Category Archives: Chemistry

Avogadro’s Number

Amedeo Avogadro was one of the most brilliant scientists of the 19th century.   He was born in Italy on August 9, 1776 and died on his 80th birthday on August 9, 1856. Avogadro studied the behavior of gasses and realized that the properties of a gas such as volume and pressure at a known temperature depended on the number of gas particles, not the type of gas molecule.
avogadro_amedeo

Avogadro proposed that the weight of a sample of gas at a known volume, pressure and temperature was related to the weight of the individual molecules of the gas.

 

 

 

In 1909, a French Physicist named Jean Perrin, proposed that a scaling factor could be found relating the weight of 1 molecule of a gas to its volume and suggested this scaling factor be named after his predecessor, Avogadro.  jean_perrin_1926

The value of this scaling factor could not be measured untill the 1950’s.

In the 1950’s, methods became available which allowed scientists to obtain extremely pure samples of Carbon-12.  Using X-ray diffraction, exact measurements were made of the spacing of Carbon atoms in Graphite.

Artwork of the crystal structure of graphite showing parallel layers of hexagonally arranged carbon atoms. Graphite as used in pencil lead, lubricants, and structural components that need strength and lightness.
Artwork of the crystal structure of graphite showing parallel layers of hexagonally arranged carbon atoms. Graphite as used in pencil lead, lubricants, and structural components that need strength and lightness.

 

 

 

graphite-x-ray-diffractionartists rendering of graphite crystal

 

 

 

 

 

X-Ray Diffraction pattern from single crystal graphite. The arrangement of the bright spots shows the hexagonal structure of the graphite crystal. The spacing of the bright spots can be related back to the  spacing between individual atoms.

In the 1990’s new techniques such scanning-tunneling microscopy were developed which allow us to directly image the atoms on the surface of a crystal.scanning-tunneling-graphite

 

Notice the hexagonal arrangement of the Carbon atoms in graphite.

 

 

Avogadro’s Number:

Knowing that Carbon 12 has 6 protons and 6 neutrons for a total of 12 Atomic Mass Units, scientists set about to make an ultra pure sample of Carbon-12, they measured out exactly 12.000…. grams and grew a single crystal of graphite.  By measuring the volume of the crystal and knowing the exact spacing of the carbon atoms (from x-ray diffraction), they were able to determine the exact number of Carbon atoms in 12 grams of graphite.  This number was calculated to be:

N = 6.022 x 10^23 and was the first accurate measurement of Avogadro’s number.

The Mole:

Avogadro’s number turned out to be a very useful number. The name mole (or mol) is used to denote the 1 molecular weight of a substance.   The names mol and mole are used interchangeably.

For example, 1 mol of Carbon contains 6.022 x 10^23 atoms of Carbon and weighs 12 grams.  1 mol of water weighs 18 grams and contains 6.022 x 10^23  molecules of H2O.

If you know the weight of a sample and its chemical composition, you can find the number of moles in a sample.
Simply total up the elements in a compound, multiply each by their atomic weight to get the molecular weight of the compound.
Next divide the weight of the sample by the molecular weight of the molecule.
example: 18 grams of H2O is 1 mole
example  36 grams of H2O is 2 moles
(notice the relation between MOLEcular weight and the unit MOLE)

Find the weight of 1 mole of each of these chemicals.
Hint: Total up the molecular weights of each component.
H2
He
12C
H2O
Al2O3

Find the weight of 2 moles of each of these chemicals.
Hint: Total up the molecular weights of each component, multiply by 2.
H2
He
12C
H2O
Al2O3

 

Balancing Chemical Reactions – Intro

Reading quiz Prentice Hall Chemistry pg 320-328

For 5 points:
Given the equation __A + __B –> __C + __D
1] Which letters represent the reactants
2] Which letters represent the products
3] Define coefficients
4] for a reaction where G, and H are reactants and L and M are products, write a skeleton equation
5] write the balanced chemical equation for Hydrogen + Oxygen yields Water

Diatomic Gas Molecules

LO: Understand the structure of a diatomic molecule.

Vocabulary: Di (Latin for 2) + atomic -> 2 atoms A molecule containing two atoms of the same element or species.

Diatomic-molecule-picture

Where are diatomic molecules found on the periodic table?

800px-Diatomic_molecules_periodic_table.svg

Some diatomic molecules have single bonds (shared electron pairs), others have two or three….

diatomic molecules

 

Here’s an easy way to remember which atoms form diatomic molecules

mnemonicdevicediatomicmolecules

Chemistry: Representative elements worksheet

Assignment: Identify one element from each of the following categories. Identify each element’s position in the S-P periodic table and write in the elements’ symbol (example: H for Hydrogen).

pdf copy of this worksheet:  Representative element wksht

Symbol / Element Name Categories:
Alkali Metal
Alkali Earth metal
Metal
Semi-metal (metalloid)
Non-metal
Halogen
Noble gas

s-p_table

Watch the element film at http://ed.ted.com/periodic-videos . Next, fill in the table using information from the video and from http://webelements.com/ . More information is available at: http://education.jlab.org/itselemental/

Chemistry Unit 1 Test

Chemistry Unit 1Test

Chemistry Unit 1 Test

This test is the Unit 1 Common Assessment

Topics covered:

Models of the atom (Bohr, Rutherford)

Electronic structure of the atom, atomic emission spectra (flame test spectroscopy / gas discharge tube spectra)

The nucleus: neutrons, protons

Isotopes

Nuclear Chemistry: Why nuclei decay, decay modes – alpha, beta, gamma, half lives

Radioactivity: Half Lives

Definition: The Half Life of a radio active material is defined as te length of time it takes for one half the sample to undergo radioactive decay.This means that after one half life only half the origonal radioactive isotope remains. The other half of the material has decayed into other isotopes or other elements (depending on the type of decay).

The half life of radioactive isotopes is determined experimentally using a Geiger Counter.

Principle of operation of a Geiger Counter

Chemistry Unit 1 Practice Quiz

Study this quiz open book, open notes.
Please explore these questions thoroughly until you understand them fully!

Chemistry Unit 1 Practice Test

Chemistry Unit 1 Practice Test

This practice test is preparation for the Unit 1 Common Assessment

Topics covered:

Models of the atom (Bohr, Rutherford)

Electronic structure of the atom, atomic emission spectra (flame test spectroscopy / gas discharge tube spectra)

The nucleus: neutrons, protons

Isotopes

Nuclear Chemistry: Why nuclei decay, decay modes – alpha, beta, gamma, half lives

Nuclear Processes

Most of the heavy elements in the universe today originated from the explosive burning of super giant stars.

 

This graphic shows how nuclear material is burned to form heavy elements in a red-giant star such as Betelgeuse near the end of its life.

Betelgeuse-in-Orion

The process of a star exploding is called a super nova.  Elements heavier than Iron appear to have been produced by the explosion of super giant stars early in the life of the universe.

supernova-fusion

With each generation of exploding stars and formation of new stars, the “metalicity” or metal content of the stars increase. The Horsehead nebula is a region in the Milky Way where new stars are being formed from the remnants of exploded stars.Horsehead_Nebula

This nuclear burning process results in many elements which have an imbalance of nucleons ie too many neutrons. These unstable elements decay into more stable states forming all the other elements in the periodic table.

In this lesson, we’ll study nuclear decay and the 3 major kinds of nuclear radiation.

________________________________________________________________

To start with the He nucleus is the most stable of all nuclear building blocks. He has 2 protons and 2 neutrons.

When an unstable nucleus decays by losing a He nucleus it is called Alpha Decay. Alpha decay lowers the atomic number of the parent nucleus by 2 (protons) and lowers the atomic mass by 4 amu (2 protons + 2 neutrons).

Another way that a nucleus can get rid of excess neutrons when a neutron loses a high energy electron and transforms into a proton. This process is called Beta Decay.

The third method of nuclear decay happens when a high energy photon or light particle is emitted. This is called Gamma decay. In Gamma decay, the nucleus loses a large amount of energy. This usually happens when a nucleus splits into two or more parts such as during Alpha Decay.

 

Emission Spectroscopy Lab

Warm up:  Do you think  the emission spectrum of Hydrogen is made of a continuous rainbow of colors or a set of individual colored spectral lines?  Why?

Hint: review your notes and your worksheet on the Hydrogen electronic transmissions, especially the Balmer Series.

These pictures show the difference between continuous, emission and absorption spectra.

Em-Abs-spectra

Why is the absorption spectrum the opposite (compliment) of the emission spectrum?

Study these specra before looking at the spectra from the gas discharge tubes.

 

EmissionSpectra-Hg-H-He

In the space below, draw the emission lines for H, He and Hg using colored pencils.

Hydrogen ________________________________________________________

 

Helium ____________________________________________________________

 

Mercury __________________________________________________________

 

Now look at the emission lines of the gas discharge tubes and put  Czech marks on your drawing  above each spectral line you were able to see.

 

 

Hydrogen Emission Spectra

LO: Understand how Hydrogen’s electron energy levels result in the Hydrogen emission spectra.

As we studied yesterday, the Bohr model describes the energy levels of an atom as a series of shells with the the inner shell being at the lpwest or ground state energy.

Here’s a graphic of Hydrogen’s energy levels, the electronic transitions between levels, the Lyman, Balmer, and Paschen series.

(thanks to: “Hydrogen transitions” by A_hidrogen_szinkepei.jpg: User:Szdoriderivative work: OrangeDog (talk • contribs) – A_hidrogen_szinkepei.jpg. Licensed under CC BY 2.5 via Commons – https://commons.wikimedia.org/wiki/File:Hydrogen_transitions.svg#/media/File:Hydrogen_transitions.svg )

Hydrogen-Emission-Lines

Make a table with 3 columns.

Column 1: write the wavelengths of the Balmer series transitions.

Column 2: For each, write the transition from n’  to n.

Column 3: For each transition, use the wavelength to identify the color of the emission line. (hint, reference the visible spectrum in your book or use the following spectrum;

visible-s

Q: Which emission line has the most energy, which has the least energy.

Periodic Table: Orbitals

Learning Objective: to understand the order in which electrons fill orbitals and how the orbitals are arranged in the periodic table.

In your notebooks, sketch the 1s, 2s and 2p orbitals.

Now sketch this graphic.

orbital-blocks

Now with a ruler, draw in columns for Groups I – VIII. Use the periodic table on the wall or in your book for reference.

There are some real mysteries here. Notice that the 3d orbitals are actually part of row 4 of the periodic table. The reason for this is that the orbitals fill in a very special order.

Copy this chart to your notes:

electron_orbital_filling_chart1308104179633

Conclusion: The 1s fills first. Then the 2s, then the 2p then the 3s, then the 3p. Note the4s fills before the 3d, 4p and 5s.
Next step: How may electrons can an s orbital hold? _____________  Why? ________
How many electrons can a set of p orbital hold? _______________   Why?________

Why do orbitals fill in such a weird order?
The answer has to do with the specific energies of the orbitals – and YES – there is definitely some overlap.

The principle that orbitals fill up according to their energy is called the “Aufbrau Principle”. In fact the lowest energy orbital (1S) is always the first to fill.

Here’s a plot of the orbits and their energy levels….

 

aufbaudgrm

Notice that the 4S orbital has a lower energy than the 3 d orbital – this means that the 4s orbital will fill with electrons before the 3d orbitals start to fill (K, Ca).

The electron configuration of an element is simply a list of which orbitals have electrons in them and how many electrons are in each orbital.

Examples: Hydrogen 1S1

Carbon  1S2, 2S2, 2P2

Class Project:
1] Now write a list of the first 20 elements by symbol (name).
2] For each element, draw a pyramid diagram to figure out the order in which the orbitals fill.
3]Write down the electron configuration for each of these 20 elements.
and write the electron configuration for each element.

Conclusion: You can now write the electron configuration for any element.

Spectroscopy

Spectroscopy is the study of materials based on the wavelength of their reflected or emitted light.

In chemistry, flame spectroscopy is used to identify the elements in an unknown material sample by heating the sample in a flame and observing the color light emitted.  visible-s

Here’s a video that explains how the flame spectrometry lab works…..

Lab: for each sample,  approximate the wavelength of emitted light by finding the corresponding color on this chart. Notice that the wavelengths are given in nanometers.

What is a nanometer?

Then calculate the energy of the emitted light using Plank’s formula;

E = hv = hc/Lambda

where h is Plank’s constant 1.67 x 10^-19 Joules and c is the speed of light 3.0 x 10^8 m/s.

Or use hc = 12,400 eV Angstroms.

Next find the energy levels of each sample element on the resource table and identify which electronic transition is accounting for most of the emitted light.