Design of the electrolyzer
An electrolytic cell (electrolyzer) consists of several key elements:
- Electrolyte: a solution or melt of a substance that ensures the presence of free ions.
- Electrodes: conductors immersed in the electrolyte. They are made of metal or graphite. In some setups, the metal casing of the vessel acts as one of the electrodes.
- Direct current source: creates the necessary potential difference for the forced movement of electrons.
During the operation of the electrolyzer, it is important to observe safety precautions. For example, when producing gases, the space above the cathode and anode is separated by a special diaphragm. This prevents the generated gases (such as hydrogen and chlorine) from mixing and eliminates the risk of a spontaneous explosion.
Processes at the electrodes
When direct current is passed through an electrolyte, ions move directionally toward the electrodes with the opposite charge:
- At the cathode (–), the process of reduction takes place. The cathode donates its electrons to particles from the solution. For example, during the electrolysis of hydrochloric acid solution, hydrogen ions ($H^+$) accept electrons and turn into gaseous hydrogen ($H_2$).
- At the anode (+), oxidation occurs. The anode takes electrons from the electrolyte particles. In the case of $HCl$, chloride ions ($Cl^-$) give up electrons, forming chlorine molecules ($Cl_2$).
Overall, the reaction for $HCl$ looks like this: $2H^+ + 2Cl^- = H_2 + Cl_2$.
Metal purification (Refining)
One of the important industrial processes is electrolytic purification using a soluble anode (using copper as an example). In this case, the material of the anode itself participates in the reaction.
An ingot of blister copper serves as the anode, and a thin plate of pure metal serves as the cathode. When current passes through, the blister copper at the anode is oxidized, giving up electrons, and passes into the solution as $Cu^{2+}$ ions. Then these ions move to the cathode, accept electrons, and deposit on it in the form of pure copper.
Impurities in this case either precipitate to the bottom or remain in the electrolyte. As a result, no chemical changes occur; only physical transfer and purification of the metal take place.
Quantitative calculations
Calculations in electrolysis are subject to stoichiometric laws, where electrons are treated as full participants in the reaction (reactants).
To calculate the number of electrons that have passed through the circuit, the formula is used: $n(e^-) = (I \cdot \Delta t) / F$ Where $I$ is the current (in Amperes), $\Delta t$ is time, and $F$ is the Faraday constant.
Under real conditions, the mass of the obtained product is often less than theoretically calculated due to side reactions. The efficiency of the process is evaluated by the current efficiency ($\eta$) indicator — the ratio of the practically obtained mass of the substance to the mass calculated from the equation.
Briefly: answers to topical questions
What Faraday's laws are used for quantitative calculations of electrolysis?
For quantitative calculations of electrolysis, Faraday's law is used, employing stoichiometric ratios and electrical measurement data. The calculations are based on the following parameters and formulas:
- Faraday constant ($F$) is a physical quantity representing the charge of 1 mole of electrons (96,485 C/mol).
- Amount of substance of electrons — calculated by the formula $n(e^-) = \frac{I \cdot \Delta t}{F}$, where $I$ is the current in amperes, $\Delta t$ is the duration of the process.
- Product mass — calculated taking into account the molar mass and the stoichiometric coefficient of electrons (e.g., $m(\text{Au}) = \frac{I \cdot \Delta t \cdot M(\text{Au})}{3F}$).
- Current efficiency ($\eta$) — the ratio of the actually obtained product mass to the theoretically calculated one ($\eta = \frac{m \cdot \nu(e^-) \cdot F}{I \cdot \Delta t \cdot M}$).
To remember the processes at the electrodes: "Anode — Oxidation" (both words start with a vowel), "Cathode — Reduction" (both words start with a consonant).
Class and exam questions
What is the main difference between electrolysis and a galvanic cell?
In a galvanic cell, a redox reaction proceeds spontaneously with the release of energy. In electrolysis, the reaction proceeds non-spontaneously and requires an input of electrical energy from the outside.
What is a soluble anode?
This is an anode that, during electrolysis, itself enters into an oxidation reaction (dissolves), giving up its ions to the electrolyte. It is used, for example, in copper refining.
Why is a diaphragm needed in an electrolyzer?
The diaphragm separates the spaces above the anode and cathode so that the evolved gases (such as hydrogen and chlorine) do not mix, preventing their explosive interaction.
What else to study on this topic
- Calculation of the mass of a gold coating (gold plating) according to Faraday's laws
- Application of electrolysis for the production of large-tonnage chemical products
- Application of protective electroplated coatings on metals
- Mechanism of dissociation of complex compounds during electrolysis
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