electrolysis of the solution
This page deals with the electrolysis of aqueous compounds. Most people will have many of these questions in their 14-16 year old chemistry class.
The role of water in the electrolysis of aqueous electrolyte solutions
Because of the presence of water, it's more complicated if you're electrolyzing a solution rather than a melt.
Water itself is a very weak electrolyte as it breaks down into hydrogen and hydroxide ions to a very small extent.
|Remarks:Of course I'm simplifying this. You should know that in these cases the hydrogen ion itself doesn't exist - it actually combines with another water molecule to create the hydroxide ion H3Europa+The status symbol (aq) indicates this.|
This means that more than one ion may reach each electrode and you can choose which one to discharge.
For example, if you electrolyze a sodium chloride solution, both sodium and hydrogen ions (from the water) will be attracted to the cathode, and both chloride and hydroxide ions (from the water) will be attracted to the anode
The table below lists some metals (and hydrogen) that tend to lose electrons. The more negative the E° value (often pronounced "E-zero"), the further to the left is the equilibrium position.
That is, the more negative the E° value, the greater the tendency for one of the elements to lose electrons and form ions.
It also means that something like lithium has little tendency to accept electrons to form atoms after ionization.
In contrast, a species with a positive E° value will be reluctant to lose an electron to form an ion, but can easily cause one of its ions to gain an electron and form a neutral element again.
So gold will not be very lively as it has a very positive E° value. It is not easy to remove electrons to make gold ions, but it is easy to convert gold ions back into gold metal.
The electrochemical series can be thought of as an extended, slightly modified series of reactions.
When it comes to electrolysis, all you really need to know is this:
|Remarks:For the purposes of electrolysis, one does not need to know where these numbers come from and where exactly the equilibrium holds.|
If you want to read more about itElectrochemical series, including the source of these numbers, which you can find at this link. This is the second in a series of Redox Potential pages that you may want to readFirst pageas well as. It is not necessary to follow the rest of the current page.
Summarize what happened
Before I consider concrete examples in detail, I would like to summarize the results. It is important to remember the pattern given in the next part.
What happens at the cathode?
Positive ions are attracted to the cathode where they pick up one or more electrons and are discharged.
Either deposit metal or generate hydrogen from water. What you get depends on where the metal is in the electrochemical series and in some cases the concentration of the solution.
The higher an element is in the electrochemical series, the more easily it loses electrons and the less willing it is to regain them. For example, it's much easier to convince copper to take electrons back to turn ions into atoms than it is to convince lithium to do the same.
What happens to the anode?
Use inert electrodes like platinum or carbon
In general, if you have halogens, you get halogens. Along with all the other common anions (negative ions), you get oxygen from the water.
But attention does play a role here. For example, if you have a concentrated solution of sodium chloride, the anode produces mainly chlorine gas. With increasingly dilute solutions, you get less chlorine and more oxygen. Very, very dilute solutions supply mainly oxygen.
where the anode is not inert
If the anode is not inert, complications can arise. We'll look at some examples further down the page.
Electrolysis of copper(II) sulphate solution using a carbon electrode
Copper is lower in the electrochemical series than hydrogen, so using the summary above you can predict that copper will be released at the cathode.
Continuing using the summary above, you would predict that oxygen would be released at the anode since there are no halogens.
That's exactly what happened.
in the cathode
Copper(II) ions and hydrogen ions are attracted to the negative electrode. Copper is lower in the electrochemical series than hydrogen, so it is the copper that accepts electrons from the cathode.
The cathode is coated with copper.
an der Anode
Sulfate and hydroxide ions are attracted to the positive electrode, but it is difficult to get the sulfate ions to donate their electrons.
Now it gets complicated because in this case there are two ways to describe the anodic reaction. It is easiest to think of it in terms of hydroxide ions.
Provided that hydroxide ions are expelled
oxygen is released.
The problem is that there are very few hydroxide ions in the cupric sulfate solution. You can solve this problem by finding that the water reactions that produce hydrogen ions and hydroxide ions are in balance. When you release hydroxide ions, the balance shifts and replaces them.
Obtain oxygen directly from water molecules
The overall effect is exactly the same as the release of hydroxide ions and the water balance shifts to replace them. Hydrogen ions are also formed as a result of the shifting equilibrium. These are of course repelled by the anode.
So which one is correct?
It almost certainly depends on the pH of the solution. In this particular case, the copper(II) sulfate solution is moderately acidic, meaning there are fewer hydroxide ions than in pure water - hence the second (water) equation is likely to be more accurate.
|Remarks:What did you do with it for testing purposes? You need to find out which version of these equations your prover is using, and then stick with it throughout - don't worry about changing them from one example to another. You need to check which ones they have used in previous work and which form they prefer in their scoring scheme. You will most likely accept both, but you have to be sure.|
Any solution containing sulfate ions (including dilute sulfuric acid) will behave the same way at the inert anode - oxygen will be released.
Nitrate ions also produce oxygen. It's easier to liberate hydroxide ions from water (or water itself, if you use this equation) than nitrate ions.
Electrolysis of sodium chloride solution with a carbon electrode
Sodium is much higher on the electrochemical series than hydrogen, so using the summary above you can predict that hydrogen will be released at the cathode.
Continuing using the summary above, you would predict that chlorine (a halogen) would be released at the anode.
The situation turns out to be a little more complicated, since the result at the anode depends on the concentration of the solution.
in the cathode
Sodium ions and hydrogen ions (from the water) arrive, but sodium is so present in the electrochemical series that its ions cannot exit where there is a choice.
If you're electrolyzing molten sodium chloride, you have no choice - you have to liberate sodium ions. But as a solution you have other options. Unfortunately, there are two different ways of looking at this problem, similar to the anode problem discussed above.
Suppose the hydrogen ions are released
Hydrogen gas is produced.
You can get over the fact that there aren't many hydrogen ions in solution by considering that water is in equilibrium when it ionizes to form hydrogen and hydroxide ions. The faster the hydrogen ions are expelled, the more water is split off to replace it.
Obtaining hydrogen directly from water molecules
As explained above in the case of similar anodes, the overall effect is exactly the same regardless of the viewing angle. They produce hydrogen gas and hydroxide ions form, which are formed along with the hydrogen ions as the water balance shifts to replace the hydrogen ions that are expelled.
So which equation should you use?
They should be tied to any equations that examiners use in their questions or in their marking schemes. In fact, they could take any of them.
Whenever you electrolyze a metal compound higher than hydrogen in the electrochemical series, you give off hydrogen, and the same argument applies. In some cases, however, the hydrogen is not released under these circumstances, which we discuss further down the page.
an der Anode
Chloride and hydroxide ions are attracted to the positive electrode. Actually, hydroxide ions are a bit easier to remove, but what you get is mostly chlorine.
|Remarks:At this level, you have to accept that to a large extent. NOsimplyI can add explanations without making this long and often complicated page worse. I don't think at this level of the chemistry exam you will explain why.|
If you come across a question from an examiner that really needs a thorough explanation, please let me know at the address on the websiteabout this pagebook page. It would be helpful if you could also tell me exactly what your examiner is asking of you.
The formation of chlorine is given by:
The formation of oxygen follows from one of the following equations:
Aqueous solutions of bromide and iodide
In both cases it can be assumed that bromine or iodine is formed at the anode. These equations are similar to the discharge of chloride ions above.
Electrolysis of sodium chloride solution with a mercury cathode
This is a great example of how the characteristics of the electrodes can make a big difference.
This was once the main industrial method of producing caustic soda, as well as chlorine and hydrogen, but has largely been replaced by more environmentally friendly methods. Many hazardous environmental pollutions have occurred in the past due to the release of mercury into the environment.
in the cathode
When sodium and hydrogen ions reach the mercury cathode, the sodium ions are expelled as metallic sodium. It dissolves in mercury, forming a solution called "sodium amalgam".
The sodium amalgam flows out of the electrolyser, reacts with water and releases mercury, which circulates through the electrolyser, producing caustic soda and hydrogen gas.
an der Anode
As expected, chlorine gas is produced.
Carbon electrode electrolysis of a zinc sulfate solution
I use zinc compounds as an example of the rather unexpected results obtained in the electrochemical series from lead to zinc in electrolytic solutions of metal compounds.
Everything is above hydrogen in the electrochemical series, so one would expect hydrogen to discharge at the cathode, not metal. This does not happen in any reasonable concentration of these metal salt solutions.
in the cathode
Zinc ions accept electrons from the cathode and form zinc atoms, which are plated on the cathode.
an der Anode
This is just another case of sulfate electrolysis, which we explore in more detail on our page on the electrolysis of copper(II) sulfate solutions.
|Remarks:Also, there is no quick and easy way to explain why zinc ions are being discharged instead of hydrogen ions, and an exam at this level is unlikely to ask you to explain it.|
If you want to know more you can google itoverpotential.You may come across phrases like “huge overpotential of hydrogen”. The term "overpotential" is used without any actual explanation. In reality it says that hydrogen is harder to discharge than one would expect given its position in the electrochemical series - we know this because in the cases we are talking about you get zinc instead of hydrogen from experiments.
So if you want to move on (which is almost certainly not necessary for this level of chemistry testing), look for a realistic explanation as to why hydrogen E° values are not applicable for zinc sulfate electrolytic solutions.
Electrolysis of a silver nitrate solution with a silver anode
This is an example of an electrode where you participate in a reaction using chemistry.
in the cathode
When you electrolyze a silver nitrate solution using silver as the anode, the silver is deposited onto the desired cathode material.
This can be used for silver plating.
an der Anode
However, instead of driving something out of the solution, the silver from the anode goes into solution at the anode in the form of silver ions, leaving electrons at the anode.
The anode loses silver, the net change is just the transfer of silver from the anode to the cathode.
Electrolysis of copper(II) sulphate solution using a copper anode
A similar change occurs when you electrolyze a cupric sulfate solution with a copper electrode. As might be expected, copper is deposited at the cathode, but instead of releasing oxygen at the anode, copper(II) ions go into solution. There is also a net transfer of copper from the anode to the cathode.
This is used to clean copper. See the section about the page for more informationcopper.You don't need the whole page - just the cleaning section.
some practical details
Of course, you can also electrolyze the solution by placing it in a beaker with two carbon electrodes and connecting the electrodes to a DC power source, such as a battery.
However, you may want to collect any gases released for testing purposes and possibly measure their volume. The last point on this page describes two simple devices that allow you to do this.
Collect all the gases so you can test them
If gas leaks from both electrodes, they must be separated and collected. This is an inexpensive and easy method.
First, both small test tubes are filled with the solution you want to electrolyze. The gases emitted by the two electrodes do not mix. If two gases are present, they can be tested separately.
In addition to the gas, metal deposited on the cathode and bromine or iodine solutions formed on the anode can also be clearly seen. Bromine solutions are light to medium orange, iodine solutions vary in color from orange to deep red depending on the iodine concentration.
|Remarks:If you electrolyze an iodide solution, you only get iodine. The released iodine actually reacts with unreacted iodide ions to form the soluble ion I3-.This causes red to appear.|
Collect all the gases so you can measure them
An easy way is to use a side arm U-tube. You can collect and measure the volume of gas released by collecting it above water into an inverted graduated cylinder or gas syringe.
An ammeter is included in the circuit because when you are measuring the volume being dispensed you almost certainly want to know what current is flowing in order to make calculations. Calculations are covered on other pages in this section.
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© Jim Clark 2017
Electrolysis of aqueous solutions is a process in which an electrical current is passed through an aqueous (water-based) solution to cause chemical reactions. This process splits the ions in the solution into individual atoms, allowing them to gain or lose electrons.What happens to the solution after electrolysis? ›
During the electrolysis, hydrogen and chloride ions are removed from solution whereas sodium and hydroxide ions are left behind in solution. This means that sodium hydroxide is also formed during the electrolysis of sodium chloride solution.What is the process of electrolysis? ›
electrolysis, process by which electric current is passed through a substance to effect a chemical change. The chemical change is one in which the substance loses or gains an electron (oxidation or reduction).What can happen to water in the electrolysis of solutions? ›
Electrolysis of acidified water
H+ ions are attracted to the cathode , gain electrons and form hydrogen gas. OH - ions are attracted to the anode , lose electrons and form oxygen gas.
A simple example of an electrolysis process is between an impure copper anode and a pure copper cathode in a temperature-controlled, quiescent solution of copper sulfate with a constant current source connecting the two electrodes .What is the end product of electrolysis? ›
The by products of electrolysis of water are hydrogen and oxygen.What do you end up with after the electrolysis of water? ›
Electrolysis is a technique used by scientists to separate a compound or molecule into its component parts. By adding electricity to water and providing a path for the different particles to follow, the water can be separated into hydrogen and oxygen.What are the 3 products of electrolysis? ›
- positive ions gain electrons from the negatively charged cathode.
- negative ions lose electrons at the positively charged anode.
In the electrolytic method, light or heavy water molecules (H2O or D2O) are resolved by electrolysis and ions of hydrogen isotopes (H+ or D+) are attracted to the surface of cathodes.What are the rules for electrolysis of solutions? ›
- H+ ions are attracted to the cathode , gain electrons and form hydrogen gas.
- OH - ions are attracted to the anode , lose electrons and form oxygen gas.
The laws state that (1) the amount of chemical change produced by current at an electrode-electrolyte boundary is proportional to the quantity of electricity used and (2) the amounts of chemical changes produced by the same quantity of electricity in different substances are proportional to their equivalent weights.What is produced at the cathode in electrolysis of solutions? ›
hydrogen is produced at the cathode if the metal is more reactive than hydrogen.What is electrolysis in simple terms? ›
Electrolysis is a process in which electrical energy, from a direct current (dc) supply, breaks down electrolytes. The free moving ions in electrolytes are attracted to the oppositely charged electrodes which connect to the dc supply.What type of reaction is electrolysis? ›
Summary. Electrolysis is a non-spontaneous reaction causing a chemical change. The definition of electrolysis is the decomposition of a compound by using electrical energy, which also aptly gives the meaning of the term. Electrical energy causes a chemical change.What are 4 examples of electrolysis? ›
Electrolysis examples: Metals like sodium, potassium, aluminium, zinc etc. are obtained from electrolysis of electrolytes containing these metals. For strongly electropositive metals, molten electrolyte is subjected to electrolysis and for other less electropositive metals, aqueous solutions can be electrolysed.What is the liquid called in electrolysis? ›
Electrolyzers using a liquid alkaline solution of sodium or potassium hydroxide as the electrolyte have been commercially available for many years.What are the products from the electrolysis of water? ›
Electrolysis of water is the process by which water is split into hydrogen and oxygen by the application of electrical energy.Does electrolysis destroy water? ›
Every water molecule includes two atoms of hydrogen and one atom of oxygen. We use a process called electrolysis to break apart water molecules into hydrogen and oxygen. Electrolysis uses an electrical current to split the molecule apart.What type of reaction is electrolysis of water? ›
Electrolysis of water is a decomposition reaction.What is the conclusion of the electrolysis of water experiment? ›
Conclusion: A decomposition reaction occurs when a single reactant breaks down to give simpler products in this electrolysis of water experiment. Under the right conditions, water decomposes to form hydrogen and oxygen. In this case, electricity is the appropriate condition for water decomposition.
On electrolysis , discharge of electrons takes place at cathode as well as anode. At anode, it undergoes oxidation by the loss of electron. At cathode it undergoes reduction by gaining electrons.What product is formed at cathode? ›
At the negative electrode (cathode), hydrogen is produced if the metal is more reactive than hydrogen. At the positive electrode (anode), oxygen is produced unless the solution contains halide ions when the halogen is produced.What not to do before electrolysis? ›
Avoid plucking or waxing for 2-3 weeks before, and avoid shaving for 3-5 days before your electrolysis appointment. In order for the needle to follow the hair follicle and more easily remove the hair, there needs to be at least 1/8th of an inch of hair above the surface of the skin.How many minutes for electrolysis? ›
An electrolysis treatment lasts anywhere between 15 minutes and one hour.Can I do electrolysis on myself? ›
Since there's a risk of permanent skin damage — even when electrolysis is performed by a professional — at-home electrolysis treatments are not something we recommend.What is the best fluid for electrolysis? ›
In general, an aqueous solution of caustic potash or soda is used as the electrolyte for water electrolysis.What is the most efficient electrolyte for electrolysis? ›
Sodium Hydroxide NaOH NaOH, also called “lye”, it is a very efficient electrolyte, highly conductive and caustic.Why is vinegar used in electrolysis? ›
This limitation on current prohibits the water from being split into hydrogen and oxygen. Vinegar is a weak acid, so it doesn't fully dissociate when dissolved in water, meaning there are fewer ions to conduct electricity.How much water is needed for electrolysis? ›
Producing hydrogen through the process of electrolysis theoretically requires 9 L of water per kg of hydrogen based on the stoichiometric values. . However, most commercial electrolysis units on the market today advertise that they require between 10 and 11 L of deionized water per kg of hydrogen produced.What is the first law of electrolysis explain? ›
Faraday's – First Law of Electrolysis
It states, during electrolysis, the amount of chemical reaction which occurs at any electrode under the influence of electrical energy is proportional to the quantity of electricity passed through the electrolyte.
Faraday's laws of electrolysis are quantitative relationships based on the electrochemical research published by Michael Faraday in 1833.What is the first law of electrolysis equation? ›
Faraday's first law of electrolysis formula
Faraday's first law of electrolysis is based on m ∝ Q, where m is the mass of the substance and Q is the quantity of charge passed.
At the cathode, reduction (electronic gain) occurs, and if the metal is less reactive than hydrogen, a metal is formed; if the metal is more reactive than hydrogen, hydrogen is produced.Is A cathode positive or negative? ›
Cathodes get their name from cations (positively charged ions) and anodes from anions (negatively charged ions). In a device that uses electricity, the cathode is the negatively charged electrode.What happens to negative ions in electrolysis? ›
During electrolysis: positive ions, cations , move to the cathode. negative ions, anions , move to the anode.What happens to the electrolyte after electrolysis? ›
Ions can move in the liquid state (after melting) or in aqueous solution (after dissolving in water). The molten or dissolved substance is called the electrolyte . Electrolysis is the decomposition of an electrolyte by an electric current. It is used to extract reactive metals from their ores .Is electrolysis a permanent solution? ›
Is electrolysis permanent? Yes, electrolysis safely and permanently removes hair from all skin tones. It is the only FDA-approved permanent hair removal treatment. Because electrolysis permanently destroys the growth cells in the hair follicles, the hair will not grow back.What happens to concentration of electrolyte after electrolysis? ›
The effect of concentration on electrolysis is explained below as: The resistance decreases as the concentration increases and this reduces the potential difference. And with the same amount of current, more metal is inserted in the cell with a greater potential difference.What is destroyed during electrolysis? ›
Next, a low-level electrical current passes through the needle or probe into your skin and destroys the hair follicle. Hair is unable to grow back in an area where the follicle has been destroyed. The process of electrolysis can be slow and can require several treatment sessions to destroy the hair follicles.Do you gain or lose electrons in electrolysis? ›
Products of electrolysis
positive ions gain electrons from the negatively charged cathode. negative ions lose electrons at the positively charged anode.
At the anode, electrons leave the electrolyte. It is connected to the positive terminal of the battery. The anions are discharged at the anode. Oxidation takes place at the anode.Does electrolysis ever fail? ›
In some cases, hair removal history and failure to follow your treatment plan can lead to unsuccessful electrolysis as well. An initial consultation can reveal the underlying factors of your unwanted hair growth and determine the most effective approach and ample solution.How many treatments until electrolysis works? ›
With Electrolysis, hair can take anywhere from 2-4 treatments at 6 weeks apart to kill it off for good. A lot of clients do quite a few growths in between the 6 week period.What are the rules for electrolysis? ›
In order for electrolysis to occur, compound must be dissolved in water, or melted to form a liquid. This means that the ions are free to move around when a current is applied. Aqueous electrolytes contain water. When a compound is dissolved in water, the resulting solution has water present.Does electrolysis cause a chemical change or physical change? ›
(B) Electrolysis is an example of a chemical change. In the electrolysis of water (Fig. 1.13 B), hydrogen and oxygen gases are produced from liquid water.What happens to lead ions during electrolysis? ›
During electrolysis: Pb 2+ ions gain electrons at the cathode and become Pb atoms. Br - ions lose electrons at the anode and become Br atoms, which pair up to form Br 2 molecules.What are the hazards in electrolysis? ›
Chemical hazards: The electrolysis process can produce harmful chemicals, such as chlorine gas, if the water used is contaminated with impurities. Exposure to these chemicals can cause respiratory problems, skin irritation, and other health issues.What are two disadvantages of electrolysis? ›
- Electrolysis for hair removal can be painful and very expensive method.
- Sometimes swelling on skin if not done electrolysis properly.
- After electrolysis, sometimes red patches develop on skin which takes around week to disappear.
The major components of this process are electrolytes, electrodes, a salt bridge and an external power source. The factors affecting the electrolysis process are the nature and material of the electrode, the presence of oxidizing and reducing agents in the electrolyte, and electrode potential.