Chem 3 exam

If a problem says… diffuses, effuses, how far, how fast, and compares two gases

use grahams law u1/u2 = SQR of m2/m1

If a problem says… before - after, one gas changes conditions

use combined gas law (cross out constants) P1V1/n1T1 = P2V2/n2T2

If a problem says… one set of conditions, and asks for moles, mm or grams

use PV=nRT

If a problem says… bond energy table

DH = bonds broken - bonds formed

If a problem says… DH* table

DH= products - reactants

If a problem says… several reactions given with DH values

use Hess’ law - rearange the reactions and then cancel and add them up

If a problem says… mass, DT and a heat capacity

q=mCDT

If a problem says… signs of DH and DS, temperature, and spontaneous language

DG= DH-DAS

5 assumptions of kinetic molecular theory (KMT)

1- particles are in continuous random motion (true)

2- the distance between particles is much larger than the particles themselves (true only at low pressure)

3- particles exert no force on eachother (not true)

4- kinetic energy depends only on kelvin temp (true)

5- collisions are elastic, so no energy is lost (not true)

reaction to always be spontaneous

negative H and positive S

reaction to never be spontaneous

pos H and a neg S

reaction to be spontaneous at low temp

neg H and neg S

reaction to be spontaneous at high temp

pos H and pos S

if the problem says…average velocity, rms, how fast for one gas at a temp

µ_rms = √(3RT/MM), with R = 8.314, MM in kg/mol, and T in K

if the problem says…a gas mixture, total pressure, partial pressure

use daltons law - Ptotal = Pa+Pb..

if a problem says.. gas collected over water, wet gas

subtract water first, use Pgas = Ptotal - PH2O, then use PV=nRT

if a problem says… mole fraction or percent of the mixture

use χ_A = n_A / n_total = P_A / P_total

if a problem says.. at STP

1 mol = 22.4 L (shortcut instead of PV = nRT)

if a problem says.. reaction with all gases, what volume of x and y L or z

Volume ratio = mole ratio (coefficients), at the same T and P

if a problem says… heat, work, expands against or internal energy

ΔE = q + w and w = −PΔV (×101.3 to convert L·atm to J)

if a problem says… calorimeter, temperature of the water rose

use q_rxn = −q_water, then ΔH = q ÷ moles for kJ/mol

if a problem says… S° table, “entropy change”

use ΔS = products − reactants (S° of elements is not zero; units are J)

if a problem says.. crossover temperature, at what temperature does it become spontaneous

Set ΔG = 0, so T = ΔH / ΔS (with ΔS in kJ)

if a problem says… vapor pressure at another temperature, boiling point at ___ torr, and DHvap given

use Clausius–Clapeyron: ln(P₁/P₂) = (ΔH_vap/R)(1/T₂ − 1/T₁), with R = 8.314 and ΔH in J

if a problem says heating or cooling through a phase change (ice - steam)

use Sloped parts: q = mCΔT. Flat parts: q = nΔH_fus or q = nΔH_vap. Add up every section.