F325 Module 2
short notes on key topic areas- some blank spaces because I used them to print off and add my own diagrams, advise doing the same as it is good practice
- Created by: Daneen
- Created on: 23-11-14 19:58
Lattice Enthalpy
Enthalpy change that accompanies the
Formation of one mole
Of an ionic compound
From its gaseous ions
Under standard conditions
It is exothermic because it is breaking bonds
Indicates strength of ionic bonding- more negative means there are strong electrostaic forces
Cannot be measured directly because it is impossible to form one mole of an ionic lattice from gaseous ions
Standard Enthalpy change of Formation
Enthalpy change that takes place when
One mole of a compound is formed
From its constituent elements
In standard states and under standard conditions
Exothermic Process
Enthalpy of atomisation
Enthalpy change that takes place when
One mole of gaseous atoms forms
From the element
In its standard states
Always an endothermic process because bonds have been broken
Ionisation energies
(First) Enthalpy change that accompanies the
Removal of one electron
From each atom in
One mole of gaseous atoms
To form one mole of
Gaseous 1+ ions
endothermic process because the electron being lost has to overcome the attraction from the nucleus in order to leave the atom
Electron Affinity
(First) Enthalpy change accompanying the
addition of one electron
to each atom
of one mole of gaseous atoms
to form one mole of gaseous 1- ions
1st is an exothermic process because the electron is attracted to the outer shell of an atom by the nucleus
2nd is an endothermic process because the electron is repelled by the 1- ion, this repulsion has to be overcome
Born-Harber Cycles
Arrows pointing up are Endothermic
Arrows pointing down are Exothermic
Enthalpy Change of Solution
Enthalpy change that takes place when
one mole of compound is
completely dissolved in water
under standard conditions
Break Down of Ionic Lattice: overcoming attractive forces between oppositely charged ions requires energy- it is the same value as Lattice Enthalpy but has a opposite sign (endothermic- positive sign)
Hydration: Gaseous ions bond to water molecules- the negatively charged ions going to slightly positive hygrogen and positively charged ions going to slightly negative oxygen, bonds are made so this is exothermic
Enthalpy that takes place when
one mole of isolated gaseous ions is dissolved in water
forming one mole of aqueous ions under standard conditions
Lattice Enthalpy from enthalpy change of solution
enthalpy changes of hydration = Lattice Enthalpy + Enthalpy of solution
Hydration and Lattice enthalpies
Lattice Hydration
Ionic size: becomes less negative as the size of the ionic radius increases- indicating weaker attraction between ions and hence weaker ionic bonding
Becomes less negative as size of ionic radius increases- less attraction to water molecules and less energy exerted
Ionic Charge: charge increases it produces a larger attraction between the positive and negative ions, ionic radius decreases from ions being closer in the lattice producing more attraction making lattice enthalpy more negative
As charge increases it has a greater attraction to water, decreasing in size and making enthalpy of hydration more negative
Entropy
The quantitative measure of the degree of disorder in a system
Entropy helps explain things that happen naturally: gas spreading through a room, heat from fire spreading through the room, ice melting in a warm room, salt dissolving in water - energy is changing from being localised (concentrated) to more spread out (diluted)
Entropy increases (more positive) when partices are more disordered
ΔSθsys = ΣSθproducts - ΣSθreactants
ΔSθtotal = ΔSθsys + ΔSθsurr
Free Energy
Free Energy Change is the balance between enthalpy, entropy and temperature for a process: ∆G=∆H+T∆S, a process can take place spontaneously when ∆G<0
Cells and Half Cells
A half cell comprises an element in two oxidation states
Electrons flow along wires from negative electrode to positive electrode
Cell Potentials
Standard electrode potential od a half cell is the e.m.f of a half cell
compared with a standard hydrogen half cell
measured at 298K, 1 mol dm-3 concentrations and 100kPa pressure
Standard cell potential: positive terminal - negative terminal
Cell Reaction: negative terminal is losing electrons, being oxidised and therefore its half equation is reversed. Electrons are then balanced. Combine both equations and cancel electrons to give overall cell reaction
feasibility of reactions
The more negative electrode potential, the greater tendancy to release electrons and shift equillibrium to the left
If increase of temperature: follows le chaterlier's principle; by shifting to the right and removing the electrons from the equillibrium making the electrode potential less negative/more positive
Will reaction actually take place?
- predictions only made about equillibrium posistion and not about reaction rate- might be extremely slow reaction because of high activation energy
- Actual conditions different to standard
- standard electrode potentials apply to aqueous equillibria- reaction may take place under different conditions
IF DIFFERENCE BETWEEN STANDARD ELECTRODE POTENTIAL IS LESS THAT 0.4V, REACTION IS UNLIKELY TO TAKE PLACE
Storage and Fuel Cells
- Non-rechargeable cells provide energy until chemicals have reacted to such an extent that voltage falls
- Rechargeable Cells can have the reaction reversed on recharging- chemicals regenerated (Nickle ad Cadium batteries used in rechargeable batteries and lithium ion and lithium polymer used in laptop chargers)
- Fuel Cells use external supplies of fuel and an oxidant which are consumed and need to be provided continuously
Hydrogen Fuel Cell:
- Reactants flow in, products flow out while electrolyte remains in the cell
- Can operate virtually continuously so long as fuel and oxygen flow into cell
Hydrogen for the future
Fuel Cell Veichles (FCVs)- hydrogen gas/ hydrogen rich fuels
Methonal used over hydrogen because
- liquid is stored easier than a gas
- can be generated from biomass
Advantages Limitations
- Less CO2
- Greater efficiency (40-60%)
- Hydrogen can be stored as a liquid at high pressures
- can be absorbed on to the surface of solid material
- low feasibility of storing liquid hydrogen
- absorbers have limited lifetime
- Fuel Cells have limited lifetime and require changing
- Fuel Cells use toxic chemicals in their production
- Hydrogen is only an energy carrier and not an energy source
Hydrogen for the future
Fuel Cell Veichles (FCVs)- hydrogen gas/ hydrogen rich fuels
Methonal used over hydrogen because
- liquid is stored easier than a gas
- can be generated from biomass
Advantages Limitations
- Less CO2
- Greater efficiency (40-60%)
- Hydrogen can be stored as a liquid at high pressures
- can be absorbed on to the surface of solid material
- low feasibility of storing liquid hydrogen
- absorbers have limited lifetime
- Fuel Cells have limited lifetime and require changing
- Fuel Cells use toxic chemicals in their production
- Hydrogen is only an energy carrier and not an energy source
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