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  • Created by: Louise
  • Created on: 02-05-13 18:32

Enthalpy Change

"The heat energy change measured under conditions of constant pressure"

  • Symbol :  H 
  • Measured in KJmol-1 (Heat energy per mole)
  •  H = Enthalpy of products - Enthalpy of reactants

Exothermic  H = negative

Endothermic  H= positive

Standard Conditions:

  • Pressure = 100KPa
  • Temperature = 298K
  • Concentrations = 1moldm-3
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Exothermic/Endothermic Reactions

Exothermic is a reaction that gives out heat (energy). 

  • When a reaction is exothermic  H (enthalpy change) is negative. 
  • Because enthalpy of products < reactants . Products have less heat energy because energy is released.
  • Energy is given out to surroundings so temperature increases.
  • Oxidation is exothermic

Endothermic is a reaction that takes in heat (energy).

  • When a reaction is enothermic  H (enthalpy change) is positive.
  • Because enthalpy of products > reactants . Products have more heat energy because energy is absorbed.
  • Energy is taken in from surroundings so temperature decreases.
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Bond enthalpies

Reactions are all about breaking/making bonds -

Energy is required when you break bonds, so making bonds is endothermic

Energy is given out when you make bonds, so making bonds is exothermic

The enthalpy change of a reaction is the overall effect of these two changes. 

The energy needed to break a bond between two atoms is the same amount of energy that is given out when that bond is formed. These "Bond enthalpies"  have specific values that differ depending on the atoms attatched to either side of the bond.

Mean Bond Enthalpies - are used on calculations because the energy required to break an individual bond can change depending on where it is. 

Mean Bond Enthalpy (Bond Dissociation Enthalpy) - Energy or heat needed to break a covalent bond averaged over several compounds.

Larger bond enthalpy=Stronger bond,  Stronger bond = Shorter Bond (Harder to break)

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Theory and Practise

Enthalpy change values will be different in theory to in practise.

This is because:

  • If not done in an isolated system, heat is lost to equipment and surroundings
  • Bond enthalpy vaues are for a specific bond and they are averaged over several compounds. MEAN bond enthalpies
  • Bond enthalpy values don't take into account the type of compound that the bond is in
  • Individual bonds can be affected by other bonds in a molecule
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Standard enthalpy change of reaction

 Hr  -Is the enthalpy change when a reaction occurs under standard conditions with all the reactants and products in their standard states.

Standard enthalpy of formation

  Hf - The enthalpy change when one mole of a compound is formed from its constituent elements in there standard states under standard conditions

Standard enthalpy of combustion

  Hc - The enthalpy change when one mole of a substance is completely burned in oxygen under standard conditions

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 H = Enthalpy of products - Enthalpy of reactants

Q= m c  T

  • Q= Heat energy [KJ]                                                    
  • m= mass (of liquid being heated, if 2 combine mass)  [Kg]                         
  • c= specific heat capacity (water is 4.18)              
  • ∆T= Change in temperature [K]
  • Note** m x c =total heat capacity

 H= Q / n

  • n=moles 
  • Use conc x vol= mol or mass/ mr = mol   (Here use the mass in g of the thing being heated, combusted etc)
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Hess Law

"The enthalpy change for a reaction is independant of the path taken"

The enthalpy change going from A   B can be found by adding the values of the enthalpy changes for reactions (A   X) + (X   Y) + (Y   B) 

Hess Cycles:

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When given the standard enthalpies of formation:

 Hf  p - r  (Products - Reactants)

When given the standard enthalpies of combustion:

 Hc  r - p  (Reactants - Products)

Doesn't matter if you're trying to calculate formation or combustion, choose the equation to use according to the data you are given. 

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