0
RATES
Cl2_production
10 k = 0.1 / (3600)  # 0.1 per hour
20 rate = k * TOT("[Cl2]")
30 moles = rate * TIME
40 save moles
END
SOLUTION
[Cl2]  1
END
USE solution 1
EQUILIBRIUM_PHASES
O2(g) -0.7 10
KINETICS
Cl2_production
-formula [Cl2] -1 Cl 2
-step 86400 in 24 steps
USER_GRAPH 1
    -headings               Time Cl Cl2
    -axis_titles            "Days" "Moles per kilogram water" ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x total_time/3600
20 graph_y TOT("Cl"), TOT("[Cl2]")
  -end
    -active                 true

[Merlin66]

Thank you :) I've modified it for the fun, still not correct but working :

SOLUTION_MASTER_SPECIES
    Cl(1)         HOCl             0     HOCl           
    Cl(-1)        Cl-              0     Cl             
    Cl(0)         Cl2              0     Cl     
    [Cl2]         [Cl2]            0     Cl2             71
SOLUTION_SPECIES
    [Cl2]    =    [Cl2]
    log_k 0.0
Cl2 + H2O = HOCl + Cl- + H+
    log_k     -2
2Cl- = Cl2 + 2e-
    log_k     -47.26
HOCl = ClO- + H+
    log_k     -7.5
2H2O = O2 + 4H+ + 4e-
    log_k     -200
RATES
Cl-_production
10 k = 8.5 / 96485 # 8.5 Amperes * 1 Seconde / Faraday

20 rate = k * TOT("[Cl2]")
30 moles = rate * TIME
40 save moles
END
SOLUTION 1
    redox     O(-2)/O(0)
Cl 5 g/kgw
Na 5 g/kgw
    [Cl2]        5 g/kgw
    O(0)      1 O2(g)      -0.7
    -water    1 # kg
END
USE solution 1
EQUILIBRIUM_PHASES
#O2(g) -0.7 10
#CO2(g) -3.481 10 # carbon dioxide at atmospheric pressure 0.033 kPa standard
KINETICS 1
Cl-_production
    -formula  Cl-  2 [Cl2]  -1
-steps     28800  in 32 steps # 8 Heures

USER_GRAPH 1
    -headings               Time Cl- [Cl2] pH
    -axis_titles            "Hours" "Moles " ""
    -initial_solutions      false
    -connect_simulations    true
    -plot_concentration_vs  x
  -start
10 graph_x total_time/3600
20 graph_y MOL("Cl-"), MOL("[Cl2]")
30 graph_sy -la("H+")
  -end
    -active                 true
END

[dlparkhurst]

You have eliminated O2(aq), which could be OK depending on what you are trying to do.

However, your -formula is not correct. Charge is not considered, so you have effectively removed [Cl2] and added Cl2. To effect the reduction, I think you want to have HCl 2 in the formula, which is equivalent to 0.5H2 + 0.5Cl2.

If you remove O2(aq) as you have done, then you could avoid defining [Cl2] and simply define the solution with Cl(0). Then you could add H2 to produce the reduction.

[Merlin66]

Calculating and entering log_k for redox half cell each time bother me.
Just want to see how Ampere of electrolysis affect pH and Cl2 production, then HOCl and OCl-
Because there's a little of Cl- (Cl(-1)) I imagine that the time of production must be limited, and more if pH fall to much.

 

[dlparkhurst]

Don't follow your comments about the half cell. For calculations without O2(aq), Cl-, HOCl, and OCl- are in equilibrium.

[Merlin66]

My comment concern the log_k wich is different for the temperature
For example: HOCl = OCl- + H+ #Dissolution Partielle fn(pH)
    log_k     -7.537 # Just for 25°C
I cannot modifiy it each time when I need to proccess electrolisys for different temperature.
And for equlibrium I didnt found a difference from nothing when I use :
SOLUTION_SPECIES
2H2O + 2e- = H2 + 2OH- #Reduction Cathode
   log_k -200
2Cl- = Cl2 + 2e- #Oxydation Anode
    log_k     -47.26
Cl2 + H2O = HOCl + Cl- + H+ #Reaction
    log_k     -2.98
HOCl = OCl- + H+ #Dissolution Partielle fn(pH)
    log_k     -7.537

I have no Cl2 production and no Cl- use.
I'm loose

[Merlin66]

And what I saw was :
HOCl -> ClO- + H+ (Hypochlorite)
Not
HOCl -> OCl- + H+ (Like in your answers, there's nothing like that)

[dlparkhurst]

I don't see a difference in your two equations. You can use either ClO- or OCl-, but you must be consistent in your definitions. The spelling is important; if you use both, they will be considered different species.

You can define temperature dependence in SOLUTION_SPECIES definitions. Look at the manual.

I don't understand your complaint. If you post your input file, I will look at it.

[Merlin66]

Ok my actual try wich doesnt work as I expect :

SOLUTION_MASTER_SPECIES
    Cl(1)         HOCl             0     HOCl           
    Cl(-1)        Cl-              0     Cl       
    Cl(0)         Cl2              0     Cl     
SOLUTION_SPECIES
#2H2O + 2e- = H2 + 2OH- #Reduction Cathode
#   log_k -86.08
2Cl- = Cl2 + 2e- #Oxydation Anode
    log_k     -47.26 #k 1.396
Cl2 + H2O = HOCl + Cl- + H+ #Reaction
    log_k     -2.98
HOCl = ClO- + H+ #Dissolution Partielle fn(pH)
    log_k     -7.537

2H2O = O2 + 4H+ + 4e- #Reduction Cathode
    log_k     -86.08

#Reactions Parasites
#4OH- = O2 + 2H2O + 4e-
#Cl- + H2O = ClO- + 2H+ + 2e-

RATES
Cl2_production
-start
10 k = 8.5 / 96485 # 8.5 Amperes * 1 Seconde / Faraday
15 mCl = TOT("Cl2")
#16 if (mCl < 0) then goto 40
20 rate = k * mCl
30 moles = rate * TIME
40 save moles
-end
END
SOLUTION 1
#    redox     O(-2)/O(0)
#    O(0)      1 O2(g)      -0.7
REACTION 1
    Halite     1
    1 moles in 1 steps
SAVE SOLUTION 2
END

USE solution 2

EQUILIBRIUM_PHASES
O2(g) -0.68 10
#CO2(g) -3.481 10 # carbon dioxide at atmospheric pressure 0.033 kPa standard
KINETICS 1
Cl2_production
    -formula  Cl2  +1.0 Cl- -2.0
-steps     28800  in 32 steps # 8 Heures
INCREMENTAL_REACTIONS true
USER_GRAPH 1
    -headings               Time Cl- Cl2 HOCl pH
    -axis_titles            "Hours" "Moles" ""
    -initial_solutions      true
    -connect_simulations    true
    -plot_concentration_vs  t
  -start
10 graph_x  TOTAL_TIME / 3600
20 graph_y mol("Cl-"), mol("Cl2"), mol("HOCl")
30 graph_sy -la("H+")
  -end
    -active                 true
END

[dlparkhurst]

Here O2(aq) is disabled and the reduction of Cl2 is implemented by adding H2 (equivalent to adding electrons). I used the phreeqc.dat database.


SOLUTION_MASTER_SPECIES
Cl(-1) Cl- 0 Cl
Cl(0) Cl2 0 Cl2
SOLUTION_SPECIES
2H2O = O2 + 4H+ + 4e- #Reduction Cathode
    #log_k     -86.08
    log_k     -186.08
2Cl- = Cl2 + 2e- #Oxydation Anode
    log_k     -47.26 #k 1.396
Cl2 + H2O = HOCl + Cl- + H+ #Reaction
    log_k     -2.98
HOCl = ClO- + H+ #Dissolution Partielle fn(pH)
    log_k     -7.537
RATES
Reduction
-start
10 k = 8.5 / 96485 # 8.5 Amperes * 1 Seconde / Faraday
15 mCl2 = MOL("Cl2")
#16 if (mCl < 0) then goto 40
20 rate = k * mCl2
30 moles = rate * TIME
40 save moles
-end
END

SOLUTION 1
-units mol/kgw
Cl(0) 1
END
USE solution 1
KINETICS 1
Reduction
    -formula  H2 1
-steps     28800  in 32 steps # 8 Heures
USER_GRAPH 1
    -headings               Time Cl- Cl2 HOCl pH
    -axis_titles            "Hours" "Moles" ""
    -initial_solutions      true
    -connect_simulations    true
    -plot_concentration_vs  t
  -start
10 graph_x  TOTAL_TIME / 3600
20 graph_y mol("Cl-"), mol("Cl2"), mol("HOCl")
30 graph_sy -la("H+")
  -end
    -active                 true
END