Chemistry 11

Last April 19, 2010 we went over two of three forces that occur in compounds.

I. Molecular Polarity and Intermolecular Bonds
- Polarity is the result of intermolecular bonds

1. London Dispursion Force (LDF)

- Experienced by all molecules
- Result of e- pushing on each other
- weakest of all forces
- as the number of electrons increase, LDF increases
- considered as a temporary force

2. Dipole - Dipole

- Dipoles are partial separation of charges
- LDF is a type of temporary charge
- some molecules have a permanent dipole
- These are polar molecules
- Polarity is determined by electron affinity (How much an atom wants electrons)
- Electron affinity is called electron negativity
- Electron Negativity is highest on the top right and lowest on the bottom left of the periodic table
- A bond between two atoms/molecules with different electronegativities results in a dipole-dipole bond
- are weak versions of Ionic bonds

Last March 28, 2010 we started the Unit on Solution Chemistry

-Solution chemistry is the study of reactions that occur in solution
-Solutions are homogeneous mixtures
-Solvents are the part present in greater amounts
-Solutes are the part present in smaller amounts

-A Solute is soluble in a solvent if it dissolves to form a homogeneous mixture
-A Saturated solution contains as much solute as possible
-An unsaturated solution can dissolve more solute
-Solubility is a measure of how much solute can dissolve (g/L, g/mL, mol/L, ppm)
-Solubility of Ba(NO3)2
in water @ 25C = 63g/100mL
in alcohol @ 25C = 1.6g/100mL

March 10, 2010

I. ATOMIC WEIGHT

-on your periodic table, the atomic weight for silver is 107.9. This is because there are more than one isotope of silver and some are lighter than others. And we recall that isotopes have different numbers of neutrons of the same element.

-51.839% has a Mass Number of 107
-48.1161% has a Mass Number of 109
-Atomic Weight is listed on your periodic table.

Each element gives off a specific color of light ( emission spectra)
-unique to each element
If electrons absorb energy they can be bumped to a higher level
When they fall to a lower level they release energy as light

Atomic Structure

Atoms are made up of parts called subatomic particles
-Protons
-Neutrons
-Electrons
Atomic number equals the number of protons

Isotopes

# of protons determine the type of element
changing the number of neutrons changes the isotope of the element
all isotopes have the same chemical properties

Mass Number

mass# is the total of protons and neutrons
Different isotopes have different mass
ex. How many protons and neutrons are in Na?
-Na has 11 protons with mass number of 23
-therefore Na has 12 neutrons and 11 protons

Covalent Bonds:
-Electrons shared between non-metals.
-To draw lewis dot diagrams:
1.)Total all valence e- in all atoms.
2.)Identify the element that can form the most bonds. Thie will be the central atom.
3.)Draw bonds netween atoms as a -------. This represents 2e-.
4.)Any e- not part of a bond are lone pairs around the atom.
5.)Check to make sure each atom has a full octet.

Atoms and Ions:
-Atoms are electrically neutral.
-Number of protons = number of electrons.
-Ions have different number of protons and electrons.
-Ions can be either positive (lost e-) or negative (gained e-)
-Cation = positive ion
-Anion = negative ion

Example 1: Br- : gained 1e-, anion

Chemical Bonds:
-A bond is an electrostatic attraction between particles.
-Bonds occur as elements try to achieve noble gas electron configuration:
1.)Nobel gases (usually) do not form compounds or bonds.
2.)In nobel gases, the outermost energy level have stable octets.
-metals lost elevtrons (oxidize)
-non-metals gain electrons (reduced)

Lewis Dot Structure:
-Atoms can be represented by dot diagrams:
1.)Dots represent electrons.
2.)Only valence electrons are shown.
-Write the atomic symbol for the atom:
1.)This represents the nucleus and filled inner electron levels.
-One dot is used to represent outer energy level electrons:
1.)One e- is placed in each orbital before any pairing occurs.
2.)Beginning with the 5th e-, pairing can occur up to a maximum of 8e-.

Ionic bonds:
-Electrons are transfered from metals to non-metals:
1.)No dots are shown on metal.
-"Charged" species are written in brackets.

Bohr Model:

-atoms are electrically neutral

-there are 2 models that can be sued to describe electron configuration:

1.) energy level model

2.) bohr model

-electrons occupy shells which are divided into orbitals:

- 2e- in the frist orbital

- 8e- in the second orbital

- 8e- in the third orbital

-atomic orbitals each ahve a specific name and shape

FG06_022.GIF.gif

hybridized orbitals:

-the first of the bohr levels is the first orbital and it holds 2e-

-the 2nd contains the 2s, 2px, 2py, 2pz orbitals, they combine (hybridize) to form one 2sp3 orbital

The earliest atomic theories arose from the greeks, Lavoisier, Prous, Dalton, JJ Thompson, Rutherford and Bohr.

Greeks

In 300 BC Democritus said atoms were indivisible particles

This is the first mention of atoms ( atomos )
It was only a conception model. It wasn't testable

There was no mention of any atomic nucleus or its constituents
Can't be used to explain chemical reactions
This theory was the most accepted view for over 2000 years

Lavosier (late 1700s)

Law of conservation of mass
Law of definite proportions
BUT his theory wasn't a true atomic theory because it didn't discuss what atoms were or how they were arranged

Proust (1799)

If a compound is broken down into its constituents, the products exist in the same ration of compound
Experimentally proved lavoisier laws

Dalton (early 1800s)

Atoms are solid, indestructible spheres ( like billiard balls)

Provides for different elements (Different Spheres)
Doesn't mention subatomic particles
Can't explain isotopes
No mention of nucleus
No Valence shells

JJ Thompson (1850s)

Raisin bun model
solid, positive spheres with negativeparticales embedded in them
He was the first to mention the negative and positive charge of electrons and protons
Introduced the idea of a nucleus
No neutrons mentions so radioactive decay can't be explained
Doesn't explain how electrons can exist outside of nucleus
Doesn't explain electrons role in chemical bonding

Rutherford(1905)

Showed that atoms have a positive, dense center with electrons outside
Rusulted in a planetary model
Explains why electrons spin around nucleus
Suggests atoms are mostly empty space
Should be unstable( electrons and protons should attract and destroy atom)
No mention of reactions
doesn't explain valence level electrons role in chemical bonds

Bohr (1920s)

Electrons must only exist in specific orbitals around nucleus
Explains how valence electrons are involved in bonding
Explains diference between ionic and covalent bonding
Resolves problem of atomic instability
Explains atomic emission spectra

Last January 29, we went through Limiting Reactants in class.

I. Limiting Reactants

- usually one reactant is used up before the other
-this reactant is call the limiting reactant
----> it stops or limits the reaction
- the limiting reactant determines the amount of products produced
- Assume one reactant is the limiting reactant and determine what quantity of the other is needed

Here is an example:

1. What is the limiting reactant when 200g of P4 reacts with 323 g of Cl2 forming Phosphorous Trichloride?

a. Determine the theoretical yield of the above reaction

Here is a video on Limiting Reactants

Last January 25, we went through mass to mass conversions.

It's really not that hard, it's just that we have to add another step in the end- which is converting moles to mass again, and that of course we can do by getting the molar mass.

Here is an example:

1. Lead (IV) Nitrate reacts with 3.5g of potassium iodide. How many grams of Lead (IV) Nitrate are required for a complete reaction?

Here is a video as well for more techniques and information about stoichiometry.

Last January 21, we did more examples on stoichemetry.

Here are some examples that we encountered in class.

1. When 2.0g of oxygen reacts with Nitrogen monoxide how many moles of Nitrogen dioxide are produced?

2. What mass of water vapor is produced when 4.0L of Hydrogen is burnt (with oxygen) at STP?

Last January 13, 2010 we discussed Calorimetry in our chem class.

I. Calorimetry
- To know the amount of energy released, three things need to be known:

Temperature Change (Delta T)
Amount of Water (kg)
Specific Heat Capacity ---> 4.10 kJ/ kg degrees Celcius (for water)

Here is the equation that we can use for solving Calorimetry problems.

Now here is an example:

1. Calculate the amount of heat recquired to warm 250.0g of water from 10 degrees to 15 degrees celcius.

II. Molar Enthalpy

- Change in heat for each mole reacted

When a candle (C25H52) is burned, heat is released according to the reaction

If 5.0g of wax is burnt, how much heat is released?

Last January 11, we discussed Heat in Chemical Reactions during our chem class.

This includes exothermic and endothermic reactions.

Exothermic: Reactions that release heat
Endothermic: Reactions that absorb heat

Heat is just a form of energy and all chemicals have stored energy. Stored energy is called Enthalpy.

Enthalpy is a chemical potential energy.

Exothermic Reactions convert enthalpy into heat.

2C8H18 + 25O2 ---> 18H2O + 16CO2 + Heat

Enthalpy is given the symbol H, hence Delta H is change in enthalpy.

Here is an image of how an enthalpy diagram looks like:

Exothermic Diagrams shows high to low enthalpy, change in enthalpy is negative and heat is released. While endothermic diagrams show the opposite of exothermic diagrams.

Endothermic diagrams shows low to high enthalpy, change in enthalpy is positive and heat is absorbed.

II. Change in Enthalpy and Moles

Coefficients can stand for moles or molecules

N2 + 3H ---> 2NH3 + 46.3 kJ

1 mole of N2 ==> 46.3 kJ
3 moles of H2 ==> 46.3 kJ
2 moles of NH3 ==> 46.3 kJ

Conversion Factors

46.3 kJ/1 mol N2 , 46.3 kJ/3 mol H2 , 46.3 kJ/ 2 mol NH3

Now, here are some examples.

1. Fin the amount of heat produced if 6.0 mol of H2 are consumed when making ammonia.

6.0 mol x 46.3 kJ/3 mol of H2 = 92.6 kJ

Mole to mole calculations:
- Coefficients in balanced equations represent the number of moles.
- They can also be conversion factors.

Example 1: When 2.1x10^-3 mol of aluminum hydroxide react with sulphuric acid, how many moles of aluminum sulphate are produced.
Step 1: Write the chemical equation -> Al(OH)3 + H2SO4 -> Al2(SO4)3 + HOH
Step 2: Balance the equation -> 2Al(OH)3 + 3H2SO4 -> Al2(SO4)3 + 6HOH
Step 3: Conversion -> 2.1x10^-3 mol of Al(OH)3 x 1 mol Al2(SO4)3 / 2 mol of Al(OH)3 = 1.0 x 10^-3 mol of Al2(SO4)3

When balancing chemical equations you must make sure that the formulas are correct. If they aren't correct, you'll have a difficult time balancing the equation(usually) and the most obvious, you'll be wrong. . ...

Example
Balance the following equation:
Na + Cl
You need to remember that chloride is one of the diatomic molecules. Therefore the equation turns into: Na + Cl2
The product of this equation will be: Na2Cl
This equals: Na + Cl2 --> Na2Cl
But this isn't balanced! So in order to balance this equation, whatever is on one side, the other side has to have.
For instance there is only 1 sodium in the first part and 2 in the other part. In order to balance this you need to have 2 sodiums in the first part, etc.
So the final equation will look like this:
4Na+ Cl2 --> 2Na2Cl

When balancing alcohols you always balance in this order: C->H->O
Example
CH3OH + O2 --> CO2 + H2O
1 C 1
4 H 2
3 O 3
If you're confused, you can always do the little chart above ^ to show how many of each element there is.
The end result would be:

2CH3OH + 3O2 --> 2CO2 + 4H2O

January 7, 2010.

So, today we learnt about the types of reactions. There are 6:

-Synthesis: A + B -> AB
EX: 2SO2 + O2 -> 2SO3

-Decomposition: AB -> A + B
EX: 2C12H22011 -> 24C + 22SH2 + 11O2

-Single replacement: A + BC -> B + AC
EX: Ca + 2KCl -> 2K + CaCl2

-Double replacement: AB + CD -> AD + CB
(Switch the two non-metals)
EX: Mg + Cl2 + K2SO4 -> MgSO4 + 2KCl

-Neutralization: Acid + base -> water + ionic salt
EX: H2SO4 + 2KO4 -> 2HOH + K2SO4

-Combustion (reaction with oxygen): 2C8H18 + 25O2 -> 16CO2 + 18H2O
EX: 2Mg + O2 -> 2MgO

Watch video here: http://www.youtube.com/watch?v=nVysOW0Lb8U&feature=related

January 5, 2010

Wow, first chemistry class of the new decade- and we already did some stuffs. These stuffs I'm referring to are Word Equations. Yeeey!

Word Equations are just chemical reaction put into words. It's as simple as that.

You just have to keep in mind the 7 diatomic molecules which are H, O, N, Cl, Br, I, F. Plus Phosphorous and Sulfur.

Here is a video that will show you how to balance word equations.

Here are some examples as well.

1. Magnesim reacts with Oxygen to produce Magnesium Oxide.

2. Copper II Sulphate Tetrahydrate decomposes to form water and Copper II Sulphate.

We should also keep in mind some Acids such as the following:

Last Class, Mr. Doktor showed us a couple examples of Chemical Reactions.

We have just finished our unit on Moles, and now we are moving up to the next one which focuses on Chemical Reactions. We really didn't talk about what chemical reactions were, but what we talked about are evidences/characteristics that helps us determine whether a chemical reaction has occured or not.

And these are: Fizzling/Bubbling, Precipate Forms, Change of Color (but not often considered as a chemical reaction), Heat and all others that concerns either the absorption or release of energy.

That's practically it. Oh BTW, I really liked the Magnesium Reaction thing, it really looked like an actual firework. Haha.

We'll upload the video as soon as possible.

Ciao!

When dealing with molar concentrations, you might be asked to give directions. In cases like this, you usually have to find the products of solution and figure out how much mass you have to add to the solution.

if you add water the number of moles in the solution stay the same
if the volume changes the solution will became less concentrated

ex.

Ello, I am Ron Weasley. I was asked to make me a 500 mL solution of MgSO4 with a concentration of 0.98M, bloody hell if I do say so meself. Bugger, what steps should I follow?

Determine the number of moles by dividing the molarity by volume
- V=500mL-->0.5L / o.98mol/L x 0.50L=0.49 mol
After finding the number of moles, determine what the mass of MgS
- Mg=1(24.3), S=1(32.1), O=4(16) }120.4 g/mol
Multiply moles by grams to find the overall mass
- 0.49mol x 120.4g = 58.99 g
After finding the mass, write the follow up steps for making the solution ex:
- weigh 59 g of MgSO4
- measure 500 mL of water
- stir solution

Dilution of Solution

when you add water, the concentration decreases
if volume is doubled then the concentration is halved

volume con'c moles
6.0 L 2.0 mol/L 12.0 mol
12.0 L 1.0 mol/L 12.0 mol

example:

I am Optimus Prime and I added 300 mL of water to 60 mL of 0.90M of HF. What is the final [HF]

In this question, it's best to use: C1V1=C2V2
C1 and V2 are the first concentration and first volume
C2 and V2 are the final concentration and volume
- C1= 0.90/ V1=300mL or 0.3L / C2=unknown/ V2=360 mL or 0.36 L
- (0.90)(0.3)=C2(0.36)/(0.36)
- 0.27/0.36=C2
- C2=0.75 M

Here is a video explaining concentration a little more. ..

Please excuse the examples.. haha

Last December 1, during our chem class- we talked about molar concentration.

A Solution is a homogenous mixture. Solute is the component present in smaller smaller amount while the solvent is the component present in larger amount.

Concentration can be defined as the ratio of the amount of solute over the amount of solvent.

Possible units for concentration are: g/mL, g/L, mg/mL, mL/L and so on. However, we would use mol/L more often.

mol/L is also known as Molarity. Molarity is the ratio of moles of solute per litres of solution.

"con'c " is another way of writing Concentration.
"[ ]" is another way of writing "The Concentration of ..."

Here are some simple equations that we would use in solving for the Molar Concentration of a solution. (You might have to click on the picture to see it better. )

One technique that can help in solving for concentration is that, read the problem carefully and write down what you know. From there on, you can solve as easy as pie! :D

Here are examples on Molar Concentration: (You might have to click on the picture to see it better.)