Electrolysis of Aqueous Solutions

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.

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!

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.

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.

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?

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

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:

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.

Fig. 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.