Chapter 20- Electrode potentials and electrochemical cells Flashcards
What is an electrochemical cell and what are they used for?
A combination of two half cells, consists of two electrodes separated by an ionic conductor (electrolyte). Cells are used to measure electrode potentials by reference to the standard hydrogen electrode
How does an electrochemical cell work?
Electron transfer between products of redox reactions at either electrode causes a current to flow and a potential difference is produced between the two electrodes
What is an electrode?
A conductor through which electricity enters or leaves a substance or region
What is the structure of an electrochemical cell?
Made of up of two chemical cells, each contains an electrode with a solution containing ions with the metal ion in it. Electrodes connected by a wire and there is a salt bridge between the solutions
What is a salt bridge and what does it do?
A porous substance soaked in a solution of inert, strong electrolyte like filter paper soaked in KNO3. Salt ions flow through the bridge to complete the cell and balance charges in solutions.
What are the rules for conventional representations of cells?
-Salt bridge of double line in middle
-Most negative potential electrode on left
-Most oxidised species next to salt bridge
-Line between different phases, use state symbols
What is standard electrode potential?
Potential difference/emf when a half cell is set up against a standard hydrogen electrode under standard conditions, 298K, 100kPa, 1M solutions
What are standard conditions for standard electrode potential?
298K, 100kPa, 1M solutions
What is the standard hydrogen electrode?
The measuring standard for half-cell potentials. By definition, it has an emf of 0.00V. It consists of 1M solution H+ ions, hydrogen gas and a platinum electrode
Why is a platinum electrode used in the standard hydrogen electrode?
Platinum is a metal and so conducts electricity, allowing for the flow of electrons and it is inert so will not react
What is the role of the more negative and more positive electrodes in a cell?
More negative electrodes are where substance is more easily oxidised and will produce the electrons while more positive electrodes are where substance is more easily reduced and will accept electrons
What is the emf of a cell?
The difference between the potential differences of the cathode and anode
How is the emf of a cell calculated?
Most positive potential minus most negative potential
What does a greater magnitude of emf show?
A greater magnitude of emf shows that the reaction taking place is spontaneous and favourable
How do substances with more positive electrode potentials behave in redox reactions?
Oxidising agents so they are reduced and so gain electrons. They oxidise species more negative than themselves
How do substances with more negative electrode potentials behave in redox reactions?
Reducing agents so they are oxidised and so lose electrons. They reduce species more positive than themselves
What is the electrochemical series?
A series of elements arranged in order of their standard electrode potentials. The more negative the potential, the more reactive the substance.
How does increasing the concentrations of solutions in a cell affect emf?
Makes cell emf more positive as fewer electrons are produced in reaction
How does increasing pressure of a cell affect emf?
Makes cell emf more negative as more electrons are produced
What are three types of commercial cells?
Rechargeable, non-rechargeable and fuel
Why are some cells rechargeable?
The reaction that occurs in the cell is reversible so the reactants can reform and they regain their cell potential
What is the most common rechargeable cell?
A lithium ion cell
What is the structure of a lithium ion cell?
Anode- lithium cobalt oxide electrode
Cathode- graphite (carbon) electrode
Electrolyte of a lithium salt in organic solvent used to carry charge
What are the half equations for a lithium ion cell?
Cathode- Li+ + e- ⇌ Li
Anode- Li+ + e- + CoO2 ⇌ Li+[CoO2]-
What is the overall reaction for a lithium ion cell?
Li + CoO2 ⇌ Li+[CoO2]-
How are lithium ion cells recharged?
A current is applied over the cell forcing electrons to move in the opposite direction causing the reactions to reverse
What is a non-rechargeable cell?
A cell that is designed to be used only once since the reactions involved are irreversible
What is a fuel cell?
An electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidising agent (often oxygen) into electricity through redox reactions. They do not need to be recharged but require fuel
What is an alkaline hydrogen-oxygen fuel cell?
A fuel cell which uses a continuous supply of hydrogen and oxygen to generate a continuous current, producing water as a waste product
Where do the hydrogen and oxygen come from for a fuel cell?
Hydrogen often obtained using fossil fuels, oxygen comes from air
What are the half equations for an alkaline hydrogen-oxygen fuel cell?
Anode (-ve): H2 + 2OH- –> 2H2O + 2e-
Cathode (+ve): O2 + 2H2O + 4e- –> 4OH-
What is the overall equation for both hydrogen-oxygen fuel cells?
2H2 + O2 –> 2H2O
so emf is the same +1.23V for both
What are some pros and cons of hydrogen-oxygen fuel cells?
Pros: Only produce water as waste product so eco-friendly, emf doesn’t drop as long as fuel keeps being supplied, very efficient
Cons: Hydrogen usually made using fossil fuels and is flammable, also expensive
