5.2.3 Redox and electrode potentials

What is a redox reaction?

a transfer of electrons from a reducing agent to an oxidising agent

Is potassium manganate a oxidising or reducing agent?

oxidising agent

What does self-indicating mean?

it changes colour when the redox reaction is complete

Write the half equation for the reduction of manganate

MnO4- + 8H+ + 5e- → Mn2+ + 4H2O

Write the formula for the manganate ion

MnO4-

What colour are manganate ions?

purple

What is the oxidation state of Mn in MnO4-

+7

What is manganate reduced to?

Mn2+

What colour are Mn2+ ions?

pale pink almost colourless

In a MnO4-/Fe2+ titration, where is the MnO4- and where is the Fe2+?

• MnO4- is in burette

• Fe2+ is in conical flask

What is the overall redox equation for the reactrion between Fe2+ and MnO4-?

5Fe2+ + MnO4- + 8H+ → 5Fe3+ + Mn2+ + 4H2O

What provides the H+ ions in the redox reaction between manganate and iron (II)

excess sulfuric acid

What is the colour change in the redox reaction between Fe2+ and MnO4-?

colourless to pale pink

Write the formula of the thiosulfate ion

S2O32-

Is thiosulfate an oxidising or reducing agent?

reducing agent

What colour is iodine in aqueous solutions?

brown

Write the overall redox equation for the reaction between thiosulfate and iodine

2S2O3 2- + I2 → 2I- + S4O6 2-

What is the indicator in the reaction between thiosulfate and iodine

starch

What colour is starch when iodine is present

blue-black

What is the overall colour change of the titration between iodine and thiosulfate

blue-black → colourless

What is the I2/S2O32- titration used in the analysis of?

• copper (ii) compounds

• household bleach containing chlorate ions

• copper alloys e.g. brass

What are standard solutions prepared in?

volumetric flask

What is used to measure exact volumes?

pipette

Where does the redox reaction take place (what glassware)

conical flask

What is used to add the oxidising agent dropwise?

burette

What are the steps to making a standard solution

1. accuratelly weigh the solid

2. rinse into a beaker and add rinsings

3. carefully transfer the solution into a volumetric flask

4. add rinsings

5. fill with distilled water till the bottom of the meniscus is in line with the graduation mark

6. invert several times to mix fully

What does the Electrochemical series give?

the standard electrode potential, E, values for reduction half equations

What does a standard electrode potential value give?

the relative ease of a reduction or oxidation process

What do E values compare?

the relative oxidising or reducing power of different species

Which is the forward reactions of E values?

reduction

What does a more positive E value indicate?

the species has a greater tendency to gain electrons and undergo reduction so equilirium shifts right

What does a more negative E value indicate?

the species has a greater tendency to lose electrons and undergo oxidation so equilirium shifts left

Which side of the E equation are oxidising agents found on?

lefr

Which side of the E equation are reducing agents found on?

right

In the equation H2O2 + 2H+ + 2e- → 2H2O what is the oxidising agent?

H2O2 and 2H+

What is used to measure standard electrode potential?

a standard half cell and hydrogen half cell

What is the standard electrode potential of hydrogen?

0

What are standard conditions?

• 298 K

• all solutions 1.00 moldm^-3

• all gasses 1 atm

What temperature is standard conditions?

298K

What concentration is standard conditions?

1.00moldm^-3

What pressure is standard conditions?

1 atm

How many types of standard half cell are there?

3

List 3 types of standard half cell

• metal/metal ion

• metal ion/metal ion

• non

Draw a metal ion/ metal half cell

What is used as the electrode in a metal ion/ metal half cell?

the metal

In a metal ion/ metal half cell where does redox take place?

surface of the metal

What is the solution containing dissolved metal ions called?

electrolyte

Draw a metal ion/ metal ion half-cell?

In a metal ion/metal ion half cell what is used as the electrode?

platinum

Why is platinum used as an electrode?

• inert

• electrical conductor

Draw a non-metal/non-metal ion half cell for Cl2

Draw a non metal/ non metal ion half cell for I2

What is used as the electrode in a non-metal/non-metal ion half cell?

platinum

What is the redox system of a standard hydrogen half cell?

2H+ + 2e- → H2

Draw the standard hydrogen half cell

Define standard electrode potential

the e.m.f of a half cell compared with a standard hydrogen half cell measured at 298K, 1 atm and with all solution concentrations of 1 moldm^-3

Give 3 features to measure standard electrode potential

• half cell must be connected to a standard hydrogen half cell by a salt bridge

• standard conditions must apply

• standard electrode potential is measured in volts from a high resistance voltmeter

Draw the apparatus that measures the standard electrode potential of Zn?

How do electrons move in a cell?

through the wire

How do ions move in a cell?

through the salt bridge and solution

What is a salt bridge usually made from?

filter paper soaked in an ionic solution which doesn’t react with the electrolytes

Give an example of ionic solutions used in salt bridges?

KNO3 or NH4NO3

What is used to record the standard electrode potential

high resistance voltmeter

What is an electrochemical cell?

when any two half cells are connected together by a salt bridge

Draw a zinc/copper cell

In a cell, which is the positive electrode?

the electrode with the more positive E value

In a cell which is the negative electrode?

the electrode which has the more negative E value

In a cell, which half cell undergoes reduction?

the positive electrode

In a cell which half cell undergoes oxidation

the negative electrode

How do you calculate standard cell potential?

E value of reduction - E value of oxidation

What is always the sign of a standard cell potential?

positive

What is the rule for a redox reaction to be feasible?

the species undergoung reduction must have a more positive E value

GIve 2 limitations of predictions made using standard cell potentials?

1. kinetcs

2. non-standard concentrations of solutions

What are electrochemical cells used as in life?

a source of electrical energy

Give 2 types of storage cells and batteries

• primary cells

• secondary cells

What are primary cells?

cells that provide electrical energy until the chemicals have fully reacted to an extent the voltage falls and the cell is flat

Are primary cells rechargeable?

no

What is a secondary cell?

the chemicals react, providing electrical energy and then the reaction can be reversed during the recharging process so that the cell can be used again

Is a secondary cell rechargeable?

yes

Give 3 examples of secondary cells?

• lead-acid car batteries

• nickel-cadmium cells in torches

• lithium-ion cells in laptops

What is a fuel cell?

a cell which uses the energy from a reaction of a fuel with oxygen to create a voltage

Why are fuel cells different to storage cells?

the fuel and oxygen flow into the cell and the products flow out so electrical energy is continuously generated

Do fuel cells need to be recharged?

no

What are modern fuel cells based on?

hydrogen or hydrogen rich fuels like methane

Give 2 types of electrolytes for modern hydrogen fuel cells

• acidic

• alkaline

What is the overall equation for a hydrogen fuel cell?

H2 (g) + 1/2O2 (g) → H2O (l)

What is the standard cell potential for a hydrogen fuel cell?

+1.23 V