Chem 2110 Mini Exam 2

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lewis structure

answer

-so named to honor the chemist Gilbert Lewis, who invented the electron-dot symbols
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covalent vs ionic
answer

-when a loser reacts with a winner (ie. group 1 metal with a non-metal), there is only one formula for the final product

 

-when non-metals form covalent compounds with each other, there could be more than one formula for the final product

 

-most compounds contain both types of bond

 

-the difference between an ionic bond and a covalent bond lies in the difference between the electronegativity of the two atoms that form the bond

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naming covalent compounds
answer

-is similar to that of ionic compounds, only more specific

 

-the first element is named like a cation, while the second element is named like an anion.

-the number of atoms of each element in the compound is clearly named by using prefixes such as mono, di, tri, tetra, penta, hexa, hepta, octa, etc

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free radicals
answer

-atoms or molecules with unpaired electrons are called free radicals

-free radicals are not stable as they are still seeking to achieve the octet status

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central atom
answer

-atom at the center of a molecule/Lewis structure
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formal charge
answer

Formal Charge = Group Number – # of unshared electrons – # of bonds

 

-the concept of formal charge is different from the concept of oxidation states of an element

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arrows
answer

– we use a curved arrow to indicate the direction of flow of electrons

-a full arrow head indicates the flow of 2 electrons, whereas a half arrow head indicates the flow of 1  electron

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polyatomic ions-
answer

-covalent molecules with an electric charge

 

* the charge of the polyatomic ion = the sum of the formal charges in the Lewis Structure

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resonance structures
answer

-all of the possible and correct Lewis structures of an ion

-we use the word resonance to describe the situation where we cannot use only one Lewis structure to adequately represent a covalent molecule

 

-in realisty, the molecule should be viewed as a ‘hybrid’ of all resonance structures

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VSEPR
answer

-Valence Shell Electron Pair Repulsion

 

-electron pairs want to stay as far apart as possible from each other

 

 

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wedge/dash bonds
answer

-wedge bond is a bond pointing out of the paper plane toward you, and a dash bond is a bond pointing out of the paper plane away from you
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dipoles
answer

-we use a dipole arrow to clearly indicate the polarization of the bond

 

-a dipole is described quantitatively by its dipole moment, which is an experimentally measurable quantity and has the "micro" symbol

 

-we use the concept of bond polarity to discuss the degree of polarization of any bond formed between two atoms

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bond polarity
answer

-we use the concept of bond polarity to discuss the degree of polarization of any bond formed between two atoms

 

-bond polarity is strongest for ionic compounds (difference greater than 1.7 electronegativity)

-next strongest for polar covalent compounds (difference between electronegativity of 0.4-1.7)

-and very small for non-polar covalent (between 0 and 0.4)

 

-an ionic bond is an extreme case of bond polarization… a complete separation of the positive and the negative end. When this happens, the dipole arrow is no longer appropriate in describing the ionic bond because the force of attraction between cations and anions in non-directional

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molecular dipole
answer

-the sum of the individual dipoles gives rise the net dipole for the molecule

 

-to acquire, must follow the rules of vector addition

 

***A molecule with a non-zero molecular dipole moment is a polar molecule. When th molecular dipole moment is zero, the molecule is non-polar even if it contains polar bonds

question

In summary, how to predict whether any covalent molecule is polar of non-polar
answer

1)Draw the Lewis Structure

2)Draw individual dipoles

3)Find a resultant dipole for each pair of individual dipoles by vector addition

4)Find the molecular dipole from the resultant dipoles by vector addition

5)Ifthe resultant dipoles do not cancel out, the molecule is polar; otherwise non-polar

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Density
answer

-For any substance, we measure its quantity by measuring its volume or mass. If we know the density of the substance, we can also calculate the volume from the mass data or vice versa

 

Density = Mass/Volume

 

-For a given substance, density changes with temperature. A perfect example would be the different densities exhibited by water vapor, water, and ice.

 

**In the metric system, the densities of solids and liquids are usually expressed as grams per cubic centimeter or grams per cubit milliliter, respectively. The density of gasses is usually stated as grams per Liter

question

Avogadro’s Number
answer

=6.022X1023X1amu = 6.022X1023;X 1.67X10-24g = 1gram

;

-Avogadro’s number is called a mole. The atomic mass in grams is called the Molar Mass, and has the unity g/mol.

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******Mole Equation******
answer

************* 

m = Mn

 

or

 

n= m/M

 

*************

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stoichiometry
answer

-isthe application of MOLE RELATIONSHIPS from balanced reaction equations to calculate, for example, how much of the reactants are required, how much of the products are formed, whatthe yield of the reaction is, etc.

 

***To get # molecules, must first have moles!!

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Percent Yield
answer

-theoretical yield – the highest possible amount to be produced in this reaction

 

actual yield = the actual amount of product gathered in the laboratory

 

Percent Yield = Actual Yield/Theoretical Yield X 100

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The Polarization of charges within a molecule is the key to understanding intermolecular forces!
answer

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hydrogen bond
answer

-the strongest of all intermolecular forces

-though is much weaker than ionic an covalent bonds

-hydrogen bonds ONLY formed between F, O, & N

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Bonds

answer

Intermolecular, covalent, or ionic

 

-all bonds result from electrostatic attractions and repulsions between positive nuclei and negatic electrons

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Dipole-Dipole
answer

Head-to-tail arrangements have a lower energy and therefore, are more common than head-to-head

-There is an overall attraction between the polar molecules

-This is called the dipole-dipole intermolecular force

 

-These forces exist in all polar molecules

-When the dipoles are far apart, however, the attractions and repulsions almost cancel each other

-Dipole-dipole foreces are only significant when the dipoles are close together

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London Forces/Van der Waals interactions
answer

-In non-polar molecules, the negative centers still shift from moment to moment

 

-Thus, at any one instant any single atom or non-polar molecule is momentarily polar, and has an INSTANTANEOUS DIPOLE MOMENT

 

-This is why even non-polar molecules such as N2 attract each other

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Polarizability

answer

 -The easier it is for electrons to move around, the farther away the negative center might be from the positive center, thus the greater the distance between the opposite charges, and the greater the instantaneous dipole moments

 

Polarizability is a measure of the ease with which a molecule creates instant dipoles

 

-In the same way that non-polar molecules polarize instantaneously, atoms also polarize momentarily. Different atoms also have different polarizability

 

**Valence electrons make the greatest contribution to the polarizability of the atom. In the periodic table, when we go doesn any group, the valence electrons locate further and further from the nucleus. Since the core charge is constant in any given group, we expect polarizability to increase as atomic size increases.

 

***Hence, a large molecule with many atoms has larger polarizability than a small molecule

 

-Polarizability increases in a molecule with increasing size and number of atoms

 

*****Greater polarizability means greater London Forces

question

Temperature-
answer

-a measure of the kinetic energy (energy to move) of the molecules in any substance
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Phase Change
answer

-For most substances, the density of their solids is greater than that of their liquids. This is because the molecules usually pack more closely together in the solid form than in the liquid form. However, ice floats in water. This means that the density of ice is less than that of water.

 

-Phase changes are the switching between different physical states of most substances

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Solutions…
answer

-A solution is a homogeneous mixture of two or more substances; these can be all gases, all liquids, all solids, or a mixture of substances in different physical states

 

-Solute- solid substance is put into a liquid (Solvent)

 

-We say that the solute is being dissolves in teh solvent and that the solute is solutble in the solvent.

 

-How well a substance dissolves in water can be measured by its solubility

 

-A solution with relatively small concentration is said to be dilute .

-A solution having a large concentration of solute is said to be concentrated

 

-When the solvent is water, the solution formed is termed an aqueous solution

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miscible
answer

-Liquids that are soluble in each other are said to be MISCIBLE

 

-Liquids that are insoluble in each other are said to be immiscible.

 

-More specifically, substances taht are soluble in water are said to be hydrophilic (like water). Substances taht are insoluble in water are said to be hydrophobic (dislike water)

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solubility
answer

-The maximum amount of a solute that can be dissolved in a given amount of water is called the solubility of the solute in water

 

***The solubility of a solid substance in water usually increases with the increase of temperature

 

 

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saturated
answer

-A solution that contains the maximum amount of a solute at a given temperature is said to be saturated

 

 

 

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recrystallization
answer

-If we make a saturated solution of a solute at a high temperature, then cool the solution down to a much lower temperature, due to the lowered solubility of the solute at the lower temperature, some solute will come out of the solution.

 

-The name ‘recrystallization’ suggests that the substance to be purified exists as crystals before being dissolved at a high temperature and formed crystals again once being pushed out of the solution at a lower temperature

question

Three types of crystals
answer

1)Molecular Solids

2)3-D Network Solids (3-D ionic)

3)Diamonds (3-D covalent)

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molecular solids
answer

-covalent molecules held together by intermolecular forces (ie. ice)

-in the case of ice, the formation of H-bonds is the key factor for its structure

 

-molecular solids melt at very low temperatures because of the relatively weak strength of the intermolecular forces

 

** the most commonly seen molecular solids at room temperature and presure are the iodine (I2), sulfur (S8), and phosphorus (P4) crystals, each having a formula that represents the smallest structural unit in the molecular solid

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3-D Network Solids
answer

-CO2 is non-polar covalent molecule, and the only intermolecular forces that exist among them are London forces. This is why dry ice forms only at temperatures below -78C

 

In these solids, the regular arrangement of ions extends indefinitely in three dimensions and no individual molecules can be recognized. The ions are held together by ionic bonds in a fashion that ensures that each ion is bonded (attracted) to as many ions of opposite charge as possible

 

****Held together by real bonds, not intermolecular forces (molecular solids)

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Diamonds
answer

-also called ‘covalent network solids’

 

-smallest unit is not a molecule, but an atom (carbon)

 

-the four bonds arrange around the central carbon in a perfect tetrahedral arrangement

-Diamon is pure carbon atoms

 

X-Ray Diffractometers are very sophisticated cameras that take photos of arrangements inside crystals

question

Measuring Concentration
answer

The concentration of commercial products is usually indicated by MASS PERCENTAGE or VOLUME PERCENTAGE:

 

Mass/Volume Percentage = mass/volume of solute(g) / (mass/volume of solute + mass/volume of solvent)

 

-a measure of concentration

 

In the lab, the most commonly encountered concentration expression is Molarity

 

Molarity = Moles of Solute / Liters of Solution

 

-units for molarity, thus, are MOLES/LITER

 

question

triple bonds
answer

-strongest and shortest bonds

 

-double bonds are less strong and slightly longer

 

-single bonds are the weakest and longest of the covalent bonds

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exceptions to octet rule
answer

-octet rule only applies to smaller elements in 2nd row

-in higher rows, other orbitals are accessible and more than 8 electrons around atom are possible

 

ie. PCl5 and SF6

 

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Polyatomic Ions
answer

-a group of atoms with a charge

 

-3 special ones:

-NH4+ ammonium

-OH hydroxide

-CN cyanide

 

MUST KNOW SLIDES 19 + 20 IN pp CH 5

question

shape
answer

-shape determines polarity, polarity determines solubility

-repelling electrons determine shape of model

 

to predict shape, must get coordination number

 

TOTAL COORDINATION # = # BONDING ATOMS + # LONE PAIRS

question

Types of Shape
answer

LINEAR

1 or 2 Coord. #s

180o

 

Trigonal Planar

3 Coord. #s

120o

 

Tetrahedral

4 C#

109.5o

 

Trigonal Bipyramidal

5C#

120/90

 

Octahedral

6C#

all 90

question

MEP
answer

-map of electrostatic potential…

-way to visualize distribution of charge in a molecule (rainbow)

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hybrid
answer

unequally weighted combination of all resonance structures
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molecular/empirical formulas
answer

molecular- the actual formula with the amount of atoms

 

empirical – uses smallest units… useful for finding percent composition

question

polarizability
answer

-predicts the magnitude of van der waals interactions

-is the ability of the electrons on an atom to respond to a changing electric field

-atoms with very loosely held electrons are very polarizable

*high polarizability = loosely held electrons

question

relative strength of all bonds
answer

cation/anion (ionic) – strongest

covalent bond

-ion-dipole

-dipole-dipole

van der waals – weakest

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phase changes
answer

-caused by heat and pressure changes

 

-characterized by changes in molecular order..

 -molecules in solid phase have the greatest order

-molecules in gas phase have greatest randomness

-more kinetic energy = more randomness

 

*phase changes occur when enough energy is acquired to overcome intermolecular forces

question

vapor pressure
answer

-when vapor pressure = rate of condensation, dynamic equilibrium is established

 

-vapor pressure is a measure of strength of intermolecular forces (force over a surface)

 

-the stronger the intermolecular force, the lower the vapor pressure

 

-different compounds reach a vapor pressure of 1 atm at different temperatures, aka boiling points

 

****boiling occurs when atmospheric pressure = vapor pressure

 

0the stronger the intermolecularforeces, the lower the vapor pressure, and the higher the boiling point

(ie. more difficult to reach atmospheric pressure because bonds are stronger)

question

Phase Diagrams

 

answer

-Boiling point always corresponds to 1atm

 

-the triple point is where all 3 phases exist simultaneously

 

* all elements have a triple point!

 

-water is strange: at higher pressure, boiling point increases and melting point decreases (V-shape)

 

-most other elements increase in boiling and melting points as pressure incresases

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