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Chemical Thermodynamics

Chemical Thermodynamics. Enthalpy. Mr. Shields Regents Chemistry U13 L07. Thermodynamics. We’re going to move from Collision Theory and Reaction Rates to an area of chemistry known as … CHEMICAL THERMODYNAMICS. In this section we’ll discuss several concepts that will

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Chemical Thermodynamics

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  1. Chemical Thermodynamics Enthalpy Mr. Shields Regents Chemistry U13 L07

  2. Thermodynamics We’re going to move from Collision Theory and Reaction Rates to an area of chemistry known as … CHEMICAL THERMODYNAMICS In this section we’ll discuss several concepts that will Help us determine whether a reaction is SPONTANEOUS Or NOT SPONTANEOUS We’ve already touched briefly on one of these concepts … ENTHALPY

  3. Thermodynamics SPONTANAITY in a chemical sense can be defined as: “ A physical or chemical change that occurs w/o outside Intervention but may require energy to begin the process” There are 2 terms in Thermodynamics play a KEY ROLE in Helping us determine if a reaction is spontaneous… DH The change in ENTALPY or the change in the heat content of a system DS The change in ENTROPY or the measure of disorder or randomness in a system

  4. Chemical Reactions • To understand a chemical reaction you need to know • three things: • How quickly does the reaction occur • i.e What is the RATE OF REACTION • Is it slow (iron rusting) or fast (TNT exploding) • 2) Will the reaction GO TO COMPLETION or do we reach • an EQUILIBIUM • - for ex. 2NO2 N2O4 • or 2Na + Cl2  2NaCl

  5. Chemical Reactions • 3) In which direction (forward or reverse) is the reaction • Spontaneous? • - i.e In which direction does the reaction want to • proceed? • For example • Shiny iron nail in water  rusty nail in • water • Rusty iron nail in water Shiny nail in water or…

  6. Chemical Reactions • Note that Spontaneous doesn’t mean instantaneous it • just means… • “the reaction will proceed on its own w/o any continuous • Outside help” • What might that “help” be? • Heat (increasing or decreasing) • Pressure (increasing or decreasing) • Electric Current • Or any other form of energy

  7. Dec. PE Inc. PE Enthalpy We know water flows “down hill” and as do skiers. This is the Spontaneous direction, but why? Flowing downhill minimizes PE and decreased PE equals Increased STABILITY

  8. Reactants Products Enthalpy The same is true for Chemical reactions. The lower the PE of the PRODUCTS relative to reactants The more stable they are. This change in PE involves the 1st of the two terms we mentioned earlier, namely the Change in ENTHALPY (DH)

  9. Hreactants DH = Hproducts Enthalpy Recall ENTHALPY (DH) is defined as the difference in PE of the Products and the PE of the reactants: DH = Hproducts - Hreactants Enthalpy is thus a measure of the energy of a substance. It is also known as The HEAT OF REACTION If ∆H is negative the reaction is EXOTHERMIC H2 +O2 H20 + Heat

  10. Hproducts = DH Hreactants Enthalpy However, if DH is positive the reaction is ENDOTHERMIC Ex: NH4Cl + Heat  NH3 + HCl Or N2 + O2 + Heat  2NO Now open your Ref. Tables Refer to Table I; Heats of Reaction

  11. Enthalpy DH is proportional To the AMOUNTof Material undergoing reaction. For example … Consider the following reaction: 4Fe + 3O2  2 Fe2O3DH = -1644 KJ/mol The above balanced equation states I need 4 moles Fe to react with 3 moles of O2 to produce 2 moles of Fe2O3 But what if I only had half the material (2 moles of Fe to react with 1½ moles of O2. How much heat would be released? Since we would only yield 1 mole of Fe2O3 we would only Produce half the heat; DH would be -1644KJ/2 = -822 KJ/mol

  12. N2 + O2 2NO Enthalpy Problem: 1) Is the following Reaction pathway Exo or Endothermic? 2) Is DH positive or negative? 3) Do the Products contain more or less energy than the reactants? 4) If only 0.25mol of N2 is available for reaction how much heat is absorbed? • 1) Endo • 2) DH is Positive • 3) they contain • more PE than • reactants • -4) 45.6 KJ

  13. Spontaneity Can we predict if a reaction is spontaneous based on The Enthalpy value? The answer is a QUALIFIED YES • - In most cases EXOTHERMIC REACTIONS are • SPONTANEOUS • - especially if DH is a LARGE Negative Value • And in most cases ENDOTHERMIC REACTIONS are • NOT SPONTANEOUS • - especially if DH is a LARGE Positive Value

  14. Spontaneity • However these assumptions are NOT ALWAYS True • One also needs to consider the contribution of the • other term we mentioned earlier • - DS, The ENTROPY term which we’ll discuss in • our next lesson

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