Avogadro Concept



Molecular Modeling With Avogadro

  1. Avogadro Quimica Concepto
  2. Avogadro's Law Concept
  3. Avogadro Mole Concept
  4. Ley De Avogadro Concepto
  5. Numero De Avogadro Concepto De Mol

The Avogadro number (or constant) has been defined in many different ways through its long history. Its approximate value was first determined, indirectly, by Josef Loschmidt in 1865. (Avogadro's number is closely related to the Loschmidt constant, and the two concepts are sometimes confused.). Amedeo Avogadro is best known for his hypothesis that equal volumes of different gases contain an equal number of molecules, provided they are at the same temperature and pressure. His hypothesis was rejected by other scientists. It is now called Avogadro's law. Secondly, when was the mole discovered? Molecular Modeling With Avogadro. This is a simple and fun, hands-on activity that demonstrates the basic concepts of computer-based molecular modeling. Using molecular mechanics, small molecules may be modeled by treating the atoms as balls and the bonds connected them as springs.

Introduction

This is a simple and fun, hands-on activity that demonstrates the basic concepts of computer-based molecular modeling. Using molecular mechanics, small molecules may be modeled by treating the atoms as balls and the bonds connected them as springs. We can then use a set of equations, which we refer to as a force field to place these atoms in the correct geometrical arrangement and calculate a relative energy that we can use to compare different conformations of these molecules.

The basic components of a typical molecular mechanics force field include:

Erelative = Ecovalent interactions + Enon-covalent interactions

The covalent and non-covalent interactions can further be broken down into the following components:

Ecovalent interactions = Ebond-stretching + Eangle-bending + Etorsion

Enon-covalent interactions = Eelectrostatic + Evan der Waal's

The equations used for the covalent interactions are Hook's law (spring harmonics). Equations used for the electrostatic interactions is Coulomb's law, and the equations used for the van der Waal's component is the 6-12 Lennard-Jones potential.

Materials

  • Windows, Macintosh or Linux Notebook PC>
  • Avogadro Molecular Modeling Software (available free of charge here)
  • 3x5 Index Cards
  • Sharpies or colored markers

Procedure

  1. Place one to three notebook computers on a table and have Avogadro already running as participants approach. It may also be advisable that the Desktop space is clear of files as well, in the event that Avogadro is closed or accidentally crashes during the dmeo and must be restarted.
  2. Decide ahead of time a small collection of potential models for participants to construct, and draw their names and chemical structures on the 3x5 index cards. The best molecules to build in this project are small organic molecules.

    Examples of good molecules to use for this demonstration include:

    • Water (H2O)
    • Acetylene (C2H2)
    • Carbon Dioxide (CO2)
    • Carbonic Acid (H2CO3)
    • Ethane
    • Ethanol (CH3CH2OH)
    • Ethylene
    • Glucose or Fructose (C6H12O6)
    • Methane
    • Propane
    • Ribose
    • 20 Amino Acids
    • Palmitic Acid
    • alpha-Linoleic Acid
  3. Allow participants a few minutes to play with the software and familiarize themselves with building small molecules. You may want to explain that the best way to build molecules is to start with their carbon chain, and build everything as if all atoms are carbons first. Then, modify specific atoms in functional groups to oxygen or nitrogen or whatever is necessary. Hydrogens do not need to be explicitly added (the software adds those automatically).
  4. Once a molecule is built on the screen, the molecule can be optimized to its correct geometry through a process called energy minimization. This process applies the force field and moves the atoms to their optimum positions based on the lowest energy conformation (you are minimizing the energy to its lowest point – hint: draw them an energy diagram as an example).

    Go to the EXTENSIONS > MOLECULAR MECHANICS menu option and select SETUP FORCE FIELD. Use the MMFF94 force field, change the number of steps to 2,000, change the algorithm to CONJUGATE GRADIENT, and the convergence criterion to 1 x 10-7, and press OK. Run the procedure by going to EXTENSION > OPTIMIZE GEOMETRY and the molecule should appear in its lowest energy conformation.

    If it does not appear in its lowest energy conformation, that means that the computer put the molecule into a LOCAL ENERGY MINIMA, as opposed to the GLOBAL ENERGY MINIMA. Molecular mechanics energy minimization can only lower energy, it can not increase energy – so you are stuck in a local energy minima. To get out of this, you need to physically move some atoms and re-minimize to recalculate the new energy.

  5. A simple experiment that can be done to illustrate the concept of minimization and local vs. global energy minima, is to have them build cyclohexane. This is easy to build as they just draw six carbon atoms and connect them all in a ring. Have them rotate this molecule around a bit to observe that the atoms are more or less arranged flat (not correct).

    Apply the energy minimization procedure, as specified above, to optimize the geometry. Most of the time, the computer will obtain the correct, 'chair', conformation of cyclohexane. Occasionally, some computers may minimize to the, 'boat', or, 'twist-boat', conformation – they are stuck in a local minima.

    See if the participant can convert between the boat and chair conformations of cyclohexane by moving atoms around and re-minimizing.

Safety

There are no safety issues with this demonstration.

Disposal of Waste Products

There are no waste disposal issues with this demonstration.


Screen shot of the chair conformation of cyclohexane, energy-minimized with Avogadro.

Mole and Avogadro’s Number

The chemical changes observed in any reaction involve the rearrangement of billions of atoms. It is impractical to try to count or visualize all these atoms, but scientists need some way to refer to the entire quantity. They also need a way to compare these numbers and relate them to the weights of the substances, which they can measure and observe. The solution is the concept of the mole, which is very important in quantitative chemistry.

Avogadro’s Number

Amadeo Avogadro first proposed that the volume of a gas at a given pressure and temperature is proportional to the number of atoms or molecules, regardless of the type of gas. Although he did not determine the exact proportion, he is credited for the idea.

A mole is a quantity that contains 6.02 *1023 atoms, molecules and ions.

Avogadro’s number is the number of particles in a mole 6.02 *1023

The mole (or mol) represents a certain number of objects. The amount of a substance that contains the same number of entities as there are atoms in 12 g of carbon-12. One mole of H2O molecules contains 6.022 x 1023 molecules. • 1 mole contains 6.022 x 1023 entities (Avogadro’s number). One mole of NaCl contains 6.022 x 1023 NaCl formula units.

Avogadro Quimica Concepto

Exactly 12 g of carbon-12 contains 6.022 x 10 23 atoms.

*** There is a new definition of mole available at C & EN article

The meaning and usefulness of the mole: number of moles of elements in a compound can also be determined from the chemical formula. For example,

the chemical formula of water is H2O. Therefor in 1 mole of H2O contains 2 mols of H and 1 mol of O from the subscript of the chemical formula.

Mol and number of particles can be easily calculated by using the conversion factor:

1mol
6.022 x 1023 or vice versa

Examples:
How many atoms are present in 3.0 mols of Ag?

Ans: Since it is mols to atoms, Avogadro’s number is the conversion factor.

3.0 mols of Ag * 6.022 x 1023 atoms of Ag = 18.067 * 1023 atoms or 1.8 *1024atoms Ag

1 mol Ag

  1. How many molecules are present in 2.93 mols of H2O?

Since it is mols to molecules, Avogadro’s number is the conversion factor.

Ans: 2.93 mols of H2O * 6.022 x 1023 molecules of H2O = 18.067 * 1023 atoms or 1.8 *1024

1 mol H2O

  1. How many mols of Hydrogen are in 1.56 mols of CH4?

Since it is mols from a compound to mols of an element, we will use chemical formula.

1.56 mols of CH4 * 4 mols H = 6.24 mols of H

Avogadro's Law Concept

  • mol CH4
    4) How many hydrogen atoms are there in 2.50 mol of glucose (C6H12O6)?
  • We will use both chemical formula and avogadro’s number to solve this problem.
    2.50 mol C6H12O6 *12 mols H * 6.022 x 1023 atoms of H =180.66 x 1023 or 1.81 x 1023 atoms
    1 mol C6H12O6 1 mol H
Avogadro Concept

Watch the following video:

Questions:

  1. How many atoms are present in 5.5 mol of Fe?

Avogadro Mole Concept

  1. How many mols of each element present in 2 .00 mols of the following compound?

C9H8O4 (aspirin)

3. How many oxygen atoms are present in 5.20 mol of Al2(SO4)3?
  1. How many molecules are present in 7.29 mols of the compound below?

5. 11* 1024 molecules are present in a glass of water. How many mols of H2O are present?

Ley De Avogadro Concepto

Ans: 1. 3.31*1024 atoms

Avogadro Concept

2.18 mols of C, 16 mols H, 8.0 mols O

3. 3. 77 *1025 atoms of oxygen

Numero De Avogadro Concepto De Mol

4. 4.39*1024 molecules

5. 5.17 mols

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