Enthalpy Changes in chemical and physical Reactions

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Presentation transcript:

Enthalpy Changes in chemical and physical Reactions

Recall the basic shapes of endothermic and exothermic Potential energy diagrams

Relate detailed PE diagram to this simplified diagram

Actual Reactions have intermediate steps that are not shown in the overall PE Diagram

Dissolving sodium hydroxide in water actually involved multiple steps to get the dH You had to find the heat gained by the water and also the moles of NaOH used. The actual value from the reference table is a much more complex set of reactions that involve knowing the Enthalpy(heat) of formation of the compound in the solid form and the enthalpies of formation for the ions that are formed. The following example makes use of the table provided to you by me, if you lose it, you can print it out from the next slide from the PDF.

Let’s calculate the heat of solution (dissolution) for sodium chloride DH0 Dissolution or Dissociation means the overall reaction when you add water is NaCl (s)  Na + (aq) + Cl - (aq) We need to use the values in the standard enthalpy of formation table to find the overall DH. Find the Standard enthalpy of formation of NaCl (s) Find the the Standard enthalpy of formation of Na + (aq) Find the the Standard enthalpy of formation of Cl - (aq) Remember to keep the signs. The overall DH is the sum of the ion products DH– DH of reactants

NaCl (s)  Na + (aq) + Cl - (aq) The overall DH is the sum of the ion products DH– DH of reactants DH o =DH Products - DH Reactants = (-239.7 kJ/mol+ -167.4 kJ/mol) - (-411.0 KJ/mol) DH o = -407.1 KJ/mol + 411.0 KJ/mol DH o = +3.9 KJ/mol Look at Reference Table I and compare the value! Pretty darn close! Find the Standard enthalpy of formation of NaCl (s) -411.0 KJ/mol Find the the Standard enthalpy of formation of Na + (aq) -239.7 kJ/mol Find the the Standard enthalpy of formation of Cl - (aq) -167.4 kJ/mol

Homework: show the calculation for the values given in Table I for sodium hydroxide and ammonium nitrate by using this same method

What type of reaction is the one seen here? How do you know? Born-Haber Cycle: A series of steps in a chemical reaction that shows Enthalpy changes What type of reaction is the one seen here? How do you know?

What is lattice energy? Lattice energy is the energy that is released in the formation of the stable ionic bonds seen within an ionic crystal. The previous Born-Haber cycle showed that the lattice energy was -756.9 kJ/mol The overall Heat of Formation of CsF was -553.5 KJ/mol. To find the overall Heat of Formation you would need to add up all the individual steps (keep the +/ - signs ) Please do this now to prove how you get the overall Enthalpy of formation. When you do this, you are applying Hess’s Law. The overall change in enthalpy(heat) can be found through summation of the individual steps in a process.

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