Question

A major source of chlorine gas, \(\mathrm{Cl}_{2}\), is from the electrolysis of brine, which is concentrated saltwater, \(\mathrm{NaCl}(\mathrm{aq})\). What is the sign of the electrode where the chlorine gas is formed? Is it negative or positive?

Short answer

Chlorine gas forms at the positively charged anode.

Step-by-step solution

Understanding Electrolysis

In electrolysis, an electric current is passed through an electrolyte to cause a chemical reaction that otherwise wouldn't happen. In the case of brine electrolysis, which involves the aqueous solution of sodium chloride, it's crucial to understand the reactions happening at each electrode to determine where chlorine gas forms.

Identifying the Electrodes

There are two electrodes in electrolysis: the anode and the cathode. Reduction occurs at the cathode, which attracts positive ions, while oxidation occurs at the anode, which attracts negative ions.

Determining Chlorine Gas Formation

Chlorine gas, Cl extsubscript{2}, forms through the oxidation of chloride ions, ext{Cl extsuperscript{-}}, which occurs at the anode. The chemical equation for this reaction is: \[2 ext{Cl extsuperscript{-}}(aq) ightarrow ext{Cl extsubscript{2}}(g) + 2e extsuperscript{-}\]

Assigning the Electrode Sign

Since oxidation occurs at the anode, the anode must be the site where chlorine gas forms. In electrolysis, electrons leave the anode, making it positively charged because it loses electrons.

Key concepts

Chlorine Gas

Chlorine gas, with the chemical formula \( \mathrm{Cl}_2 \), is a diatomic molecule composed of two chlorine atoms. It is a pale green gas with a pungent smell. Chlorine gas plays a significant role in industrial and chemical processes, one of which is the electrolysis of brine.

• Chlorine gas is used for disinfecting drinking water, which helps prevent diseases by killing bacteria and other microorganisms.
• It is also used to produce hydrochloric acid and as a bleaching agent in the paper and textile industry.

Given its reactivity, chlorine gas is not usually found in nature in its elemental form, but rather in compounds, most commonly sodium chloride (table salt). During the electrolysis process, it is liberated in a pure gaseous form at the anode, where oxidation occurs.
Being a corrosive and highly reactive substance, chlorine gas must be handled with care to prevent harmful health effects such as respiratory issues and skin irritation.

Brine

Brine is a highly concentrated solution of salt (sodium chloride, \( \mathrm{NaCl} \)) in water. It is widely used in various chemical applications, including the electrolysis process to produce chlorine gas.

• The concentration of salt in brine affects the efficiency of the electrolysis process since it determines the number of available \( \mathrm{Cl}^- \) ions for oxidation.
• High concentration brine provides a sufficient number of ions for the electrochemical reactions to take place efficiently.

In industrial setups, brine is preferred because it serves as an abundant source of the chloride ions necessary for producing chlorine gas and also sodium hydroxide \( \mathrm{NaOH} \), another valuable industrial chemical, at the cathode.

Electrode Reactions

During electrolysis, reactions occur at two electrodes: the anode and the cathode. These reactions are responsible for generating products like chlorine gas and sodium hydroxide from brine.

• The cathode is the site of reduction, where positively charged ions gain electrons. In brine electrolysis, hydrogen gas and \( \mathrm{NaOH} \) form at the cathode.
• The anode is the site of oxidation, where negative ions lose electrons. In this case, \( \mathrm{Cl}^- \) ions are oxidized to form chlorine gas.

Each electrode is necessary for balanced electrochemical reactions. Without proper electrode reactions, the electrolysis process cannot result in the desired separation and formation of chemical substances.

Oxidation

Oxidation is a chemical process where a molecule, atom, or ion loses electrons. This process is vital in electrochemistry, including the electrolysis of brine to produce chlorine gas.
In the context of electrolysis, oxidation happens at the anode. Here, chloride ions \( \mathrm{Cl}^- \) donate electrons, transforming into molecular chlorine gas \( \mathrm{Cl}_2 \).

• The overall oxidation reaction for the formation of chlorine gas is: \[ 2 \mathrm{Cl}^- (aq) \rightarrow \mathrm{Cl}_2 (g) + 2e^- \]

Understanding oxidation allows us to identify not only the creation of chlorine gas but also recognize that the process releases electrons, which balances the overall charge during electrolysis. Thus, making sense of electrode polarities and why the anode is positively charged.

Anode

The anode is one of the two electrodes in an electrolysis setup and is critically significant in the formation of chlorine gas. The anode attracts anions, or negatively charged ions, within the solution.

• In brine electrolysis, the chloride ions \( \mathrm{Cl}^- \) migrate toward the anode.
• At the anode, the oxidation process occurs, converting \( \mathrm{Cl}^- \) ions into chlorine gas \( \mathrm{Cl}_2 \).
• This reaction releases electrons into the circuit, thereby making the anode positively charged.

Therefore, the anode is not only instrumental in the production of chlorine gas but also plays a fundamental role in allowing the electric current to continue circulating through the system and maintaining the electrochemical reactions. Recognizing the function of the anode is essential for understanding electrolysis and the generation of various chemicals in this process.