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41 Cards in this Set

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  • Back
Meltin points vs. Polarity
As the polarity increases, the melting points will increase.
Separate compounds by general polarity and mp: alkanes, alkenes, and aromatic compounds
alkanes - least polar - lowest mp
alkene - middle polar and mp
aromatic - highest mp
Symmetry and branching do what to mp? Why?
- Symmetry raises MP and branching lowers them.
- The more symmetrical a molecule is, the better it packs in the crystal.
(T/F) Forces that stabilize a moclecule more in the solid than in the liquid state will cause a molecule to have a higher melting point.
What do you do for melting point comparisons?
1. separate the compounds by general polarity.
2. examine each grouping for trends in polarity and or molecular symmetry.
What do you do for boiling point comparisons?
1. Look for unusually heavy molecules.
2. Look for highly polar functional groups.
3. Look for the effect of dispersion forces.
4. Look for trends in teh symmetry of molecules.
(T/F) Moelcular weight is one of the key determinants of bp.
True, something extraordinarily heavy is going to be harder to boil than something that is lighter.
How does polarity of functional groups relate to bp?
- Higher the polarity, the higher the bp b/c they increase the attractions of molecules for each other.
- Alcohols are capable of hydrogen bonding.
What is the strongest type of intermolecular attraction?
Hydrogen bonding.
(T/F) Dispersion forces are the only kind of intermolecular attractions that cause nonpolar molecules to stick together.
True. (If the molecule is longer, therefor it'll have more opportunities to track other molecues)
How does symmetry relate to bp?
- A more symmetrical molecule will be more compact and thus have less effective surface area. THerefore it has less opportunities for it to engage in dispersion-type attractions with other molecules, making it a lower bp compound.
- Less symetrical ==> higher BP
What are the two factors that affect relativing bps btw substances?
molecular weight and intermolecular forces.
In terms of bps, what is the most important factor?
Rate Law from Experimental Results strategy
1. Write down the general form of the rate law.
2. determine the order of the reaction wrt each reactant (start with the ones where the concentrations are constant for all but one of the reagents).
3. White down the rate law with the correct orders.
What does the rate constant k depend on?
Rate Law from Reaction Mechanisms
1. Identify the slow step in the reaction and write down the rate law expression for taht step.
Exothermic system
delta(H)<0 (?); therefore add more heat will get more of the reactants b/c you have to think of heat as a product.
Liquids and solids are: compressible or incompressible?
incompressible; so altering the pressure at which the reaction occurs should have no effect.
Ionic species: what do you have to watch out for in terms of Le Chatelier's Principle?
Ionic species can dissociate in water depending on their electrolytic strength and may result in multiples of the original concentration of solid.
Gas: what do you have to watch out for in terms of Le Chatelier's Principle?
an increase in pressure favors teh side of the reaction with fewer molecules of gas
Determining the rate law from reaction mechanisms
1. identify the slow step in the reaction and write down the rate law expression for that step.
2. If intermediates exist in the rate law from step 1, use prior steps to solve for their concentration and eliminate them from the rate law.
3. Combine constants and simplify the rate law.
In reaction mechanisms, what is the goal?
to eliminate the concentraiton of intermediates b/c they are usually high-energy species that exist only briefly.
When can you use the rate law?
for rate limiting steps and steps of the mechanisms that are in equilibrium.
Thermodynamic stability of the product depends on what?
On the energy of the product; NOT the activation energy.
What determines the rate of formation?
the difference in energy between the starting material and the top of the hump leading to each product. (activation energy)
Is a catalyst consumed during a reaction?
No, it speeds up a reaction (it lowers the hump).
delta(G) = delta(H) - T * delta(S)
-mneumonic: Good Honey Tastes Sweet
At equilibrium, what does delta(G) equal?
What are the three equations for delta(G)?
delta(G) = delta(H) - T delta(S) | units: kJ/mol
delta(G)_knot = -R T ln K_eq | units: kJ/mol
delta(G) = delta(G)_knot + R T ln Q | units: kJ/mol
At equilibrium, what does Q equal to?
What is the reaction quotient equation based on the following reaction:
2NO (g) + Cl2 (g) => 2NOCl (g)
Q = [P_NOCl]^2 / ( [P_NO]^2 [P_Cl2] )
What should you be weary of when writing the reaction quotient equation?
Only aqueous species and gases should be considered in the equation.
- When dealing with gases, consider the concentration.
- When dealing with gases, focus on their partial pressures.
For combustion reactions, the carbon source has the structure _______
Bond dissociation energy
energy required to break a particular type of bond in one mole of gaseous molecules.
Hess's Law
delta(H_rxn) = delta(H_b(reactants)) - delta(H_b(products)) = total energy input - total energy released
Signs for bonds broken and bond formation based on bond dissocation energy
When bonds are broken, the enthalpy is the bond dissocation energy.

When bonds are formed, its the negative of the bond dissociation energy.
Heat of formation
heat absorbed or released during the formation of a pure substance from the elements at a constant pressure.
What does a negative/positive heat of formation entail?
- negative: heat is released to form the product.
- positive: heat is required to from the product.
Heat of formation Hess's law?
General equation for combustion.
____ + O2 => CO2 + H2O
Enthalpy is a state function, what does this mean?
regardless of the path you take to get from the reactants to the products, the change in enthalpy will be the same.