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ENERGETICS (Hess's Law (Standard enthalpy change of Formation (Example…

- - - - Method 1 (Enthalpy Cycle)
        
        Draw the diagram of the reaction (the enthalpy cycle)
        
        Look at the direction of the formation of the main equation as shown below:
        
        Then, solve the question by using the cycle as your guide
        
        The same direction, add,different direction, subtract
      - Method 2 (Manupilating Equation)
        
        Look at the equation given:
        Reaction 1 2H2(g)+O2(g) → 2H2O(l) ΔH1=−572kJmol−1
        Reaction 2 2H2(g)+O2(g) → 2H2O(g) ΔH2=−484kJmol−1
  - - - Using ∆Hr=Σ∆Hf (products) − Σ∆Hf (reactants)
        Σ∆Hf (products)=(4×90)+(6×−242)= −1092 kJmol−1
        Σ∆Hf (reactants)=4×−46=−184 kJmol−1
        Note: ∆Hf for O2(g) is zero because it is an element in its standard state.
- - - - Heat: form of energy that flows from a higher temp. to a lower. temp
      - Temp: measure of the average kinetic energy of particles
      - Internal energy: total amount of energy (kinetic + potential
    - - Ex(it)othermic
      - En(ter)dothermic
    - - Neutralisation
        
        Assuming C of reactant alike with water
        
        Heat loss to the surrounding
        
        C of the cup not taking into account
      - Combustion
        
        Evaporation of alcohol and water
        
        Heat loss to the surrounding
        
        Incomplete combustion of alcohol
      - Solution
        
        Absorption of heat from surrounding
        
        Assuming C of mixture with water
        
        Mass of salt does not taken into account in mass of solution