Chemistry Chapter 1
Thomson's Experiments Showed
"Particles" emerged from one disc (the cathode) and moved to the other (the anode). These particles could be deflected by electrical fields in a direction that would indicate they were negatively charged. The particles could also be deflected by magnetic fields. The particles carried the electrical charge - that is if the ray was bent, for example by a magnetic field, the charge went with it. The metal that the cathode was made of did not affect the behavior of the ray - so whatever the composition of the ray - it appeared to be independent of the element that it came from.
Dalton's Atomic Theory
1. Elements are composed of small indivisible, indestructible particles called atoms. 2. All atoms of an element are identical and have the same mass and properties. 3. Atoms of a given element are different from atoms of other elements 4. Compounds are formed by combinations of atoms of two or more elements. 5. Chemical reactions are due to the rearrangements of atoms, atoms (matter) are neither created nor destroyed during a reaction.
Avogadro's Number
6.02 x 10^23.
What is the difference between an element and a compound?
A compound contains atoms of different elements chemically combined together in a fixed ratio. An element is a pure chemical substance made of same type of atom.
What is a Force?
A push or a pull (in chemistry an "interaction").
How big do you think an atom (H) is?
About 0.1 nanometer (0.1 x 10-9 m).
Elements
Atom is smallest unit of an element. Elements are ordered on the periodic table. 91 (or 98) elements are naturally occurring - number is arguable and is not a detail we are interested in.
Rutherford's Experiment Showed
Atom was mostly empty space. Small dense positive nucleus in the center of the atom.
How do you know atoms exist? What is the evidence?
Atomic force microscopy (AFM) and scanning tunneling microscopy (STM) can provide images at the atomic level.
Thomson's Plum Pudding Atom Model
Atoms contain electrons "embedded" in the atom.
What distinguishes one element from another? How do the atoms of different elements differ?
Atoms of elements differ depending on the number of protons in the nucleus and electrons in the electron cloud.
What types of bonds and/or interactions are represent for H2 in the solid phase?
Both covalent bonds and London dispersion forces.
London Dispersion Forces
Caused by fluctuations of electron density in the molecule (or atom). Adjacent molecule - gets induced dipole. Present between ALL molecules (neutral species). We use δ- or δ+ to mean a partial negative or positive charge. Increase with the size of the particle (number of electrons). Increase with the surface area. Part of a range of intermolecular forces (between particles).
Which has atoms in it?
Cells, Air, and Gold.
Gold Foil Experiment
Conducted by Ernest Rutherford in which alpha particles that were shot at gold foil were deflected when they hit the positive center of gold atoms. The nucleus was discovered as a result of this experiment.
Potential Energy Distance Curve
Constant change between attraction and repulsion. When attraction and repulsive forces are equal, PE is at a minimum (the well).
What interactions and/or bonds are overcome when hydrogen at 30K warms up to 6000K?
Covalent bonds.
Law
Describes a phenomenon (what).
JJ Thomson
Discovered the electron (Nobel Prize 1906).
To calculate limiting reactant
Do not compare masses directly. Calculate the amount of product possible from each reactant - the smallest amount must be from limiting reactant.
What is stronger: gravity or electromagnetic forces?
Electromagnetic.
Potential Energy
Energy associated with the position of a system of objects in a field. You can't have potential energy without a field (gravitational, electric, magnetic). You can't have potential energy without two or more objects (in the field). Sometimes we call this "stored energy" (but this can be a bit misleading). We will use a model where we say the potential energy is stored in the field.
Which statement about the breaking of a single chemical bond is true?
Energy is absorbed. Breaking bonds requires an input of energy (to the system) - this can be by adding thermal energy - increases the kinetic energy of the molecules by collisions with other particles.
Which statement about the forming of a single chemical bond is true?
Energy is released. Forming bonds releases energy (to the system) - this moves energy from the system to the surroundings.
Theory
Explains the phenomenon (why).
What is a limiting reagent?
Finite amount of one of the reagents, which limits how much product can be formed.
Cathode Ray Tube
Glass tube consisting of two electrodes under very low pressure. When a high voltage is applied, cathode emits "particles" that move toward anode.
The 4 fundamental types of forces
Gravity, Electromagnetic, Strong, and Weak.
Diatomic Molecules
H2, N2, F2, O2, I2, Cl2, Br2 (Have No Fear Of Ice Cold Beer).
Neutrons
Harder to detect! Why? Neutral in charge - not manipulated with electric and magnetic fields. Reside in the nucleus. Slightly heavier than protons.
Which do you think would have higher melting and/or boiling points: He or Xe?
Helium has a melting point of 1K, and xenon has a melting point of 161K.
What is the difference between an atom and a molecule?
In a molecule, atoms are bonded together by single, double, or triple bonds. An atom has a nucleus surrounded by electrons.
Which type of interaction is stronger: LDF or covalent bond?
Intermolecular Forces (IMF) include van der Waals interactions (London Dispersion Forces are one example). All IMFs are between atoms or neutral molecules. Bonds are more permanent. They are stronger and harder to break. Bonds occur within a molecule.
Scientific Model
It can be a drawing or a graph or a diagram or an equation, physical or mental. We use models to help us make sense - predict what would happen. They make testable, quantifiable predictions. (We could imagine lots of different models that would help us understand when we add cold liquid to hot).
As the helium atoms approach what happens to the potential energy?
It decreases.
As the helium atoms get very close (overlapping electron clouds) what happens to the kinetic energy?
It decreases.
Is a molecule or compound bigger than an atom?
It depends on the elements of the molecule/compound and atom.
What is an alpha particle?
It has two protons and two neutrons. It is the nucleus of a helium atom.
As the helium atoms approach what happens to the kinetic energy?
It increases.
As the helium atoms get very close (overlapping electron clouds) what happens to the potential energy?
It increases.
Why do you think it takes a temperature of 6000K to break the interaction between two hydrogen atoms - but only 4K to break the interaction between two helium atoms?
It requires a lot more energy to break covalent bonds than LDF.
As the helium atoms approach what happens to the total energy?
It stays the same.
As the helium atoms get very close (overlapping electron clouds) what happens to the total energy?
It stays the same.
What is preventing the He atoms from flying apart? (why do they stick together?)
LDF
Why do the atoms move toward each other?
LDF.
What is the difference between Law and Theory?
Law is the description of the phenomenon while the Theory explains the phenomenon.
The boiling point of helium is 4K. When helium boils, what interactions and/or bonds are overcome?
London dispersion forces.
The boiling point of hydrogen is 20K. When hydrogen boils, what interactions and/or bonds are overcome?
London dispersion forces.
Molecule vs. Compound
Molecule - contains more than 2 atoms Compound - contains more than 2 types of elements All compounds are molecules, but all molecules are not compounds.
Systems
Part of the universe you are looking at. Can monitor energy changes between system and surroundings. Energy is never lost - it can be transferred or transformed (First Law of Thermodynamics).
Discovery of the Electron
Particle rays bend toward (+) plate. Therefore, they are negatively charged particles. Mass of particles was about ~2,000 times lighter than H atom!
Rutherford's Planetary Model
Protons condensed in nucleus with surrounding electrons.
How big is a mole?
Represents a large quantity. 6. 022 x 10^23 of atoms, moles, molecules, ions, etc.
Gravity
Responsible for attraction between objects that have mass.
Electromagnetic
Responsible for attraction/repulsion between objects that have electric charge.
Isotope
Same number of protons and electrons, but vary in number of neutrons
Ernest Rutherford
So atoms have electrons - but where are they?
Compared to He, the London Dispersion Forces between Xe atoms are expected to be
Stronger because the electron cloud in Xe is "floppier".
What happens when two atoms of He approach each other?
The attractive Coulombic interaction causes the two helium atoms to approach but if they get too close, the electron clouds start to repel each other.
Scientific Theory
The best available explanation of the existing evidence/data/observations. Explains how and/or why. Makes testable predictions. Is falsifiable (ie can be proven false by experiments and data). May change over time as new evidence becomes available.
How do forces relate back to chemistry?
The electromagnetic force is important and we can understand atomic interactions in terms of Coulombic attractions and repulsions.
If He atoms get too close
The electron clouds start to repel each other.
Kinetic Energy
The energy associated with motion
How do the He atoms "know" the temperature is rising?
The energy is transferred from other atoms that have collided with the walls of the container (that were directly heated).
What happens when you add thermal energy (raise the temperature) to the helium atoms? Why?
The kinetic energy of the atoms increases, they move faster, collide and/or vibrate with more energy. Eventually, they vibrate strongly enough (have enough kinetic energy) to overcome the attractive force between the two He atoms and they fly apart.
Atomic Number
The number of protons in the nucleus of an atom of an element.
What is the evidence from Thomson's experiment that demonstrated that ALL atoms contain electrons?
The particles behaved identically regardless of the identity of the cathode (where they were emitted from).
What is the evidence from Thompson's experiment that all atoms contain electrons?
The particles were identical regardless of the identity of the cathode (where they were emitted from).
Diatomic Hydrogen
The two H atoms have formed a NEW chemical species - a hydrogen molecule (H2). It has different chemical and physical properties than hydrogen atoms (H).
As the He atoms approach
There is an attractive interaction.
Why do He atoms oscillate? (go back and forth)
There is not enough energy in the system to break the LDF
What is the relationship between thermal energy and temperature?
Thermal energy is a measure of the average kinetic energy of the atoms. Temperature T is directly related to average kinetic energy (1/2mv2).
If the puck and the charge are opposite charges what happens?
They attract each other.
If the puck and the charge are the same charge what happens?
They repel each other.
Why do we use balanced equations?
They tell us the ratio of how the compound reacts with atoms, ions, molecules, moles, etc.
Ångstrom model
Think of the electron cloud as "fluffy". It can fluctuate momentarily so that the electron density is distorted.
What is the concept of a mole and what is it used for?
Translation between molecular and macroscopic scale.
How many molecules must be present for LDFs to exist?
Two or more
What kinds of energy do you know?
Two types of energy: Kinetic and Potential. All other forms of energy can be related to these two at the molecular level.
Stoichiometry
Use mass/mole conversions to compare and predict amounts of products and/or reactants in reactions.
How would you explain the difference between an atom and an element?
Very different properties.
Energy
We don't know - but definitions often are circular. Any change in matter is accompanied by a change in energy. Changes in energy are caused by changes in forces. Energy is conserved - (of course, this depends on the system you define). Units SI - Joule (kg m2 s-2). 1 calorie = 4.184 J.
Formation of Covalent Bonds
When two H atoms approach - they are attracted much more strongly than two He atoms. They form a covalent bond.
Could this represent a H2 in the solid phase?
Yes.
Could this represent a molecule of H2?
Yes.
What are the units for molar mass?
g/mol. Add up all atomic masses - Formula Mass (amu).
Energy can be
transferred (from one object to another) and transformed (eg from potential to kinetic energy).
Van der Waals radius
½ the distance between atoms at the potential minimum (where the forces of attraction and repulsion are equal).
So what does the atom look like?
Ångström model: Electrically neutral. Very small nucleus (~ 2 x 10-15 m). Nucleus contains protons (+1 charge, 1 amu) and neutrons (0 charge. 1 amu). "Cloud" of electrons - takes up most space (~ 20 - 200 x 10 -12 pm). Electrons (-1 charge, mass ~ 1/3000 amu).