Chemistry A2 Level

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Chemistry A2 Level
2011-12-31 06:02:26
chemistry alevel energy

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  1. Lattice enthalpy
    • enthalpy change that accompanies the formation of one mole of an ionic compound from its gaseous ions under standard conditions
    • exothermic change, negative sign
  2. Key enthalpy changes
    • Formation: one mole of a compound is formed from its constituent elements in there standard states under standard conditions
    • Atomisation: one mole of gaseous atoms is formed from its element in its standard state
    • First ionisation energy: one mole of gaseous 1+ ions is formed from one mole of gaseous atoms
    • Second ionisation energy: one mole of gaseous 2+ ions is formed from one mole of gaseous 1+ ions
    • First electron affinity: one mole of gaseous 1- ions is made from one mole of gaseous atoms
    • Second electron affinity: one mole of gaseous 2- ions is made from one mole of gaseous 1- ions
    • Neutralisation: one mole of water is formed by the reaction between an acid and a base under standard conditions
    • Hydration: one mole of aqueous ions is formed from gaseous ions
    • Solution: one mole of solute is dissolved in sufficient solvent that no further enthalpy changes occurs on further dilution
  3. Born-Haber cycles
  4. Enthalpy change of solution
  5. Ionic charge and radius
    • Higher charge: higher attraction, more exothermic, more negative
    • Smaller radius: more attraction, more exothermic
  6. Entropy
    • a measure of the disorder of a system
    • system becomes energetically more stable when it becomes more disordered, increase in entropy
    • Physical state: solids have lowest entropy, gases have highest
    • Dissolving: increases entropy
    • Change in number of gaseous molecules: more particles/moles, more entropy
  7. Free-energy change
    • the tendency of a process to take place depends on temperature T, the entropy change in the system S, and enthalpy change H, with the surroundings
    • free-energy change, G is the balance between enthalpy, entropy and temperature for a process
    • Spontaneous reaction: free-energy has to be 0 or negative (for endothermic S must be positive, TS>H)
  8. Redox
    • redox: a reaction in which both reduction and oxidation take place
    • oxidation number: a measure of the number of electrons that an atom uses to bond with atoms of another element
    • half-reaction:
    • oxidising agent: a reagent that oxidises (takes electrons from) another species
    • reducing agent: a reagent that reduces (adds electrons to) another species
  9. Electrode potentials
    • Standard electrode potential: the emf of a half cell compared with a standard hydrogen half cell, measured at 298K with solution concentrations of 1 mol dm-3 and a gas pressure of 100 kPa (1 atm)
    • How to measure SEP using a hydrogen electrode: hydrogen electrode is always on the left, has a value of 0V, whole EP=RHS-LHS
    • most negative side is where oxidation happens
    • Metal/metal ion half cells:
    • Non-metal/non-metal ion half cells: plantinum electrode is placed in the solution
    • Metal ion/metal ion half cells: inert platinum electrode allows electrons to pass into or out of the half cell
  10. Feasibility of reactions
    • the cell potential is feasible when over +0.40
    • changing the concentration shifts the equilibrium to oppose the change
    • rate of reaction may be slow, so may appear not to happen
    • may have a high activation energy, stop it happening
  11. Storage and fuel cells: hydrogen fuel cells
    • energy storage cells are like electrochemical cells
    • a fuel cell uses the energy from the reaction of a fuel with oxygen to create a voltage
    • 1. hydrogen is oxidised to H+ ions/protons at the anode
    • 2. the polymer electrolyte only allows the H+ ions across forcing e- to travel round the circuit
    • 3. electric circuit creates a current
    • 4. oxygen is reduced at the cathode
    • 5. water is produced
    • Catalyst: platinum
  12. Storage and fuel cells: FCVs
    • Fuel cell vehicles (FCV)
    • fuel cells develloped to be fuelled by hydrogen gas (emits only water) or by hydrogen-rich fuels (methanol, natural gas or petrol) (relese small amounts of pollutants)
    • Advantages of FCV's: less pollution, less CO2 and more efficient
  13. Storage and fuel cells: storage of hydrogen
    • as a liquid under pressure (very low temperature required)
    • adsorbed onto the surface of a solid material
    • absorbed within some solid materials
  14. Limitations of hydrogen fuel cells
    • platinum catalysts are expensive
    • toxic chemicals used
    • fuel cells have a limited life span
    • disposing of a fuel cell is expensive
    • high production costs
    • if stored as a gas it's explosive
    • if a liquid need expensive storage equipment
    • limited life of adsorbers and absorbers
    • manufacturing hydrogen takes energy
  15. Hydrogen economy
    • may contribute largely to future energy
    • But....
    • many people have concerns about the safety and reliability of hydrogen
    • hydrogen fuel supplies are expensive to set up, so it needs to be cheaper
    • clean energy sources need to be used to produce hydrogen