Last Minute Cram - Physical Flashcards
equilibrium is…
- a reaction where the rate of the forward reaction is equal to the rate of the reverse.
- where concentration of the reactants and products are constant.
factors which affect equilibrium
temperature
pressure
concentration
factors not affecting equilibrium
catalyst
what are stoichiometric coefficients?
molar ratios
to determine equilibrium composition of a mixture, you need…
equilibrium amount of only one of the species
what is the equilibrium constant (K)?
a value used to characterise the composition of the reaction mixture.
there are several forms of the equilibrium constant which all vary with what?
temperature
Kc
the equilibrium values are expressed as concentration of mol/L
Kp
the equilibrium values are expressed as partial pressures (used for reactions involving gases)
equilibrium law
expression of the amounts of products and reactants in an equilibrium.
Kc = [products]^p / [reactants]^r
at a constant temperature
what is the value of Kc affected by?
a change in temperature
what is the value of Kc not affected by?
- a change in concentration of reactants or products
- a change in pressure
- adding a catalyst
heterogeneous reaction
where the reactants are in different states.
homogeneous reaction
where the reactants are in the same state.
what do you not include in the equilibrium expression?
any term for a solid because any pure solid is given a constant of 1.
if using partial pressures, what is different about the equilibrium expression?
Big ‘P’ instead of square brakcets
will the values of Kp and Kc be the same or different in general?
different
to obtain a high K value…
you would have to have a top heavy equation so there are more products.
to obtain a low K value…
you would have to have a bottom heavy equation and therefore reactants would be favoured.
If the value of K is around 1…
neither products or reactants are favoured.
what change does concentration have on eqm position and K value?
EQM position changes
No change to value of K
what change does pressure have on eqm position and K value?
EQM position changes
No change to value of K
what change does temperature have on eqm position and K value?
EQM position changes
K value changes
what change does a catalyst have on eqm position and K value?
No change to EQM position
No change to K value
If a solute is added to two immiscible liquids and the solute is soluble in both…
Some of the solute will dissolve in each of the solvents.
When the solute is shaken the solute distributes itself between the two liquids in a definite ratio called the partition coefficient.
partition/distribution coefficient is…
called K and has no units and is temperature dependant.
both concs need same units
you will get more efficient extraction if…
you do more than one extraction and use smaller volumes.
eg: 4x5 is better than 2x10 or 1x20
chromatography works providing…
the partition coefficients are different.
chromatography relies on…
a mobile phase and stationary phase
Rf value
the distance that the component has travelled compared to the distance the solvent has travelled (solvent front)
what can be done to ensure Rf value stays constant?
is all conditions are reproduced exactly i.e. type of paper, solvent and temperature
Rf=
distance travelled by the component / distance travelled by the solvent
gas liquid chromatography
mobile phase = gas
stationary phase = liquid held on an inert solid support of relatively small particle size
stationary phase in column (usually coiled and placed in oven)
carrier gas comes out column, passes into detector which measures changes to composition of gas
suitable gas to use as a mobile phase
inert (non-reactive)
eg: helium
hydronium ion
H3O+
acid
any substance capable of donating a proton
base
any substance capable of accepting a proton
In Bronsted-Lowry terms, the ionisation of water is represented by:
H20 + H20 –> H30+ + OH-
acid + base–>conjugate acid + conjugate base
conjugate acid
comes from the base
acts as an acid
the species formed when a base accepts a proton
conjugate base
comes from the acid
acts as a base
the species left when an acid donates a proton
when an acid donates a proton, the species left is called the…
conjugate base of that acid
when a base accepts a proton the species formed is called the…
conjugate acid of that base
the number of H+ and OH- ions in water are…
equal
Kw at 25 degrees
1x10^-14
H20 –> H+ + OH-
exo or endo?
endo
H20 –> H+ + OH-
increase/ decrease in temp
increase - eqm moves to right
decrease - moves to left
pH equations
pH=-log[H+]
[H+]=10^-pH
pH + p0H =
14
the strength of an acid is determined by…
the extent to which it ionises
strong acids
completely ionise in aqueous solutions
strong bases
completely dissociate into ions in aqueous solutions
examples of strong acids
HCl
HNO3
H2S04
most other acids are weak
examples of strong bases
NaOH
KOH
Ca(OH)2
most other bases are weak
weak acid
an acid that only slightly dissociates in a water solution
only a small percent of acid molecules donate their hydrogen
example of weak acid
ethanoic acid
CH3COOH (5% ionisation @25 degrees)
weak bases
most bases are weak
only dissociate slightly in water solution
example of weak base
NH3
Ka equation
Ka=[H30+][A-] / [HA]
Ka=[H+]^2 / [C]
measure of the strength of an acid
Ka values of the strongest weak acids
10^-4
1 in every 10000 molecules break up to form ions
equations to link pKa and Ka
pKa=-logKa
Ka=10^-pKa
calculating pH from the pKa
pH=1/2pKa-1/2logc
dissiciation of any base B in aqueous solutions can be represented by the equation…
B+H20–>BH+ + OH-
hydrogen ion transferred from water to the base
dissociation constant of B
the dissociation constant Ka of the conjugate acid BH+
relating Ka and Kb
Ka=10^-14/Kb
KaxKb=10^-14
what does the ka value of the conjugate acid gove information about?
the strength of the base
what happens to the base as the value of Ka increases?
base gets weaker
what happens to the base as the value of Ka decreases?
base gets stronger
what are salts?
ionic compounds that return from the neutralisation reaction of an acid and a base
soluble salts will dissociate in solution to form…
ions
why do some salts dissolve to form acidic/alkaline solutions?
due to the ability for the ions within the solution to dissociate
strong acids/bases fully dissociate
weak acids/bases only partially dissociate
what salt will form when a strong acid and base dissolve?
pH7
fully dissociate
what salt will form when a weak acid and strong base dissolve?
alkaline solution
what salt will form when a strong acid and weak base dissolve?
acidic solution
why do H+ and Cl- have no tendency to join but NH4+ and OH- do?
HCl is a strong acid so will fully dissociate
NH4+ are ions of a weak base and will associate with OH-
what are indicators used for?
to determine the end-point in an acid-alkali titration
what are indicators?
dyes whose colours are sensitive to pH
usually a weak acid
dissociation of an indicator (weak acid)
HIn + H2O –> H3O+ + In-
“choose a suitable indicator” question
use table in data booklet
make sure it covers the correct pH range
theoretical point for an indicator colour change
[HIn] = [In-]
and therefore Kin=[H3O+]
i.e. pKin=-log[Kin] (pKin=pH)
In practice point for indicator colour change
colour change is only distinguishable when [HIn] and [In-] differ by a factor of 10
pH=pKin +/- 1
(log10=1)
when is a suitable indicator not able to be chosen?
weak acid and weak alkali since the pH does not change rapidly enough at the end-point (titration curve does not have an almost vertical section)
what is a buffer solution
one in which the pH of the solution remains approximately constant when small amounts of acid or base are added or the solution is diluted in water
an acidic buffer consists of…
a solution of a weak acid and one of its salts with a strong alkali
a basic buffer consists of…
a solution of a weak base and one of its salts with a strong acid
adding a base to an acidic buffer
in an acidic buffer solution, the weak acid supplies more hydrogen ions when the existing ones are removed by the base.
adding acids to an acidic buffer
The salt of the weak acid provides the conjugate base to react with the hydrogen ions when small amounts of acid are added.
first law of thermodynamics
energy is always conserved
Hess’ law: overall reaction enthalpy is the sum of the reaction enthalpies of each step
ΔH=Σ ΔHproducts-ΔHreactants
standard enthalpy of combustion
enthalpy change when one mole of a substance is completely burned in excess oxygen
what are standard conditions
one mole of a substance at one atmosphere pressure and any specified temperature (in kelvin)
standard enthalpy of formation
the enthalpy change when one mole of a compound is formed from its elements in their standard states
calorimetry
describes the quantitative determination of the change in heat energy which occurs during a chemical reaction
spontaneous process
mainly exothermic
sometimes endo
most common endo are change of state
what are spontaneous endothermic processes always accompanied by
an increase in disorder
second law of thermodynamics
whenever energy is transformed from one fork to another. entropy increases and energy decreases
entropy analogy
low entropy = tidy room
high entropy = messy room
entropy
a measure of the disorder in a system
entropy change with temperature
as temperature increases, entropy increases because particles vibrate more and gaps open up, allowing particles to rotate
entropy and complex molecules
entropy increases as the moles of vibration increases
when does entropy increase?
- many substances dissolve to form solutions
- when a mixture is formed
- number of moles increases
signs for bond making and breaking
breaking +
making -
the conversion of energy into work
never 100% efficient
work
unavailable energy
part of the heat change that is never converted into work
ΔH=
free energy + entropy energy
ΔS=
ΔS(system) + ΔS(surroundings) = +ve
Σ S(products)-Σ S(reactants)
units of entropy change
joules per kelvin
free energy change
combination of enthalpy and entropy change
ΔG=
ΔH-TΔS
negative value for ΔG
reaction is likely to happen spontaneously
positive value for ΔG
reaction unlikely to happen unless external energy is available to do work on the system
feasible ΔG=
0 or below
what does the speed of a reaction depend on?
the rate determining step (slowest step)
in a reaction aA+bB–>cC
rate=
k[A]^m[B]^n
rate constant k is only constant if…
temperature and activation energy stay constant
what are m and n in rate equation
the order of reaction for each of the reactants
- *has to be determined experimentally**
- no relationship to molar ratios*
zero order reactions
rate remains constant
[A] does not change the rate
rate=k[A]^0
rate=k
first order reactions
[A]∝rate
as [A] doubles, rate doubles
Rate=k[A]
second order reactions
as [A] doubles, rate quadruples
Rate=k[A]^2
how to work out overall order of reaction
add up the order of reaction for each of the reactants
calculating rate constant
k=rate/[A][B] (or whatever order of reaction is)
sub in values from any line in table
calculating units for rate constant
sub in units to rate equation for each term and simplify
rate against concentration graphs
zero order reaction - horizontal line
first - diagonal line through origin
second- curved line with increasing gradient
concentration against time graphs
zero - diagonal line from high on conc. axis
first - curved line from high on conc. axis
second - steeper curved line from high on conc. axis
what will not be part of the rate determining step
a zero order reactant
one not involved in the slow step so will have no effect on rate of reaction
one molecule of s reactant involved in the r.d.s is…
first order w.r.t. that reactant
two molecules of a reactant involved in the r.d.s. is…
second order w.r.t. that reactant
