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Demo of FeSCN 2+ eqm Add Fe3+ Add SCN2+ Add NaOH

Demo of FeSCN 2+ eqm Add Fe3+ Add SCN2+ Add NaOH. LeChatelier’s Principle (p.50t). LeChatelier’s Principle (p.50t). LeChatelier’s Principle says that if a closed system at eqm is subjected to a change, the system will try and counteract that change (do the opposite).

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Demo of FeSCN 2+ eqm Add Fe3+ Add SCN2+ Add NaOH

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  1. Demo of FeSCN2+ eqm • Add Fe3+ • Add SCN2+ • Add NaOH

  2. LeChatelier’s Principle (p.50t)

  3. LeChatelier’s Principle (p.50t) • LeChatelier’s Principle says that if a closed system at eqm is subjected to a change, the system will try and counteract that change (do the opposite). • Cross off the paraphrase. • This principle will allow us to predict the effect of changing temperature, concentration and pressure on an eqm system. • In each case we will look at the eqm: 2 NO(g) + Cl2(g)  2 NOCl(g) + 76 kJ

  4. Effect of Temp Change (p.50b) 2 NO(g) + Cl2(g)  2 NOCl(g) + 76 kJ • If the temperature is decreased (heat lost): • How do you undo that stress? • Which way does the eqm shift? • If the temp. is increased, the system will shift left to consume that extra heat. • Think about the demo with the brown and colorless gas and the color change with temperature. • Hebden doesn’t really discuss why this happens. Here’s the reason:

  5. Reasoning for Shift w/ Changing Temp • The system is initially at eqm so both forward and reverse rxn rates are equal. • The rxn is exothermic in the forward direction (draw PE diagram). • If temp is decreased, both forward and reverse rxns will slow down. • Which rxn is affected (slowed down) more? • Which rate is now faster? • Which way will the eqm shift? • What happens to the rates as the system shifts? • The opposite is true if temp is increased

  6. Graphical Analysis of Temp Change (p.51t) • 2 NO(g) + Cl2(g)  2 NOCl(g) + 76 kJ

  7. Effect of Conc Change (p.51m) 2 NO(g) + Cl2(g)  2 NOCl(g) + 76 kJ • If the concentration of Cl2 is increased: • How do you undo that stress? • Which way does the eqm shift? • Again, Hebden doesn’t really discuss why. Here’s the reason:

  8. Reasoning for Shift w/ Changing Conc • 2 NO(g) + Cl2(g)  2 NOCl(g) + 76 kJ • The system is initially at eqm so both forward and reverse rxn rates are equal. • If [Cl2] is increased, • What happens to the forward rate? • What happens to the reverse rate? • Which way will the eqm shift? • What happens to the [ ] of each species? • What happens to the rates as the system shifts? • The opposite is true if [Cl2] is decreased

  9. Graphical Analysis of Conc Change (p.52t) • 2 NO(g) + Cl2(g)  2 NOCl(g) + 76 kJ

  10. Effect of Pressure Change (p.52m) 2 NO(g) + Cl2(g)  2 NOCl(g) + 76 kJ • If the pressure of the system is increased (or volume is decreased): • How do you undo that stress? • How do you do that? • Which way does the eqm shift? • Again, Hebden doesn’t really discuss why. Here’s the reason:

  11. Reasoning for Shift w/ Changing Pressure • The system is initially at eqm so both forward and reverse rxn rates are equal. • If pressure is increased: • What happens to all concentrations? • What happens to both rates? • Which rxn is affected (sped up) more? • Which way will the eqm shift? • What happens to the rates as the system shifts? • The opposite is true if pressure is decreased.

  12. Graphical Analysis of Pressure Change (p.53t) • 2 NO(g) + Cl2(g)  2 NOCl(g) + 76 kJ

  13. Effect of Adding a Catalyst (p.53m) • A catalyst lowers the overall activation energy for both forward and reverse reactions. • Therefore, a catalyst speeds up both rxns equally • There is no shift in an equilibrium when a catalyst is added. • If a system is not at eqm yet, a catalyst can help it reach eqm faster.

  14. Summary (p.53b) • Changing the temperature: All concentrations slowly change and there is no sudden change on the graph. • Changing the concentration: One species will suddenly jump up or down. • Changing the pressure: All concentrations will immediately jump or drop. • Click for link to animation

  15. Industrial Uses of LeChat’s Principle (p.57t) • Video of the Haber Process.

  16. Homework • Hebden #17, 19, 21, 23, 24, 26-30. • Quiz coming up on p. 37-56 (in 3 classes).

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