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 

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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

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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

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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 

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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 

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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

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Born-Harber Cycles

Arrows pointing up are Endothermic

Arrows pointing down are Exothermic

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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

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Lattice Enthalpy from enthalpy change of solution

enthalpy changes of hydration = Lattice Enthalpy + Enthalpy of solution

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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 

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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

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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


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Cells and Half Cells

A half cell comprises an element in two oxidation states 

Electrons flow along wires from negative electrode to positive electrode

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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

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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

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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
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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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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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