Chem 1212 Unit 1

What are Intermolecular forces?

• Forces between two molecules

• They hold two or more separate molecules together

• Electrostatic interactions between molecules

• Unlike intramolecular forces, which are forces within a molecule

• Intra is stronger than inter

How do IMFs affect states of matter?

• State of matter is determined by the strength of the IMFs

• Weak IMFS and high KE are characteristic of gases

• Strong IMFs and low KE are characteristic of solids

What a\is the order of strength with IMFs

• Weakest to strongest

• London dispersion forces

• Dipole-induced dipole forces

• Dipole-Dipole interactions

• Hydrogen bonding

• Ion-dipole forces

What does it mean for a molecule to be polar?

Polar molecules have a net dipole moment, meaning that there is a permanent inequality in the distribution of electrons in the electron cloud

• This results from polar bonds, when two atoms have different electronegativities

• The molecules must also be asymmetrical

What are London dispersion forces?

• LDFs are temporary, weak IMFs that occur when electrons in adjacent molecules or atoms move around

• This creates a temporary, induced dipole in a molecule, resulting in electrostatic attraction

Where are LDFs present?

• LDFs are present in ALL atoms and molecules

• They are the main and only interactions between two non-polar molecules

What increases the strength of LDFs?

• Increasing polarizability

• Increasing atomic size

• Increasing molecular weight

• Increased surface area of a molecule

What are dipole-induced dipole forces?

• This occurs when polar molecules induce temporary dipoles in non polar molecules, resulting in DeBye forces

• These are stronger than LDFs

The strength of these forces increases as

• The strength of the dipole on the polar molecules increases

• The molar mass of the non polar molecule increases

What are dipole-dipole forces?

• This occurs when the permanent dipoles on two polar molecules interact and attract

• These are stronger than DeBye forces

What is hydrogen bonding?

• These are a special type of dipole-dipole bond in which an H atom (on the FON) is attireacted to the lone pair of another N,O, or F atom on another molecule

• The atoms involved are very small, making these bonds very strong

• Denoted by a dotted line

What are Ion-dipole forces?

• This occurs when an ionic compound dissolves into a polar solvent (water most of the time)

• When an ion reacts with a polar molecules

• These ionic compounds dissociate and the ions will attach to the dipole of the polar molecules

• Strongest IMF

Rank the states of matter based on IMF strength

• Gases have the weakest

• Solids have the strongest

• Liquids are in the middle

How does a phase change occur?

Phase changes occur when

• KE overcomes IMF strength (Melting, boiling)

• IMF strength overcomes KE (condensing, freezing)

How do IMFs affect boiling point?

• As IMF strength increases, boiling point temperature increases

What is vaporization? How do IMFs affect it?

• Vaporization occurs when some molecules in a liquid have a greater KE than the average KE, these molecules will break off and turn to a gas

• This is an endothermic process

• Vaporization rate increases as temp increases, liquid surface area increases, and as IMF strength decreases

What is volatility?

• Volatility is how easily liquids evaporate

• Volatile liquids evaporate easily and have weak IMFs

• Non-volatile liquids have strong IMFs

What is enthalpy of vaporization? Of condensation?

• Hvap is the energy required to vaporize 1 mole of a liquid at 1 atm of pressure

• Vaporization is endothermic, so this value will always be positive

• Enthalpy of condensation is the opposite of vaporization, so its enthalpy is the opposite of Hvap

What is dynamic equilibrium in liquids?

• In a closed system, liquid molecules will evaporate through vaporization, but eventually they will collide with the liquid condense back again

• Once the rate of vaporization and condensation is equal, the system has reached dynamic equilibrium

What is vapor pressure? How do IMFs affect it?

Vapor pressure is the pressure exerted by a vapor in dynamic equilibrium in a closed system

• The vapor pressure of a Susbstance increases as:

• IMF strength decreases, because this will increase the rate of vaporization

• Temperature increases, meaning more KE and more vaporization

• Volume DOES NOT affect vapor pressure

What is surface tension? What affects it?

• The amount of energy required to increase the surface area of a liquid (or to break through the surface)

• Water molecules at the surface are pulled inward by IMFs, meaning they are packed close together

• Surface tension increases as IMF strength increases and as temp decreases

Explain capillary action

• The ability of a liquid to flow up a narrow tube

• This results from

• Cohesive forces: the attraction between like molecules in a liquid

• Adhesive forces: the attraction between the molecules in a liquid and the molecules in the surface of a tube

How does capillary action affect a meniscus?

• If cohesive forces are stronger than adhesive, then there will be a convex meniscus (bulge going upward)

• If adhesive forces are stronger than cohesive forces, then the meniscus will be concave

What is viscosity? What affects it?

• The resistance of a liquid to flow

• Increases as IMFs increase and as temperature decreases

• Also increases as molar mass increases and as molecules get more complex shapes

What is miscibility? What affects it?

• If to liquids mix together and form a homogeneous mixture, they are miscible, they can dissolve in eachother

• If two liquids form a heterogenous mixture, they are immiscible with eachother

• Polar substances are miscible with other polar substances

• Non-polar substances are miscible with other non-polar

• Polar and non-polar are immiscible

What are the two groups of solids?

• Crystalline solids

• Arranges in structures with long-range order or patterns

• Amorphous solids

• Have disorder in their structure

What is a crystal lattice? What are they composed of?

Crystal lattice

• 3D arrangement of atoms, ions, or molecules within a crystalline solid

Unit Cell

• The smallest collection of atoms, ions, or molecules that make full crystal lattices

• Building blocks

Lattice Point

• Areas of the unit cell that represent atoms or ions

What is a cubic unit cell? What are the three types?

• Unit cell in the shape of a cube

• Simple cubes, body centered cubes, and face centered cubic

What is a simple cubic unit cell? How many atoms do they have?

• A unit cell consisting of one atom at each corner

• Each corner atom is shared by 8 unit cells, so 1/8 of the atoms are in each cell

• 1/8 × 8 = 1 atom in a simple cubic cell

• Coordination number is how many atoms surround each atom or ion in a unit cell

• Simple unit cell coordination number is 6

What is a body-centered cubic (BCC) unit cell?

• Consists of one atom at each corner and one atom in the center of the cube

• 2 atoms per unit cell

• Coordination number: 8

What is a face-centered cubic (FCC) unit cell?

• Consists of one atom in each corner and one atoms in the center of each face

• Face atoms are shared between two cubes, so 1/2 in each cube

• 4 atoms per cell

• Coordination number: 12

What are the different types of crystalline solids?

• Molecular solids

• Ionic Solids

• Atomic solids

• Non-bonding

• Mettalic

• Network covalent

What are molecular solids?

• Lattice sites are occupied by molecules (Ex: H20)

• The molecules have strong covalent bonds, but weak IMFS between the molecules

Weaker IMFs means

• Molecular solids have moderately low melting points

What is ionic solids?

• Solids with lattice points occupied by ions

• Oppositely charged ions have very strong electrostatic forces (ionic bonds) between them

Properties

• Very high melting point

• Hard and brittle

• Low conductivity when solid

• High conductivity when aqueous

What is lattice energy? What affects it?

• (deltaH lattice) is a measure of the strength of forces between ions in an ionic solid

Can be defined as

• Energy released when forming the solid from gaseous ions (exothermic)

• Energy required to break the solid into separate gaseous ions (endothermic)

Lattice energy increases as

• Ionic charge increases

• Ionic size decrases

• Charge is much more important than size

• Greater lattice energy = higher melting point

What are atomic solids? What are the different types?

• A solid with lattice stirs occupied by atoms

Different types differ by the types of bonds they exhibit

• Metallic solids

• Covalent network solids

• Non-bonding solids

What are metallic solids?

• Solids held together by metallic bonds

Electron sea model

• A positively charged atomic metal nuclei stays still while a sea of delocalized electrons are shared through the entire solid

Properties

• Very good conductors of heat and electricity

• Malleable and ductile

• Variable hardness and melting points

What are covalent network solids?

• Solids held together by covalent bonds

• These can be elements or compounds

MEMORIZE THESE

• C(diamond), C(graphite), SiO2 (quartz), SiC

Properties

• Very high melting points

• Harder than metallic solids

• Do not conduct electricity well

What are non-bonding solids?

• Generally solids of noble gases

• They are held together by intermolecular forces onlyl

• Very low melting points

What is a phase change?

• A change in the physical form of a substance, but not its chemical identity

• Solid to liquid = melting

• Liquid to solid = freezing

• Solid to gas = sublimation

• Gas to solid = deposition

• Liquid to gas = vaporization

• Gas to liquid = condensation

What does a phase diagram do? What are the phases?

• It shows a substances phase changes based on temperature and pressure

Solid

• Low temperature

• High pressure

Liquid

• Moderate temperature

• Moderate pressure

Gas

• High temperature

• Low pressure

What are the triple point and critical point on a phase diagram?

Triple point

• Temperature and pressure at which all three states of matter coexist in equilibrium

Critical point

• Temperature and pressure at which the vapor pressure curve ends

Supercritical fluid

• Past the critical point, liquid and gad phases are indistinguishable

What is the molar enthalpy of a phase change?

• These describe the change in enthalpy required to change the phase of one mole of a substance

• Hvap = -Hcond

• Hfus(melt) = -Hfreez

• Hsub = -Hdep

What is a heating curve? What are the equations involved?

• A heating curve also visualizes phase changes

• When a substance remains in a single phase, adding heat increases the temperature as KE

• When a phase change is occurring, the temperature does not change - all heat goes towards changing potential energy

What is a reaction rate expression?

• The change in the concentration of a reactant or propduct with respect to time

How do you write out reaction rate expressions?

How do you write rate expressions if the stoichiometry isn’t 1-to-1?

What is a rate law? A differential rate law?

• A rate law is a mathematical equation or expression that describes how the concentration of reactants changes as a reaction progresses

• Products do not appear in rate laws

• A differential rate law describes the relationship between reaction rate and reactant concentration

What are zero, first, and second order reactions?

Zero order

• The rate of reaction does not change with the concentration of reactants

• The units of k are M/s

First order

• The rate of reaction is directly proportional to the reactant concentration

• As the reaction proceeds, the rate will decrease because Concettaration decreases

• Units of k are 1/s

Second order

• The rate of reaction is directly proportional to the square of the reactant concentration (exponential)

• Much more sensitive to reactant concentration

• Units of k are 1/M x s

What is the reaction order of a reactions with multiple reactants?

• Overall order = x + y + z

• Orders can also be fractions or even negative

What is an integrated rate law?

• This rate law of a reaction describes the relationship between the concentrations of the reactants and time

• This is found by integrating the differential rate laws (just memorize the formulas or look at formula sheet)

Explain zero order integrated rate laws

• The reaction rate is proportional to a constant

Explain first order integrated rate laws

• The reaction rate is proportional to the concentration of A

Explain second order integrated rate laws

• The reaction rate is directly proportional to A squared

What is half life? What are the half life equations?

• The half life of a reaction is the time required for the concentration of a reactant to reach one half of its initial value

What does collision theory state?

• Reactants must collide in order to react with each other

• Not all collisions lead to reactions because the reactants must collide with the proper orientation to form products

• Molecules must collide with enough KE to break chemical bonds and react

What factors increase collisions in a gas phase reaction?

• Increasing the number of reactant particles

• Increasing the temperature, causing particles to move faster

Boltzmann distribution of energy

• The higher the temperature, the greater the fraction of molecules with more KE than the activation energy

What is a potential energy diagram?

• This shows the change in potential energy as reactants are converted into products

What is activation energy?

• The minimum energy necessary to form a product during a collision between reactants

• The height of the hill in a potential energy diagram

• Higher activation energy means slower reaction rate

What is a transition state in a reaction?

• This is a high energy and unstable state called the activated complex between reactant and product

• Partially formed and broken bonds

What is the Arrhenius Equation?

• It relates the rate of a reaction to temperature

• The frequency factor (A) is reaction specific and considers the fraction of effective to ineffective collisions

What is a reaction mechanism?

• The reaction mechanism is the entire series of individual chemical steps by which an overall chemical reaction occurs

• The number of reactants involved in each step is called its molecularity

What is an intermediate?

• Each step in a reaction mechanism is a elementary step

• A reaction intermediate is a substance that is formed in one elementary step and consumed in another

What is a unimolecular step?

• An elementary step where a single reactant species produces one or more molecules of product

• Always first order

What are bimolecular steps?

• Elementary steps where two molecules or atoms collide to form an activated complex, resulting in products

• Always second order

What are termolecular steps?

• Elementary steps where three molecules, atoms, or ions collide, resulting in products

• The probability of this happening is extremely low

• Always third order

What is a rate determining step?

• This is a reaction step that occurs slower than all of the other steps

• Products cannot form any faster than the slowest step

• This step will have the highest activation energy

What do reactions with initial slow steps tell you about the rate law?

• The first step would be the rate determining step, so its rate law is the same as the overall reaction

For reactions with initial fast steps, still use the slow step as the rate law, but get rid of any intermediates

What is a catalyst?

• A substance that increases the rate of a reaction by lowering the activation energy without being consumed

• Catalysts provide an alternate reaction pathway and mechanism

What are the different types of catalysts?

Homogeneous catalysts

• Exist in the same phase as the reactants

Heterogeneous catalysts

• Exist in a different phase from the reactants

Biiological catalysts

• These are enzymes ending in -ase

Higher activation energy on a potential energy diagram means lower k and slower reactions

The rate constant k is talking about how fast a reaction happens. If K2 is 3 and K1 is 1, then K2 reaction is 3 times faster