Enthalpy and Hess's cycles

A review of enthaply changes and hess's cycles

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  • Created by: R_Hall
  • Created on: 10-11-12 14:20

Enthalpy Changes

  • A reaction that gives out energy and heats the surroundings is exothermic, and a reaction that takes in energy and cools the surroundings is endothermic
  • An enthalpy change (ΔH) is an energy change under constant pressure
  • Exothermic reactions have -ΔH value, energy is lost to the surroundings. Endothermic reactions have +ΔH value, energy is gained from surroundings
  • Standard conditions are temperature of 298K (25C), 1 mole concentration solution and 1 atmosphere pressure
  • Enthalpy of formation is the enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states, under standard conditions
  • Enthalpy of combustion is the enthalpy change when 1 mole of a fuel is burnt completely in oxygen, under standard conditions
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Heat Transfer

  • Enthalpy changes can be calculated by measuring the temperature change of water when it is heated (or cooled) by the reaction. We also need to know the mass and specific heat capacity of the water
  • Energy transferred= c x m x ΔT
  • c is specific heat capacity (J), m is mass (g) and ΔT is the temperature change (°C)
  • In order to calculate the enthalpy change from the energy transfer, you need to convert the energy transfer for volume reactants used into the energy transfer for 1 mole
  • In enthalpy experiments, it is difficult to avoid energy from being transferred to the surroundings ("heat loss"). Because of this, we have to assume that there is no other energy loss.
  • We ignore the heat transferred to the calorimeter (used to insulate the reaction) and energy transferred to any solids in the calorimeter
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Percentage Uncertainty

  • When any physical measurement is made, there is always some kind of experimental uncertainty associated with the value obtained
  • Percentage uncertainty is a measure of uncertainty of a reaction
  • The error in measuring a quality (precision error)= + or - 1/2 of the smallest scale division
  • Percentage error= error/ measurement x 100
  • For example, the smallest division on a 1dp balance is 0.1g, so the error is + or - 0.005g.  You measured out 1g of a substance. 0.005/1 x 100= 0.5%
  • Percentage errors can be combined by simply adding them together
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Hess Cycle

  • Hess's Law- the enthalpy change for any chemical reaction is independent of the route by which the chemical reaction takes place
  • So long as your starting a finishing points are the same, the enthalpy change will also be the same
  • There is a direct route and an indirect route for the enthalpy change for the reaction below, both routes end up at the same point
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Bond Enthalpies

  • A chemical bond is an electrical attraction between atoms or ions.
  • When you break a bond, you have to overcome these attractive forces. The quantity of energy needed to break a bond in a molecule is the bond dissociation enthalpy or bond enthalpy
  • The bigger the enthalpy value, the stronger the bond
  • The shorter the bond, the stronger the bond-stronger attraction between atoms, so more energy needed to break them apart
  • Double bonds require more energy to break than single bonds, and triple bonds need more than double bonds
  • Bond enthalpies  given are often averages from several compounds, so the results of calculations are not always precise
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