CHEM111 CHAPTER 8-12 Flashcards
Thermodynamics
Study of energy and temperature changes
Potential energy
Stored energy that matter possesses due to its position, condition and/or composition (cohesive force)
Kinetic energy
Energy of motion (disruptive force)
What happens when a substance absorbs heat energy
The particles within the substance vibrate and move faster
Equation that represents energy, heat and work
ΔE=q+w. Triangle E represents change in energy, q is heat, and w is work
Exothermic change
Releases heat energy (-q)
Endothermic change
Absorbs energy (+q)
Law of conservation of energy
Also called the first law of thermodynamics that states energy cannot be created or destroyed (ΔEsystem)=-(ΔEsurroundings)
Specific heat
Relationship between heat, mass, and temperature. It is the amount of heat required to raise the temperature of one gram of material by one degree Celsius. s=q/mΔT
Coffee cup calorimetry
When water is placed in an insulated cup covered with a cork lid and a thermometer. The system to be studied is placed inside the coffee cup and the reaction or change is allowed to take place
Bomb calorimetry
Measures the heats of reactions for fuels and other high energy chemical changes. T substance is placed in a heavy steal container and outside of the container is a second container filled with water and equipped with a thermometer
Extensive property
Energy absorbed or released in a reaction
Fuel value
The amount of heat energy that can be released by a combustion reaction of a certain substance (expressed in terms of energy per unit gram)
Reaction enthalpy
Amount of heat energy that is absorbed or released in a chemical reaction at constant pressure represented by triangle Hrxn
Physical state depends on (4):
- Chemical identity of the matter (what it is)
- Temperature of the matter
- Pressure the matter is subjected to
- The intermolecular forces that hold it together
Q
Representation of total energy released or absorbed by a system
Specific heat capacity
When heat capacity is given per gram of substance
Molar heat capacity
When heat capacity is given per mole of substance
Sublimination
When a material goes from solid to gas without going through the liquid state (endothermic - energy required to break intermolecular bonds)
Deposition
When a material goes from gas to solid without going through the liquid state (exothermic - energy released as intermolecular bonds form)
Ionic bonds
Attraction between oppositely charged ions
Covalent bonds
Attraction between two nonmetals and results when two atoms share electrons
Octet rule
States atoms are stabilized by 8 electrons in their valence level
Expanding octet
A bonding arrangement in which an atom has 10 or 12 valence electrons. This is ONLY possible with elements in rows 3-7 on periodic table
Single covalent bond
Two atoms share one pair of valence electrons
Double covalent bond
Two atoms share two pairs of valence electrons
Triple covalent bond
Two atoms share three pairs of electrons
Coordinate covalent bond
Both electrons of a shared pair originate from the same atom
Strength of bond
The more electron pairs being shared, the greater the strength of bond. increase as it goes single -> double -> triple
What element is never the central atom
Hydrogen because only two valence electrons. Or Fluorine
Localized electron model
Assumes all electrons are in pairs thus it does not handle odd-electron molecules very easily
Steps to drawing Lewis Structures (4)
- Add up all the valence electrons
- Frame the structure using single bonds (typically the atom that is nearer to the lower left of the periodic table is the central atom)
- Fill octets of the outer atom first and add lone pairs
- Fill the octet on the central atom
Formal charges
the charge that would reside on the atom if all of the bonding electrons were shared equally.
Resonance structures
Set of Lewis structures that show how electrons are distributed around a molecule or ion. Resonance structures are used when a single Lewis structure cannot adequately depict the structure (for 2nd or 3rd covalent bonds, and lone pairs)
Valence shell electron pair repulsion (VSEPR)
Model based on the idea that electrons repel each other, and therefore they occupy regions that are as far away from each other as possible
Electronic geometry
Arrangement of electrons around a central atom
Molecular geometry
Arrangement of atoms within a molecule
Two VSEPR electron pairs
linear
Three VSEPR electron pairs
Trigonal planar (120 deg) - Bent
Four VSEPR electron pairs
Tetrahedral (109.5 deg) - Trigonal pyramidal
Five VSEPR electron pairs
Trigonal Bipyramidal
Six VSEPR electron pairs
Octahedral
What shape is a water molecule
Tetrahedral
Electronegativity
Measure of the ability of that atom to attract shared electrons towards itself and is affected by atom size, nuclear charge, and the number of non valence electrons
Pauling scale
Numerical scale of electronegativities based on bond-energy calculations for different elements joined by covalent bonds
Pure covalent bond
Bond between two identical atoms or atoms of the same electronegativity (equal sharing of electron)
Polar covalent bond
Covalent bond between atoms of intermediate difference in electronegativity (unequal sharing of electrons)
Greater difference in electronegativity = ____
The greater the polarity of the bond
Bond polarity
Measure of the inequality in sharing of electrons in a bond
Dipole
An overall polarity in which different sides of a molecule have slight positive and negative charges
Which element is the most electronegative
Fluorine. It has the strongest pull for electrons
Which elements are the least electronegative
Cesium and francium. It has the weakest hold on electrons
Bond summary (3):
- Covalent: Difference in electronegativity less than 0.5
- Polar covalent: Difference between 0.5 and 2.0
- Ionic: Difference greater than 2.0
Types of intermolecular forces (3):
- Hydrogen bonding
- Dipole-dipole forces (van der waals forces)
- London dispersion forces (van der waals forces)
Dipole-dipole forces
Attractive forces between the positive end of one polar molecule and the negative end of another polar molecule. Can interact with each other electrostatically (positive end of one molecule lines up with negative end)
Hydrogen bonding
Strong dipole-dipole forces when H is attached to a highly electronegative elements such as N, O, or F (very polar bond)
London dispersion forces
Results from the motion of electrons that creates temporary dipoles in molecules.
Strength of intermolecular forces least to greatest
London dispersion –> dipole-dipole –> hydrogen bonding
Key pointers of hydrogen bonding (2):
- Requires donor and acceptors (donor is H bound to an N, O, F, and acceptor is a lone pair on an N, O, F)
- Intermolecular hydrogen bonding also seen in alcohols (organic molecule containing O-H group) and amines (organic molecule containing N-H group)
Intermolecular forces
Usually [weak] forces of attraction or repulsion between individual molecules