Equilibrium

1. Equilibrium • When a chemical system is at equilbrium the rate of the forward rxn is equal to the rate of the reverse rxn • If no changes are made to its conditions this would in theory go on forever • However, there are ways that we can change equilibrium

2. Le Chatelier’s Principle • Henry-Louis Le Chatelier,  (1850 -1936) • French chemist who is best known for Le Chatelier’s Principle • Allows us to predict the effect a change of conditions (such as temperature, pressure, or concentration of reaction components) will have on a chemical reaction

3. Le Chatelier’s Principle • If a system that is at equilibrium is changed (temperature, concentration, pressure) processes will occur that tend to counteract that change • This means the rxn will shift in a way to try and “undo” what was added • Add heat  rxn will shift so it will use up added heat • Remove heat  rxn will shift so it can produce heat • Increase [ ]  rxn will shift so it will decrease [ ]

4. Temperature • A + B + heat C + D • If we increase the temperature of this system, we are adding heat • To counteract the equilibrium will move in such a way as to use up heat. • heat is on the left, the forward reaction uses up heat • equilibrium will shift toward the right. • A new equilibrium will be established in which there is more C and D and less A and B than in the original equilibrium.

5. Temperature • When the temperature is increased, the equilibrium will shift away from the side with the heat term • A + B + heat 2NO2 • Shift right • New equilibrium where concentrations will be higher on the side with out heat and lower on the side with heat

6. Temperature • Now, if the temperature was decreased, the equilibrium would shift in such a way that would produce heat (to counteract the change). • Equilibrium shifts toward the side with the heat term (in other words, produce heat) • A + B + heat C + D • Shift LEFT

7. Temperature • When the temperature is decreased, the equilibrium will shift toward the side with the heat term. • A + B + heat 2NO2 • Shift left • New equilibrium where concentrations will be higher on the side with heat and lower on the side without heat

8. Concentration • H2(g) + I2(g) 2HI(g) • If we add some H2 to a flask containing this mixture at equilibrium, [H2] will immediately increase • In order to counteract this change, the equilibrium will shift to the right in order to "use up" some of the extra H2. (In other words to decrease the [H2]).

9. Concentration • If the concentration of a substance in an equilibrium system is increased the equilibrium will shift toward the other side of the equation, in order to counteract the change • H2(g) + I2(g) 2HI(g) Add H2 equilibrium will shift right

10. Concentration • H2(g) + I2(g) 2HI(g) • Let's say now that we somehow take away some I2 • [I2] will immediately decrease. • In order to counteract this change, the equilibrium will shift to the left in order to increase [I2] again

11. Concentration • If the concentration of a substance in an equilibrium system is decreased the equilibrium will shift toward the side of the equation with that substance, in order to counteract the change • H2(g) + I2(g) 2HI(g) Remove H2 equilibrium will shift left

12. Partial Pressure • Changing the Partial Pressure of a gas in an equilibrium system has the same effect as changing the concentration of that gas. • When we have a gas mixture, partial pressure is the pressure exerted by one gas in the mixture • The pressure exerted by all gases in the container is the total pressure

13. Partial Pressure • If the pressure of a substance is increased, equilibrium shifts toward the other side of the equation • H2(g) + I2(g) 2HI(g) • Increase pressure of H2 shifts right • If the pressure of a substance is decreased, equilibrium shifts toward the side of the equation with that substance • H2(g) + I2(g) 2HI(g) • Decrease pressure H2 shifts left

14. Total Pressure • The more moles of gas in a certain volume, the higher the pressure. • 1 molecule of gas compressed into 2 molecules • Higher number of molecules exerting more pressure against the container

15. Total Pressure • If we decrease the volume of a container we increase the pressure • Smaller space for more collisions • If we increase the volume of a container we decrease the pressure • Large space less collisions

16. Total Pressure • If the total pressure of a system at equilibrium is increased, the equilibrium will shift toward the side with less moles of gas (as shown by coefficients) • N2(g) + 3H2(g) 2NH3(g) • If the total pressure on this system is increased, the equilibrium would shift to the right (the side with fewer moles of gas) • This counteracts the imposed change by reducing the pressure • New equilibrium established higher for 2NH3

17. Total Pressure • If the total pressure of a system at equilibrium is decreased, the equilibrium will shift toward the side with more moles of gas (as shown by coefficients) • N2(g) + 3H2(g) 2NH3(g) • If the total pressure decreased, the equilibrium shift to the left (the side with more moles of gas) • New equilibrium established for reactants

18. Video • https://www.youtube.com/watch?v=7zuUV455zFs • https://www.youtube.com/watch?v=XhQ02egUs5Y

19. Temperature • The reaction: A + B C ∆H = -56 kJ is exothermic (∆H is negative), so heat is given off or written on the right. • A + B C + 56 kJ (heat) • The reaction: D + E F + G ∆H = 43 kJ is endothermic (∆H is positive), so heat is absorbed, or written on the left • D + E + 43 kJ F + G

20. Temperature • Exothermic rxn • heat is given off to the right • Endothermic rxn • Heat is absorbed by left