Chapter 20 - Electrochemical Cells Flashcards
What do standard electrode potentials do
They allow a way of comparing how easily different substances give away electrons
What happens to oxidising and reducing agent ability as electrode potential gets bigger
Oxidising agent ability increases
Reducing agent ability decreases
What happens to oxidising and reducing ability as electrode potential gets smaller
Reducing agent ability increases
Oxidising agent ability decreases
How do electrode potentials show the direction of a redox reaction
The half equation which has the biggest electrode potential will oxidise the other equation
Write both half equations as reductions
If the reaction given is going the wrong direction then it is not feasible
What are the limitations of electrode potential values
Electrode potential values can tell you if a reaction is feasible but not if it will happen
It doesn’t take into account activation energy
In what form are all the half equations for electrode potentials written in
They are written as reduction half equations
What are oxisiding agents
Electron acceptors
What are reducing agents
Electron donors
In a reduction half equation which side is the oxidising and reducing agent
Reducing agents are on the right
Oxidising agents are on the left
What is a electrochemical cell
It is 2 half cells connected by an external circuit and a salt bridge
What are redox couples/half cells
The metal atoms in contact with their aqueous metal ions
How are half cells written
The oxidised species first
What way do electrons flow in an electrochemical cell
From the negative electrode to the positive electrode
What is used to measure the potential difference between 2 half cells
Add a high resistance voltmeter in the external circuit which measures it but stops the reaction
What is the electromotive force of a cell (e.m.f)
The potential difference of the 2 half cells
Tells us the power from the combination of different half cells
What does the salt bridge do
It allows the ions from the ionic substance to flow to maintain charge balance / complete the circuit
How is a salt bridge made
Filter paper soaked in an electrolyte solution that doesn’t react with the contents of the beakers e.g. saturated potassium nitrate solution
How do you use conventional cell notation
Place the negative electrode ions and atoms on the left and the positive electrode ions and atoms on the right
Electrons flow from left to right
How is e.m.f measured of a half cell
A half cell is connected to the standard hydrogen electrode so the value can be compared to other half cells
What is the standard hydrogen electrode
It is an electrode with a electrode potential of 0.00V
Pt l H2 l H+ ll
How is the equation for e.m.f of a electrochemical cell
electrode potential (RHS) - electrode potential (LHS)
What does the Platinum electrode do in a half cell
It provides an inert surface for electron transfer to occur
What means a reaction is feasible
the e.m.f value must be positive
What is a battery
It is a number of cells connected together
What are the 3 types of cells
Non-rechargeable cells
Rechargeable cells
Fuel Cells
What are Non-rechargeable batteries
They are cells where the voltage falls as chemicals are used up
What are Zinc-Carbon cells like
Walls of battery used up as battery is used so prone to leakage
Cells used for applications of small, intermittent currents
What are the types of Non-rechargeable cells
Zinc-Carbon Cell
Alkaline cell
What are alkaline cells like
These are used for continuous high currents
Higher current and longer life than zinc-carbon cells
What are rechargeable cells
They are cells where the reactions are reversible
They are reversed by applying an external current
What are the types of rechargeable cells
Li ion cell
Nickel-Cadmium cell
Lead - Acid Battery
What are Li ion cells like
Most common type of rechargeable cell
Used in mobile phones and laptops
What are the half equations for the Li ion cell
Li+ + CoO2 + e- > LiCoO2
Li+ + e- > Li
CoO2 + Li > LiCoO2
What happens to the equation when the cell is being re-charged
The equation is reversed
What are the Nickel-Cadmium cell like
Expensive but recharge many times
Replace non-rechargeable cells
What is the Lead-Acid battery like
They are used as car batteries
Used to start the car
Recharged when engine is running
What is a fuel cell
The fuel is hydrogen and oxygen
The fuel is continually supplied from an external source
Operate in both acidic and alkaline conditions
Very efficient and don’t need to be recharged
What is the biggest challenge for fuel cells
Finding a cheap source of hydrogen not from fossil fuels
What are the two types of fuel cells
Acidic fuel cells
Alkaline Fuel Cells
What are the half equations for the alkaline fuel cells
O2 + 2H2O + 4e- > 4OH-
2H2O + 2e- > H2 + 2OH-
O2 + 2H2 > 2H2O
What is the overall equation for the acidic fuel cell
2H2 + O2 > 2H2O
What are the advantages and disadvantages of Non-rechargeable cells
Cheap
Small
Prone to leakage
Short Life
Hard to dispose of
What are the advantages and disadvantages of Rechargeable cells
Can be used many times
Cheaper overall from multiple uses
Expensive initially
Waste into land fill
What are the advantages and disadvantages of Fuel Cells
Environmentally friendly as product is water
Quiet
Hydrogen is flammable and expensive
Hard to find hydrogen not from fossil fuels
Less portable
What are the conditions for measuring e.m.f with a standard hydrogen electrode
100kPa
298K
All solutions at 1 mol/dm3
What is the property of the non-reactive porous separator in a non-rechargeable cell
Allows ions to flow
What is the function of the carbon rod in a non-rechargeable cell
Allows electrons to flow
What is the meaning of an electrochemical series
It is a list of electrode potentials in numerical order
