Question

Consider a salt bridge voltaic cell represented by the following reaction: $$ \mathrm{Ca}(s)+2 \mathrm{H}^{+}(a q) \longrightarrow \mathrm{Ca}^{2+}(a q)+\mathrm{H}_{2}(g) $$ (a) What is the direction of the electrons in the external circuit? (b) What electrode can be used at the cathode? (c) What is the reaction occurring at the anode?

Short answer

Question: In a voltaic cell with a chemical reaction between Calcium and Hydrogen ions, determine (a) the direction of electron flow in the external circuit, (b) a suitable electrode for the cathode, and (c) the reaction occurring at the anode. Answer: (a) The direction of electron flow in the external circuit is from the anode (Calcium electrode) to the cathode (Hydrogen electrode). (b) A suitable electrode for the cathode is Platinum (Pt). (c) The reaction occurring at the anode is the oxidation of Calcium, as it loses two electrons: Ca(s) → Ca²⁺(aq) + 2e⁻.

Step-by-step solution

Identify the half-reactions at the anode and cathode

First, let's identify the half-reactions occurring at the anode and cathode. The overall cell reaction is given as: $$ \mathrm{Ca}(s)+2 \mathrm{H}^{+}(a q) \longrightarrow \mathrm{Ca}^{2+}(a q)+\mathrm{H}_{2}(g) $$ To identify the half-reactions, we will break the overall cell reaction into two half-reactions, one involving oxidation and the other involving reduction. Oxidation half-reaction: $$ \mathrm{Ca}(s) \longrightarrow \mathrm{Ca}^{2+}(a q)+2e^{-} $$ Reduction half-reaction: $$ 2 \mathrm{H}^{+}(a q) +2e^{-} \longrightarrow \mathrm{H}_{2}(g) $$ Now we can see that the oxidation half-reaction occurs at the anode, and the reduction half-reaction occurs at the cathode.

Determine the electron flow direction in the external circuit

Electrons flow from the anode to the cathode in the external circuit. Since oxidation occurs at the anode and reduction at the cathode, we have: Anode (oxidation): Calcium loses two electrons (anode is the source of electrons) $$ \mathrm{Ca}(s) \longrightarrow \mathrm{Ca}^{2+}(a q)+2e^{-} $$ Cathode (reduction): Hydrogen ions gain two electrons (cathode is the sink of electrons) $$ 2 \mathrm{H}^{+}(a q) +2e^{-} \longrightarrow \mathrm{H}_{2}(g) $$ So, the direction of electron flow in the external circuit is from the anode (Calcium electrode) to the cathode (Hydrogen electrode).

Suggest a suitable electrode for the cathode

A suitable electrode material should not be affected by the reduction half-reaction occurring at the cathode, and it should conduct electricity efficiently. A good choice for the Cathode would be Platinum(abbreviated as Pt). Platinum is highly chemically inert and an efficient conductor of electricity. It is also an effective catalyst for the hydrogen evolution reaction.

Write the anode reaction

We have already identified the half-reaction occurring at the anode. The anode reaction is the oxidation of Calcium, as it loses two electrons: $$ \mathrm{Ca}(s) \longrightarrow \mathrm{Ca}^{2+}(a q)+2e^{-} $$

Key concepts

Electrochemistry

Electrochemistry is an intriguing field of chemistry that deals with the interrelation of electrical and chemical changes in matter. It encompasses the study of chemical reactions that produce an electric current and those that can be driven by applying an electrical potential.

Central to its understanding is the concept of voltaic or galvanic cells, which convert chemical energy into electrical energy through spontaneous oxidation-reduction reactions. These cells are composed of two different metals, known as electrodes, immersed in an electrolyte solution. One metal acts as the anode, where oxidation occurs, while the other serves as the cathode, the site for reduction reactions.

The electrochemical series, a list of standard electrode potentials, provides insights into the reactivity of different elements, and it's crucial when predicting the outcome of these oxidation-reduction reactions. In a typical classroom setting, a zinc-copper voltaic cell is a common example used to help students visualize the flow of electrons from the more reactive zinc anode to the less reactive copper cathode.

Electrochemistry is not restricted to theoretical concepts; it has a plethora of practical applications ranging from batteries and fuel cells to electrolysis and corrosion prevention. Understanding electrochemistry is fundamental for advancements in energy storage and conversation technologies.

Galvanic Cell Reactions

Galvanic cell reactions, often demonstrated in a salt bridge voltaic cell, are electrochemical processes that generate electricity through spontaneous chemical reactions. These reactions are divided into two half-reactions: oxidation, which occurs at the anode, and reduction, which takes place at the cathode.

In a salt bridge voltaic cell, a salt bridge serves the crucial role of closing the circuit by allowing the flow of ions, while maintaining electrical neutrality in each half-cell. This prevents the solutions from mixing, but provides a pathway for ions to balance the charge as electrons travel through the external circuit from anode to cathode.

The understanding of these reactions is vital for designing batteries and other energy-harvesting devices. By knowing the specific half-reactions involved, engineers can select appropriate electrode materials and electrolyte solutions to optimize the performance of the galvanic cell. For instance, in the salt bridge voltaic cell, platinum is often chosen as the cathode material due to its inert and conductive properties, as in the exercise where it is suggested for the cathodic reaction involving hydrogen ions.

Oxidation-Reduction Reactions

Oxidation-reduction reactions, or redox reactions, are the essence of electrochemistry. They involve the transfer of electrons between substances and are characterized by one species being oxidized and another being reduced.

Oxidation refers to the loss of electrons, resulting in an increase in the oxidation state of an element, which occurs at the anode. Reduction involves the gain of electrons, leading to a decrease in the oxidation state, and occurs at the cathode. These changes can be tracked using oxidation numbers to determine which elements are oxidized and which are reduced during the reaction.

Redox reactions are not solely limited to voltaic cells but occur in numerous biological processes and industrial applications. For example, cellular respiration and photosynthesis are driven by complex redox reactions. In industry, redox reactions are employed in processes such as metal extraction, waste water treatment, and chemical manufacturing.

By deeply understanding these reactions, students can better grasp the principles of electrochemistry and their implications. In the example exercise, calcium is oxidized at the anode, losing electrons, while hydrogen ions are reduced at the cathode, gaining electrons, illustrating the fundamental concept of redox in a voltaic cell setting.