CEM 141 week 2
CEM 141 week 2 Cem 141
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This 6 page Class Notes was uploaded by Leah DiCiesare on Saturday September 17, 2016. The Class Notes belongs to Cem 141 at Michigan State University taught by J. Hu in Fall 2016. Since its upload, it has received 48 views. For similar materials see General Chemistry in Chemistry at Michigan State University.
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Date Created: 09/17/16
Lecture Notes: 9/129/16 o Electromagnetic Force is mediated by electric and magnetic fields These forces act at a distance Electromagnetic force is much stronger than gravity and can be attractive or repulsive Electrostatic force between charged particles; special case of electromagnetic force Like charges repel; unlike charges attract q1q2 Coulomb's Law: Fα 2 q1 nd q2 are the charges, r is the distance r between the two particles Gravitational forces are two small to be detected at atomic/molecular level Ex. Surface of ball is electron clouds so the edge is negatively charge; when two surfaces come in contact they repel each other and there is a tiny space in between If something is not moving there is either no forces, or the forces are equal and opposite Can sit on chair because gravity=electromagnetic forces at that point o Forces influencing energy If you want to change energy state of a substance, you have to apply a force Don't know exact nature of energy Conservation of energy cannot create or destroy energy (in a closed system the energy levels stay the same) Energy is typically defined as "the potential to do work" (not accurate) doing work requires forces, applying forces requires energy we get a circular definition 3 statements are true: Any change in matter is accompanied by a change in energy Changes in energy are caused by changes in forces Energy is conserved (depends on system defines) 2 2 Unit of energy J (kgm /s ); 1 calorie = 4.184 J o Systems Needed to study energy System: part of universe you are looking at Surroundings: everything else in universe Can monitor energy changes between system and surroundings Energy is never lost or created (transferred or transformed) Two types of fundamental energy: kinetic and potential All other forms can be related to these two at atomic/molecular level KE = 1/2mv associated with motion Potential energy: energy associated with the position of a system of objects in a field Lecture Notes: 9/129/16 Can't have without a field Need 2 or more objects Not correct to say "the moon has potential energy" Potential energy is associated with the system of the moon and the earth interacting through a gravitational field Sometimes called "stored energy" (misleading) Potential energy is stored in the field not in the object(s) Energy can be transferred (from one object to another) or transformed (one type of energy to another) Connects to chemistry atoms are made up of charged particles; understanding relations between atoms Make molecules, make solids from liquids, make gases into liquids London Dispersion Force o Instantaneous uneven distribution of the atom o o δ = partial charge o Electrons of 2nd atom are attracted to 1st atom o 2nd dipole is induced o Caused by fluctuations of electron cloud in molecule or atom o Adjacent molecules or atoms gets induced dipole o Present between all molecules or atoms (neutral species) o Attractive force o Energy when atoms approach Total energy: no change overtime because it is an isolated system, no exchange with surroundings Kinetic energy: increases first, then decreases Potential energy: decreases first, then increases Why: unlike charges attract, like charges repel When they get to overlapping, the force determining potential energy becomes the repelling force; moving toward each other was attractive force No stable atomatom interaction because there's too much energy enter 3rd atom which takes some of the energy Lecture Notes: 9/129/16 o Why do the atoms oscillate?: Why don't they stop? And why don't they move further apart? This is an isolated system no energy exchanged with surroundings Transformation between KE and PE can last forever There is enough energy to make the atoms go back to their starting positions in each cycle Removing energy could stop them: third atom colliding with twoatom system, the third atom then moves faster (KE transfer), total energy of twoatom system decreases; amplitude of oscillation is smaller (atom interaction is stabilized). Energy is transferred to a third atom through collision. In order to form a stable atom interaction we need to remove energy from the system There is not enough energy to make them move further apart. Adding energy would make them move further apart Increasing total energy makes atom system unstable A convenient way to add or remove energy is to change the temperature of the system heating (surroundings provide energy) or cooling (surrounding absorb energy) Increase temperatures: the kinetic energy of atoms increases and they move faster, collide or vibrate with more energy Enentually they vibrate stongly enough to overcome London Dispersion attractive forces, so they fly apart Temperature is directly related to average kinetic energy not all the atoms at a given temperature have the same KE Thermal energy is a measure of the average kinetic energy of the atoms Textbook Notes: 1.51.7 1.5 Evidence for Atoms o 1st scientific theory on atomic structure by John Dalton Published 1805 Elements are composed of small, indivisible, indestructible particles call atoms All atoms of an element are identical and have the same mass and properties Atoms of a given element are different from atoms of other elements Compounds are formed by combinations of atoms of two or more elements Chemical reactions are due to the rearrangements of atoms, and atoms (matter) are neither created not destroyed during a reaction 1.6 The Divisible Atom o Amedeo Avogardo proposed that under conditions of equal temperature and equal pressure, equal volumes of gases contained equal numbers of particles (molecules), and that the densities were proportional to the weight of the individual molecules o Josef Loschmidt in 1865 combined that with the assumption that atoms and molecules move as elastic objects (billiard balls) enabled him to calculate the force a molecule would exert when travelling at a particular speed o Probing the Substructure of Atoms Luigi Galvani in the late 1700s discovered that animals can produce and respond to electricity; assumed that electrical activity was involved in normal animal movements Alessandro Volta developed first modern battery alternated sheets of 2 different metals with discs soaked in salt water produced a steady electrical current J.J. Thompson used cathode ray tubes (glass tubes with wires embedded in them and connected to metal discs) Rays emerged from one disc (cathode) and moved to the other (anode) "cathode" rays were deflected by electrical fields in a direction that indicated that they were negatively charged Rays could also be deflected by magnetic fields Rays carried electrical charge Metal that the cathode was made of did not affect the behavior of the ray Conclusion: these particles must come from within the atoms of the cathode Discovery of electron Plum pudding model: electrons embedded in a positively charged area 1908 Ernest Rutherford Textbook Notes: 1.51.7 Examined alpha particles (positively charged; only the nucleus of a Helium atom) behavior as they were fired at a thin sheet of metal Some particles were deflected rather than going straight through Positively charged particles were being repelled by positive part of the atoms Positive part of atoms is very small and most of the atom space is empty Planetary model: small nucleus with positively charged particles and nearly all atomic mass and it was encircled by electrons Positive particle called proton 1932 James Chadwick identified the neutron, also in the nucleus but without a charge 1.7 Interactions Between Atoms and Molecules o Interactions Between Atoms: A Range of Effects Attraction and repulsion of charged particles is electromagnetic force q1q2 Coulombs Law: Fα r2 1 and 2 are the charges, r is the distance between the two particles Fα m1m2 Gravity: r2 m1 nd m 2are the masses, r is the distance between the two objects Gravity is weaker than electromagnetic Gravity is only attractive, electromagnet is attractive and repulsive Atoms as a whole are neutral but parts of them are charged and the electrons are always moving As electrons move, at any given instants they are more on one side of the atom than the other making is partiaδly ( ) charged; one side is partially negativeδ( ) where the electrons are, the other side is partially positive δ +) Instantaneous dipole As dipole approaches another atom, the second atom's electron distributions is affected and leads to an induced dipole As two atoms approach they are increasingly attracted to each other to a point, at that point they start repelling each other because their likecharged particles are too close together o Force and Energy: an overview Force: interaction between objects that cause a pull (attraction) or a push (repulsion) between those objects An interaction leads to an energy change in the system of interacting objects How do forces influence energy? Any changes in force are accompanied by changes in energy Textbook Notes: 1.51.7 Kinetic energy is energy of motion (KE=1/2mv )2 Potential energy is the energy of position or stored energy Energy is a property of a system not of an object Potential energy depends on the distance between two objects An object in motion can have energy associated with it (KE), but potential energy must always be associated with the system of interacting objects Electromagnetic force Similarities between electromagnetic and gravitational forces Act at a distance Mediated by fields Inversely proportional to the square of the distance between the two objects Differences Electromagnetic is stronger Gravity is always attractive; electromagnetic can be attractive or repulsive Electromagnetic force is determined by charge of particles; affects atom and molecule interactions As charged particles get closer the force is stronger (attractive or repulsive) Equations are good for idealized situations with point charges not with atoms which attract and repel each other o Interacting Atoms: Forces, Energy Conservation and Conversion Total mass and energy of an isolated system is conserved As atoms approach each other they are attracted by London Dispersion forces As kinetic energy increases, potential energy decreases and vice versa Stabilizing interaction reduces the potential energy of the system Destabilizing interaction raises the potential energy of the system
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