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Chapter 1-A Review of General Chemistry

STUDY
PLAY
What are constitutional isomers?
-compounds that have the same molecular formula but differ in the way that atoms are connected
What does the term valence describe?
-the number of bonds usually formed by each element
How many bonds does hydrogen prefer to form?
-1
How many lone pairs does hydrogen prefer to form?
-0
How many bonds does carbon prefer to form?
-4
How many lone pairs does carbon prefer to form?
-0
How many bonds does nitrogen prefer to form?
-3
How many lone pairs does nitrogen prefer to form?
-1
How many bonds does oxygen prefer to form?
-2
How many lone pairs does oxygen prefer to form?
-2
How many bonds does F prefer to form?
-1
How many lone paris does F prefer to form?
3
How many bonds does neon prefer to form?
-0
How many lone pairs does neon prefer to form?
-4
How many bonds do halogens (F, Cl, BR, or I) prefer to form?
-1
How many lone pairs do halogens prefer to form?
-3
How many electrons does the first shell of an atom hold?
-2
How many electrons does the second shell of an atom hold?
-8 electrons
What are organic compounds?
-are derived from living organisms
-contain carbon atoms
What are inorganic compounds?
-derived from non-living things
-lack carbon atoms
What is a covalent bond?
-a bond formed through the sharing of electrons between atoms
How is the number of valence electrons determined?
-by consulting the periodic table of elements
-the number of valence electrons in an atom is identified by its group number in the periodic table
What happens (in terms of energy) when electrons are shared to form a bond?
-there is a decrease in energy, indicated by a negative value in delta H
What happens as atoms move closer to ideal separation distance?
-energy continues to be lowered
At some point, what happens?
-the force of repulsion between the nuclei beings to overwhelm the forces of attraction, causing the energy of the system to increase
What does the lowest point on an energy diagram represent?
-the lowest energy (most stable) state
What does this state determine?
-both the bond length and the bond strength
What is the octet rule?
-atoms bond in such a way so as to achieve the electron configuration of a noble gas
-outer shell will have 8 electrons (with the exception of H)
What is a lone pair?
-a pair of unshared or nonbonding electrons
What is a formal charge?
-a charge associated with any atom that does not exhibit the appropriate number of valence electrons
How do you calculate formal charge on an atom?
1. determine the appropriate number of valence electrons for an atom (by consulting the periodic table)
2. determine whether the atom exhibits the appropriate number of electrons
What are the three categories that chemists classify bonds into?
-covalent
-polar covalent
-ionic
What do these categories emerge from?
-the electronegativity values of the atoms sharing a bond
What is electronegativity?
-a measure of the ability of an atom to attract electrons
What is the electronegativity of carbon?
-2.5
Of nitrogen?
-3.0
Of oxygen?
-3.5
Of flourine?
-4.0
Of chlorine?
-3.0
Of sulfur?
-2.5
What electronegativity trends does the periodic table exhibit?
-increases from the right to the left
-increases from bottom to the top
What if the difference in electronegativity between atoms is less than 0.5?
-the electrons are considered to be equally shared between the two atoms, resulting in a covalent bond
What if the difference in electronegativity between atoms is between 0.5 and 1.7?
-the electrons are not shared equally between the atoms, resulting in a polar covalent bond
What occurs when polar covalent bonds are formed?
-electrons tend to shift away from lower electronegativity atoms to higher electronegativity atoms
What is this withdrawl of electrons called?
-induction
What does induction cause?
-the formation of partial positive and partial negative charges
What happens if the difference in electronegativity is greater than 1.7?
-electrons are not shared at all, resulting an ionic bond
What is an ionic bond the result of?
-the force of attraction between the two oppositely charged ions
What is an atomic orbital?
-a region of space where there is a high probability (90-95%) of finding an electron; a region of space that can be occupied by electron density
Describe the shape/properties of electrons?
-electrons are not purely particles, they have wavelike properties as well
What is a wave equation?
-an equation that is constructed to describe the total energy of a hydrogen atom
What does it take into account?
-the wavelike behavior of an electron that is in the electric field of a proton
What is the wave equation solved to give?
-a series of solutions called wavefunctions
What does each wavefunction correspond to?
-an allowed energy level for the electron
What are the three possible phases of an electron's wavefunction?
-(+), typically represented by the color red
-(-), typically represented by the color blue
-(0), known as a node
What are the three atomic orbitals that most organic compound utilize?
-1s
-2s
-2p
How many electrons can occupy the 1s orbital? How many nodes does it have?
-can be occupied by 2 electrons
-has no nodes
How many electrons can occupy the 2s orbital? How many nodes does it have?
-can be occupied by 2 electrons
-has 1 node
How many two p orbitals are there?
-3
How many electrons can occupy each 2p orbital? How many nodes does it have?
-can be occupied by 2 electrons
-has 1 node
What are degenerate orbitals?
-orbitals with the same energy level
What do the Aufbau principle, the Pauli exclusion principle, and Hund's rule help to determine?
-the order in which atomic orbitals are filled
What is the Aufbau principle?
-the lowest-energy orbital is filled first
What is the Pauli exclusion principle?
-each orbital can accommodate a maximum of two electrons that have the opposite spin
What is Hund's rule?
-when dealing with degenerate orbitals (such as p orbitals), one electron is placed in each degenerate orbital first, before electrons are paired up
Two waves/atomic orbitals that approach each other can interfere in which two possible ways?
-constructively; or
-destructively
What does constructive interference produce?
-a wave with a larger amplitude
What does destructive interference result in?
-waves canceling each other, which produces a node
According to valence bond theory, what is a bond?
-the sharing of electron density between two atoms as a result of the constructive interference of their atomic orbitals
When two hydrogen atoms bond, where is the electron density of this bond located?
-primarily on the bond axis
What type of bond is this?
-a sigma bond
What is the definition of a sigma bond?
-a bond that is characterized by circular symmetry with respect to the bond axis
What is molecular orbital theory?
-a description of bonding in terms of molecular orbitals, which are orbitals associated with an entire atom rather than an individual atom
What are some of the key distinctions between atomic orbitals and molecular orbitals?
-atomic orbitals are regions of space associated with an individual atom while a molecular orbital is associated with an entire molecule
According to MO theory, what happens when two atomic orbitals overlap constructively?
-they cease to exist, and are replaced by two molecular orbitals that are each associated with the entire molecule
In simple hydrogen bond, what two molecular orbitals can be formed?
-bonding MO
-antibonding MO
What is the bonding MO a result of?
-constructive interference of the original two atomic orbitals
Is the bonding MO high in energy or low in energy? Does it have any nodes?
-low in energy
-has no nodes
What is the antibonding MO a result of?
-destructive interference of the two original atomic orbitals
Is the antibonding MO high in energy or low in energy? Does it have any nodes?
-high in energy
-has one node
Why are MOs a more complete analysis of bonds?
-because they include both constructive and destructive interference
What must the number of MOs created be equal to?
-the number of atomic orbitals that were used
What type of bonds does MO theory better describe?
-double bonds
What are hybridized atomic orbitals?
-atomic orbitals that are combinations of different orbital types (such as s and p)
Is the hybridization process a real physical process that the orbitals undergo?
-no
-it is a mathematical procedure that is used to arrive at a satisfactory description of the observed bonding
What type of hybridization does the C in CH4 undergo?
-sp3
What orbitals are combined to form the sp3 orbitals?
-2s orbital and the 2px, 2py, and 2pz orbital
How do the 4 sp3 orbitals compare in energy?
-they are degenerate (equal)
Why must the orbitals be equal in energy?
-to form four equal-energy symmetrical C-H bonds
What type of hybridization do the carbons in CH2CH2 undergo?
-sp2
What orbitals are combined to form the sp2 orbitals?
-2s orbital and 2 2p orbitals
What then remains?
-one 2p orbital
What can the 2 2p orbitals of CH2CH2 do?
-form a pi bond
Where does the pi overlap occur?
-above the plane of the molecule and below the plane of the molecule
What does this mean?
-that the pi bonds are not closely tied to the plane of the molecule
Describe the MO theory of pi bonds
-p orbitals can overlap constructively to form a pi bonding MO
-p orbitals can overlap deconstructively to form a pi anti-bonding MO
Describe the anti-bonding MO
-an extra node is formed, the molecule is at high energy
What is the molecular formula of ethane?
-CH3CH3
What is the number of sigma bonds and pi bonds that are used by the carbons in ethane?
-4 sigma bonds
What is the molecular formula of ethylene?
-CH2CH2
What is the number of sigma bonds and pi bonds that are used by the carbons in ethylene?
-3 sigma bonds, 1 pi bond
What is the molecular formula of acetylene?
-CHCH
What is the number of sigma bonds and pi bonds that are used by the carbons in acetylene?
-2 sigma bonds, 2 pi bonds
Rank the C-C bond length of ethane, ethylene, and acetylene increasing order
1. acetylene
2. ethylene
3. ethane
Rank bond energy of ethane, ethylene, and acetylene increasing order
1. ethane
2. ethylene
3. acetylene
What is an atom's steric number used for?
-to determine an atom's hybridization/geometry
What is an atom's steric number equal to?
-number of sigma bonds plus number of lone pairs
What does the steric number indicate?
-the number of electron pairs (bonding and nonbonding) that are repelling each other
What does this repulsion cause?
-the electron pairs to arrange themselves in three-dimensional space so as to achieve maximal distance from each other
What is this principle called?
-valence shell electron pair repulsion (VSEPR) theory
What will the hybridization of a central atom be with a steric number of 4?
-sp3
What will the hybridization of a central atom be with a steric number of 3?
-sp2
What will the hybridization of a central atom be with a steric number of 2?
-sp
What will the hybridization of a central atom be with a steric number of 1?
-not applicable
What electron group geometry will all sp3 hybridized atoms form?
-tetrahedral
What molecular geometry will be formed by an sp3 hybridized atom with no lone pairs?
-tetrahedral
With one lone pair?
-trigonal pyramidal
With two lone pairs?
-bent
What electron group geometry will all sp2 hybridized atoms form?
-trigonal planar
What molecular geometry will be formed by an sp2 hybridized atom with no lone pairs?
-trigonal planar
With one lone pair?
-bent
What electron group geometry will all sp hybridized atoms form?
-linear
What molecular geometry will all sp hybridized atoms form?
-linear
What is dipole moment?
-a measurement that is used to calculate the degree of induction
What is induction?
-shifting of electrons within their orbitals towards atoms with greater electronegativities
How is dipole moment calculated?
-by multiplying the amount of partial charge by the distance of separation (between the partial positive and partial negative charges)
What does measuring the dipole moment of a particular bond allow for?
-the calculation of the percent ionic character of that bond
When dealing with a compound that has more than one polar bond, what must be done?
-you must take the vector sum of individual dipole moments
What is this vector sum called?
-the molecular dipole moment
To be polar, what must a molecule have?
-polar bonds; and
-asymmetric geometry
What happens if a molecule does not have asymmetric geometry?
-polar bonds will cancel each other out
What are intermolecular forces?
-electrostatic, attractive forces between individual molecules
What are electrostatic forces?
-forces that occur as a result of attraction between opposite charges
What are the three classifications of electrostatic attractions for neutral molecules?
-dipole-dipole interaction
-hydrogen bonding
-london dispersion forces/fleeting dipole-dipole interaction
Rank these three classifications in order from strongest to weakest
-hydrogen bonding
-dipole-dipole interactions
-london dispersion forces
What are dipole-dipole interactions?
-attractions that occur between a partial positive charge on one molecule and a partial negative charge on another molecule
What does the resulting net attraction result in?
-elevated melting point
-elevated boiling point
Is a hydrogen bond an actual bond? If not, what is it?
-no, it is not a bond
-it is a specific type of dipole-dipole interaction
What type of molecules hydrogen bond?
-very polar molecules that contain hydrogen and
either O, N, F, or Cl
Describe hydrogen bonding
-when a hydrogen atom is connected to an electronegative atom, the hydrogen will bear a partial positive charge
-this positive charge can interact with a lone pair from an electronegative atom of another molecule
What are compounds with H atoms that are capable of forming H-bonds called?
-protic
How do the strength of hydrogen bonds compare to covalent bonds?
-H bonds are 20 times weaker
What do H bonds result in?
-higher melting temperatures
-higher boiling points
What are london dispersion forces?
-fleeting dipole-dipole interactions
What will the constant random motion of electrons in a molecule sometimes produce?
-an electron distribution that is not evenly balanced with the positive charge of the nuclei
What does this result in?
-a temporary dipole, which can induce a temporary dipole in a neighboring molecule
What does this result in?
-a fleeting attraction between the two molecules
Why are london dispersion forces stronger for higher molecular weight hydrocarbons?
-because these compounds have larger surface areas that can accommodate more interactions
What does this result in?
-compounds of higher molecular weight will generally boil at higher temperatures
Why are london dispersion forces stronger for non-branched hydrocarbons?
-because a branched hydrocarbon generally has a smaller surface area than its corresponding straight-chain isomer
Why do polar compounds generally mix well with other compounds?
-because polar compounds are all capable of H-bonding and/or strong dipole-dipole interactions
Why do non-polar compounds generally mix well with other nonpolar compounds?
-because no strong attractions would have to be broken to allow them to mix
Why won't a nonpolar compound readily dissolve in water?
-water is polar and is capable of forming H bonds
-water will only break its H bonds to form new H bonds
-non-polar molecules cannot form H bonds
What are soaps composed of?
-a polar group on one end of the molecule and a non-polar group on the other end
What does the polar group represent?
-the hydrophilic/water loving part of the molecule
What does the non-polar group represent?
-the hydrophobic/water fearing part of the molecule
How do soap and oil interact?
-oil molecules are surrounded by the hydrophobic tails of the soap molecules, forming a micelle
-the surface of the micelle is comprised of all of the polar groups, rendering the micelle water soluble
-->voila, the oil can now be dissolved in water!
What does drycleaning utilize?
-a non-polar solvent to dissolve non-polar compounds
Drawing lewis structures-first step
-draw a skeleton structure showing arrangement of atoms within the molecule
And then?
-determine the total number of valence electrons in the molecule
And then?
-connect the structure, and determine how many valence electrons remain
And then?
-place electrons on outside atoms to satisfy a noble gas configuration (octet rule)
And then?
-place remaining atoms on the central atom
And then?
-calculate the formal charge for the atoms in the molecule