Electrochemical Cells Flashcards
Half Cell
Rod of metal in a solution of its own ions where an equilibrium is set up
Suggest how current flows between electrodes in an electrochemical cell
- electrons flow from negative electrode where oxidation occurs
- to positive electrode where reduction occurs
- (flow from most reactive to less reactive metal)
Suggest why a salt bridge is used in preference to a wire
Avoid further metal/ion potentials in circuit
Suggest purpose of salt bridge
- allows ions to move in salt bridge
- charge balance / completes circuit
Describe a salt bridge
Inverted U tube stoppered with cotton filled with potassium nitrate solution / soaked filter paper
Standard Electrode Potential
- potential of a cell consisting of an electrode to release electrons when attached to hydrogen electrode
- provides a measure for reducing power of metal electrode
Describe hydrogen electrode
- inverted test tube supplied with hydrogen gas
- platinum foil attached to platinum wire (unreactive)
- electrode potential of hydrogen electrode is 0V by definition
Suggest standard conditions for electrode potential
298K, 100kPa, 1 moldm-3 (only mention relevant)
Suggest purpose of platinum in hydrogen electrode
- H2 cannot conduct electricity
- platinum provides surface for transfer of electrons
- increasing surface area of platinum increases rate of reaction
Ecell or EMF
Ecell(R)- Ecell(L)
NOT positive minus negative!
How to tell if reaction in electrochemical cell is feasible
Ecell is positive
Describe lithium ion battery
- negative electrode is carbon (graphite)
- positive electrode is lithium cobalt oxide
- solid polymer electrolyte
-ve : Li -> Li+ + e-
+ve: Li+ + CoO2 + e- -> LiCoO2
Suggest how lithium ion battery is recharged
- external voltage applied greater than voltage of cell
- drives electrons in opposite direction
- reactions reversed to reform reactants
Describe alkaline hydrogen-oxygen fuel cell
- platinum catalyst electrodes
- NaOH electrolyte
- ve : H2 + 2OH- -> 2H2O + 2e-
+ve : O2 + 2H2O + 4e- -> 4OH-
(remember H2>H2O and O2>OH- for electrolyte then write ionic equations and replace H+ with OH-)
Suggest why emf of hydrogen cell is constant and why it does not need recharging
- continuous supply of reagents
- concentrations of H2/O2 constant
Suggest how hydrogen is stored
- as liquid under pressure
- absorbed onto solid metal hydride
Evaluate hydrogen cell
Advantages
- no pollution (only H2O)
- higher efficiency
- renewable
- hydrogen readily available from electrolysis of water
Disadvantages
- difficulty storing and transporting due to safety (H2 is highly flammable so explosive)
- H2 from crude oil which is finite
- electrolysis of water is expensive
- energy required comes from combustion of fossil fuels
Suggest why aluminium electrode is rubbed with sandpaper prior to use
To remove oxide layer
Suggest why certain solutions are not suitable for use in salt bridges
- react with ions in solution
- change concentrations of ions
- hence emf changes
Conventional Cell Representation
- vertical solid line for phase boundary
- double vertical line for salt bridge
- species with HIGHEST OXIDATION states closer to salt bridges
- positive electrode on RHS
- do NOT balance 2H+ for H2 electrode just simply H+
Suggest why emf of cell changes when electrodes are connected and current flows
- concentrations of ions change
- so no longer standard conditions
Suggest conditions a salt bridge must satisfy
- must allow ions to flow between half cells
- must not react with chemicals used in cell
- must not be oxidised or reduced in process
Suggest why salt bridges are plugged with cotton at both ends of the U tube
- prevent two-half cell solutions from mixing
- only allow passage of ions
Explain how salt bridges work and describe what would happen to galvanic cell if salt bridge was removed
- at negative half cell, oxidation causes solution to become positively charged so negative ions from salt bridge flow into solution
- at positive half cell, reduction causes solution to become negatively charged so positive ions from salt bridge move into solution
- main charge neutrality in cell
- without salt bridge charges in half cells become imbalanced and cell voltage will drop to zero
