ELECTRODE POTENTIALS

What is a cell?

• made up of 2 half-cells

• connected with a salt bridge (KNO3)

• metal electrodes and a solution of a compound containing the metal

Why does a voltage formed (in a zinc, copper cell)?

• Zinc has more of a tendency to oxidise to the Zn2+ ion and release electrons

• more electrons will build up

• a potential difference is created

Why do you use a high resistance voltmeter?

stops the current from flowing in the circuit, meaning its possible to measure the maximum possible potential difference

What is a salt bridge?

• used to connect up the circuit (free moving ions conduct the charge)

• made from a piece of filter paper soaked in potassium nitrate solution

• salt should be unreactive with the electrodes and electrode solutions

What happens if current is allowed to flow?

• occurs if the voltmeter is removed and replaced with a bulb

• reactions will occur separately at each electrode

• voltage will fall to zero as the reactants are used up

What happens at the positive electrode?

reduction

What happens at the negative electrode?

oxidation

What’re the rules for electrochemical cell representation?

• solid vertical line represents different phases

• double line represents the salt bridge

• more positive half cell is written on the right

• oxidised form is put next to the double line

What is the electrochemical cell standard representation for Zn and Cu?

What is used if a system doesn’t include a metal?

a platinum electrode, which provides a conducting surface for electron transfer

What’re properties of a platinum electrode which means they can be used?

• unreactive

• conducts electricity

What is the standard hydrogen electrode?

• potentials of electrodes are measured by comparing their potential to the standard hydrogen electrode potential

• potential of 0V

What is the hydrogen electrode equilibrium?

H2(g) —> 2H+ (aq) + 2e-

What is the cell representation of the standard hydrogen electrode?

Pt| H2(g) | H+(aq)

What’re the conditions of the standard hydrogen electrode?

• hydrogen gas at a pressure of 100kPa

• solution containing hydrogen ions at 1.0mol dm^-3 (from HCl)

• temperature of 298K

• platinum electrode

Why are secondary standards used?

• the standard hydrogen electrode is difficult to use so different standards are used which are easier

• they’re calibrated against the standard hydrogen eelctrode

What is the standard electrode potential?

the potential difference measured when an electrode system is connected to the hydrogen electrode system and standard conditions apply

What’re standard conditions of a cell?

• all ion solutions at 1 moldm ^-3

• temperature of 298K

• gasses at 100kPa

• no current flowing

What happens to the more positive cell in a cell equation?

it will always go forwards

What does a more positive E mean?

• more likely to be reduced and act as oxidising agents

What does a more negative E mean?

more likely to be oxidised and act as reducing agents

What would be the exam style answer to ‘use electrode data to explain why fluorine reacts with water. Write an equation for the reaction that occurs?

• As E F2/F- > E O2/H2O, F2 will oxidise H2O to O2

• 2F2(g) + 2H2O (l) —> 4F- (aq) + 4H+ (aq)

What happens to the Ecell if the 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

What does a more positive Ecell mean?

more likely the reaction is going to occur

What happens to the Ecell when the concentration of reactants increases?

increases E cell

What happens to Ecell if you increase the temperature on an exothermic cell?

causes a decrease in Ecell as the equilibrium reaction would shift backwards

What does a positive Ecell indicate?

• the reaction might occur

• if the reaction has a high activation energy the reaction will not occur

What are the different types of cells?

• non-rechargeable

• rechargeable

• fuel cells

What makes a cell non-rechargeable?

when the reactions that occur within them are non-reversible

What makes a cell rechargeable?

• forward reaction occurs on discharge giving out charge

• charging causes the reaction to reverse

What’re fuel cells?

use the energy from the reaction of a fuel with oxygen to create a voltage

What is the equations at the electrodes for the hydrogen fuel cell in alkaline conditions?

• negative : H2 + 2OH- → 2e- + 2H2O

• positive : 4e- + 2H2O + O2 → 4OH-

• overall : 2H2 + O2 → 2H2O

Why do fuel cells maintain a constant voltage overtime?

they’re continuously fed with fresh O2 and H2, maintaining constant concentration of reactants

What conditions are used in hydrogen fuel cells?

• higher temperatures which increase the rate however causes Ecell to fall as the reaction is exothermic

• higher pressure will counteract this

What’re advantages of fuel cells over petrol or diesel powered vehicles?

• less pollution and CO2

• greater efficiency

What’re limitations of hydrogen fuel cells?

• expensive

• hard to store and transport hydrogen safely

• limited lifetime

• high production costs

• use of toxic chemicals in production

How can hydrogen be stored in fuel cells?

• liquid under pressure

• absorbed on the surface of a solid material

• absorbed within a solid material

What’re advantages to ethanol fuel cells?

• ethanol can be made from renewable sources in a carbon neutral way

• raw materials to produce ethanol by fermentation are abundant

• ethanol is less explosive and easier to store than hydrogen

• new petrol stations would not be required

What’re the half cell and full equation to represent a litium ion cell?

• negative = Li —> Li+ + e-

• positive = e- + Li+ + CoO2 —> Li+[CoO2]-

• overall= Li + Li+ + CoO2 —> Li+ + Li+[CoO2]-