5.2 Energy

• Oxidising agent?


  electron acceptor, in a redox reaction it is reduced
• Reducing agent?


  is an electron donor, in a redox reaction it is oxidised
• Procedure of redox titration?


  -fill burette with a standard solution of potassium manganate(VII)

  -pipette a known volume of iron(II) sulfate solution into a conical flask

  -add an equal volume of dilute sulphuric acid

  -add the potassium manganate until endpoint is reached

  -colourless to permanent pink

  -perform a rough titre until you have 2 concordant results
• Potassium Manganate (VII)

  5e- + MnO4- + 8H+ --> Mn2+ + 4H2O
• Iron(II) sulfate

  Fe2+ --> Fe3+ + e-
• Sodium thiosulfate

  2S2O32- + I2 --> 2I- + S4O62-
• Sodium thiosulfate end point:

  -starch is added when solution goes pale yellow from orange/brown

  -blue/black to colourless at end
• Standard Electrode potential?

  EMF of a half cell compared to the standard hydrogen half cell

  Hydrogen electrode has a potential of 0.00V

  
• Type of electrode:

  solid - normal block shape

  solution - dissolved in solution with Pt electrode

  gas - same as hydrogen cell
• Standard cell potential:

  =Epot of more positive - Epot of more negative
• Oxidation occurs at negative electrode
• Reduction occurs at positive electrode
• Standard Conditions?


  298K

  1moldm-3

  100kPa
• More negative E?


  reducing agent and is oxidised

  1/2 cell is reverse reaction
• More positive E?

  oxidising agent and is reduced

  1/2 cell in forward reaction
• Standard Copper-Zinc Cell:

  
• Reaction may take place in half cells if:

  -a larger difference of E values

  -standard cell potential is over 0.4V as this may mean that the reaction wont go to completion
• Concentration effect on half cells?


  -more positive half cell has conc>1 eqm moves to the right and cell becomes more positive and the Ecell will increase as bigger difference in half cell values

  -more negative half cell has conc>1 eqm moves right and half cell becomes more positive but the Ecell will decrease as smaller difference in half cell values

  -if water is added to more positive half cell then eqm moves to the left and half cell becomes more negative and Ecell will decrease as smaller difference in half cell values
• Limitations of electrode potentials:

  -many reactions arent done in standard conditions which affects the values

  -many reactions arent carried out in aqeuous conditions and potentials dont apply to any other state

  -using electrode potentials tells us about the eqm not the rate of reaction
• Lithium Storage cells advantages?


  high energy density
• Lithium storage cells disadvantage?


  -risks of toxic chemicals

  -flammable
• Fuel cells?


  creates an electrical voltage from the chemical reaction of a fuel with oxygen
• Lattice Enthalpy?


  enthalpy change when one mole of an ionic lattice is formed from its gaseous ions under standard conditions
• Enthalpy of atomisation?

  enthalpy change when one mole of gaseous atoms is formed from the element in its standard state under standard conditions
• Enthalpy of first ionisation energy?


  enthalpy change when one mole of gaseous atoms form one mole of gaseous unipositive ions under standard conditions
• Enthalpy of electron affinity?


  enthalpy change when one mole of gaseous atoms each gain an electron to form negative ions with a charge of 1- under standard conditions
• Enthalpy of solution?


  enthalpy change when one mole of a solute dissolves in a large excess of water under standard conditions
• Enthalpy of hydration?


  enthalpy change when one mole of isolated gaseous ions reacts with water to produce one mole of aqueous ions
• Factors effecting lattice enthalpy:

  -sizes of ions, smaller ions have greater attraction so the lattice enthalpy with these ions is more negative then others

  -charges of ions, greater charge has greater attraction wich makes the lattice enthalpy with these ions more negative
• Factors effecting hydration enthalpy:

  -size of ions, smaller ions have greater attraction to water molecules so these ions are more exothermic then bigger ions so enthalpy is stronger

  -charge of ions, greater charge means smaller ion and therefore a greater attraction to water molecules so enthalpy of hydration increases becoming more exothermic
• Entropy?


  measure of the amount of dispersal energy in a system which is greater the more disordered a system
• Factors effecting entropy:

  -change in physical state, gas has more dispersal energy then a liquid or solid, solid has the least amount of dispersal energy

  -change in number of gaseous molecules, going from 2 moles of gas in a reaction to 4 means more entropy
• A positive change in entropy is favourable
• Entropy change?


  Entropy of products - entropy of reactants
• Free energy?


  G=H-TS

  where;

  G is free energy

  H is enthalpy change

  T is temperature

  S is entropy change
• For a reaction to be feasible G<0
• Feasible?


  used to describe whether a reaction is able to happen
• Free energy if?


  -H(-) and TS(+) then G is always negative so feasable

  -H(-) and TS(-) then G is negative at low temps so only feasble at low temp

  -H(+) and TS(+) then G is negative at high temp so only feasable at high temp

  -H(+) and TS(-) then G is always positive so not feasable