Thermochemistry : 9.2 Hess Law

• Hess's Law of Heat Summation
  

When reactants are converted to products, the change in enthalpy is the same whether the reaction take step in one step or in a series of step.

    ΔH₁ =ΔH₂ + ΔH₃

• Application Using Hess's Law
  
  • To find ΔH of any reaction for which we can write on equation, even if it is impossible to carry out.
    
  • Two methods of calculation
    
    • Algebraic method
      
    • Energy cycle method
      
• Algebraic Method
  
  • Important steps:
    
    • Identify the target equation whose ΔH unknown.
      
    • Rearrange the equations given mole of reactants and products are on the correct side.
      
    • Add the equation to obtain the target equation all other substances must cancel.
      
• Standard Enthalpy of Formation (ΔH_f)
  
  • Heat change when 1 mol of a compound is produced from its elements in their standard states.
    
• ΔH_f
  of Elements
  
  • In its standard states
    
• ΔH_f of Compounds
  
  • Most compound have negative ΔH_f
    
    • The compound is more stable than its components elements.
      
• Determining ΔH_rxn From ΔH_f
  
  aA + bB = cC + dD
  
  H_rxn = [cΔH_f (C) + dΔH_f (D) – aΔH_f (A) + bΔH_f (B)]
  
  H_rxn = ( sum of ΔH_f of all of the products ) – ( sum of ΔH_f of all of the reactant )