Electrolysis Flashcards
What happens when a metal plate is placed in a solution containing its ions?
“Some of the metal atoms from the metal plate will ionize and go into solution as positively charged ions
What happens if reaction (1) is favored in a metal/metal ion system?
“The metal plate or electrode becomes negatively charged with respect to the solution or electrolyte. “
What happens if reaction (2) is favored in a metal/metal ion system?
“The electrode becomes positively charged with respect to the electrolyte. “
What is electrode potential?
“A potential difference
What is the net reaction when a copper plate is dipped into a copper(II) tetraoxosulphate(VI) solution?
“$Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$ “
What happens to the copper plate and solution in the $Cu^{2+}(aq)/Cu(s)$ system?
“The copper plate becomes positively charged on the surface due to a deficit of electrons
What is the net reaction when a zinc plate is dipped into a zinc tetraoxosulphate (VI) solution?
“$Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$ “
What happens to the zinc plate and solution in the $Zn(s)/Zn^{2+}(aq)$ system?
“The zinc plate becomes negatively charged on the surface due to an excess of electrons
What does the electrode potential of a given system depend on?
“The overall energy change
What is standard electrode potential ($E^0$)?
“The potential difference set up when a metal is in contact with a one-molar solution of its ions at $25^{\circ}C$. “
Why is a reference standard needed to measure electrode potential?
“It is not possible to measure the absolute value of an electrode potential directly; instead
What is the standard reference electrode for measuring electrode potentials?
“Hydrogen has been chosen internationally as an arbitrary standard
How is the standard electrode potential of a metal ions/metal system expressed?
“In terms of the potential difference (in volts) which exists between the metallic electrode and the standard hydrogen electrode. This is known as the standard electrode potential of the metal ions/metal system on the hydrogen scale. “
When is the electrode potential value positive?
“If electrons flow from the hydrogen electrode to the metal electrode. “
When is the electrode potential value negative?
“If electrons flow from the metal electrode to the hydrogen electrode. “
What does the standard hydrogen electrode consist of?
“Hydrogen gas at $25^{\circ}C$ and one atmospheric pressure
How is the standard electrode potential of a metal ions/metal system defined?
“The potential difference set up between the metal and a one-molar solution of its ions at $25^{\circ}C$ (arbitrarily taking the standard electrode potential of the hydrogen ions/hydrogen gas system as 0 volt). “
What apparatus is used to measure standard electrode potentials?
“The apparatus in fig. 11.4. “
What is the standard electrode potential of the $Cu^{2+}(aq)/Cu(s)$ system?
“+0.34 V (voltmeter needle deflects to the left). “
What are the half-cell reactions in the $Cu^{2+}(aq)/H_2(g)$ system?
“Platinum in hydrogen electrode: $H_2(g) \rightarrow 2H^+(aq) + 2e^-$ (Oxidation); Copper electrode: $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$ (Reduction); Overall reaction: $Cu^{2+}(aq) + H_2(g) \rightarrow Cu(s) + 2H^+(aq)$. “
What is the table of standard electrode potentials?
“Table provided in the document “
What is the purpose of electrochemical cells?
“To produce an electric current. “
How is an electric current set up between two electrodes?
“A flow of electrons (i.e.
What is an electrochemical cell (voltaic or galvanic cell)?
“A device in which chemical energy is converted to electrical energy. It consists of two half-cells where an oxidation half-cell reaction occurs at the electrode in one half-cell
What happens at the zinc electrode in a $Zn/Cu$ electrochemical cell?
“The atoms at the zinc electrode undergo oxidation and lose two electrons each to form zinc ions which go into the solution. The zinc electrode becomes negatively charged and functions as the negative electrode or anode. “
What happens at the copper electrode in a $Zn/Cu$ electrochemical cell?
“The excess electrons from the zinc electrode flow to the electron-deficient copper electrode where they are accepted by the copper(II) ions. The copper(II) ions are reduced to copper atoms which are deposited on the copper electrode. The copper electrode becomes positively charged and functions as the positive electrode or cathode. “
What is the function of a discharging cell (like a lead accumulator)?
“When connected to an external circuit
What happens at the anode of a discharging lead accumulator cell?
“During the process of discharging
What happens at the cathode of a discharging lead accumulator cell?
“The electrons from the anode are accepted at the cathode where the lead(IV) oxide and the hydrogen ions from the electrolyte undergo reduction to produce lead(II) ions and water. The lead(II) ions then combine with the tetraoxosulphate(VI) ions from the electrolyte to become deposited at the cathode as lead(II) tetraoxosulphate(VI). “
What happens during the discharging process of a lead accumulator?
“The density of the acid decreases to 1.15 g cm-3 due to the absorption of hydrogen and tetraoxosulphate(VI) ions from the electrolyte
What happens when both electrodes of a lead accumulator are completely covered with lead(II) tetraoxosulphate(VI) deposits?
“The accumulator will stop discharging a current. To make it produce a current again
What happens during the recharging process of a lead accumulator?
“The chemical reactions at the respective electrodes in the cell are reversed because it is an electrolysis. The electrode which is the anode in the charged cell becomes the cathode
What are the reactions at the cathode and anode during recharging of a lead accumulator?
“At the cathode (recharging cell): $PbSO_4(s) \rightarrow Pb^{2+}(aq) + SO_4^{2-}(aq) + 2e^- \rightarrow Pb(s)$; At the anode (recharging cell): $PbSO_4(s) + 2H_2O(l) \rightarrow Pb^{2+}(aq) + SO_4^{2-}(aq) \rightarrow PbO_2(s) + 4H^+(aq) + 2e^-$. “
What happens after recharging a lead accumulator?
“The density of the acid in the cell returns to its initial value of 1.25 g cm-3 due to the release of the same amount of ions (i.e.
What is the electrochemical series?
“An arrangement of the standard electrode potentials of the various half-cells in order
What are the significant features of the electrochemical series?
“The good reducing agents (strongly electropositive elements) are at the top
How is the direction of current flow determined in an electrochemical cell?
“The direction of the current flow from a simple electrochemical cell is dependent on the relative positions of the two metallic electrodes on the electrochemical series. The metal higher up the series will serve as the negative electrode (anode) while that lower down will serve as the positive electrode (cathode). “
How is the electromotive force (e.m.f.) of an electrochemical cell calculated?
“It is given by the algebraic difference between the electrode potentials of the two metallic electrodes. A big e.m.f. means a powerful cell. So
What is the relationship between the electrochemical series and the chemical reactivity of metals?
“The electrochemical series is sometimes known as the activity series of metals because the metals in the series are arranged in decreasing order of chemical reactivity. The strongly electropositive metals like sodium and potassium are more reactive than the weakly electropositive metals like silver and gold. “
What is the relationship between the electrochemical series and the chemical reactivity of non-metals?
“The non-metals in the series are arranged in increasing order of chemical reactivity. A strongly electronegative non-metal like chlorine is more reactive than a less electronegative non-metal like iodine. “
What is the relationship between the electrochemical series and compound formation?
“Two elements are more likely to form a stable compound if they are far apart in the series. Elements which are close to one another in the series will either not combine at all or form very unstable compounds. Hence
How does the electrochemical series relate to the occurrence of metals in nature?
“The metals above hydrogen in the series do not occur in the free state in nature. Instead
How does the electrochemical series predict displacement reactions?
“The series shows at a glance the order in which elements will displace one another. Metals will displace other metals which are below them in the series. The ease with which one metal can displace another is also dependent on their relative positions in the series. The further apart they are in the series
What is the relationship between the electrochemical series and redox reactions?
“Displacement reactions are also redox reactions. From the electrochemical series
What is the rule for preferential discharge of ions in electrolysis?
“During electrolysis
What are the limitations of the electrochemical series?
“Although the electrochemical series gives us a lot of important information
What is corrosion?
“The process by which most metals react with the oxygen and moisture in the surrounding air to form oxides
Why is the corrosion of iron significant?
“Because iron is the most common metal that is found around us. The corrosion of iron is known as rusting
What are the reactions at the anode and cathode during the rusting of iron?
“At the anode: Iron is oxidized to give iron(II) ions
What is the overall process of rust formation?
“The iron(II) and hydroxide ions diffuse away from the anodic and cathodic regions and settle down as iron(II) hydroxide
How can rusting be prevented by cathodic protection?
“By coating an object made of iron with
