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Gen Chem II chap 11

QuestionAnswer
Intermolecular forces are forces that act between stable molecules. It is responsible for the state of a sample of matter= solid liquid, gas.
the solid and liquid states have a much higher density than the gas state. Therefore the molar volume of the solid and liquid states is much smaller than the gas state.
The solid and liquid states have similar what? densities. Generally the solid state is a little denser. Notable exception
The molecules in the solid and liquid state are in close contact with each other, while the molecules in a gas are far apart.
The molecules in a gas have complete freedom of motion. Their kinetic energy(energy in motion) overcomes the attractive forces between the molecules.
The molecules in a solid are locked in place, they cannot move around, though they do vibrate, they don’t have enough kinetic energy to overcome the attractive forces.
The molecules in a liquid have limited freedom – they can move around a little within the structure of the liquid. They have enough kinetic energy to overcome some of the attractive forces, but not enough to escape each other.
The properties of solids, liquids, and gases can be explained based on the kinetic energy of the molecules and the attractive forces between molecules.
Kinetic energy tries to give molecules freedom of motion
Kinetic energy gives molecules what degrees of freedom? translational, rotational, vibrational.
Attractive forces try to keep the molecules together.
Kinetic energy depends only on the temperature. KE = 1.5 kT
The close packing of the particles results in solids being incompressible.
The inability of the particles to move around results in solids retaining their shape and volume when placed in a new container; and prevents the particles from flowing.
Some solids have their particles arranged in an orderly geometric pattern – we call these crystalline solids. Ex=salt and diamonds. Other solids have particles that do not show a regular geometric pattern over a long range – we call these amorphous solids. Ex=plastic and glass.
Explaining the properties of a liquid. What can their density be compared to? Do they have a definite or indefinite volume? they have higher densities than gases because the molecules are in close contact. They have an indefinite shape because the limited freedom of the molecules allows them to move around. Have a definite VOLUME.
Are liquids compressible? no! Molecules closely spaced are not easily compressible. Molecules widely spaced are.
Changes between states. How does a solid go to a liquid? A liquid to a soli? heat. Cold.
Changes between states. Liquid to a gas? heat or reduction of pressure.
Changes between states. Gas to a liquid? cool or increase pressure.
What is the definition of intermolecular forces? are forces that act between stable molecules. It is responsible for the state of a sample of matter
Intermolecular attractions are due to attractive forces between opposite charges. + ion to – ion. + end of polar molecule to - end of polar molecule. H-bonding is especially strong. Even nonpolar molecules will have temporary charges.
larger the charge = stronger attraction
longer the distance = weaker attraction
The stronger the attractions between the atoms or molecules, the more energy it will take to separate them.
Boiling a liquid requires we add enough energy to overcome the attractions between the molecules or atoms.
The higher the normal boiling point of the liquid, the stronger the intermolecular attractive forces. It takes more energy to break these bonds.
What electron distribution results in a temporary dipole? fluctuations in the electron distribution in atoms and molecules result in a temporary dipole.
A region with excess electron density has what charge? partial (─) charge
A region with depleted electron density has what charge? partial (+) charge
The attractive forces caused by these temporary dipoles are called dispersion forces. Aka London Forces. All molecules and atoms will have them. Dispersion forces is when in any instant, electrons can be unevenly distributed.
As a temporary dipole is established in one molecule, it induces what in the surrounding molecules? dipole in all the surrounding molecules. This attracts the molecules to one another.
The magnitude of the induced dipole depends on several factors the SHAPE, POLARIZABILTY of the electron, volume of the electron cloud.
larger molar mass = more electrons
more electrons = larger electron cloud
larger electron cloud = increased polarizability
increased polarizability = stronger attractions
more surface-to-surface contact = larger induced dipole = stronger attraction
Noble Gases are all nonpolar atomic elements.
How is the strength of attractive forces between molecules linked to the boiling point? The stronger the attractive forces between the molecules, the higher the boiling point will be.
Intermolecular forces are much weaker than bonding forces.
Intermolecular forces and bonding forces Intermolecular forces are the result of smaller charges interacting at a great distance. Bonding forces are large charges (protons and electrons) interacting at very close distances.
The magnitude of dispersion forces depends on how easily the electrons in the atom or molecule can move or POLARISE in response to an instantaneous dipole, which in turn depends on the SIZE or VOLUME or the electron cloud.
Dispersion forces increase with increasing molar mass. SHAPE affects it too!
As the molar mass increases, the number of electrons increase. Therefore the strength of the dispersion forces increases.
What factors determine the magnitude of dispersion forces? The MOLAR MASS, MOLECULAR SHAPE.
Dipole-dipole forces are always present in POLAR molecules. Polar molecules have PERMANENT DIPOLES that interact with the permanent dipoles of neighboring molecules. If molecules have dipole-dipole forces, they have a higher boiling and melting point.
What example of attractive force is this? The positive end of one permanent dipole attracts the negative end of another. this attraction is the dipole-dipole force.
Polar molecules have high what? They have what attractive forces? boiling and melting points relative to nonpolar molecules with similar mass. Have dipole-dipole forces and dispersion forces.
Miscibility the ability to mix without separating into two states of liquid. The polarity of molecules composing liquids is also important in determining the miscibility.
In general, polar liquids are miscible with other polar liquids but are NOT miscible with nonpolar liquids.
Electronegativity increases as we move across a row in the periodic table.
Electronegativity decreases as we move down a column on the periodic table.
How to determine if a molecule is polar or not A polar molecule is a molecule that has a net dipole moment due to its having UNSYMMETRICAL POLAR BOND. There are two factors that go into determining if a molecule is polar or not. To determine if a molecule (or ion) is polar or non-polar, you must deter
To determine if a molecule (or ion) is polar or non-polar, you must determine both factors 1) The polarity of the individual bonds in the molecule. 2) The shape or geometry of the molecule.
To determine if a given individual bond is polar, you need to know the ____ of the two atoms involved in the bonds? electronegativity of the two atoms involved in that bond. To find the electronegativities of all the elements.
If the electronegativity of the two atoms has a difference of _____ or less, then the bond is non-polar 0.3 or less, then the bond is non-polar.
If the electronegativity difference is greater than ____ but less than _____, then it is polar. greater than 0.3 but less than 1.7, then the bond is polar.
If the two values have a difference greater than _____, then the bond is ionic 1.7, then the bond is ionic, which is just very very polar.
Once you know which bonds in the molecule are polar and which are non-polar you must use the shape of the molecule. You need the shape because two polar bonds, if oriented correctly can cancel each other out (like two equally strong people pulling in opposite directions on a rope -- nobody moves).
If all bonds are non-polar, then the whole molecule is non-polar regardless of its shape.
If there is symmetry in the molecule so that the polarity of the bonds cancels out- what is an example of this? then the molecule is non-polar. A common example of this is carbon dioxide, or CO2.
CO2 structure The molecule is linear, and its Lewis dot structure is like this
Another example of this is CCl4 where each carbon-chlorine bond is polar, but the molecule is non-polar. Here, how they cancel out isn't as obvious, but they do. CCl4 is a tetrahedral molecule, and the 4 C-Cl polar bonds cancel each other out.
If there are polar bonds but and their symmetry does not cancel each other out, the overall molecule is polar. Water is a typical example of this. The two O-H bonds are oriented in a V-shape, and so the don't cancel out.
Hydrogen Bonding, what is it? POLAR molecules containing HYDROGEN atoms bonded directly to small electronegative atoms like FLOURINE, OXYGEN, NITROGEN. FON. It is sort of a super dipole-dipole force. This is a strong attraction between the hydrogen and fluorine, and nitrogen. It occur
What are four examples of intermolecular bonds? Hydrogen bonds, dispersion forces, dipole-dipole force, ion-dipole force.
What is a good example of hydrogen bonding? Water. H2O
When a very electronegative atom is bonded to hydrogen, it strongly pulls the bonding electrons toward it Since hydrogen has no other electrons, when it loses the electrons, the nucleus becomes DESHIELDED. Exposing the H proton. The exposed proton acts as a very strong center of positive charge, attracting all the electron clouds from neighboring molecules.
Dipole-dipole force, class definition a molecule with a positive and negative end. (One pulls more e- to it than the other). Flourine is the most greedy of all the elements to want to grab e-. For example= Br-Cl. Cl is closer to the greedy F on the periodic table Cl pulls the electrons closer
Ion-dipole force occurs when an ionic compound is mixed with a polar compound. It is between an ion and a dipole. It is especially important in aqueous solutions of ionic compounds.
Summarizing intermolecular forces-dispersion forces are present in all molecules and atoms and INCREASE with INCREASING MOLAR MASS. These forces are always weak in small molecules but can be significant in molecules with HIGH MOLAR MASS.
Summarizing intermolecular forces- dipole-dipole forces are present in polar molecules.
Summarizing intermolecular forces-hydrogen bonds are the strongest… of the intermolecular forces that can occur in pure substances (second only to ion-dipole forces in general) are present in molecules containing H and FON.
Summarizing intermolecular forces-ion-dipole forces are present in mixtures of ionic compounds and polar compounds. These are VERY STRONG and are especially important in aqueous solutions of ionic compounds.
Liquids tend to minimize their surface area.
Surface tension the surface tension of a liquid is the energy required to increase the surface area by a unit amount.
surface tension is a property of liquids that results from the tendency of liquids to ______ their surface area. minimize their surface area.
In order to minimize their surface area, liquids form what? drops that are spherical. The surface layer acts like elastic skin.
How is surface tension related to intermolecular forces? Surface tension decreases as intermolecular forces decrease.
Because they have fewer neighbors to attract them, the surface molecules are _____ than those in the interior. Do they have a higher or lower potential energy. less stable than those in the interior. Have a higher potential energy.
The surface tension of a liquid is the energy required to increase the surface area a given amount.
What does intermolecular forces have to do with surface tensions? The stronger the intermolecular attractive forces, the higher the surface tension will be.
Raising the temperature of a liquid does what to the surface tension? reduces its surface tension. Raising the temperature of the liquid increases the average kinetic energy of the molecules. The increased molecular motion makes it easier to stretch the surface.
1 poise = 1 P = 1 g/cm∙s
 often given in centipoise, cP
• larger intermolecular attractions = larger viscosity
• higher temperature = lower viscosity
Viscosity the resistance of a liquid to flow.
How is viscosity measured? it is measured in a unit called the poise (P), 1 P(poise) as 1 g/cm*s.
Viscosity is greater in substances… Viscosity is greater in substances with stronger intermolecular forces because if molecules are more strongly attracted to each other, they do not flow around each other as freely.
Capillary action the ability of a liquid to flow against gravity up a narrow tube when taking a blood sample.
Capillary action results from a combination of two forces cohesive forces and adhesive forces.
Cohesive forces the attraction between molecules in a liquid. The cohesive forces cause the liquid to stay together.
Adhesive forces the attraction between these molecules and the surface of the tube. The adhesive forces cause the liquid to spread out over the surface of the tube.
Vaporization the process by which thermal energy can overcome intermolecular forces and produce a state change from liquid to gas.
The higher the temperature, the greater the average energy of the collection of molecules.
The molecules with higher energy, have enough energy to break free from the surface, where the molecules are held less tightly than in the interior due to fewer neighbor to neighbor interactions, and into the gas state.
What does having a larger the surface area have to do with the rate of vaporization? the faster the rate of evaporation. This will allow them to escape the liquid and become a vapor
What is the process of vaporization? The transition from liquid to gas is called vaporization.
The transition from gas to liquid is condensation.
How does weaker intermolecular forces and vaporization have to do with each other? The weaker intermolecular forces allow more molecules to evaporate at a given temperature, against increasing the rate of vaporization.
Volatile liquids that vaporize easily are called volatile. Acetone is more volatile than water.
Nonvolatile are liquids that do not vaporize easily. Motor oil is virtually nonvolatile at room temperature.
Summarizing the process of vaporization 1. The rate of vaporization increases with increasing temperature. 2. The rate of vaporization increases with increasing surface area. 3. The rate of vaporization increases with decreasing strength of intermolecular forces.
Heat of vaporization the amount of heat required to vaporize one mole of a liquid to gas is its heat.
Is vaporization endothermic or exothermic? is an endothermic process. vaporization requires input of energy to overcome the attractions between molecules.
Is condensation an endothermic or exothermic reaction? is an exothermic process.
What is the Heat of Vaporization, DHvap? the amount of heat energy required to vaporize one mole of the liquid is called the Heat of Vaporization, DHvap. Sometimes called the enthalpy of vaporization.
Is Heat of Vaporization endothermic or exothermic? always endothermic, therefore DHvap is +
What is the heat of vaporization of water at its normal boiling point is 100 degrees Celsius is +40.7 kJ/mol
How many kJ is in 1 mole? 1 kJ= 1 mole
The heat of vaporization is always positive because the process in endothermic- energy must be absorbed to vaporize a substance. The heat of vaporization is somewhat temperature dependent.
The heat of vaporization of a liquid can be used to calculate the amount of energy required to vaporize a given mass of the liquid.
Dynamic Equilibrium eventually the rate of condensation and the rate of vaporization become equal. Condensation and vaporization continue at equal rates and the concentration of water vapor above the liquid is constant. Evaporation and condensation are still occurring.
When two opposite processes reach the same rate so that there is no gain or loss of material, we call it a dynamic equilibrium. This does not mean there are equal amounts of vapor and liquid – it means that they are changing by equal amounts.
The vapor pressure of a particular liquid depends on two things the intermolecular forces present in the liquid and the temperature.
vapor pressure remember using Dalton’s Law of Partial Pressures the pressure exerted by the vapor when it is in dynamic equilibrium with its liquid is called the vapor pressure
the weaker the attractive forces, the _____ the vapor pressure the higher the vapor pressure. The higher the vapor pressure, the more volatile the liquid
Weak intermolecular forces result in volatile substances with high vapor pressures because the intermolecular forces are easily overcome by THERMAL ENERGY.
Strong intermolecular forces result in nonvolatile substance with low vapor pressures.
We can describe the tendency of a system in dynamic equilibrium to return the equilibrium with the following general statement When a system in dynamic equilibrium is disturbed, the system responds so as to minimize the disturbance and return to a state of equilibrium.
A system in dynamic equilibrium can respond to changes in the conditions when conditions change, the system shifts its position to relieve or reduce the effects of the change
If the pressure above a liquid, vapor system in equilibrium is decreased some of the liquid evaporates, restoring the equilibrium pressure. If the pressure is increased some of the vapor condenses, bringing the pressure back down to the equilibrium pressure.
When the temperature of a liquid is increased, what does it do to its vapor pressure? its vapor pressure rises because the higher thermal energy increases the number of molecules that have enough energy to vaporize.
Boiling point definition the boiling point of a liquid is the temperature at which its vapor pressure equals the external pressure.
The normal boiling point definition the normal boiling point of a liquid is the temperature at which its vapor pressure equals 1 atm.
The normal boiling point of pure water is 100 degrees Celsius. At lower pressure, water boils at a lower temperature.
When liquid water is boiling it cannot go above 100 degrees Celsius. It cannot go over its boiling point.
The lower the external pressure, what does it do to the boiling point of a liquid? The lower the boiling point of the liquid
The vapor pressure increases with increasing temperature, however, the relationship is NOT LINEAR. In other words, doubling the temperature results in more than a doubling of the vapor pressure.
In the Clausius-Clapeyron Equation, Pvap is the vapor pressure.
In the Clausius-Clapeyron Equation, B is a constant that depends on the gas.
In the Clausius-Clapeyron Equation, Heat of vaporization is the heat of vaporization.
In the Clausius-Clapeyron Equation, R is the gas constant. 8.314 J/mol*K.
In the Clausius-Clapeyron Equation, T is the temperature in kelvins.
The Clausius-Clapeyron Equation gives a linear relationship between the natural log of the vapor pressure and the inverse of temperature.
The Clausius-Clapeyron Equation is a convenient way to measure what? LEADS TO A CONVENIENT WAY TO MEASURE THE HEAT OF VAPORIZATION. We just measure the vapor pressure of a liquid as a function of temperature and create a plot of the natural log of the vapor pressure verses the inverse of the temperature. We can then determ
There is also Clausius-clapeyron equation two point form that we can use with just two measurements of vapor pressure and temperature to determine the heat of vaporization.
Clausius-clapeyron equation two point form, we can use this form of the equation to predict what? the VAPOR PRESSURE of a liquid at any temperature if we know the ENTHALPY of VAPIRIZATION and the NORMAL BOILING POINT.
Clausius-clapeyron equation two point form can be used with just two measurements of _____ and ____? vapor pressure and temperature
Can also be used to predict the vapor pressure if you know the ___ and the ___ heat of vaporization and the normal boiling point.
Remember, the vapor pressure at the normal boiling point is 760 torr.
At some temperature, the meniscus between the liquid and vapor disappears and the states commingle to form what? a supercritical fluid . supercritical fluid have properties of both gas and liquid states.
The temperature required to produce a supercritical fluid is called the critical temperature.
The pressure at the critical temperature is called the critical pressure.
At the critical temperature or higher temperatures, the gas cannot be _______ to a liquid condensed to a liquid, no matter how high the pressure gets.
Surface molecules with sufficient energy may break free from the surface and become a gas – this process is called sublimation.
The capturing of vapor molecules into a solid is called deposition.
The solid and vapor phases exist in dynamic equilibrium in a closed container at temperatures… at temperatures below the melting point. Therefore, molecular solids have a vapor pressure.
Melting=_____ Melting=fusion.
As a solid is heated, its temperature rises and the molecules vibrate more vigorously
What happens to solids when they melt? Once the temperature reaches the melting point, the molecules have sufficient energy to overcome some of the attractions that hold them in position and the solid melts (or fuses).
Once the temperature reaches the melting point, all the added heat goes into what? melting the solid – the temperature stays constant.
Once all the solid has been turned into liquid, the temperature can again start to rise.
Is melting an endothermic or exothermic reaction? melting is an endothermic process. melting requires input of energy to overcome the attractions between molecules.
Freezing is an exothermic process melting requires input of energy to overcome the attractions between molecules.
The amount of heat energy required to melt one mole of the solid is called the Heat of Fusion, DHfus. Sometimes called the enthalpy of fusion.
Heat of fusion is endothermic or exothermic? always endothermic, therefore DHfus is +. POSITIVE! Somewhat temperature dependent.
Heat Curve for water on flashcard 7 and on!
What is the heat of fusion of ice to liquid? 6.02 kJ/mol
Heat of fusion from liquid to gas? 40.7 kJ/mol
• describe the different states and state changes that occur at various temperature - pressure conditions
A map of the state or phase of a substance as a function of pressure (on the y-axis) and temperature (on the x-axis) is what? LOOK AT PG 486! Phase diagram. Pressure and temperature.
What does a line represent on a phase diagram? state changes.
On a Phase Diagram, what is the vapor pressure curve? liquid/gas line. Both states exist simultaneously.
On a Phase diagram, the critical point is the furthest point on the… vapor pressure curve.
Triple point on a phase diagram is the temperature/pressure condition where all three states exist simultaneously.
The critical point on a phase diagram represents the temperature and pressure above which a supercritical fluid exists.
For most substances, freezing point increases as _____increases pressure increases.
Water does what when it freezes? it expands.
Crystalline solids have a very _________ arrangement of their particles regular geometric
How is the arrangement of the particles and distances between them determined? by x-ray diffraction
What happens when light encounters two slits separated by a distance comparable to the wavelength of the light? Constructive and destructive interference.
What is constructive interference and destructive interference? constructive interference is two wavelengths that add up to be greater. Destructive interference is two wavelength which cancel each other out.
What is X-ray diffraction and what does it do to the particles of crystals? in this technique, a crystal is struck by beams of x-rays, which then are reflected. The wavelength is adjusted to result in an interference pattern between the particles. THE EXACT PATTERN OF DIFFRACTION OR REFLECTION REVEALS THE SPACINGS BETWEEN THE PLA
What does Bragg’s law do? for a given wavelength of light incident on atoms arranged in layers, we can measure the angle that produces constructive interference and then compute ‘d’ the distance between the atomic layers.
Anytime we talk about x-ray diffraction, what do we use? Bragg’s law.
when allowed to cool slowly, the particles in a liquid will arrange themselves to give the maximum attractive forces, this will do what to the energy? What will the result be? it will minimize the energy. The result will generally be a crystalline solid.
The arrangement of the particles in a crystalline solid is called the crystal lattice.
The smallest unit that shows the pattern of arrangement for all the particles is called the unit cell.
Unit cell, what is its dimension? How many layers of particles does it contain? unit cells are 3-dimensional, usually containing 2 or 3 layers of particles. Unit cells are repeated over and over to give the macroscopic crystal structure of the solid.
Each particle in the unit cell is called a lattice point.
Lattice planes are planes connecting equivalent points in unit cells throughout the lattice.
In a unit cell, the number of other particles each particle is in contact with is called its coordination number.
Higher coordination number means more interaction, therefore stronger _________ holding the crystals together stronger attractive forces holding the crystal together
the packing efficiency is the PERCENTAGE OF VOLUME in the unit cell occupied by particles.
The higher the coordination number, the more efficiently the particles are packing together.
Cubic unit cell. What are the angles in the corners? Are the length of all the edges equal? all 90° angles between corners of the unit cell. The length of all the edges are equal.
If the unit cell is made of spherical particles, how much of each corner particle is within the cube? How much of each particle on a face is within the cube? How much of each particle on an edge is within the cube? 1/3 of each corner particle is within the cube. 1/2 of each particle on a face is within the cube. 1/4 of each particle on an edge is within the cube.
The simple cubic unit cell has how many atoms on its corners? it has 8 on all the corners. HOWEVER, only 1/8 of each atom on the corners is on there!
How many atoms does a unit cell contain? Only ONE.
What is the radius for simple cubic unit? l=2r
What is the radius for body-centered cubic unit cell? 4r/square root of 3.
What is the radius for the face-centered cubic unit cell? l=2square root of 2r.
How to calculate the density of solid crystalline aluminum that is a face-centered cubic unit cell? by d=m/V. ‘m’= stands for the mass of unit cell and number of atoms in unit cell*mass of each atom. ‘V’= volume of unit cell, (edge length)^3.
Closest-packed structures, with spheres, it is more efficient to do what with the rows? Offset each row in the gaps of the previous row than to line-up rows and columns.
AA Pattern The second layer atoms can sit directly over the atoms in the first – called an AA pattern.
AB pattern The second layer can sit over the holes in the first – called an AB pattern.
ABA pattern The third layer atoms can align directly over the atoms in the first – called an ABA pattern.
ABC pattern Or the third layer can sit over the uncovered holes in the first – called an ABC pattern.
Classified by the kinds of units found