March 31,11 Polar Molecules

Polar Molecules:-Polar molecules have an overall charge separation
-Unsymmetrical molecules are usually polar
-Molecular dipoles are the result of unequal sharing of electrons in a molecule

Predicting Polarity:
-If a molecule is symmetrical the pull of e is usually balanced
-Molecules can be unsymmetrical in two ways
1)different atoms
2)different number of atoms

http://www.youtube.com/watch?v=LKAjTE7B2x0

March 28, 2011 - Bonding, Bonds and Electronegativity (Angelo)

Types of Bonds:
- there are three main types of bonds
     1. Ionic (metal  -  non-metal)
          - electrons (will now be abbreviated to e.) are transferred from metal to non-metal
     2. Covalent (non-metal  -  non-metal)
          - e. are shared between non-metals
     3. Metallic (metal)
          - holds pure metals together by electrostatic attraction

Electronegativity:
- electronegativity (will now be abbreviated to "en.") is a measurement of an atom's attraction for e. in a bond (NOTE: there are NO UNITS for electronegativity)
     - Fluorine = 4.0
     - Chlorine = 3.0
     - Cesium = 0.8
- atoms with greater en. attract e. more
- Polar Covalent bonds form an unequal sharing of e.
- Non-Polar Covalent bonds form from equal sharing

Bonds:
- the type of bond formed can be produced by looking at the difference in en. of elements
     - if (en. > 1.7), it is an Ionic bond
     - if (en. < 1.7), it is a Polar Covalent bond
     - if (en. = 0), it is a Non-Polar Covalent bond


EXAMPLES:


1. Predict the type of bond formed.

                         F  -  F

Identify the electronegativity of each element: The electronegativity of Fluorine is 3.98.
Find the difference by forming and solving an equation: 3.98 - 3.98 = 0.00
State what type of bond was formed: Because the electronegavitity of this bond is 0.00, we may conclude that this is a Non-Polar Covalent bond.

2. Predict the type of bond formed.

                         Mg  -  S


Identify the electronegativity of each element: The electronegativity of Magnesium is 1.31, and the electronegativity of Sulphur is 2.58.
Find the difference by forming and solving an equation: 1.31 - 2.58 = -1.27 = 1.27 (NOTE: find and use the absolute value)
State what type of bond was formed: Because the electronegativity of this bond is 1.27, we may conclude that this is a Polar Covalent bond

3. Predict what time of bond was formed. Then identify the positive and negative sides of the following bond.

                         Ca  -  Se


Identify the electronegativity of each element: The electronegativity of Calcium is 1, and the electronegativity of Selenium is 2.55.
Find the difference by forming and solving an equation: 1 - 2.55 = -1.55 = 1.55
State what type of bond was formed: Because the electronegativity of this bond is 1.55, we may conclude that this is a Polar Covalent bond.
State which side is negative, and which side is positive: The electronegativity of Selenium is greater than the electronegativity of Calcium. From this, we may conclude that Selenium is the negative side and Calcium is the positive side because the electron will be attracted to Selenium.
Represent this with a diagram: δ + Ca  -  Se - δ

Acid-Base Reactions (brian)

strong acids & strong bases:
-strong acids(SA) dissociate to produce H+ ions
-stong bases(SB) dissociate to produce OH- ions
-when a SA and SB mix they form water and an ionic salt
-total volume changes
NaOH + HCl > NaCl + H2O

pH & pOH:
-pH is a measure of the hydrogen ions present in a solution
pH = -log[H+]
-pOH is a measure of hydroxide ions present in a solution
pOH = -log[OH-]

 

 

Ion Concentrations (brian)

Dissociation:
-ionic compounds are made up of 2 parts
 a) cation - positively charged particle
 b) anion - negatively charged particle
-when ionic compounds are dissolved in water the cation and anion separate from each other
-this process is called dissociation
-when writing dissociation equation the atoms and charges must balance
-the dissociation of sodium chloride is
NaCl > Na+ + Cl-
-if the volume does not change then the concentration of individual ions depends on the balanced coefficients in the dissociation equation

Example: BaSO4 > Ba2+ + SO4 2-
Example: 0.250M solution of KOH
               KOH >     K+      +  OH-
             0.250M  0.250M  0.250M

Dilutions (brian)

Diluting solutions:-when two solutions are mixed the concentration changes
-dilutions is the process of decreasing the concentration by adding a solvent (usally water)
-the amount of solute does not change
n1=n2-because concentration is mol/L we can write C=n1Vand
n=CV
so C1V1=C2V2

Example: determine the concentration when 200ml of 0.20M HCL is diluted to a final volume of 400ml
               (0.20mol/l)(0.200L)=C2(0.400L)
               C2=0.1M

Titrations (Zac)

-a titration is an experimental technique used to determine the concentration of an unknown solution.
Terms & Equipment
-buret - contains the known solution. used to measure how much is added
-stop cock- valve used to control the flow of solution from the buret
-pipet - used to accurately measure the volume of unknown solution
-erlenmeyer flask - container for unknown solution
-indicator - used to identify the endpoint of the titration
-stock solution - known solution
Eg. Zac completed a titration .330 M NaOH with 15.00 mL samples of HI of unknown concentration.  The data he gathered is below.  Determine the concentration of HI.

Trial                                1                     2                     3                      4
Final Reading (mL)        11.9                22.8                34.2                43.1
Initial reading (mL)          .7                  11.9                22.8                34.2 
Volume Used (mL)        11.2               10.9                11.4                 8.9*
*insignificant due to inaccuracy
NaOH + HI = NaI + HOH

Solutions & Molarity (Zac)

-Solutions are homogeneous mixtures composed of a solute and a solvent
-solute is the chemical present in the lesser amount (whatever is dissolved)
-solvent is the hemical present in the greater amount (whatever does the dissolving)
-Chemicals dissolved in water are aqueous
eg. NaCl (aq)
-Concentration can be expressed in many different ways (g/L, mL/L, % by volume, % by mass, mol/L)
-the most common way is mol/L which is also called Molarity
-mol/L = M
-[HCl] = concentration of HCl
Eg. Determine the concentration of 16.0 g of C6H12O6 in 0.750 L of water

16.0g x 1mol/180g x 1/.75L = .119 M