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Enthalpy (p.43t)

Spontaneity of Reactions (or how do you know whether reactions will go one way or to equilibrium?) (p.43t). Enthalpy (p.43t). We’ve already discussed the fact that reactions would prefer to go “downhill” (i.e. in the exothermic direction). But is this the only factor that drives reactions?

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Enthalpy (p.43t)

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  1. Spontaneity of Reactions (or how do you know whether reactions will go one way or to equilibrium?) (p.43t)

  2. Enthalpy (p.43t) • We’ve already discussed the fact that reactions would prefer to go “downhill” (i.e. in the exothermic direction). • But is this the only factor that drives reactions? • It can’t be because: • Endothermic reactions can also happen (although far less common) • Some exothermic reactions do not go to completion.

  3. Randomness (p.44t) • The other factor is randomness. • Randomness comes about when there are many possible scenarios. • Let’s look at dice (picture on p. 44) • The dice are nicely ordered but what happens if they are shaken? • Could they ever be rolled that same way? • Why do we call what happens on a roll, “random”?

  4. Entropy (p.45t) • The same is true of molecules. • A solid is very organized but if you dissolve it in water it becomes very random. • Highly RANDOM states are favored over highly ORDERED states, simply because there are more possible random states. • Entropy is the amount of randomness in a system. • Entropy always tries to go to a maximum (maximum randomness).

  5. Conclusion (p.45b) • For any rxn, there are 2 driving forces: • Enthalpy always tries to go to a minimum (go in the exothermic direction). • Entropy always tries to go to a maximum (go to maximum randomness). • How do we recognize maximum randomness in a reaction?

  6. Randomness of Phases (p.46b) • Solids have zero randomness • Pure liquids are much more random • Solutions are more random than pure liquids • Gases are much more random than all other phases. • gases >> solutions > liquids >> solids • If there is only one phase in a reaction, the side with the most molecules is the most random.

  7. Examples of Entropy (p.47t) • Which side is more random (reactants or products) in each of the following? • CaC2(s) + 2H2O(l) C2H2(g) + Ca(OH)2(aq) • A(g) + B(s)  2C(g) + D(s) • Conclusion: The side of a rxneqn favored by the tendency to maximum randomness (max. entropy) is the side containing THE MOST PARTICLES OF THE MOST RANDOM PHASE.

  8. Entropy and Enthalpy Examples (p.47b) • Which side do entropy and enthalpy favor in each of the following? • C2H2(g) + 2Cl2(g) C2H2Cl4(l) + 386 kJ • Maximum entropy favors reactants (gases) • Minimum enthalpy favors products (exothermic rxn) • Reaction goes to equilibrium • CH4(g) + O2(g)  CO2(g) + 2H2O(g) + 394 kJ • Maximum entropy favors products (more gases) • Minimum enthalpy favors products (exothermic rxn) • Reaction goes to completion

  9. Entropy and Enthalpy Examples cont. (p.48t) • Which side do entropy and enthalpy favor in each of the following? • 4Au(s) + 3O2(g) + 162 kJ  2Au2O3(s) • Maximum entropy favors reactants (gas) • Minimum enthalpy favors reactants (exothermic rxn) • Reaction doesn’t happen at all.

  10. Homework • Hebden #14, 15 odd letters, 16 odd letters.

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