Chemistry SM-1232Week 10 Lesson 3

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademySpring 2008

Class Today• We’ll have next Friday off• I’ll give you a take home quiz after class on Friday• Tests and quizes are not yet graded. I’ll have them after this

weekend.• Disturbing Equilibrium: concentration change, volume change,

temperature change• Ksp • Wiki project to be discussed on Friday!

Equilibrium Control

• Chemists can control equilibrium by changing the “dials” available to us.

• But it’s like sailing a boat. If you want to turn right you put the rudder in the opposite direction.

• Le Chatelier’s Principle! When a chemical system at equilibrium is disturbed, the system shifts in a direction that minimizes the disturbance

Le Chatelier’s Principle: concentration changes• N2O4 2 NO2

• If we add in more NO2 we’ll force the reaction further to the left.

• If we add in more N2O4 we’ll force the reaction further to right

Le Chatelier’s Principle: concentration changes• 2 BrNO 2 NO + Br2

• What happens if we add in BrNO into the reaction chamber?

• What happens if we add in Br2? NO?

Le Chatelier’s Principle: volume changes

• Pressure and volume are inversely related• If there is more pressure there is less volume• If there is more volume there is less pressure

• Consider N2 + 3H2 2NH3

• All gases take up the same amount of space, but count how many moles of gas are on the left side. Count how many are on the right side.

Le Chatelier’s Principle: Volume Changes

• N2 + 3H2 2NH3

• If we increase the pressure the system wants to relieve the stress.

• In order to relieve the stress from pressure we can shrink volume.

• In which direction would the reaction shrink volume? So, if we increase pressure, which side of the reaction will be favored?

Le Chatelier’s Principle: Volume Changes

• N2 + 3H2 2NH3

• If we increase the volume the system wants to relieve the stress.

• In order to relieve the stress from a volume increase we can grow volume of gas.

• In which direction would the reaction move if the goal was to increase gas volume?

Le Chatelier’s Principle: Changing Temperature• Exothermic• Out heat• A + B C + D + HEAT• Endothermic • In Heat• A + B + HEAT C + D

Le Chatelier’s Principle: Changing Temperature• A + B C + D + HEAT• Which side will be favored if I add more heat in?• Need help think about condensing water:

H2O(g) H2O (l) + heat

• A + B + HEAT C + D• Which side will be favored if I take heat out?• Think about boiling water.• H2O(l) + heat H2O (g)

E Chatelier’s Principle: Changing Temperature• N2 + 3H2 2 NH3 + Heat

• What happens if you remove heat?

• N2O4 + heat 2 NO2

• What happens if you remove heat?

Solubility

• Solubility is an equilibrium expression too.• I told you there is a tug of war between the

forces that would break up a solid and the forces that would hold it together

• We have a number we can ascribe to talk about how much a solid dissolves

Ksp

• Consider CaF2(s) Ca2+(aq) + F-(aq)

• Write the Keq

• Now take away the solid component• We call that the Ksp

Ksp

• CaF2(s) Ca2+ (aq) + 2F-(aq)

• You should have found Ksp= [Ca][F-]2

• Same rules apply here for interpreting the results.

• Greater than one favors products• Less than one favors the reactants• In other words if it’s less than 1 it wants to

stay a solid!

Ksp

• We typically don’t look for Ksp of things that are very soluble, so you’ll almost always see number that are much smaller than one.

• Write the Ksp for BaSO4, Mn(OH)2, and Ag2CrO4

Ksp to Molar Solubility

• AgCl(s) Ag +(aq) + Cl-(aq) Ksp= 1.77e-10• 1 mol = [AgCl]= [Ag+] = [Cl-] = [X]

• Ksp = [Ag+][Cl-]• Ksp = [X][X]• Ksp = [X]2• 1.77e-10 = [X]2• [X]= ?

You Try

• BaSO4 (s) Ba2+ + SO42-• Ksp 1.03 e-5

• What is [Ba2+]?

Consider H2O

• 2 H2 + O2 2H2O• Keq = 3.2e81 !!!

• But if you stick Hydrogen and Oxygen in a balloon what happens?

Activation Energy

What happens at Activation?

Activation Energy and Rate

• Reactions that have high activation energies have really slow rates.

• The number of atoms that can overcome the activation energy is related to how much kinetic energy molecules have, which is related to temperature

• Increase the temp and you typically increase the number of atoms that can overcome the activation energy.

We have to add energy

• We have to get over the hump!• After we get over the hump then energy can

be released!

Virtual Demonstration

• Oh NOES!!!!!

• We’re going to do this at the end of class or else you’re too giddy.

Catalysts!

• Catalyst is a substance that increases the rate of chemical reactions, but is not consumed by the reaction. A catalyst works by lowering the activation energy for the reaction making it easier for reactants to get over the energy hump!

Catalysts

Good and Bad Catalyst

• Cl + O3 ClO + O2• ClO + O Cl + O2• Overall, O3 + O 2 O2

• That’s bad! Chlorine in the atmosphere reacts with Ozone and turns it into just Oxygen.

Enzymes

• Enzymes are biological catalysts made for very specific tasks!

• Sucrase is a good example.• It turns Sucrose and Water into Glucose and

Fructose, which our bodies can use very easily for energy.

• Sucrase targets sucrose specifically and does not target other molecules.

Chapter 15

• You need to know Keq, Ksp, activation energy, catalysts, and enzymes!

• Quiz will be posted online• It’s due on Monday. Hopefully I’ll have

everything graded and updated on the Wiki by then.

Wiki• Water and Energy Wiki• We are going to apply your new knowledge to world problems• The first part of the assignment is to go watch an

Inconvenient Truth, again• For the actual wiki. You are going to pick a topic of generating

clean water or renewable energy• You will research that topic and write a 5,000 word wiki about

it. • Use detail, site your sources, get quotes from relevant

scientists be creative.• You are researching a solution to world problems

Wiki

• Work in partners or groups of 3• I prefer everyone to be in the same class, but

if you absolutely have to you can pick someone outside of this time slot

To do

• Get your wiki started• Choose your wiki topic• Read the beginning of Chapter 16• Watch Inconvenient Truth• Finish your quiz• Check on Monday for your class grade so far