Entropy

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  • Created by: Shannon
  • Created on: 02-04-15 14:11
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  • Entropy
    • The measure of the disorder of a system
      • The entropy of a substance is determined by how random its arrangement is and the energy of the molecules in that substance
        • Solid -> Liquid -> Gas (increasing entropy)
        • Perfect crystals at zero kelvin have zero entropy
        • More complex substances have a higher entropy than simple substances in the same physical state
    • Entropy change of the system
      • When a chemical reaction takes place, the entropy of products will be difference from the entropy of reactants
      • = sum of the entropy of products - sum of the entropy of reactants
    • Entropy of the surroundings
      • = -energy change / temperature
    • Increases in temperature
      • Increase temperature for an exothermic reaction causes a decrease in the total entopy
        • Reaction less thermodynamically favourable
      • Increased temperature for an endothermic reaction causes an increase in total entropy
        • Reaction more thermodynamically favourable
      • Entropy of surroundings decreases in magnitude
    • Feasibility of a reaction
      • A successful reaction depends on the balance between the entropy of the system and entropy of the surroundings
      • Reactions are spontaneous as long as the total entropy is positive
    • Thermodynamics vs kinetics
      • A reaction may be thermodynamically feasible however, a high activation energy for the reaction may prevent it from occuring
        • Reactants are said to be kinetically inert
    • Predicting solubility
      • Dependent on lattice energy, enthalpy of solution and enthalpy of hydration
      • Lattice energy = the energy change when 1 mole of ionic compound is formed from its gaseous ions under standard conditions
        • Affected by...
          • Ionic charge - greater the charge the greater the attraction between cations and anions
      • Enthalpy of solution = the enthalpy change when 1 mole of compound dissolves in a stated amount of water under standard conditions
      • Enthalpy of hydration = the enthalpy change when 1 mole of ions dissolve in water, under standard conditions, to produce a solution of concentration 1 moldm-3
        • Affect by...
          • Ionic charge - the greater the charge the greater the dipole force of attraction between ion and H2O molecules
            • Hydration enthalpy increased
              • Ionic radius - the smaller the radius the greater the force of attration
          • Ionic radius - the smaller the radius the greater the force of attration

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