2. 1 Enthalpy Mr. ShieldsRegents Chemistry U13 L07

3. 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

4. 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… D H  The change in ENTALPY or the change in the heat content of a system D S  The change in ENTROPY or the measure of disorder or randomness in a system

5. 4 Chemical Reactions To understand a chemical reaction you need to know three things: 1)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. 2NO 2  N 2 O 4 or 2Na + Cl 2  2NaCl

6. 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…

7. 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? -H-Heat (increasing or decreasing) -P-Pressure (increasing or decreasing) -E-Electric Current -O-Or any other form of energy

8. 7 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 Enthalpy Dec. PE Inc. PE

9. 8 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 1 st of the two terms we mentioned earlier, namely the Change in ENTHALPY ( D H) Enthalpy Reactants Products

10. 9 Recall ENTHALPY ( D H) is defined as the difference in PE of the Products and the PE of the reactants: D H = H products - H reactants Enthalpy H reactants H products D H = 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 H 2 +O 2  H 2 0 + Heat

11. 10 However, if D H is positive the reaction is ENDOTHERMIC Enthalpy Ex: NH 4 Cl + Heat  NH 3 + HCl Or N 2 + O 2 + Heat  2NO H reactants H products = D H Refer to Table I; Heats of Reaction Now open your Ref. Tables

12. 11 Enthalpy Consider the following reaction: 4Fe + 3O 2  2 Fe 2 O 3 D H = -1644 KJ/mol The above balanced equation states I need 4 moles Fe to react with 3 moles of O 2 to produce 2 moles of Fe 2 O 3 But what if I only had half the material (2 moles of Fe to react with 1½ moles of O 2. How much heat would be released? Since we would only yield 1 mole of Fe 2 O 3 we would only Produce half the heat; D H would be -1644KJ/2 = -822 KJ/mol D H is proportional To the AMOUNT of Material undergoing reaction. For example …

13. 12 Enthalpy Problem: 1) Is the following Reaction pathway Exo or Endothermic? 2) Is D H positive or negative? 3) Do the Products contain more or less energy than the reactants? 4) If only 0.25mol of N 2 is available for reaction how much heat is absorbed? - 1) Endo - 2) D H is Positive - 3) they contain more PE than reactants -4) 45.6 KJ N 2 + O 2  2NO

14. 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 D H is a LARGE Negative Value - And in most cases ENDOTHERMIC REACTIONS are NOT SPONTANEOUS - especially if D H is a LARGE Positive Value

15. 14 Spontaneity However these assumptions are NOT ALWAYS True - One also needs to consider the contribution of the other term we mentioned earlier - D S, The ENTROPY term which we’ll discuss in our next lesson