Chesmistry U2M3 - Enthalpy Changes

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  1. Exothermic
    • Increase temperature of surrounding
    • Enthalpy of chemical system decreases as /\H is negative becuase the system has lost energy to surroundings
    • Example is the combustion of hydrocarbon fuels
  2. Endothermic
    • Decreases the temperature of surroundings
    • Increases the enthalpy of chemical systems as /\H is positive because the system has gained energy from surroundings
    • Example is the decomposition of metal carbonates e.g. CaCO3
  3. Enthalpy Profile Diagrams
  4. Enthalpy Profile Diagrams
    • Chemical reaction can be regarded as braking bonds followed by making bonds. Braking bonds needs energy, so energy must be put into the reaction
    • The minimum energy needed to start a reaction is called activation energy
    • After that bonds a re made which release energy
    • When bonds are broken energy is taken in positive
    • When bonds are made, energy is released, negative
    • If the bonds made are stronger than the bonds broken then there is a overall decrease in enthalpy and the reaction is exothermic
    • If the bonds broken are stronger than the bonds made there is an overall increase in enthalpy and the reaction in endothermic
  5. Types of Enthalpy Change
    • r=change of reaction
    • c=chanhe of combustion
    • f=change of formation
  6. Standard Enthalpy change of formation definition
    • Enthalpy change when 1 mole of a compound is formed from its constituent elements in their standard states under standard conditions.
    • When standard state=0
  7. Standard enthalpy change of combustion
    Enthalpy change when 1 mole of a substance (element or compound) reacts completely with oxygen under standard conditions
  8. Standard enthalpy change of reaction
    The enthalpy change that accompanies a reaction under standard conditions in the molar quantities expressed in a chemical equation, all reactants and products being in their standard states
  9. How to calculate enthalpy changes
    • Energy change in surroundings = m x c x T(difference in Temperature)
    • m=mass
    • c=4.2 (A constatnt)
  10. 50cm^3 of concentrated sulfuric acid 20.0mol dm^-3, was added to 950cm^3 of water at a temperature of 19Celcius. The solutionwas stirred gently until maximum temperature was reached at 28C. What is the enthalpy change for this reaction?
    • STEP 1 : Write the equation. m x c x T=energy change
    • STEP 2 : Write down values for symbols m, c and T. m=950+50=1000cm^3 c=4.2J T=28-19=9C
    • STEP 3 : Do the calculation energy change = 1000 x 2.4 x 9= 37000J=37.8kJ
    • It is an exothermic reaction so it has a negative sign -37.8kJ
    • What is the enthalpy change per mole of sulfuric acid?
    • STEP 4 : Calculate the amount in moles of Sulfuric acid
    • amount(mol)=Vol(cm^3)/1000 x concentration
    • 50/1000 x 20 = 1.00 mol
    • STEP 5 : Divide the /\H value by moles =37.8/1 =37.8kJmol^-1
  11. Hess's Law
    • Hess' Law states that for any chemical change the enthalpy change is the same whatever the route taken
    • /\H1=/\H2+/\H3
  12. Calculating enthalpy changes indirectly using hess's Law

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Chesmistry U2M3 - Enthalpy Changes
2012-04-16 20:54:14
U2M3 Enthalpy Changes

U2M3 - Enthalpy Changes
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