Enthalpy Change

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  • Enthalpy Changes
    • Enthalpy is a measure of the energy content of a system
      • We can measure enthalpy changes that occur when a chemical or physical change takes place
    • Processes can be classified as exothermic (heat given out), or endothermic (heat taken in)
      • In an exothermic reaction a lower energy state is produced
        • The energy that is no longer locked up in the system is released as heat, so the temperature rises
          • e.g. Combustion, oxidation reactions, respiration
      • In an endothermic reaction a higher energy state is produced
        • Energy needs to be taken in from the surroundings, so either the reaction must be actively heated or the temperatures
          • e.g. Thermal decomposition of calcium carbonate, photosynthesis
    • Standard conditions
      • Pressure = 1 atmosphere
      • Temperature = 25 degrees centigrade
    • Enthalpy change of reaction
      • Standard enthalpy change of reaction is the enthalpy change when the number of moles shown in the equation react to form the products , under standard conditions
    • Enthalpy change of combustion
      • Standard enthalpy change of combustion is the enthalpy change when 1 mole of a substance is burned completely in oxygen under standard conditions
    • Enthalpy change of formation
      • Standard enthalpy change of formation of a substance is the enthalpy change when 1 mole of a substance is formed from it's elements under standard conditions
    • Reversible reactions
      • If the enthalpy change for a reaction is known, then the enthalpy change for the reverse reaction has the same magnitude, but with the opposite sign
    • Determining enthalpy changes experimentally
      • Heat evolved = mc(delta)t
        • m is the mass of substance heated up or cooled down
        • C is the specific heat capacity
        • delta t is the temperature change
        • Summary of steps
          • Find heat change (J) = mc(delta)t
            • Convert units of heat change to kJ: divide by 1000
              • Find heat change per mol: Divide by mol reacting
                • Untitled

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