Energetics 

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  • Energetics
    • Enthalpy
      • ENTHALPY is a measure of the amount of heat energy contained in a substance. It is stored in the chemical bonds and intermolecular forces as potential energy.
        • If enthalpy change in a reaction is negative, energy has been released to the surroundings = reaction is exothermic
        • If enthalpy change is positive, energy has been absorbed to the surroundings = reaction is endothermic
        • Enthalpy of combustion = reactants - products
        • Enthalpy of formation = products - reactants
      • Heat change    Q=mc?T
        • the specific heat capacity gives heat needed to increase the temperature of unit by mass by 1K
          • SPECIFIC HEAT CAPACITY     c= q/?T       heat capacity= heat change/ temperature change
      • the standard enthalpy of formation of a substance is the enthalpy change that occurs when one mole of a substance is formed from its elements in their standard states under standard conditions
    • Hess's Law
      • Hess's law states that the enthalpy change for any chemical reaction is independent of the route, provided the starting conditions and  final conditions, and reactants and products are the same
        • it allows us to calculate enthalpy changes for the reactions that cannot be measured directly in the laboratory
          • enthalpy circles
            • Reversing the direction of the reaction reverses the sign of ?H
    • Bond enthalpies
      • MEXO: making bond is exothermic
      • BENDO: breaking bond is endothermic
      • Energy needed to break one mole of bonds in gaseous molecules under standard conditions
        • enthalpy change of a reaction can be calculated by using bond enthalpies - Remember to take in consideration coefficients
    • Ozone depletion
      • the double bond in O2 is stronger than the 1.5 bond in ozone and so is broken by radiation of higher energy and shorter wavelength
        • Ozone is formed from free radical substitution
      • E (photon) = hv
    • Ionisation energies and electron affinities
      • first ionisation energy is the minimum energy required to remove one mole of electron from one mole of gaseous atoms    Na(g)--> Na?(g) + Cl? (g)
      • first electron affinity is the enthalpy change when one mole of gaseous electrons is added to one mole of gaseous atoms      Cl(g)+e?--> Cl?
    • Gibbs free energy
      • ?G must be negative for a reaction to be spontaneous
        • ?G (system) = ?H(system) - T?S(system)

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