Module 5 Section 2: Energy Flashcards
(120 cards)
Combine the two half equations
Why are transition metals good oxidising and reducing agents
They are good at changing oxidation state
So they’ll give out or receive electrons
Also change colour so it’s easy to spot when the reaction is finished
How potassium manganate is used as an oxidising agent
Contains manganate (VII) ions (MnO4 -) in which manganese has an oxidation state of +7
Can be reduced to Mn2+ ions during a redox reaction
MnO4 - is purple and [Mn(H2O)6]2+ is pale pink, but looks colourless so the colour change in the reaction is purple to colourless
How is potassium dichromate used as an oxidising agent
Contains dichromate(VI) ions (Cr2O7 2-) in which chromium has an oxidation state of +6
They can be reduced to Cr+3 ions during a redox reaction
Cr2O7 2- is orange and [Cr(H2O)6]3+ looks green so colour change is from orange to green
How to work out the concentration of a reducing agent
Titrate a known volume of it against an oxidising agent of known concentration
Can work the other way
How to carry out redox titration with Fe2+ as a reducing agent and MnO4 - as a oxidising agent
Measure out a quantity of the Fe2+ reducing agent
Add dilute sulfuric acid to the flask (this is an excess to make sure there are enough H+ to allow oxidising agent to be reduced)
Gradually add the aqueous MnO4 - to the reducing agent using a burette, swirling the conical flask as you do so
Stop when the mixture in the flask just becomes tainted with the colour of the MnO4 - (end point) and record volume of oxidising agent added
Run more titrations and calculate mean volume of MnO4 -
How can iodine - sodium thiosulfate titrations be used
They are a way of finding the conc of an oxidising agent
The more concentrated an oxidising agent is, the more ions will be oxidised by a certain volume of it
How to prepare for a titration of iodine and sodium thiosulfate
Use a sample of oxidising agent to oxidise as much iodide as possible by measuring out 25cm3 of potassium iodate solution (KIO3 and oxidising agent)
Add this to excess acidified potassium iodide solution (KI) where the iodate(V) ions oxidise some of the iodide ions to iodine
Then find out how many moles of iodine have been produced by titrations the solution with a known conc of sodium thiosulfate (Na2S2O3)
Method of titration of iodine with sodium thiosulfate
Take flask containing solution of oxidised iodide ions (iodine and water)
Add sodium thiosulfate from a burette to the flask drop by drop
Colour fades to pale yellow (hard to see at end point) so add 2cm3 of starch solution to detect presence of iodine so solution goes dark blue showing there still iodine there
Add sodium thiosulfate one drop at a time until blue colour disappears
When this happens, all the iodine has just been reacted
Then calculate number of moles of iodine in the solution
How to do titrate calculation to find number of moles of iodine produced
How to calculate the concentration of oxidising agent using the original equation (6.6x10-4 moles of iodine and 25cm3 oxidising agent)
Definition of lattice enthalpy
Lattice enthalpy is the enthalpy chance when 1 mole of an ionic lattice is formed from its gaseous ions under standard conditions of 298K and 100kPa
What is lattice enthalpy
Ionic compounds can form regular structures called giant ionic lattice
The positive and negative ions are held together by electrostatic attractions
Lattice enthalpy is when gaseous ions combine to make a solid lattice
What does lattice enthalpy measure
This measures the ionic bond strength
The more negative the lattice enthalpy the stronger the bonding
What makes the lattice enthalpy more exothermic
The higher the charge on the ions the more energy is released when an ionic lattice forms
Due to stronger electrostatic forces between the ions
This means that the lattice enthalpy will be more negative
The smaller the ionic radii of the ions involved, the more exothermic the lattice enthalpy
Smaller ions have a higher charge density and the smaller ionic radii means the ions can sit closer together in the lattice
What is the born Haber cycle used for
Lattice enthalpy can’t be measured directly so this cycle is used to calculate the enthalpy change if you took another less direct route
Meaning of the born Haber cycle diagram
Route 1:
Formation of gaseous atoms
• changing the elements in their standard states into gaseous atoms (endothermic process)
Formation of gaseous ions
• changing gaseous atoms into positive and negative gaseous ions (endothermic)
Lattice formation
• changing gaseous ions into the solid ionic lattice (this is lattice enthalpy and is exothermic)
Route 2
Converts elements in their standard states directly to the ionic lattice
Includes one enthalpy change which is enthalpy or formation and is exothermic
Standard enthalpy change of atomisation
The enthalpy change that takes place for the formation of one mole of gaseous atoms from the element in it’s standard state under standard conditions
What sign is in front of the enthalpy of atomisation
Standard enthalpy of atomisation is always an endothermic process so + value
Bonds are broken to form gaseous atoms
What is electron affinity
This is the opposite of ionisation energy
Electron affinity measures the energy to gain electrons
Always exothermic as the electron being added is attracted to nucleus
Definition of the first electron affinity
The enthalpy change that takes place when one electron is added to each atom in one mole of gaseous atoms to form one mole of gaseous 1- ions
Diagram example of successive electron affinities of oxygen