Chem Exam 4 - Buffers, Titrations, Enthalpy

True or False: At the equivalence point, the moles of a weak acid = the moles of its conjugate base

False

True or False: If a gas cylinder is compressed with a constant external pressure, wsys is positive.

true

True or False: Energy will be released for a chemical reaction if the sum of the energies of the bonds

formed is greater than the sum of the energies of the bonds broken

true

what combos make a buffer?

WA, CB

WB, CA

partial: WA, SB and WB, SA

you can only use HH when ___

x-approximation is valid

at the equivalence point, moles titrant =

initial moles of analyte

at the half-equivalence point, pH =

pKa

buffer effectiveness ______ as difference (must be 0.10-10) in relative amounts of HA and A- increase

decreases

buffer effectiveness ______ as HA and A- become less concentrated

decreases

strong acids

HCl, HBr, HI, HNO3, HClO4, H2SO4

strong bases

LiOH, KOH, NaOH, RbOH, CsOH, Ca(OH)2, Sr(OH)2, Ba(OH)2

Ka increases as

acid strength increases, pH decreases, pKa decreases

weak conjugate acids and bases have ____ effect on pH

no

SCAs (cations of weak bases) make solutions:

more acidic

If SCA and SCB, the solution will be slightly acidic if

Ka of SCA > Ka SCB

to buffer against SA

high [CB]

to buffer against SB

high [WA]

to buffer against both

same [ ] WA and CB

the pKa of the chosen acid must be within _____ unit(s) from the desired pH

1

@ the equivalence point

moles SB titrant = moles SA initially present

indicators must be

intentionally chosen

titrant

the substance you’re adding

analyte

the substance of unknown concentration

what is on the y-axis of a titration curve

pH

what is on the x-axis of the titration curve

volume

when does pKa = pH

at the half-equivalence point

pre-equivalence point

use ICE table

post-equivalence point

calculate [OH-] or [H+]

at the equivalence point

if titrant is a strong base: use Kb

if titrant is a strong acid: use Ka

First Law of Thermodynamics

the total energy of the universe is constant

open system

allows the exchange of both energy and matter with its surroundings (open thermos)

closed system

allows the exchange of energy but not matter with its surroundings (closed thermos)

isolated system

allows the exchange of neither matter nor energy with its surroundings (perfectly insulated thermos)

Internal Energy (E) 

equals heat + work or kinetic + potential of all particles that make up the system

E is a ______ function

state

delta E negative

exothermic

delta E positive

endothermic

+w

work is done on the system

-w

work is done on surroundings

expansion (volume of system increases)

-w

compression (volume of system decreases)

+w

specific heat capacity

the amount of heat required to change the temperature of 1g of substance by 1 degree Celsius

qsolution =

-q rxn

feels warm

exothermic

feels cold

endothermic

Hess’ Law

if a chemical rxn can be written as a series of steps, then enthalpy of reaction for the overall reaction is equal to the sum of heats of reaction for each step

bond enthalpy

the energy change associated with the breaking of one mole of a particular bond in a gas phase

bond enthalpies is the _______ accurate method to determine enthalpy of reaction

least

only the bond energies of ________ can be precisely measured

diatomic molecules/homonuclear molecules

naught

standard states and standard conditions (1 atm, 25 degrees Celsius)

standard state solids

pure substances, I2, Cgraphite

standard state gases

noble gases, H2, O2, F2, N2, Cl2

standard state solutions/ions

concentrations of 1.0 M

standard state liquids

Br2, Hg

enthalpy of reaction

enthalpy of products - enthalpy of reactants

enthalpy of formation of an element in a standard state

0

standard enthalpy of reaction 

the enthalpy change for a process in which all species are in their standard states

standard enthalpy of formation of compounds

the change in enthalpy when 1.0 mol of compound forms from its constituent elements in their standard states