Chemistry 162 Ch.12,Ch.13
Collision Theory of Chemical Reactions
-According to collision theory, chemical reactions occur as a result of collision between reacting molecules. -According to this theory, the rate of chemical reactions rate depends directly on three factors:
q=energy factor
-Collisions that result in a chemical reaction are called effective collisions. -To be effective, colliding molecules need to have a certain minimum amount of energy. -The minimum amount of energy needed to initiate a chemical reaction is called the activation energy. -Activation energy has the symbol E and is in units of J/mol or kJ/mol . -A bigger E for a chemical reaction translates into a smaller q. This slows the reaction down.
Continued.....
-In a solution, solute particles replace some of the solvent molecules at the surface. -This interferes with the ability of solvent to vaporize and reduces the rate of vaporization. But the rate of condensation stays the same.
Chemical Kinetics
-Is the study of how fast reactions take place. This is the rate of a chemical reaction. -Some happen almost instantaneously, while others can take millions of years. -Increasing the rate of a reaction is important to many industrial processes and well as the proper operation of living things.
Measuring Chemical Reaction Rates
-Monitor the decrease in concentration of a reactant with time or the increase in concentration of a product with time. -Then analyze the concentration vs. time data---rate -Two ways to measure concentration with time: 1. May quench reaction. Remove sample and analyze 2. Measure a property of a reactant or product that depends on concentration. This may be done continuously, without interrupting the reaction. Example: conductivity, refractive index, color (colorimetery), viscosity, magnetism, etc.
Osmotic Pressure
-Osmosis is the selective flow of solventmolecules through a semipermiable membrane from a solution of low concentration into a solution of high concentration.
Reaction Coordinate Diagrams
-This shows the energy of molecules during the course of a reaction. -The E is an energy barrier(bump) in the diagram between reactants and products. -Colliding molecules must posses E to react. -The first part of an effective collision is the formation of a high energy loosely bonded, activated complex (or transition state) at the top -Subsequently, the activation complex breaks apart to form products.
A closer look...ideal solutions
-Total vapor pressure curve is linear -Means the solute and solvent have very similar intermolecular forces. -Solute molecules are just taking the place of the solvent molecules. -This type of solution is called an ideal solution Example: Hexane(C6H14) heptane:(C7H16)
Nonideal Solution
-When solvent-solvent/solute-solute and solvent solute interactions are different, nonideal solution result. -When the solute/solvent interactions are stronger, the total vapor pressure of the solution will be less than predicted by Raoult's law. -These solutions show negative deviation from Raoult's law. Example: Nitric acid + Water
Nonideal Solutions
-When solvent-solvent/solute-solute and solvent-solute interactions are different, nonideal solution result. -When the solute/solvent interactions are weaker, the total vapor pressure of the solution will be greater than predicted by Raoult's law. -These solutions show positive deveation from Raoult's law. Example: Ethanol (C2H5OH) + Water
Colligative Properties
-are properties that depend on the number of solute particles in solution - Colligative properties do not depend on the nature of the solute particles.
Properties are
1. Vapor pressure lowering 2. Boiling point elevation 3. Freezing point depression 4. Osmotic pressure
Example of a reaction with a small activation energy
2NI3(s)-----N2(g)+3I2(g)
Example
A balance
Example of a reaction with a large activation energy......
C(s)+O2(g)------CO2(g)
Chemical Kinetics Ch.13
Chemical Kinetics Ch.13
Vapor Pressure Lowering
Consider a Mixture of Two Components Solvent +Solute There are two important cases 1. Solvent and solute are both volatile Hexane +Heptane 2. Solvent is volatile but solute is not (e.g., sucrose in water). water + sucrose
Colligative Properties of Electrolytes
Electrolytes undergo dissocation when dissoved in water, so the number of "particles" (ions) you get is more that expected from the concentration of the electrolyte. The van't Hoff factor(i) accounts for this effect.
Continued.........
Eventually, equalibrium is re-estimated, but awith a smaller number of vapor molecules. -The vapor pressure of a solution is lower than the vapor pressure of the pure solvent. -THIS IS VAPOR PRESSURE LOWERING
Qualitative Interpretation of Vapor Pressure Lowering
If a solute is nonvolatile, only the solvent contributes to the vapor pressure of a solution. Dynamic Equilibrium Pure Solvent
Molarity (M)
Is the number of moles of solute dissolved in 1 liter of solution
Boiling Point Elevation
Solutions boil at a higher temperature than the pure solvent
Freezing Point Depression
Solutions freeze at a lower temperature than the pure solvent.
P=probability or steric factor
Some molecules must be oriented in a certain way to favor a chemical reaction. Chemical reactions with important orientation (steric) requirements have small values for P.
The van't Hoff Factor
The van't Hoff factor is usally smaller than predicted due to interactions between ions. These interactions include the following: For strong electrolytes.... -Short range Coulombic forces-result in ion pairs -Long range Coubombic forces-redistribute the ions -These interactions increase with concentration -For weak electrolytes, incomplete dissociation reduces the number of ions from that expected from the formula unit.
Z=Collision Frequency
Where collision frequency is the number of collisions occuring per second (collisions/s) -This one is common sense -The reaction rate is directly proportional to the number of collisions per second. -Since the number of collision is directly related to the concentration of reactants, Z can also be regarded as the concentration factor. -However, not all collision result is a chemical reaction...this leads to the other factors.
Example
Volumetric flask
Semipermeable Membrane
allows solvent to flow through it, but not solute
Osmotic pressure
is often used to determine the molar masses of large molecules
Molality (m)
is the number of moles of solute dissolved in 1kg (1000g) solvent
Osmotic Pressure
of a solution is the pressure required to stop osmosis
Raoult's Law
states that the partial pressure of a volatile component of a solution is given by the vapor pressure of the pure component times the mole fraction of the component in the solution.
