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Electrolysis of Aqueous Solutions

Have you ever wondered what electrolysis is, or how we can carry out one, and what are its characteristics? Well, keep reading this article to get a better understanding and more information about the electrolysis of aqueous solutions.

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Electrolysis of Aqueous Solutions

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Have you ever wondered what electrolysis is, or how we can carry out one, and what are its characteristics? Well, keep reading this article to get a better understanding and more information about the electrolysis of aqueous solutions.

  • In this article, you will find out the definition of electrolysis on aqueous solutions.
  • How to set up the electrolysis experiments, including the required practical.
  • What are the typical outcomes expected for the electrolysis of several example solutions
  • The rules governing the reactions at each electrode, their names, and reactions at each inert electrode.

Definition of Electrolysis of Aqueous Solutions

When discussing electrolysis, there are many different forms of compounds you can apply it to, such as molten states or aqueous solutions.

Electrolysis of aqueous solutions refers to the electrolysis of substances (usually ionic) which are dissolved in water.

But what advantage does the addition of water to the system have on the electrolysis of the compound? Water can provide water molecules for mobility, and also OH- and H+ ions for reactions to take place. You will see this in the examples we will talk about below!

Take a look at our previous articles on Electrolysis to get a reminder about general rules of this topic, and reactivity series.

Electrolysis of Aqueous Solutions: Rules

Here we will discuss the general rules you can apply to all contexts where electrolysis is performed on aqueous solutions. The most important consideration you have to remember about is if the compound is dissolved in water. If it is, then you can apply the following rules to know what the outcome of an electrolysis reaction will be.

Reaction at the Positive Electrode (Rules)

At the positive electrode (the anode) two possible negative ions accumulate: this can be either the hydroxide ions from water (OH-) or the anion of the dissolved compound in question. Regarding the anions present in the solution, there also can be two types of them: here they can be monoatomic anions (such as halides), or a polyatomic anion.

Halides are negative ions of the halogens (group 7 in the periodic table) such as chlorine (Cl-) or fluorine (F-). Polyatomic anions include sulphates (SO42-) or carbonates (CO32-).

So which of the two is preferentially oxidised at the anode during electrolysis?

If the anion is simple, such as a halide, then it will be preferentially oxidised.

If the anion is complex, such as a polyatomic anion, then OH- will be oxidised.

This is due to the case that polyatomic ions are not easily oxidised and discharged during electrochemical reactions.

Concentration plays an important role during the electrolysis at the anode. Usually, the anion (if it is simple) will be preferentially oxidised and discharged. This will be the case for normal and high concentrations of the aqueous solution.

If the solution is of very low concentration or is a dilute concentration, then the hydroxide ions are preferentially oxidised, as they outnumber the anions. So at low concentrations, OH- will be discharged even when other ions are present.

Reaction at the Negative Electrode (Rules):

At the negative electrode, only positive ions will be deposited. In an aqueous solution, there can be two possible positive ions: this will be either the positive ion (cation) of the compound or a hydrogen ion (H+).

So if you have a mixture of hydrogen ions and the cations of your compound in solution at the negative inert electrode, which one of the two will be preferentially reduced?

Remember that in electrolysis, reduction occurs at the positive electrode (cathode). As at the cathode, electrons are being gained by the aqueous substance.

To know which ion will be produced at the cathode, you need to refer to the reactivity series.

The reactivity series is a table of elements (mostly metals) which are arranged according to their reactivity. At the top of the table are the elements which are the most reactive, and as you go down the reactivity of elements decreases. You can use it to compare how elements are in relation to each other based on reactivity, and more importantly, how if they can displace each other in electrochemical contexts.

Reactivity Series. Electrolysis of Aqueous Solutions. StudySmarterFig. 1: Reactivity Series. Source: bbc.co.uk

So how can you use the reactivity series to know which element will be preferentially produced at the positive electrode? Here is the rule:

hydrogen will be reduced if the metal is more reactive than hydrogen

the metal will be reduced if it has a lower reactivity than hydrogen

Thus, you should always look at the reactivity of the cation in question in relation to hydrogen on the list. Why is this the case? As an element having a lower reactivity will be easier (thermodynamically) to reduce, preferential reduction at the cathode occurs to the element with the lowest reactivity.

Summary of Rules

  1. Compound has to be dissolved in water.
  2. Positive electron (anode): hydroxide is oxidised, unless the anion in solution is monoatomic and of high concentration.
  3. Negative electrode (cathode): hydrogen is produced, unless the metal has a lower reactivity

Electrolysis of Aqueous Solutions Experiment: Investigate the Electrolysis of a Solution (RP)

Here we will cover the required practical "Investigate the electrolysis of a solution"

Set up a simple electrolysis reaction cell (diagram in next section), and prepare the following 5 aqueous solutions to perform electrolysis on: copper chloride, sodium chloride, potassium nitrate, dilute sodium bromide, and copper sulfate.

What are your predictions for what will be produced at each electrode under electrolysis? Start the appropriate voltage and run the reactions, always keeping an eye out for the the the

In one of the sections below, we will cover how to tell which product is being made at each electrode with simple indicator tests.

Based on the rules we set out above, you can predict the following elements to be discharged at each electrode:

Aqueous solution of:At cathodeAt Anode
copper chlorideCopperChlorine
sodium chlorideHydrogenChlorine
potassium nitrateHydrogenOxygen
dilute sodium bromideHydrogenOxygen
copper sulfateCopper
Oxygen

Electrolysis of an Aqueous Solution Diagram

Here you can see a diagram of how to set up your required practical, or any electrolysis reaction involving an aqueous solution, here the example involves a concentrated aqueous solution of sodium chloride (NaCl). The key is to have all the components listed and to arrange them accordingly. In the next section (methods) you will learn how to use the following diagram to set up your experiment.

The test tubes (vials) above each electrode are there to capture gas. If you are performing an experiment that does not depend on the evolution of gas, or even if you don't need to track the gas production, then it is not required in the diagram. The most important bit is the completed circuit, which is comprised with the inert electrodes being connected through the external circuit and the aqueous solution.

The dissolved compound, usually ionic, which comprises the aqueous solution is what completes the circuit. The mobility of charged particles acts similarly to the flow of electrons in a wire - they transmit electricity and charge with the input of external energy.

Electrolysis Diagram. Electrolysis of aqueous solutions. StudySmarterFig. 2: Diagram of an electrochemical cell. Diagram: revisechemistry.uk

Electrolysis of Aqueous Solutions Method

You can perform electrolysis on aqueous solutions with the following procedure. Follow the steps below after you have the solutions you want to perform electrolysis on:

  1. Fill a large beaker with your desired solution (you want to perform electrolysis on).
  2. Insert two inert electrodes (use graphite or platinum electrodes).
  3. If you are collecting gases which are produced at each electrode, place a vial above each electrode.
  4. Connect the electrodes with the wire and apply a voltage.
  5. Observe the evolution of gases or compounds at each electrode and record your observations.

Using the following steps outlined above, you can set up the electrolysis reaction and adapt the experimental set up for any type of experiment you will be performing

Indicators for Electrolysis of an Aqueous Solution

In this section, we will cover how you can test the products being made at each electrode. You can use this knowledge in the required practical on investigating the electrolysis of various solutions.

The most common products you will come across are either metal deposits, hydrogen gas, oxygen gas, or chlorine gas. Below are some tests you can do to know which product is formed at which electrode.

ElectrodeProduct Being ProducedIndicator Test
CathodeMetal DepositPhysical accumulation of product on electrode (can do a weight test).
CathodeHydrogen GasIf you light a splint, a squeaky pop sound will be produced from the gas collected in the vile.
AnodeOxygen GasIf you put a glowing splint into the vial with the collected gas, it will relight.
AnodeChlorine Gas
A damp litmus paper will turn red and be eventually bleached.

Electrolysis of aqueous solutions - Key takeaways

  • Electrolysis of an aqueous solution involves applying a voltage to a dissolved ionic compound.
  • The positive electrode is called the anode and is facilitates oxidation.
    • At the positive electrode the anion will be discharged only if it is monoatomic and of high concentration. (Otherwise, oxygen gas is produced).
  • The negative electrode is called the cathode, and it facilitates reduction.
    • At. the negative electrode the metal is discharged if it has a lower reactivity than hydrogen on the reactivity series. (Otherwise hydrogen gas is produced).
  • The aqueous solution acts to complete the circuit between the electrodes due to the mobility of charged particles (ions).

Frequently Asked Questions about Electrolysis of Aqueous Solutions

Electrolysis of aqueous solutions involves applying a voltage to ionic solutions through two inert electrodes to observe the oxidation and reduction reactions at each electrode. 

At the positive electrode (the anode) oxidation occurs, while at the negative electrode (the cathode) reduction will occur. 

In electrolysis, at the negative electrode the metal is produced if it is lower in the reactivity series than hydrogen (otherwise hydrogen gas is produced), while at the positive electrode, the anion is discharged if it is of high concentration and monoatomic, if not then oxygen is produced. 

The products of electrolysis include either hydrogen gas or metal at the negative electrode, or oxygen or anion gas at the positive electrode. 

You can predict the products of electrolysis based on the rules of the products discharged at each electrode, as well as by looking at what reaction is happening at each electrode. 

Test your knowledge with multiple choice flashcards

How do you call the positive electrode in electrolysis of aqueous solutions?

Which one is the cathode?

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