1.11-Electrode potential Flashcards
What is an electrochemical cells
. A cell has two half cells
. The two half cells have to be connected with a salt bridge
. Simple half cells with consist of a metal, and a solution of a compound contain that metal
. These two half cells will produce a small voltage if connected into a circuit
Why do you use a high resistance voltmeter
.The voltmeter needs to be of very high resistance to stop the current from flowing in the circuit
. in this state you can measure the macimum possible potential difference
.The reactions wont be occuring becasue the very high resistance voltmeter stops the current from flowing
Why does a voltage form
.When one of the compounds is a better reducing gent
.it releases electrons onto the other half cell,
. this will cause a build up of electrons which allows for a potential difference
. this can then be measured via a high resistance voltmeter
Explain what a salt bridge is and why it is used in an electrochemical cell
The salt bridge is used to connect up the circuit, the free moving ions conduct the charge
A salt bridge is usually made from a piece of filter that is soaked in salt solution usually potassium nitrate
The salt should be unreactive with the electrodes and electrode solutions
A wire is not used because the metal wire would set up its own electrode system with the solution
What happens if the current is allowed to flow
If the voltmeter is removed and replaced with something else like a bulb then a current flows
The reactions will occur seperately at each electrode, the voltage will fall to zero as the reactants are used up
What do the positive and negative sides of the electode undergo
The positive side always undergos reduction
The negative side always undergos oxidation
Explain how someone would draw a cell diagram
. The solid vertical line represents the boundary between phases
. The double line represents the salt bridge between the two half cells
. The voltage produced is indicated
. The more positive half cell is written on the right if possible
What would you do for a system that does not include a metal
If there is no metal that can act as an electrode then you must use a platinum electrode instead that is included in the cell diagram
It provides a conducting surface for electron transfer, a platinum electron is used because it is unreactive and conduct electricity
How do you measure the electrode potential of a cell
. It is not possible to measure the potential of a half electrode by itself, if is only possible to measure the potential difference between two electrodes
. It has to be connected to a half cell with a already known potential
. We use the standardy hydrogen electrode which then allows us to assign relative potentials
What are the conditions in a standard hydrogen electrode
. Hydrogen gas at a pressure of 100kPa
. Solution containing the hydrogen ion 1.0 mol dm-3
. Temperature at 298K
. Platinum electrode
What is the SHE
The standard hydrogen electrode is assigned the potential of 0 volts, and is the standard when finding the elctrode potential of anything else
Why do we use standard conditions
Standard conditions are needed because the position of the redox equilibrium will change with conditions
What are the secondary standards
.The SHE is difficult to use, so often a different standard is used which is easier to use
. These are themselves calibrated by SHE
.This is known as using a secondary standard
. Common ones ar silver and silver chloride
What are the standard conditions for a standard electrode potential
. All ions solutions at 1 mol dm-3
. Temperature 298K
. Gases at 100kPa pressure
. No current flowing
What is the formula for calculating the EMF of a cell
E cell = E rhs - E lhs
What does a more negative electrode represent in terms of redox
A more negative electrode means that that it is a better reducing agent
How can you use electrode data to prove that a reaction occurs
Work out the ecell to and use it to prove your answer, if the value is positive then the reaction is feasible
Effects of conditions on cell voltage Ecell
. The effects of changing conditions on E cell can be made by applying le chatelier principle
. If they 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
. Ecell is just a measure about how far from equilibrium the cell reactions lies, the more positive, the more likely the reaction occurs
What is the effect of concentration on Ecell
. Looking at cell reactions is a straight forward application of le chatelier
. So increasing concentration of reactants would increase Ecell and decreasing them would cause Ecell to decrease
What is the effect on temperature on the Ecell
Most cells are exothermic in the spontaneous direction so applying le chatelier to a temperature rise to these would result in a decrease in Ecell because the equilibrium reactions would shift backwards
If an Ecell is positive it indicates that a reaction might occur, there is still a possibiliy that the reaction will not occur if the activation energy is high
Where are cells used
Electrochemical cells can be used as a commercial source of electrical energy, cells can be non rechargable , rechargable and fuel cells
What is a fuel cell
A fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
Fuel cells will maintain a constant voltage over time because they are continously fed with fresh O2 and H2, these maintain constant concentrations of the reactants
What is the advantage of a fuel cell over conventional petrol or diesel power
There will be less pollution and less CO2, pure hydrogen emits only water whilst hydrogen rich fuels only produce small amounts of air pollutants and CO2
What are the limitations of hydrogen fuel cells
.Expensive
.Storing and transporting hydrogen in terms of safety, feasibility of a pressured liquid and a limited life cycle of a solid absorber
. Limited lifetime and high production costs
. Use of toxic chemicals in their production
How can be stored in fuel cells
. As a liquid under pressure
. Absorbed on the surface of a solid material
. Absorbed within a solid material
What is an ethanol fuel cell
. Ethanol fuel cells have certain advantages over hydrogen fuel cells
. Ethanol can be made from renewable sources in a carbon neutral way
. Raw materials to produce ethanol are abundant
. New petrol stations would not be requireed as ethanol is a liquid fuel
