1.11-Electrode potential

Question 1

What is an electrochemical cells

Answer 1

. 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

Question 2

Why do you use a high resistance voltmeter

Answer 2

.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

Question 3

Why does a voltage form

Answer 3

.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

Question 4

Explain what a salt bridge is and why it is used in an electrochemical cell

Answer 4

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

Question 5

What happens if the current is allowed to flow

Answer 5

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

Question 6

What do the positive and negative sides of the electode undergo

Answer 6

The positive side always undergos reduction

The negative side always undergos oxidation

Question 7

Explain how someone would draw a cell diagram

Answer 7

. 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

Question 8

What would you do for a system that does not include a metal

Answer 8

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

Question 9

How do you measure the electrode potential of a cell

Answer 9

. 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

Question 10

What are the conditions in a standard hydrogen electrode

Answer 10

. Hydrogen gas at a pressure of 100kPa
. Solution containing the hydrogen ion 1.0 mol dm-3
. Temperature at 298K
. Platinum electrode

Question 11

What is the SHE

Answer 11

The standard hydrogen electrode is assigned the potential of 0 volts, and is the standard when finding the elctrode potential of anything else

Question 12

Why do we use standard conditions

Answer 12

Standard conditions are needed because the position of the redox equilibrium will change with conditions

Question 13

What are the secondary standards

Answer 13

.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

Question 14

What are the standard conditions for a standard electrode potential

Answer 14

. All ions solutions at 1 mol dm-3
. Temperature 298K
. Gases at 100kPa pressure
. No current flowing

Question 15

What is the formula for calculating the EMF of a cell

Answer 15

E cell = E rhs - E lhs

Question 16

What does a more negative electrode represent in terms of redox

Answer 16

A more negative electrode means that that it is a better reducing agent

Question 17

How can you use electrode data to prove that a reaction occurs

Answer 17

Work out the ecell to and use it to prove your answer, if the value is positive then the reaction is feasible

Question 18

Effects of conditions on cell voltage Ecell

Answer 18

. 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

Question 19

What is the effect of concentration on Ecell

Answer 19

. 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

Question 20

What is the effect on temperature on the Ecell

Answer 20

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

Question 21

Where are cells used

Answer 21

Electrochemical cells can be used as a commercial source of electrical energy, cells can be non rechargable , rechargable and fuel cells

Question 22

What is a fuel cell

Answer 22

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

Question 23

What is the advantage of a fuel cell over conventional petrol or diesel power

Answer 23

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

Question 24

What are the limitations of hydrogen fuel cells

Answer 24

.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

Question 25

How can be stored in fuel cells

Answer 25

. As a liquid under pressure
. Absorbed on the surface of a solid material
. Absorbed within a solid material

Question 26

What is an ethanol fuel cell

Answer 26

. 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