Chem 2 Exam 1 Flashcards
What is an intramolecular force?
- Bonds within a molecule.
- Usually strong.
- Determine the chemical properties
What is an intermolecular force?
- Attraction between molecules.
- Usually weak.
- Determine the physical properties.
What are the types of intermolecular forces (IMFs)?
- London Dispersion Forces
- Dipole-Dipole Attraction
- Hydrogen Bonding
- Ion-Dipole Forces
What are London Dispersion Forces?
- Operate between all species (neutral, charged, polar, or nonpolar).
- Results from uneven distribution of electrons.
What do the strengths of London Dispersion Molecules depend on?
- Size of molecules (molar mass)
- Molecular shape (compact of elongated)
Effect of size on a large molecule (london dispersion)
Larger molecules —> more polarizable —> stronger london forces —> high MP, BP
Effect of size on small molecule (london dispersion)
Smaller molecules —> less polarizable —> weaker london forces —> low MP, BP
Compact molecular shape (london dispersion forces)
- more compact
- small area for interaction
- weaker london forces
- lower boiling point
Elongated molecular shape (london dispersion forces)
- less compact
- larger area for interaction
- stronger london forces
- higher boiling point
What is Dipole-Dipole Attraction?
- occur only in polar molecules, which contain permanent dipoles
- the positive end of one permanent dipole attracts the negative end of another through dipole-dipole forces
How to find polar molecules?
- different atoms asymmetrically attached to central atom
- only one lone pair on central atom
- lone pairs are asymmetrically placed around the central atom
What is Hydrogen Bonding?
- very strong special type of dipole-dipole interaction
- occurs between H and F, O, or N
What are Ion-Dipole Forces?
- Occurs when an ionic compound is mixed with a polar molecule
- ions are attached to the dipole of polar molecules
- higher the charge on ion, stronger ion-dipole interaction
- smaller size of ion, stronger ion-dipole interaction
IMFs from strongest to weakest?
- ion dipole forces
- hydrogen bonding
- dipole-dipole attraction
- london dispersion forces
What are the three properties that depend on the strengths of intermolecular forces?
- Viscosity
2 Surface Tension - Capillary Action
What is Viscosity?
measures a liquids resistance to flow
- ex. liquid with lower viscosity flow freely (water, gasoline,…) liquid with higher viscosity do not flow freely (honey, syrup…)
What factors affect viscosity and why
- Intermolecular Forces - stronger IMFs = higher viscosity
- Temperature - higher temp = lower viscosity
- Size and Shape of Molecule - large and complex structure = higher viscosity
What is Surface Tension?
The tendency of liquids to minimize their surface area.
- liquid beads to yield the minimum surface are (surface molecule interacts with 4 neighbors while interior molecule interacts with 6 neighbors)
What Factors Affect Surface Tension and Why
- Intermolecular Forces - stronger IMFs = higher surface tension
- Temperature - surface tension decreases as temp increases
What is Capillary Action?
The rise of liquid in a narrow tube (i.e.a capillary) immersed in the liquid
- due to attraction of the liquid molecules to the surface of the tube
- the attraction to the surface draw the liquid up tube
What is Solid to Gas Called and Vice Versa?
- Solid to gas = sublimation
- gas to solid = deposition
Vaporization
- liquid to gas
- endothermic process
- energy needed to overcome IMFs
- ∆Hvap always positive
What is ∆Hvap Heat of Vaporization?
the amount of heat required to vaporize one mole of a liquid to gas
Condensation
- gas to liquid
- exothermic process
- energy is released
- ∆Hcond always negative
∆Hcond = −∆Hvap
Rate of Vaporization Increases with….?
- Increasing Temp: average kinetic energy increases and more molecules have enough energy to over the IMFs
- Increasing Surface Area: the surface molecules are held less tightly, thus have greatest tendency to evaporate
- Decreasing Strength of IMFs: less attraction forces to hold molecules in liquid state
What is Dynamic Equilibrium?
Reciprocal processes are occurring at equal rates
- system at dynamic equilibrium: rate of forward reaction = rate of backward reaction
- occurs in closed systems where molecules cant escape
- the double arrow indicates dynamic equilibrium
What is Vapor Pressure?
pressure exerted by a vapor in equilibrium with a solid or a liquid at a given temperature in a closed system
What Affects Vapor Pressure of a liquid?
- Temperature
- IMFs
How does Temperature Affect Vapor Pressure?
At higher temps:
- molecules have higher thermal energy
- more molecules have enough energy to evaporate
- number of molecules in gas phase is higher
- vapor pressure is high
- INCREASING TEMPERATURE INCREASES VAPOR PRESSURE
How do IMFs Affect Vapor Pressure?
- Weaker IMFs —> high vapor pressure
- Stronger IMFs —> Low vapor pressure
What is Boiling Point?
Temperature at which liquids vapor pressure = the external pressure
- during boiling, temp is constant
- variations in atmospheric pressure will change boiling point (high altitude –> lower atmospheric pressure –> lower BP)
What is normal boiling point?
1 atm or 760 torr or 101.3 kPa
Effects of IMFs on Boiling Point
- weak IMF —> higher vapor pressure —> low BP
- strong IMF —> lower vapor pressure —> high BP
IMF, Rate of Vaporization, Vapor Pressure, and BP
- Weak IMF —> Higher rate of vaporization —> high vapor pressure —> low boiling point
- Strong IMF —> lower rate of vaporization —> low vapor pressure —> high boiling point
What does each symbol stand for in ln(P2/P1) = -∆𝐻vap/R (1/T2 - 1/T1) ?
- P1 = vapor pressure at temp. 1
- P2 = vapor pressure at temp. 2
- ∆𝐻vap = heat of vaporization (J/mol)
- R = gas constant, 8.314 J/mol x K
- T1 = temp. 1 in kelvin
T2 = temp. 2 in kelvin
What is a phase diagram?
Pressure(y) - temperature(x) graph
- shows the effects of both pressure and temp. on phase changes
- Three main regions: solid, liquid, gas
What is Triple Point (phase diagram)?
temp. and pressure where 3 phases coexist in equilibrium
What is Critical Point (phase diagram)?
highest temp. and pressure at which a pure material can exist in gas/liquid equilibrium
- CRITICAL POINT IS ALWAYS HIGHER THAN TRIPLE POINT
What is Supercritical Fluid?
substance at a temp. and pressure above its critical point and density near its liquid density
- have properties between those of gas and liquid
What is a solution?
homogenous mixture of two or more substances
- solution = solvent + solute
What is a solvent?
the component present in high concentration
- water is solvent —> aqueous solution
What is a solute?
the component present in low concentration
Strong Electrolytes
- Substance Dissociation = completely
- conductivity = high
- ex. strong acids, strong bases, soluble ionic compounds
Weak Electrolytes
- Substance Dissociation = partially
- Conductivity = low
- ex. weak acids, weak bases
Nonelectrolytes
- Substance Dissociation = does not dissociate
- Conductivity = no conductivity
- ex. sugar, alcohols, organic compounds
What is Solubility?
solubility of a solute in a particular solvent is the maximum concentration that may be achieved under given conditions when the dissolution process is at dynamic equilibrium
- concentration of dissolved solute remain constant
- amount of undissolved solute remain constant
What is the solubility formula?
solubility = g solute needed to make saturated solution / 100 g solvent
What is a saturated solution?
Maximum concentration of solute possible for a given temp. and pressure
- a solution containing solute that is equal to solubility
- additional solute will NOT dissolve
What is an unsaturated solution?
A solution containing less solute than the equilibrium amount
- additional solute will dissolve
What is a supersaturated solution?
A solution containing more solute than the equilibrium amount
- unstable
- a non-equilibrium state
What are immiscible liquids?
Two mutually insoluble liquids
- get two separate phases
What are miscible liquids?
Two liquids that dissolve in one another in all proportions
- get single phase
- form homogeneous solution
What is the rule for formation of homogenous solutions?
“like dissolves like”
- polar solvents dissolve polar solute
- nonpolar solvents dissolve nonpolar substances
What is the effect of temp on solid solubility?
For most solids, the silubility in water increases with increasing temp.
What is the effect of temp on gas solubility?
The solubility of the gas in water decreases with increasing temp
What is the effect of Pressure on gas solubility?
The higher the partial pressure of a gas above a liquid is, the more soluble the gas in the liquid is
- low pressure gas solubility is low
- high pressure gas solubility is high
Factors Affecting Solubility - solid
- temp increases —> solubility increases
- pressure increases —> no effect
Factors Affecting Solubility - gas
- Temp increases —> solubility decreases
- Pressure increases —> solubility increases
Molarity (M) formula?
moles of solute / Liters of solution
Molality (m) formula?
moles of solute / kilograms of solvent
Mole Fraction (x) formula?
moles of solute / (mole of solute + moles of solvent)
What is a Colligative Property?
- Property of a solution
- Depends only on the concentration of the solute
- Do no depend on the chemical identity of the solute
What are the colligative properties that we will discuss?
- vapor pressure lowering
- boiling point elevation
- freezing point depression
- osmotic pressure
Colligative Properties - Nonelectryolyte
Substance that does not dissociate into ions when dissolved in water
Colligative Properties - Electrolyte
Substance that dissociates into ions when dissolved in water
What is Van’t Hoff Factor and what is it for electrolytes and nonelectrolytes?
i = moles particles in solution / moles of formula units dissolved
- for nonelectrolyte (molecular compounds) i = 1
- for electrolytes (soluble ionic compounds, acids) i > 1
What does vapor pressure lowering mean?
The vapor pressure of the solution of nonvolatile solutes is lower than the vapor pressure of the pure solvent
What are nonvolatile solutes?
solutes that cannot evaporate from solution
What is Raoult’s Law?
quantifies the vapor pressure of a solution
What are Raoult’s law for volatile components?
- if both solvent and solute are volatile, they both contribute to the overall vapor pressure of solution
Psolution=Psolvent+Psolute - the total vapor pressure of solution containing i volatile component is:
Psolution=PA+PB+PC… - vapor pressure of each component is given by Raoults law:
Psolution=XAPOA+XBPOB+XCPOC…
What is boiling point elevation, ∆Tb?
elevation of the boiling point of a liquid by addition of a solute
- ∆Tb = BPsolution - BPpure solvent
What is freezing point depression, ∆Tf ?
lowering of the freezing point of a liquid by addition of a solute
- ∆Tf = FPpure solvent - FPsolution
Boiling Point Elevation?
Boiling point of a solution always higher than pure solvent
- BPsolution = BPpure solvent + ∆Tb
Freezing Point Depression?
freezing point of solution always lower than pure solvent
- FPsolution = FPpure solvent - ∆Tf
What is osmotic pressure?
the opposing pressure required to prevent the movement of solvent molecules through a semipermeable membrane
- TT = iMRT
What is chemical kinetics?
The study of reaction rates (speeds)
- some chemical reactions are fast and some are slow
What are chemical reaction rates?
The change in the amount of a reactance or product per unit of time
Concentrations as a function of time
- Concentration of reactant decreases with time
- Concentration of product increases with time
Rate Expressions
- negative sign (-) for reactants
- positive sign (+) for products
What are measuring rates?
-reaction rate changes during reaction—>not constant
- often initially fast when lots of reactant present
- slower and slower as reactants are depleted
- Measured by average rate, instantaneous rate, and initial rate
What are factors affecting reaction rates?
- chemical nature of reacting substances
- physical states of reactants
- temp of reactants
- concentrations of reactants
- presence of a catalyst
What does the chemical nature of the reacting substances mean?
the rate of a reaction depends on the nature of the participating substances
- ex. calcium reacts moderately with water while potassium reacts quickly with water
What does the physical states of reactants mean?
Reactants must contact in order to react. Therefore, reactions happen in liquid solution or gas phase
- Homogeneous solution: all reactants in same phase –> occurs rapidly
- Heterogeneous reaction: reactants in diff. phases. Only meet at interface. So, surface area determines reaction rate. Higher surface area = higher rate
What does the temp. of reactants mean?
Chemical reactions typically occur faster at higher temps.
- ex. many chemical processes, reaction rates are approximately doubled when temp is raised by 10 degrees celcius
What does concentrations of reactants mean?
Generally reaction rates usually increase when the concentration of one or more of reactants increases
What does the presence of a catalyst mean?
Rate-accelerating agents
- ex. enzymes
What are rate laws?
Mathematical expressions that describe the relationship between the rate and the concentration of its reactants
What is rate law and order of reaction?
Exponents specify the order of reaction with respect to each reactant
How do you find the overall order of reaction?
sum of order of each reactant in the rate law
- m+n
What is the relationship between the order of a reaction and the changes in concentration and rate?
- Zero order reaction: nothing changes
- First order reaction: when [A] doubles = rate is double. When [A] triples = rate is triple. When [A] quadruple = rate is 4 times
-Second order reaction: When [A] doubles = rate is 4 times. When [A] triples = rate is 9 times. When [A] quadruple = rate is 16 times
What does concentration and time do?
Rate law tells us how rate of reaction varies with concentration
How do we determine reaction order using graphs?
- if [A] vs time is linear then reaction order is zero
- if ln[A] vs time plot is linear, then reaction order is one
- if 1/[A] vs time plot is linear then reaction order is 2
What is the half-life of a reaction?
the time required for one-half of a given amount of reactant to be consumed
- half-life of the reaction depends on the order of reaction
What does half-life show?
Shows inverse relation between the half-life, t1/2 and rate constant k
- faster reactions exhibit larger rate constants and shorter half-lives
- slower reactions exhibit smaller rate constants and longer half-lives
What is the Collisions Theory?
model that emphasizes the energy and orientation of molecular collisions to explain and predict reaction kinetics
What is the collisions theory based on?
- the rate of a reaction is proportional to rate of reactant collisions
- molecules must be oriented properly when they collide
- collisions must have minimum kinetic energy for successful reaction
What is the collisions model?
-colliding molecules must have enough energy to cross the barrier
- energy barrier = activation energy, Ea
What is activation energy?
the minimum energy necessary to form a product during a collision between reactants
What is the Arrhenius equation?
mathematical relationship between the rate constant and the activation energy of a reaction
- k = Ae(-Ea/RT)
- A is related to frequency of collisions and the orientation of the reacting molecules
- e(-Ea/RT) is related to the fraction of collisions that has adequate energy to overcome the activation barrier
What is a reaction intermediate?
a species that are produced in one step and consumed in a later step. Appear in mechanism but not in overall reaction.
What is rate determining step?
step that occurs much more slowly than the other steps
What needs to be met to validate a mechanism?
- elementary steps must sum to the overall reaction
- The rate law predicted by the mechanism (by slow step) must be consistent with the experimentally observed rate law
- each elementary step should have an order to three or less
What is a catalyst?
substance that increases rate of chemical reaction but is not consumed by reaction
What does adding a catalyst to a chemical reaction do?
- provide an alternative mechanism
- activation energy decreases
- rate constant increases
- reaction rate increases
Where is catalyst found?
used up in early step and regenerated in later step
