Chapter 23 Redox And Electrode Potentials Flashcards
What is an electrochemical cell?
A cell which converts chemical energy into electrical energy as a result of the movement of electrons.
Electrochemical cells use chemical reactions that transfer electrons between species under redox reactions
What are electrochemical cells made from?
Two separate half cells
Each half cell is one of the two half equations within the redox reaction so half cells contain an element in two different oxidation states.
State the different types of half cell
Metal/ Metal ion half cell
Ion/ Ion half cell
Gas half cell
What is a Metal/ Metal ion half cell?
A metal rod dipped in a solution of its aqueous metal ions
The metal rod enables electrons to move to or from the half cell
The Half cell is attached to a S.H.E
What is an Ion/ ion half cell?
Aqueous ions of the same element in two different oxidation states
An inert Pt electrode is used to transport electrons to and from the half cell instead of a metal rod as this would provide three different oxidation states.
The Half cell is attached to a S.H.E
What is a Non-metal/ Non-metal ion half cell?
A gaseous non-metal element is bubbled over a Pt electrode which is immersed in a solution containing aqueous non-metal ions of the same element at 1.00moldm3.
The Half cell is attached to a S.H.E
What is the Standard Hydrogen electrode?
A half cell consisting of Hydrogen gas in equilibrium with H+ ions at 1.00 moldm^3.
An inert Pt electrode is in contact between the Hydrogen gas and H+ ions and is enclosed between a glass bell
What is the Standard Hydrogen electrode used for?
The S.H.E is used as a reference electrode in order to measure the standard electrode potential of another half cell because it has 0V.
What are electrode potentials?
How readily a chemical substance gains electrons
What does a more positive electrode potential show?
A greater tendency to gain electrons meaning it becomes reduced and is able to oxidise another species.
What is the Standard electrode potential?
The emf of a half cell measured against the standard hydrogen electrode under standard conditions of 298K, 100kPa and all solution concentration at 1.00moldm3.
What is the purpose of the salt bridge in a half cell?
Strip of filter paper soaked in KNO3 solution which allows a flow of ions to balance the charge difference that builds up between the beakers.
What is the equation to calculate cell potential?
Cell potential= most positive — least positive
State the standard conditions
100kPa
298K
All solutions at a concentration of 1.00 moldm3
How do electrons flow between the half cells?
Electrons flow away from the least positive half cell to the most positive half cell
What is the most positive half cell known as?
Cathode
What is the least positive half cell known as?
Anode
At what electrode does reduction take place?
Cathode
Red cat
What are the limitations of predicting reactions of half cells
-Non standard conditions
If cell operates under non standard conditions than the electrode potentials will be different to standard electrode potential values.
So as cell operates the SEP values change
-Rate of reaction
If Rate of reaction is slow it may appear the reaction is not happening
-Activation energy
If activation energy too high it may prevent a feasible reaction from taking place
What is an oxidation number?
The measure of the number of electrons involved in bonding to a different element
What is the sum oxidation numbers equal to?
The overall charge of the species
What does the oxidation number represent in half equations?
The total change in oxidation number equals the number of electrons involved
What does the oxidation number represent in redox reactions?
The change in oxidation number for an oxidised species equals the change in oxidation number of the reduced species
What is the oxidation number for pure element?
0
What is the oxidation number of Group 1,2 or 3 element in a compound?
Group 1 is +1
Group 2 is +2
Group 3 is +3
What is the oxidation number of F?
-1
What is the oxidation number of H usually?
+1 except when with Group 1 element/ metal its -1
What is the oxidation number of O usually?
Usually -2 except when:
Bonded with F it becomes +2
Or in a peroxide it becomes -1
What is the oxidation number of Cl usually?
-1 except when bonded to
F or O Cl becomes positive
How are oxidation numbers used to find the systematic name of a compound?
Roman numerals are used to indicate the oxidation number of an element that can exist in more than one oxidation state
What is a half equation?
Represents the transfer of electrons between a species in a redox reaction
State the rules when writing half equations
Balance all atoms except H and O
Balance O with H2O
Balance H with H+
Balance charge with e-
Write the half equation for the reduction of Manganate (VII) ion (MnO4-) to the Mn^2+ ion
MnO4- ——> Mn2+
1) Balance O with H2O and H with H+
8H+ + MnO4- ——> 4H2O + Mn2+
2) Balance charge with electrons
8H+ + MnO4- + 5e- ——> 4H2O + Mn2+
State the rules when writing half equations that take place under alkaline conditions
Calculate the change in oxidation number of the species and add electrons involved
Balance the charge using OH- ions as the reaction is taking place under alkaline conditions
Balance the atoms using H2O
Describe the procedure used during the redox titration of Manganate (VII)
- A standard solution of KMnO4 is added to a burette
- Using a pipette, a measured volume of solution is transferred to a conical flask
-An excess of dilute H2SO4 is added to the flask to provide H+ ions
-A trial titration is carried out
-KMnO4 is added from the burette and oxidises the solution under analysis
-The MnO4- ions are reduced to Mn2+ and lose their purple colour
-Once all the chemical under analysis has been oxidised the next drop of KMnO4 remains purple after swirling
-Titre is recorded and is repeated until two concordant results are collected within 0.10cm^3 of each other
Describe the procedure used during the redox titration of thiosulfate
-React the oxidising agent with aqueous iodide ions (I-) to liberate Iodine. This would be observed as a light brown solution in conical flask
- The mixture is then titrated against sodium thiosulfate solution of known concentration
-As the iodine reacts, the iodine colour becomes paler
-When the a pale yellow colour is observed just before the end point, starch is added. The
Mixture would now appear blue/black.
-The blue-black colour then disappears when all the iodine has reacted to generate a colourless solution
What is the important rule to remember when calculating thiosulfate titrations?
Mole ratios don’t exist between two equations which means that the moles of Iodine are exactly the same
