April 18 2011 - Atomic Mass and Isotopes

Heeeeeeeeeeeey everybody, its me the one and only, Wes here talking about atomic mass and isotopes.  WARNING: you are about to hit a big wall of text.   



                              One way scientists measure the size of something is by its mass. Scientists can even measure very, very tiny things like atoms. One measure of the size of an atom is its "atomic mass". Almost all of the mass of an atom (more than 99%) is in its nucles, so "atomic mass" is pretty much a measure of the size of the nucleus of an atom.  The nucleus of an atom is made up of protons and neutrons. Protons and neutrons are almost exactly the same size. If you add up the number of protons and neutrons in the nucleus of an atom, you get that atom's atomic mass. A simple hydrogen atom has just one proton and zero neutrons. Its atomic mass is 1. The most common kind of carbon atom has 6 neutrons and 6 protons. It has an atomic mass of 12.  All atoms of a certain element have the same number of protons. Oxygen atoms always have 8 protons; carbon atoms all have 6 protons. Most atoms come in different types called isotopes. Isotopes have different numbers of neutrons. The most common isotope of carbon has 6 neutrons and 6 protons. Its atomic mass is 12. A rare,radioactive isotope of carbon has 8 neutrons. Its atomic mass is 14 ( = 6 protons + 8 neutrons).  In chemistry, the number of protons in an atom is more important than the number of neutrons. Scientists call the number of protons the "atomic number". Normal atoms have the same number of electrons as protons. The number of electrons is the main thing that controls how atoms behave in chemical reactions. Scientists use the letter "Z" to stand for atomic number and the letter "A" to stand for atomic mass.

Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes. For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons. If you want to refer to a certain isotope, you write it like this: ^AX_Z. Here X is the chemical symbol for the element, Z is the atomic number, and A is the number of neutrons and protons combined, called the mass number. For instance, ordinary hydrogen is written ¹H₁, deuterium is²H₁, and tritium is ³H₁. Now elements can only have a certain number of isotopes.  there are "preferred" combinations of neutrons and protons, at which the forces holding nuclei together seem to balance best. Light elements tend to have about as many neutrons as protons; heavy elements apparently need more neutrons than protons in order to stick together. Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they're unstable


VIDEO TIME!!!!!

April 14 2011

Sup Wes here putting mah name in all the blogs I do from now on.  Learned some atomic theory today, some pretty interesting stuff, I suppose....  So in chemistry, atomic theory is a theory of the nature of matter, which states that matter is composed of discrete units called atoms, as opposed to the obsolete notion that matter could be divided into any arbitrarily small quantity. It began as a philosophical concept in ancient Greece (Democritus) and India and entered the scientific mainstream in the early 19th century when discoveries in the field of chemistry showed that matter did indeed behave as if it were made up of particles.


I made a little something, something to help explain the different atomic theories.

Scientist and date	Name of model/Idea of theory	Importance/Improvement from last model
Democritus    300 BC	Atom the indivisible particle	Talks about the atom as the smallest particle of matter. Defines the atom as an indivisible particle Explains certain natural occurrences such as the existence of elements
Dalton   1800	The solid sphere model Atoms are seen as solid, indestructible spheres	Explains a lot of chemical properties such as how atoms combine to form molecules Explains chemical change better than the Particle Theory Confirms the basic Laws of Chemistry: Conservation of Mass & definite Proportions
J.J. Thomson  1850	The raisin bun Model Atoms are solid spheres made-up of a solid positive mass (or core) with tiny negative particles embedded in the positive core.l	Infers on the existence of electrons and protons Introduces the concept of the nucleus Infers on the relative nuclear density and atom mass of different atoms
Rutherford  1905	The Planetary Model Gold leaf experiment proves that the nucleus is positive and the electrons are   outside the nucleus.	First real modern view of the atom Explains why the electron spins around the nucleus (Bohr's Contribution)
(Neils Bohr) Bohr- Rutherford  1920	Electrons in Definite energy Levels around  the nucleus Used atomic spectra to prove that electrons are placed in definite orbitals (called shells) around the nucleus.	Explains the role of valence electrons in bonding Relegates the number of valence electrons to the Periods of a periodic table Fully explains ionic and covalent bonding Places electrons in definite energy levels 2 e- in the first 8 e- in the second 8 e- in the third

^ That to easy for you? Then try this out, harder, more complicated , no way your gonna understand this (not really) explanation.

April 4 2011 - Percent Yield and Percent Purity

The percent yield, which serves to measure the effectiveness of a synthetic procedure, is calculated by dividing the actual yield of the product (in grams) by the theoretical yield (in grams).

Percent Yield   =	Actual Yield of Product (in grams) x 100%
	Theoretical (maximum possible) Yield of Product

Lithium chlorate decomposes upon heating to form lithium chloride and oxygen gas according to the equation:

2 LiClO3 ---> 2 LiCl + 3O2

A) Determine the mass of oxygen gas that could be produced by the decomposition of 24.8 g of lithium chlorate.

24.8 g LiClO3 x ( 1 mole LiClO3 / 89 g/mole ) x ( 3 moles O2 / 2 moles LiClO3) X ( 32 g/mole /1 mole O2)=13.2 g

B) In a student experiment, 4.00 g of the gas was produced in a flask containing 24.8 g of the reactant. Determine the percent yield of the student's reaction.

% yield = actual yield / theoretical yield x 100
= 4 g O2 / theoretical yield [ANSWER in A] x100
= 4/13.2 x 100=30.3%

Percent purity is the ratio of the mass of pure substance to the mass of impure substance. 

Percent Purity =	Mass of the Pure Product
	Mass of the Impure Product Obtained

                                                                                                        


Suppose you have 0.500 g of a substance you believe is pure and you determine that it contains 0.300 g of the substance you are looking for.  So your sample is (0.300/0.500) x 100% pure = 60%


                                                        

Stiochiometry

Stoichio, in greek, pretains to elements.
-Works with quantative analysis of chemical reactions and is about measuring the amounts of elements & compounds involved in a reaction. This is part of the study of reactions.

1 AgNO₃ + 1 KCl ‑‑‑‑> 1 AgCl  + 1 KNO₃

_{In stiochiometry, this balanced equation can also show the mole ratio of 1:1:1:1}

* Note that the equations all must be balance. Otherwise, the mole ratio will be totally wrong.

-The coefficient is of the balanced equations is equaled to the number of moles reacted or produced.

-What you need over what you have

eg. 4NH3 + 5O2 ------> 6H2O + 4NO

# of moles of H2O = ? ---------------> 2.00 moles O2 (6 mol H2O/5 mol O2) = 2.40 moles of H2O

Feb 21 - Enthalpy Calculations

H : Energy change in the reaction. Units - kJ/mole

Eg.  CH₄ + 2O₂  -->  CO₂ + 2H₂O  + 812kJ

H can be expressed as:

-812kJ/1mole CH₄ or -812kJ/2mole O₂ = -406kJ/1mole O₂

-812kJ/1mole CO₂ or -812kJ/2mole H₂O = -406kJ/1mole H₂O

Ex. calculate the energy released when 0.35 moles of  H₂O is produced

0.35 moles H₂O * (-406kJ/1mole H₂O) = -142.1kJ

                             2Sig Fig                    = -140kJ

Ex. How many moles of CH₄ are needed to produce 2100kJ of energy?

2100kJ * (1mole CH₄/-812kJ) = -2.586

                                                = -2.6

                                                is  2.6 since mole cannot be negative

Link to Enthalpy Calculation : http://www.saskschools.ca/curr_content/chem30/modules/module3/lesson3/lesson3.htm

Feb16, Exothermic and Endothermic

Exothermic- a reaction where it realeases energy to the surroundings
Endothermic- a reaction where it absorbs energy from the surroundings


-Molecules held together by chemical bonds
          -Add energy to break bonds
          -Give off energy to join together

Endothermic- reaction takes more energy to break bonds than it gives off to form bonds

Exothermic - reaction takes less energy to break bonds than it gives off to form bonds

Enthalpy which has a symbol H and a unit joules, is the heat contained in the system

Energy Diagrams

-Chart the potential energy of the chemicals as they change from reactants to products

-Reactants start with certain amount of energy. Energy is added to start the reaction and then nergy is released as reaction proceeds

-The relative amounts of energy determine if a reaction is endothermic or exothermic

Endothermic-





Exothermic-

1. Energy of reactans = total potential energy of all reactants in the reaction

2. Energy of products = total potential energy of all products in the reaction

3. Energy of the activated complex = potential energy of the "transition state" between reactants and products

4. Activation energy = the energy that must be added to get the reaction to progess (reactants to activated complex)

5. ΔH (change in enthalpy) = the change in potential energy during the reaction. It is the energy of products

 - energy of reactants

Energy in Equation

-Exothermic reactions have energy term on right hand side and a negative ΔH

Eg.  CH₄ + 2O₂  -->  CO₂ + 2H₂O  + 812kJ

                                                           (ΔH)

Endothermic reactions have the energy term on the left hand side and positive ΔH

Videos that's helpful and entertaining

Links that might help : http://www.emsb.qc.ca/laurenhill/science/exo.html

February 4 - Lab 5B

So today we did lab 5b, which had to do with chemical reactions.

The 4 main chemical reactions that were observed in this lab were synthesis, decomposition, single replacement, and double replacement.

The purpose of the lab was to :

1. observe a variety of chemical reactions
2. interpret and explain observations with balanced chemical equations
3. to classify each reaction as one of the four main types

So in this lab, we did seven different experiments, and each one would be classified as a synthesis, decomposition, single replacement, or double replacement reaction.  

Reaction 1: burning a copper wire.

Reaction 2: putting an iron nail in copper (II) sulfate solution.

Reaction 3: heating up copper (II) sulfate pentahydrate in a test tube.

Reaction 4: add water to product of reaction 3.

Reaction 5: combine calcium chloride with sodium carbonate.

Reaction 6: put mossy zinc into hydrochoric acid solution.

Reaction 7: mix hydrogen peroxide solution with manganese (IV) oxide.  Then put glowing splint of wood into mouth of test tube.