Final review/reminders

Fill in the blanks in the table:

 

State	Density	Shape	Volume
Gas
Liquid
Solid

State	Density	Shape	Volume
Gas	Low	Container	Container
Liquid	High	Container	Definite
Solid	High	Definite	Definite

How to know which ion is bigger?

More shells + more protons = bigger ion

(same number of electrons → same number of shells, so look at how many protons)

Electronegativity values for C, N, H, O, F, B

C: 2.5

N: 3.0

H: 2.1

O: 3.5

F: 4.0

B: 2.0

Which state(s) of matter can be compressed?

ONLY gas

Sublimation

Solid → gas

Deposition

Gas → solid

Molecules with stronger intermolecular forces tend to have what kind of boiling points? 

Higher boiling points

Which intermolecular force is stronger?

 

Adenine and Thymine with 2 H-bond donors and acceptors

 

Guanine and Cytosine with 3 H-bond acceptors and donors

 

Both had H-bonds so the forces are equivalent

Guanine and Cytosine with 3 H-bond acceptors and donors

Rank the types of intermolecular forces from weakest (1) to strongest (4)

Hydrogen bonding

 

               3             

Dipole-Dipole

 

               2             

Ion-Dipole

 

               4             

London Dispersion Forces

 

               1             

Hydrogen bond

• an unusually strong dipole-dipole interaction

• intermolecular force

• occurs between an H atom in a polar bond (usually N-H, O-H, F-H), and an electronegative atom (usually O, N, F)

Is vaporization endothermic or exothermic?

Endothermic

Heat of vaporization

Enthalpy change needed to evaporate 1 mol of a liquid at its boiling point

Heat of vaporization equation

ΔH vapor = H vapor — H liquid

Is condensation exothermic or endothermic?

Exothermic

Condensation equation

ΔH vaporization = —ΔH condensation

Evaporation vs condensation

evaporation: liquid → gas

Condensation: gas → liquid

dynamic equilibrium

When the rates of evaporation and condensation are equal → leads to stable vapor pressure

Le Chatelier’s principle

When a system in dynamic equilibrium is disturbed, vapor pressure adjusts to minimize/counteract the disturbance

Boiling point

Temperature where vapor pressure is equal to external pressure (gas above liquid)

Normal boiling point

Temperature when a liquid boils, when the external pressure is 1 atm

Vapor pressure and temperature relationship

Directly proportional

Vapor pressure and intermolecular force relationship

inversely proportional

Critical point

At a certain temperature, the interface between a liquid and a gas disappears creating a supercritical fluid (not quite liquid, not quite gas)

Critical temperature + symbol

Tc

The temperature above which a gas cannot liquify

Critical pressure

Minimum pressure required to liquify a gas at its critical temperature

Heat of fusion

Enthalpy change needed to melt 1 mol of a solid at its freezing point

Heat of fusion equation

ΔH fusion (melting) = H liquid — H solid

Which one is bigger: heat of fusion or heat of vaporization?

Heat of vaporization

Molar heat of sublimation

enthalpy change needed to sublime (solid →gas) 1 mol of a solid

Molar heat of sublimation equation (Hess’s law)

ΔH sublimation = ΔH melting/fusion — ΔH vaporization

Vaporization

liquid → gas (aka boiling)

Pressure and density relationship

directly proportional

Intermolecular force and melting point and lowering point relationship

directly proportional

dipole-dipole strength and boiling point relationship

directly proportional

Is the molar mass of water high or low?

High

 Which conditions must be true for gases to behave ideally?

 

Low pressure and High temperature

You begin with a 3.5L gas sample at 120ºC and 1.4atm. You increase the pressure to 4.7 atm and then cool the sample to 50ºC. What is the final volume of the sample?

Use combined gas law: PV/T = P2V2/T2

Match the following terms to their appropriate units

Molar Mass

Density

Molar Density

Molar Mass

 

               grams/mols             

Density

 

               grams/mL             

Molar Density

 

               mols/L             

Practice

Solve for molar mass

Molar mass = (d)(R)(T)/(P)

combined gas law:

PV/T = P2V2/T2

molar mass equation:

dRT/P

How to find limiting reactant?

Calculate n with ideal gas law. the smaller n is the limiting reactant, and the larger is the excess.

How to solve?

To find how many gases are left:

1. Find the limiting reactant

• Calculate n with ideal gas law. the smaller n is the limiting reactant, and the larger is the excess.

2. Find the excess reactants (the one that is not limiting)

3. the product is also excess

left over: the product and excess reactant (ammonia and nitrogen)

How to solve?

The logic is that because there is not an equal amount of H2 and N2 available, you cannot equally use both of them to make 2NH3.

You need to find the limiting reactant, then set up a ratio using ONLY that reactant. The reason is because there is an excess reactant, you don’t need to worry about whether it will run out or not.

1. The limiting reactant is H2 (Calculate n with ideal gas law. the smaller n is the limiting reactant, and the larger is the excess.)

2. The excess is N2

3. 3 mol H2/2 mol NH3 = 0.07 mol H2/?mol NH3 → then you solve it (you don’t include N2 in the ratio bc it is excess, so you don’t need to worry about it running out)

How to find mol fraction?

mol of molecule A/total mols

How to find partial pressure?

(n of substance A)(RT)/V

Solve

The logic of this problem is that because H2 is a limiting reactant, its mol fraction AFTER the reaction is 0 because it ran out. therefore, you compare the mol fraction of ammonia to nitrogen.

Solve

The logic of this problem is that equilibrium means the gases will mix, so you add the volumes together. Also, you will need to calculate a new number of mols for the excess reactant bc it is getting ‘eaten up”, bc when the gases mix the number of mols changes bc the reaction is taking place.

you do this by: 0.07 H2 (from calculation) /3 H2 (from equation) = 0.0245 mol N2 used

So, 0.22-0.0245 mol = 0.1962 mol N2 (the ACTUAL mol value you use)

kinetic molecular theory of gas

molecules have finite mass but no volume

molecules are neither attracted to nor repulsed by others

which conditions increase gas molecules attraction to each other?

• low temp

• high mass

• high intermolecular forces

Van der waals equation

image

What is equal to 1J?

0.239 calories, 1kgm²/s²

Is density intensive or extensive?

Intensive

Heat

A way of exchanging energy that leads to a change in temperature

Thermal energy

The energy associated with temperature

System does work ON surroundings →

Surroundings does work ON systems →

Answer

A 102g piece of silver with heat capacity 0.235 J/gºC absorbs 56.7 J of heat, if its initial temperature was 5.0ºC what is the temperature after absorbing this heat?

Reminder: if everything is the same unit, you don’t have to convert anything to liters or Kelvin (keep this in mind for specific heat capacity problems)

Answer: around 7.36

What is the sign for work in the following situations?

a. A balloon expands positive

b. Gas in a rigid container (constant volume) is warmed negative

c. A weight is placed on the top of a cylinder causing the volume to decrease  negative

Trick: look at the equation sheet for work

“constant volume” = means the volume is 0

a → negative

b → zero

c → positive

Bomb calorimetry volume

0

Coffee cup calorimetry pressure

0

When work is small…

heat, enthalpy, and internal energy are around equal

Draw O2 lewis structure

O2 = 2(6) = 12 valence electrons

O-O

• Count formal charge: Valence electrons—(number of lone dots+number of lines) → do this for each individual atom, make sure it’s 0. and add the charges together

• Make sure it meets the octet rule: for each atom, count all the electrons (but this time, each line is equal to 2 electrons, not 1 electron)

  • O-O doesn’t meet this, so add one more line

• O double bond O → correct

How to Determine Enthalpies of Reaction from Bond Energies

1. Draw the CORRECT lewis structures

2. Count the number of types of bonds in the reactants and add all of the energies together

3. Count the number of types of bonds in the products and add all of the energies together

4. Energy of reactants—energy of products = final answer

IF GIVEN A TABLE THAT SHOWS REACTIONS:

(look at OChem tutor YouTube video)

How to know if a compound has a standard enthalpy of zero?

If the element is in its natural state

ex) Pb → lead, so natural state is solid, so if' it’s in solid form it’s standard enthalpy of zero

Combustion reactions

O2 is reactant, CO2 and H2O are products

all are exothermic

molecules that O2 reacts with are called the fuel

How to calculate standard enthalpy of formation

Sum of products — sum of reactants

(if a compound is in its most stable state, then you write it as 0)

Endo vs exo

• Endothermic reactions absorb heat energy from their surroundings, causing a decrease in temperature

• exothermic reactions release energy into their surroundings, causing a temperature increase

Endo vs exo: enthalpy

Endo: positive enthalpy

exo: negative enthalpy

how to rank compounds based on vapor pressure

weaker IMFs result in higher vapor pressure, while stronger IMFs mean lower vapor pressure

Label each process as endothermic or exothermic

Vaporization:

Deposition:

Fusion:

Condensation:

Melting:

Sublimation:

Vaporization: Endothermic

Deposition: Exothermic

Fusion: Endothermic

Condensation: Exothermic

Melting: Endothermic

Sublimation: Endothermic

Sublimation

solid → gas

Deposition

gas → solid

Draw a phase diagram

Image