Electrolysis. Cell . Chapter 7 Electrolyte solution. Chemical energy. Electrical energy. 7 Electrolyte solution. Main content:. Concepts in electrochemistry and Faraday’s Law Ions transference and its number Electric conductance Theory about Strong electrolyte solution.
CellChapter 7 Electrolyte solution
Electrical energychemical energy
Electrochemistry principally researches the conversion of chemical energy into electrical energy and the relevant rules in this conversion.
1. The first kind of conductor
Electron conductor (such as metals and graphite)
A.Conduct by the moving of the free electron
B.The conductor isn’t changed in the process of
C.Resistance is raised with the raising of the temperature
D.All the amount of the electricity conducted is
undertook by electron
⒉ The second kind of conductor
(such as electrolyte solution and melt electrolyte)
A.Conduct by the inversely moving of the positive and
B.There are chemical reactions in the process of
C.Resistance is descended with the raising of the
D.All the amount of the electricity conducted is
undertook by positive and negative ions
* Solid electrolyte such as AgBr and PbI2 belong to ion conductor, but because its conduction mechanism is very complicated and the ability of its conduction is not high, so in this chapter we mainly talk about electrolyte solution.
Positive electrode :
The electrode which electric potential is higher is called anode, current flows from anode to cathode. Among primary cells the positive electrode is cathode and among electrolytic cells the positive electrode is anode.
The electrode which electric potential is lower is called cathode, current flows from cathode to anode. Among primary cells the negative electrode is anode and among electrolytic cells the negative electrode is cathode.
The cathode where the reduction occurred is called cathode, among primary cells cathode is positive electrode and among electrolytic cells cathode is negative electrode.
The cathode where the oxidation occurred is called Anode, among primary cells Anode is negative electrode and among electrolytic cells Anode is positive electrode.
Oxidation occurred, this electrode is anode, electrons flow from electrode Zn to electrode Cu, Zn electric potential is lower, and it is negative electrode.Galvanic cell
Reduction occurred, this electrode is cathode, current electrode Cu to electrode Zn, Cu electric potential is higher, and it is positive electrode.
The electrode that is connected with the positive electrode of external electrical power is negative electrode and it occurred reduction reaction, so it is cathode
The electrode (2) that is connected with the negative electrode of external electrical power is positive electrode and it occurred oxidation reaction, so it is anode.
Efficiency of current =
Efficiency of current =
⒈ The quantity of substance, which occurred chemical reaction in the surface of the electrode, is in direct ratio with the given quantity of electricity.
⒉ Pass the electricity through the series connection circuit of some electrolytic cells, when electric charge of the particle gotten is the same, the substances occurred chemical reactions in the entire electrode have the same mole, the quantity of the substances separated out is in direct ratio with its mole quantity.
Select the gains or losses electrons as z, quantity of electricity as Q, and the moles n of the substance that occurs chemical reaction in the electrode are:
The mass m of the substance that occurs chemical reaction in the electrode is:
Faraday constant is equal to the quantity of electricity of 1mol electric charge.
Three solution parts: anode, middle and cathode.
Before passing through electricity, there are 5mol positive ions (+) and 5 negative irons (–) in every part.
All ions are 1 valence, when passing through quantity of electricity of 4 mole electrons, 4 moles negative ions in the anode are oxidized, 4 moles positive irons in the cathode are reduced.
1．Ifr+=r-, the task of conduction are both 2 moles, on the surface of the imaginable planes AA and BB, there are both 2 moles negative and positive ions passing through.
When the electricity is over, the concentration of the solution in the anode and cathode parts are the same, but 2 moles less than the original solution, the concentration in the middle part is unchanged.
2． If r+=3r-, the task of conduction of positive ions are 3 moles, the negative ions is 1 mole. On the surface of AA and BB, there are 3 moles positive and 1mole negative ions passing through in inverse direction.
When the electricity is over, in the concentration in anode parts 3 moles positive and negative ions decrease, and in cathode 1 mole positive and negative ions decrease, the concentration in the middle part is unchanged.
1. All the moles of the positive and negative ions that transfer to the cathode and anode are equal to the total quantity of the electricity passing through.
The speed of ions in the electric field is expressed by formula:
dE/dl: electric potential gradient
U+electric transference rate of positive ions
U-electric transference rate of negative ions
Ionic mobility: the speed of the ions when the electric potential gradient is in 1 unit. Its unit is ㎡·s-1·V-1.
the transference number of ion B:
The ratio between the current of ion B carrying and the total current
Because the speeds of positive and negative ions are different, they carry different electric charge, so they undertake differentproportion of quantity of electricity when they transfer.
If the solution has only one electrolyte:
If the solution has many electrolytes, :
There are two parallel electrodes, the distance is l, the surface area A, and the external voltage is E. Put the electrolyte MxNy (concentration c) between two electrodes, the dissolve degree is α, c unit: mol·m-3.
The speed of positive ion is r+, in unit time there is (cxαAr+) mol substance passing through any cross section, thequantity of electricity transferred is (cxαAr+)z+F, because it is in the unit time, so
The same reason
（ ，Electric potential gradient is the same）Transference number definition
Because the solution is electric neutral, so
of the resistance, the unit is or s.
Electric conductance G is in direct proportion with the area A and is in reverse ratio with the length l of the conductor.7.3.1 electric condutance
k: electrolytic conductivity,
the electric conductance in unit length and unit area, the unit is
Electrolytic conductivity is the reciprocal of the resistance rate:
molar electrolytic conductivity Λm: the electric conductance of the solution, unit length, 1 mole electrolyte solution，unit is S·m2·mol-1.
Λm=k Vm =k/c
Vmis the volume of the 1mole electrolytic solution, unit is m3·mol-1,
c is the concentration, unit is mol·m3.
Wiston electric bridge, measurement of resistance,
AB is even slippery string resistance; it is the changeable resistance, M is the electric conductance pool where the measuring solution is put. The resistance will be measured.
I is the high frequency intercourse electricity power. G is the earphone or the oscillograph.
Connect the electricity power, adjust C point, and make the circuitry DGC has no current, the drop of electric potential of point D and C is equal, the electric bridge is in equilibrium. According to the relationship of some resistance,
Cell constant:unit is
Material mass (1 mole) in the solution is given. When the concentration reduces, interaction between particles weakens, the speed of positive and negative ions quickens. So Λmmust rise.
Λm increase with decreasing of concentration. when concentration descend below 0.001mol·dm-3, the relationship of Lm (Λm) and is linearity,
Put the line outside to , we can get molar electrolytic conductivity of infinitely diluted solution.
Lm slowly rises with decreasing of concentration, when the solution is very watery, the relationship of Lm and is linear. Below to a certain degree, Lm rapidly rises Such as CH3COOH.
Germany scientist, Kohlrausch discovered a law. In the infinitely diluted solution, every ion moves independently and it is not effected by other ions. The molar electrolytic conductivity of the infinitely diluted solution can be regarded as the sum of the two kinds of ions.
To the strong electrolyte, when the concentration is not so large, we can approximately use this formula.
To the strong electrolyte we can approximately use this formula:
Pure water itself has weak dissociation,the concentration of and both are k the table, we can get:
So, electric conductance of the pure water isApplication of the electric conductance
（1） measurement of the water purity
In fact, if thethe water of electric conductance less than , 1x10-4 S m-1 is thought as pure water
Use to draw a chart，from the intercept and slope,we can get and this is Ostwald’s dilution law.(2) Calculate dissociation constant of weak electrolyte
AB = A- + B-
Start: c 0 0
Eq: c(1-a) ca ca
In the process of titration,the concentration of ions change unceasingly， conductance titration changes all the time too, we can use the turning point of conductance titration to ensure the titration end point.
Mean activity of ions and mean activity coefficient of ions
The inter-attraction theory of the strong electrolyte ions
ideal solution so,Mean activity of ions and mean activity coefficient of ions
Chemical potential of non-electrolyte:
1-1valence electrolytes (HCl).
To the electrolyte of
mean activity of ions
mean activity coefficient of ions
mean molality of ions
Calculate From mB of electrolyte:
To the 1-1 valence electrolyte:
In main factors of affecting activity coefficient of ions, the effect of the valence is more obvious. In 1921, Lewis put forward the concept of ionic strength.
mB is the real concentration of the ion B, If it is weak electrolyte, we must multiply the degree of ionization. The unit of I is the same to m.
van’t Hoff factor
The colligative of the electrolyte is larger than non-electrolyte of the same concentration. Van’t Hoff use a factor to express their deflection, which is called Van’t Hoff factor or coefficient.
Debye-Huckel thought that every ion in the solution is embraced by the ions of different electricity. Because of the interaction of the positive and negative ions, the ions distribute not evenly.
If the central ion is positive ion, there are many negative ions around it, a part of electricity of charge is counteracted, but the left electricity of charge form a ball-shaped ionic atmosphere at r distance from the central ion.
Accord to the concept of ionic atmosphere, Debye Huckel deduced the formula (Debye Huckel’s limiting law), of the ion activity coefficient of the strong electrolyte dilutive solution.
Zi is the electric charge of ion i, I is ionic strength, A is a constant related to temperature and solvent, the A of the water can be checked in the certain table.
Debye Huckel’s limiting law usually use this formula:
This formula is suitable for the diluted strong electrolyte system and the system in which ions can be regarded as particle electric charge. Themean activity coefficient of ions calculated from this formula is theoretical, use the electromotive force method we can get the actual value.
If the radius of the ion is very big and it can’t be regarded as a system of particle electric charge, we can ameliorate Debye Huckel’s limiting law as below:
a is the average available diameter of the ion, it is about 3.5×10-10m, B is the constant related to temperature and solvent, in the water solution at 298 K,
Because every ion has a ionic atmosphere around it, at the impact of the external electric field, the positive and negative ions transfer at contrary direction, the original ionic atmosphere breaks, the new ionic atmosphere builds, there is a time interval, we call it Relaxation effect.
At the relaxation time, the ionic atmosphere is not symmetrical, it generate resistance to the motion of the central ion. This strength make the ion speed descend and it make the molar conductivity descend.The picture is in the next page.
In solution, ions are always solvent, at the impact of the external electric field, the solvent central ion and the solvent ionic atmosphere transfer at contrary direction, which adds the adherence and prevent the motion of the ions, so it make the ion transference speed and molar conductivity descend, this is called electrophoretic effect.
Consider the relaxation effect and electrophoretic effect; we can deduce a formula that is used to calculate the discrepance of the molar conductivity between the electrolyte at one certain concentration and at the infinitely diluted solution, it is called
p and q are the descendent amount of Lm caused by relaxation effect and electrophoretic effect, this theory well explain the following experience formula