Enthalpy Change

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  • Enthalpy Changes
    • Enthalpy (H) is a measure of the energy content of a system.
      • We cannot measure the absolute value of the enthalpy of a real system but we can measure enthalpy changes that occur when a chemical of physical change takes place
        • The unit of enthalpy change is kJ mol-1 and must always have a + or - sign to show whether the reaction is exothermic (-) or endothermic (+)
    • Processes can be classified as exothermic (heat given out) or endothermic (heat taken in)
      • Exothermic: Temperature change is negative
        • The reaction produces a lower energy state. The energy that is no longer locked up in the system is released as heat, so the temperature rises
        • Many chemical reactions are exothermic. Some of the most important examples are oxidation reactions, e.g.
          • The combustion of methane in air
          • The oxidation of carbohydrates such as glucose in respiration
      • Endothermic: Temperature change is positive
        • The reaction produces a higher energy state. Energy needs to be taken in from  the surroundings, so either the reaction must be actively heated or the temperature falls
        • Examples include;
          • The thermal decomposition of calcium carbonate
          • Photosynthesis
    • Standard Conditions
      • Pressure; 1 atmosphere (100kPa)
      • 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 reversible reaction has the same magnitude but the opposite sign
    • Determining Enthalpy Changes Experimentally
      • Heat Evolved = mC(delta)T
        • m is the mass of the substance (in g) being heated up or cooled down. This is nearly always water
        • C is the specific heat capacity of the substance being heated or cooled
        • (delta)T is the temperature change
        • Using mC(delta)T to find the enthalpy change
          • Find heat change: mC(delta)T
            • Convert units of heat change to kJ: Divide by 1000
              • Find heat change per mol: Divide by mol reacting
                • Decide on the sign of enthalpy change


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