Energetics and Enthalpy Changes

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Enthalpy Changes and Energy Level Diagrams

  • When a reaction occurs at a constant pressure and gives out or takes in energy, there is an enthalpy change between the system (reactants and products) and its surroundings.
  • An exothermic reaction has a negative enthalpy change. The products are at a lower energy level than the reactants. Energy has been given out by the system to the surroundings.
  • An endothermic reaction has a positive enthalpy change. The products are at a higher energy level than the reactants. Energy is absorbed by system
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Enthalpy Changes and Energy Level Diagrams (cont)

                           (http://www.chemgapedia.de/vsengine/media/vsc/en/ch/12/oc/substitution/sn_e_konkurrenz/a4_1_sn_e_temperatur/energiediagramm_cx.gif)

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Measuring Enthalpy Changes

  • We use standard conditions to make experimental measurements comparable.
  • The standard conditions are: 1atm pressure at 298K, reacting substances are all in their normal physical states at these conditions, solutions have a concentration of 1 mol/dm-3.
  • An enthalpy change measured under these conditions is called standard enthalpy change.
  • This is expressed per mole of substance reacting, with units KJ/mol.
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Measuring Enthalpy Changes (cont)

  • Standard enthalpy of a reaction is the enthalpy change for a reaction in the molar quantities from its chemical equation, under standard conditions.
  • Standard enthalpy of formation of a compound is the enthalpy change when 1 mole of a compound is formed from its elements under standard conditions.
  • Standard enthalpy of combustion is the enthalpy change when 1 mole of substance is burnt completely in oxygen under standard conditions. 
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Measuring Enthalpy Changes (cont)

  • Standard enthalpy of atomization is the enthalpy change when 1 mole of gaseous atoms is formed from its element under standard conditions.
  • Standard enthalpy of neutralization is the enthalpy change when 1 mole of water is formed by the reaction of an acid+alkali under standard conditions. 
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Measuring Enthalpy Changes (cont)

  • The energy transferred in a reaction can be calculated from a calolimetry experiment.
  • Energy change in surroundings= m x c x change in T
  • m is mass (of water), c is specific heat capacity
  • Units are KJ
  • This number divided by the number of moles of the substance gives the enthalpy change in KJ/mol. 
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Measuring Enthalpy Changes (cont)

  • The data book value for an alcohol burning enthalpy change will be greater than the experimental value:

- Not all energy produced by burning alcohol is transferred to the water.                                                    - Some heat is absorbed by the calorimeter.                   - Some heat is lost to surroundings or from surface of the water.                                                                          - The fuel has not completely combusted

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Using Hess's Law

  • Hess's law states that the total enthalpy change for a reaction is independant of the route taken, provided the initial and final conditions are the same. 
  • Hess's law is used to calculate the standard enthalpy change for reactions that are difficult to measure or cannot be measured directly
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Bond Enthalpy

  • Bond enthalpy is the energy associated with one mole of a particular bond.
  • It is the energy required to break one mole of bonds, or the energy released when forming one mole of bonds.
  • Mean bond enthalpy is an average bond enthalpy for a particular bond in a range of different compounds
  • All atoms have to be gaseous.
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Bond Enthalpy (cont)

  • Mean bond enthalpies are used to determine theoretical enthalpy changes for a reaction.
  • Enthalpy change of reaction= mean bond enthalpy of reactants - products.
  • Enthalpy changes calculated using mean bond enthalpies are often different to experiment data because the calculation uses average bond enthalpies. Also, mean bond enthalpies apply to gaseous atoms, so if it is a liquid you have to add enthalpy of vaporisation
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Bond Enthalpy (cont)

  • The larger the bond enthalpy, the stronger the bond.
  • Therefore a large bond enthalpy makes a compound very unreactive.
  • The bond enthalpy also tells us which bond will break first in the reaction (weakest), and how fast the reaction might be. 
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