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Chapter 20: Problem 90

(a) What is an electrolytic cell? (b) The negative terminal of a voltage source is connected to an electrode of an electrolytic cell. Is the electrode the anode or the cathode of the cell? Explain. (c) The electrolysis of water is often done with a small amount of sulfuric acid added to the water. What is the role of the sulfuric acid? (d) Why are active metals such as Al obtained by electrolysis using molten salts rather than aqueous solutions?

Short Answer

Expert verified
(a) An electrolytic cell is an electrochemical cell where an electric current drives a non-spontaneous redox reaction, causing the separation of elements or compounds. (b) The electrode connected to the negative terminal of a voltage source in an electrolytic cell is the cathode, where the reduction process occurs. (c) Sulfuric acid is added to water during electrolysis to increase the conductivity of the solution, making the process more efficient. (d) Active metals like Al are obtained using molten salts in electrolysis rather than aqueous solutions because molten salts allow for more efficient and effective separation of the metal, leading to a purer end product.

Step by step solution

01

(a) Definition of an Electrolytic Cell)

An electrolytic cell is a type of electrochemical cell in which an electric current is passed through a solution containing ions, causing a chemical reaction to occur. The energy from the electric current is used to drive a non-spontaneous redox reaction leading to the separation of elements or compounds.
02

(b) Electrode at Negative Terminal)

In an electrolytic cell, the electrode connected to the negative terminal of the voltage source is the cathode. Here, reduction takes place, meaning that electrons are gained by the ions in the solution. This causes the ions to gain electrons and form neutral atoms or molecules, which then deposit onto the cathode or remain in the solution as reduced species.
03

(c) Role of Sulfuric Acid in Electrolysis of Water)

During the electrolysis of water, a small amount of sulfuric acid (H2SO4) is often added to the water to increase the conductivity of the solution. Pure water has a low conductivity due to the low concentration of ions in the solution. Sulfuric acid is a strong electrolyte, meaning it dissociates into ions and improves the flow of electric current through the solution, making the process of electrolysis more efficient.
04

(d) Reason for Using Molten Salts in Active Metal Electrolysis)

Active metals like aluminum are obtained through electrolysis using molten salts rather than aqueous solutions because they are highly reactive and tend to form stable compounds with water or other components of the solution. In molten salts, these metals exist as ions without the presence of water or other interfering species. This allows for a more efficient and effective separation of the active metal during the electrolysis process, producing a pure end product. Additionally, in molten salts, there is a higher concentration of the desired ions, which improves the overall efficiency and rate of the electrolysis reaction.

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Most popular questions from this chapter

(a) Which electrode of a voltaic cell, the cathode or the anode, corresponds to the higher potential energy for the electrons? (b) What are the units for electrical potential? How does this unit relate to energy expressed in joules?

For each of the following balanced oxidation-reduction reactions, (i) identify the oxidation numbers for all the elements in the reactants and products and (ii) state the total number of electrons transferred in each reaction. $$ \begin{array}{l}{\text { (a) } 2 \mathrm{MnO}_{4}^{-}(a q)+3 \mathrm{S}^{2-}(a q)+4 \mathrm{H}_{2} \mathrm{O}(l) \longrightarrow 3 \mathrm{S}(s)+} \\ {\quad 2 \mathrm{MnO}_{2}(s)+8 \mathrm{OH}^{-}(a q)} \\ {\text { (b) } 4 \mathrm{H}_{2} \mathrm{O}_{2}(a q)+\mathrm{Cl}_{2} \mathrm{O}_{7}(g)+2 \mathrm{OH}^{-}(a q) \longrightarrow 2 \mathrm{ClO}_{2}^{-}(a q)+} \\ {\quad 5 \mathrm{H}_{2} \mathrm{O}(l)+4 \mathrm{O}_{2}(g)} \\\\{\text { (c) } \mathrm{Ba}^{2+}(a q)+2 \mathrm{OH}^{-}(a q)+\mathrm{H}_{2} \mathrm{O}_{2}(a q)+2 \mathrm{ClO}_{2}(a q) \longrightarrow} \\ {\quad \mathrm{Ba}\left(\mathrm{ClO}_{2}\right)_{2}(s)+2 \mathrm{H}_{2} \mathrm{O}(l)+\mathrm{O}_{2}(g)}\end{array} $$

Metallic magnesium can be made by the electrolysis of molten MgCl. (a) What mass of Mg is formed by passing a current of 4.55 A through molten MgCl \(_{2},\) for 4.50 days? (b) How many minutes are needed to plate out 25.00 \(\mathrm{g} \mathrm{Mg}\) from molten \(\mathrm{MgCl}_{2}\) using 3.50 \(\mathrm{A}\) of current?

A voltaic cell consists of a strip of cadmium metal in a solution of \(\mathrm{Cd}\left(\mathrm{NO}_{3}\right)_{2}\) in one beaker, and in the other beaker a platinum electrode is immersed in a NaCl solution, with \(\mathrm{Cl}_{2}\) gas bubbled around the electrode. A salt bridge connects the two beakers. (a) Which electrode serves as the anode, and which as the cathode? (b) Does the Cd electrode gain or lose mass as the cell reaction proceeds? (c) Write the equation for the overall cell reaction. (d) What is the emf generated by the cell under standard conditions?

During the discharge of an alkaline battery, 4.50 g of Zn is consumed at the anode of the battery. (a) What mass of \(\mathrm{MnO}_{2}\) is reduced at the cathode during this discharge? (b) How many coulombs of electrical charge are transferred from Zn to \(\mathrm{MnO}_{2} ?\)
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