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Electrode Potentials

A-Level AQA Chemistry

This pack of flashcards encompasses principles of electrochemical cells, redox equilibria, standard electrode potentials, and calculation methods for electromotive force (EMF). It covers the roles and reactions of components in various types of cells, including hydrogen, nickel-cadmium, and lithium-ion, as well as the implications of changing conditions on cell potential.

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"Describe the effect of changing conditions on Ecell."	"Ecell is a measure of how far from equilibrium the cell reaction lies. The more positive the Ecell, the more likely the reaction is to occur."
"What is the effect of current flow on Ecell?"	"If current is allowed to flow, the cell reaction will occur and the Ecell will fall to zero as the reaction proceeds and the reactant concentrations drop."
"Define Le Chatelier's principle."	"Le Chatelier's principle states that if a system at equilibrium is subjected to a change in conditions, the system will adjust to minimize the effect of the change."
"How does increasing the concentration of reactants affect Ecell?"	"Increasing the concentration of reactants would increase Ecell."
"Describe the effect of temperature on Ecell."	"Most cells are exothermic in the spontaneous direction, so applying Le Chatelier's principle to a temperature rise would result in a decrease in Ecell."
"What are the different types of electrochemical cells?"	"Electrochemical cells can be non-rechargeable (irreversible), rechargeable, and fuel cells."
"What is the overall reaction for a dry cell?"	"Mn2O3 + 2NH3 + H2O + Zn2+ -> Ecell = +1.51V"
"Describe the purpose of secondary nickel-cadmium cells."	"Secondary nickel-cadmium cells are used to power electrical equipment such as drills and shavers."
"Explain why chlorine undergoes a redox reaction with water."	"Chlorine undergoes a redox reaction with water because it has a more positive electrode potential (E°) compared to the oxidation potential of water. This means that chlorine can oxidize water to produce oxygen gas."
"Describe the use of lithium ion cells."	"Lithium ion cells are used to power cameras and mobile phones."
"Explain the role of powdered graphite in lithium ion cells."	"Powdered graphite acts as a support medium in lithium ion cells, allowing the ions to react in the absence of a solvent like water."
"What is the purpose of fuel cells?"	"Fuel cells use the energy from the reaction of a fuel with oxygen to create a voltage."
"Describe the limitations of hydrogen fuel cells."	"Expensive, high production costs, limited lifetime, use of toxic chemicals in production."
"What are the advantages of fuel cells over conventional petrol or diesel-powered vehicles?"	"Less pollution, less CO2 emissions, greater efficiency."
"How can hydrogen be stored in fuel cells?"	"As a liquid under pressure, adsorbed on the surface of a solid material, absorbed within a solid material."
"Define electrolysis and its role in obtaining hydrogen."	"Electrolysis is the process of using electricity to split water into hydrogen and oxygen. It is used to obtain hydrogen for fuel cells."
"Describe the advantages of ethanol fuel cells compared to hydrogen fuel cells."	"Ethanol can be made from renewable sources, ethanol is less explosive and easier to store than hydrogen, no need for new petrol stations as ethanol is a liquid fuel."
"Define a secondary standard electrode."	"A secondary standard electrode is a standard electrode that has been calibrated against the primary standard (standard hydrogen electrode)."
"What is the purpose of a salt bridge in an electrochemical cell?"	"A salt bridge is used to connect the two half-cells in an electrochemical cell and allows for the flow of ions to maintain charge neutrality."
"Define electrode in the context of electrochemical cells."	"An electrode is a solid conductor (usually a metal) that facilitates the transfer of electrons between the electrolyte and the external circuit."
"How does a high resistance voltmeter prevent current flow in a circuit?"	"A high resistance voltmeter restricts the flow of current by offering a large resistance to the flow of electrons, preventing them from passing through the circuit."
"Describe what happens when current is allowed to flow in an electrochemical cell."	"When current flows in an electrochemical cell, the reactions occur separately at each electrode. The most positive electrode undergoes reduction, while the most negative electrode undergoes oxidation."
"What does the solid vertical line represent in a cell diagram?"	"The boundary between phases, such as solid (electrode) and solution (electrolyte)."
"Why is a platinum electrode used in systems that do not include metals?"	"A platinum electrode is used because it is unreactive and can conduct electricity."
"How is the more positive half cell represented in a cell diagram?"	"The more positive half cell is written on the right side of the cell diagram, if possible."
"Define electrode potential."	"Electrode potential is the potential difference between an electrode and a reference electrode, measured in volts."
"What is the purpose of a reference electrode in measuring electrode potential?"	"A reference electrode, such as the hydrogen electrode, is used to assign a relative potential of zero volts and compare it to other electrodes."
"Describe how the electrode potential of a half-cell is measured."	"The electrode potential of a half-cell is measured by connecting it to another half-cell of known potential and measuring the potential difference between the two half-cells."
"Describe the conditions required for the standard hydrogen electrode."	"The conditions required for the standard hydrogen electrode are hydrogen gas at a pressure of 100 kPa, a solution containing hydrogen ions at 1.0 mol dm-3, and a temperature of 298K."
"What are the common secondary standards used in place of the standard hydrogen electrode?"	"The common secondary standards used are silver/silver chloride electrode (E = +0.22 V) and calomel electrode (E = +0.27 V)."
"What is the purpose of a salt bridge in a cell with a standard hydrogen electrode?"	"The salt bridge is used to maintain electrical neutrality in the cell by allowing the flow of ions between the half-cells."
"Describe the equilibrium of the hydrogen electrode."	"The equilibrium of the hydrogen electrode is H2 (g) ⇌ 2H+ (aq) + 2e-. The hydrogen gas is oxidized to form hydrogen ions and electrons."
"Describe how to calculate the EMF of a cell."	"To calculate the EMF of a cell, use the equation Ecell = Erhs - Elhs, where Erhs is the electrode potential of the right-hand side half cell and Elhs is the electrode potential of the left-hand side half cell."
"What is the purpose of using electrode potentials in calculating the EMF of a cell?"	"The purpose of using electrode potentials is to determine the direction of spontaneous change for redox reactions."
"Define oxidation and reduction in terms of electrode potentials."	"Oxidation occurs at the more negative half cell, where electrons are given off (lost), while reduction occurs at the more positive half cell, where electrons are absorbed (gained)."
"Describe the relationship between electrode potentials and the direction of spontaneous change in a redox reaction."	"The more negative half cell will always oxidize (go backwards), while the more positive half cell will always reduce (go forwards). A spontaneous change will always have a positive Ecell."
"What is the significance of the series of standard electrode potentials?"	"The series of standard electrode potentials helps determine the strength of reducing and oxidizing agents. The most powerful reducing agents are found at the most negative end of the series, while the most powerful oxidizing agents are found at the most positive end."
"Describe why fluorine reacts with water."	"Fluorine reacts with because it has a more positive electrode potential (E°) compared to the oxidation potential of water. This means that fluorine can oxidize water to produce oxygen gas."

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Describe the effect of changing conditions on Ecell.

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Ecell is a measure of how far from equilibrium the cell reaction lies. The more positive the Ecell, the more likely the reaction is to occur.

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