Question

(a) What is electrolysis? (b) Are electrolysis reactions thermodynamically spontaneous? Explain. (c) What process occurs at the anode in the electrolysis of molten \(\mathrm{NaCl} ?\) (d) Why is sodium metal not obtained when an aqueous solution of \(\mathrm{NaCl}\) undergoes electrolysis?

Short answer

(a) Electrolysis is a non-spontaneous process in which electrical energy is used to drive a chemical reaction. It involves passing a direct electric current through an electrolyte, causing the transfer of electrons between ions and the production of new substances at the electrodes. (b) Electrolysis reactions are not thermodynamically spontaneous, as they have a positive Gibbs free energy change (\(\Delta G > 0\)) and require external energy to proceed. (c) During the electrolysis of molten NaCl, the anode oxidation process occurs, with chloride ions being oxidized into chlorine gas, releasing an electron: \(2 \mathrm{Cl}^{-} \rightarrow \mathrm{Cl}_{2(g)} + 2e^{-}\). (d) Sodium metal is not obtained during electrolysis of an aqueous NaCl solution because water has a higher reactivity and lower reduction potential than sodium ions, leading to the formation of hydrogen gas and hydroxide ions instead: \(2 \mathrm{H}_{2\mathrm{O}(l)} + 2e^{-} \rightarrow \mathrm{H}_{2(g)} + 2 \mathrm{OH}^{-}\).

Step-by-step solution

(a) Definition of Electrolysis)

Electrolysis is a process in which electrical energy is used to drive a non-spontaneous chemical reaction. In this process, a direct electric current is passed through an electrolyte (a substance that conducts electricity in its molten state or an aqueous solution), causing the transfer of electrons between the ions involved in the reaction. This leads to the production of new substances at the various electrodes (anode and cathode).

(b) Spontaneity of Electrolysis Reactions)

Electrolysis reactions are not thermodynamically spontaneous. They are driven by the input of electrical energy or external energy from an external source, such as a battery or power supply. Spontaneous reactions are those that occur without requiring any external input of energy and usually have a negative Gibbs free energy change (\(\Delta G < 0\)). Conversely, non-spontaneous reactions, such as electrolysis, have a positive Gibbs free energy change (\(\Delta G > 0\)), meaning that they require an input of energy to proceed.

(c) Process Occurring at the Anode During Electrolysis of Molten NaCl)

During the electrolysis of molten sodium chloride (NaCl), a redox reaction occurs, which involves a transfer of electrons between ions. The process that occurs at the anode (positive electrode) is the oxidation of chloride ions (Cl\(^{-}\)) into chlorine gas, releasing an electron. The overall half-reaction at the anode can be written as: \[2 \mathrm{Cl}^{-} \rightarrow \mathrm{Cl}_{2(g)} + 2e^{-}\]

(d) Non-Formation of Sodium Metal During Electrolysis of Aqueous NaCl Solution)

When an aqueous solution of sodium chloride (NaCl) undergoes electrolysis, sodium metal is not obtained due to the higher reactivity of water compared to sodium ions. Instead, water undergoes reduction at the cathode (negative electrode) to produce hydrogen gas and hydroxide ions. The overall half-reaction at the cathode can be written as: \[2 \mathrm{H}_{2\mathrm{O}(l)} + 2e^{-} \rightarrow \mathrm{H}_{2(g)} + 2 \mathrm{OH}^{-}\] Since water has a lower reduction potential than sodium ions (\(\mathrm{Na}^{+}\)), it is more likely to get reduced to form hydrogen gas. Consequently, sodium metal does not form during the electrolysis of an aqueous solution of NaCl.

Key concepts

Thermodynamically Spontaneous Reactions

Understanding thermodynamically spontaneous reactions is crucial when diving into the principles of electrochemistry. These reactions are capable of occurring on their own without any external energy source. Imagine a ball rolling down a hill; it requires no push because the slope (nature) propels it forth. Similarly, spontaneous chemical reactions have a natural 'drive' to occur.

The spontaneity of a reaction is determined by its Gibbs free energy change (ay{G}). A negative ay{G} signifies a spontaneous reaction under given conditions. Conversely, electrolysis, such as splitting water into hydrogen and oxygen, is not naturally spontaneous and requires an electric current as a push to proceed. This is like pushing the ball up the hill; effort (energy) is required.

Significance in Electrolysis

Electrolysis is fascinating as it forces non-spontaneous reactions to occur by using electrical energy. Without this intervention, such reactions would remain as mere possibilities. The process not only demonstrates the reversibility of spontaneity but also highlights the pivotal role of energy in chemical transformations.

Redox Reaction

The magic behind electrolysis lies within a special kind of chemical reaction known as a redox reaction. Redox, short for reduction-oxidation, involves the movement of electrons from one species to another. It's a dance of charge, with one partner gaining electrons (reduction) and the other giving them away (oxidation).

Applying this to electrolysis, when splitting molten sodium chloride (NaCl), the chloride ions offer their electrons to the anode, undergoing oxidation, while at the cathode, a corresponding reduction occurs as sodium ions accept electrons to become sodium metal.

Understanding Anode and Cathode Activities

At the anode, oxidation of chloride ions forms chlorine gas, while at the cathode, sodium ions gain electrons to become sodium metal. In the case of an aqueous NaCl solution, water, being a better electron acceptor than sodium ions, gets reduced instead, leading to the production of hydrogen gas and leaving sodium ions in solution.

Gibbs Free Energy

At the core of predicting reaction spontaneity is the concept of Gibbs free energy (ay{G}), introduced by Josiah Willard Gibbs in the 19th century. It's an incredible tool that combines the enthalpy (heat content) of a system, its entropy (degree of disorder), and the temperature to decide whether a reaction will occur spontaneously.

Interpreting the Sign of ay{G}

When ay{G} is negative, it's a green light for spontaneity; the reaction can proceed without external help. A positive ay{G}, however, is a stop sign, indicating that the reaction needs an energy boost to happen. In electrolysis, the process is endergonic (absorbs energy) with a positive ay{G}, hence the necessity for an electrical current to drive the reaction forward. This direct correlation between the sign of ay{G} and the reaction's ability to proceed is a fundamental principle in chemistry, serving as a guide to understand and predict the flow of chemical processes.