Chem II unit 2
what is the key to solving titration problems
* if just given a random volume then determine if it is before after or at equivalence and then determine what will be present at that point on the titration chart * from there you can write the equation, then find the concentrations of species that you need according to your equation * initial, and at equivalence point use RICE- since weak acid or salt present * before equivalence uses Bdelta A and RICE- since need to see how much weak we have first * after just uses concentration since all weak components are gone * 1/2 way point just uses pka of the weak acid
How do you find pH after the equivalence point
* pH will be dominated by excess base or acid added depending on what is being titrated * Take the volume from that point and subtract from the volume at equivalence point * that left over mL is the amount of base or acid added * multiply that by the molarity of the titrant to get the moles at that point * divide that moles by the total volume that is now there (original we started with before titration + the amount of titrant that was added) * -log of that concentration = pH or pOH
How do you find pH at halfway point
* pH= pka so can just use -log of the Ka of the acid given * if graphing this, the volume is equal to 1/2 of the volume at equivalence
how would you find the pH of a titration given a random volume
* start by finding the equivalence point so you know if the volume is before or after equivalence * if the volume given is less than the equivalence point volume you write your equation with the acid and base reacting, use Bdelta A table to find number of moles of the acid and base- titrant found by multiplying molarity x volume given and the other one is given in the problem, subtract the lesser one to then be able to make a RICE chart with the weak acid reacting with water , be sure to convert back to concentration using total volume * if volume is larger than equivalence point- then subtract the equivalence point volume from that * find the number of moles in that amount of excess titrant * then find concentration by dividing by TOTAL volume * then do -log of that concentration * the titrant is strong acid or strong base so no RICE chart is needed
How do you calculate the pH at the equivalence point
* write out the equation for acid + base to see what salt you have *then take that salt that is the component that would contribute to pH and write it out in its own equation with the water to make conjugate acid or base and OH or H * write that equation out with a RICE chart * find concentration of that salt ion- by using same moles as added from titration ( M x volume of titrant to find moles) divided by the TOTAL volume in the solution (initital volume you started with + the volume of titrant added- may have to find volume of titrant added which you do by seeing how many moles you need and then how many mL of that solution you need to get that moles * Make sure to then use correct Ka or Kb according to the equation * find H+ or OH concentration and thus the pH
how do you find pH before the equivalence point
* write out your equation of titrant reacting with the weak acid or base * find moles in the volume, by multiplying the M x volume to get moles * USE BdeltaA table * write out initial moles of each species in the equation- amount of moles of weak acid/base to start with and the moles of the titrant we calculated from our given volume and molarity * subtract the lesser one * NOW make a RICE chart with the leftover weak acid or base and write out that equation -remember to convert from MOLES to molarity- DIVIDE by TOTAL volume * use Ka of the weak acid and solve for x from the RICE CHART * find pH from x concentration
Delta G standard for cell
-nFEcell, n is number of moles of electrons transferred, F is faradays constant (96,485 j/mole) (Volts (j/C)
steps to solving buffer action equation
1. write equation by identifying if a base or acid is added to the buffer and then if the acid or base in the buffer will react 2. convert any concentrations you have to moles so that anything on the Bdelta A chart is in moles 3. identify the limiting reactant and whichever one is limiting subtract that concentration from both species on the left and add it to the species on the right 4. see if you have OH or H left after, convert that in moles back to molarity then use the pH equation or pOH equation to solve for pH, if no OH or H left over do a rice chart- REMEMBER to convert back to molarity FOR THE RICE CHART- use the species that is produced, if it is an acid remember it will produce H3O + and its conjugate base and a base will produce H2O and its conjugate acid
what plot gives straight line for 2nd order reaction
1/[A] vs. t
ex: Calculate the mass of Au that can be plated in 25 minutes using 5.5 A for the half reaction Au3+ + 3 e- makes Au(s)
25min x60s/1 min x 5.5 C/s, x 1 mol e/ 96,485 C x 1mol Au/3 mole e- x molar mass of Au
what is the frequency factor
A- stands for the frequency of collisions between reactant molecules at a standard condition
Units of A and V
Amps are equal to 1 C/s and Volts are equal to 1 J/C
are q and W state function
NO
what plot gives straight line for 0 order reaction
[A] vs. t
If the pKa of HCHO2 is 3.74 and the pH of an HCHO2/NaCHO2 solution is 3.89, which of the following is TRUE?
[HCHO2] < [NaCHO2] because 3.89 is higher than 3.74 there will be more of the basic form present, so since NaCHO2 is acting as the base and HCHO2 is the acid, the basic component concentration will be higher than the acidic component concentration
common ion effect
adding a salt NaA (where A- is the common ion) shifts the position of the equilibrium to the left which lowers the H3O+ ion concentration causing the pH to be higher than the pH of the acid solution alone
common ion and solubility
addition of a soluble salt that contains one of the ions of the insoluble salt decreases the solubility of that insoluble salt
electrolytic cell
an electrochemical cell used to cause a chemical change through the application of electrical energy, forces current in NONSPONTANEOUS direction
line notation
anode is on the left and cathode is on the right side, double verticle lines separate the anode and cathode, single line on each side to show separation between starting element and the product, and include phases
what all can be used to make buffers
aside from using the weak acid or weak base with its conjugate salt, you can also use a weak acid and strong base, with that strong base usually being half the concentration- because it will create a salt and the conjugate base of the weak acid
average rate
asked to find the average rate of something over a certain number of seconds- then do the general reaction equation 1/ coefficient (conc A at time 2- conc A time 1)divided by time 2 - time 1, then when writing out the answer define the span of time used to define that rate so 0-100 seconds
how can you predict the pH of a solution that is at the equivalence point
based on the nature of the salt that is present- if it is a conjugate base of weak acid it is basic if it is conjugate acid of weak base it is acidic if it is anything else then it is neutral
buffer action vs. buffer solution problem
buffer action is just the added acid or base and whichever species it is reacting with with a single arrow, and a base reacting with acid makes water, a buffer solution has both parts of buffer with water and a double arrow
what is cathode vs anode
cathode is positive end where reduction occurs, anode is negative end where oxidation occurs
how do you choose the components you want for your buffer system
choose a weak acid with pKa= to desired pH
how do we determine if precipitation will occur
compare Q for the current solution concentrations to the value of Ksp
relationship between delta E and q
delta E= q at constant volume,
relationship between enthalpy of system and q at constant pressure
delta H= q at constant pressure
how do you write the reaction for a buffer reaction
determine if the species added is a strong acid or a strong base, then react with with the opposite thing in the buffer, for example if NaOH is added that is the strong base so the reaction will be OH- + the acid that is present in the buffer (single arrow) conjugate base and H2O
differential rate law vs. integrated rate law
differential rate law is rate vs. concentration integrated rate law= concentration vs. time data
what is endpoint and equivalence point
end point is when there is a color change with the indicator, the equivalence point is when the reaction is complete and all the acid or base has been neutralized- they are not the same but hopefully would happen at the same time
writing balanced equation for a cell given two reduction equation
find which one should be flipped in order to make most positive, then write that equation flipped, then add the two equations together and balance them so that the number of e- transferred matches, given the concentrations of each of the components can calculate the cell at nonstandard conditions using E=Ecell- .0592/n (log (q) where Q is products/reactants concentrations make sure to include exponents
units for k
for 0 order= m/s, first order is 1/s, second order is 1/(mxs)
how do we draw a voltaic cell
given 2 reactions to be used in it and the standard reduction potentials, use the standard reduction potentials to decide which one can be flipped in order to add them to make a positive E, then look at which one is the oxidation reaction and which one is the reduction, then the anode is oxidation, reduction is at cathode, and then the flow of electrons- always move from anode to cathode, in the salt bridge, show the positive ions arrow towards the cathode, the negative ions in the salt bridge towards the anode
heterogeneous vs. homogeneous catalysts
homogeneous catalysts are in the same phase as the reactant, heterogeneous catalysts are in a different phase than the reactant
what is kinetics
how fast a reaction occurs in time, NOT if it is spontaneous or not
how do you know what equation to use to find the pH of a buffer
identify the weak acid, the conjugate base that is in the salt, and just use the weak acid + H2O (arrow ) conjugate base as the anion + H3O +
how to predict if a metal will dissolve in an acid
if the acid can OXIDIZE THE METAL so the reduction of the H+ is more positive than the reduction of the metal ion- look at the reduction of both of them from the standard reduction table and see which is more positive, so you would have the reduction of H+ and the oxidation of the metal, from neutral metal to positive metal cation
how to know if you should use integrated rate law or general rate law
if the question gives concentrations of reactants and a rate then use general rate law, if it asks for the average rate law from concentrations and times given then just use the general one, if it gives you half life and asks to find k or gives k and asks to find concentration then use integrated rate law
how do we know if reaction mechanism is possible
if the rate law of the slowest step matches the experimental rate law then the mechanism can be true
le chateliers principle with galvanic cells
if the reactant concentration is increased past standard conditions the voltage will increase, if the product concentration is increased past standard conditions then the voltage will decrease
how can you determine the form a compound will be in based on the solution it is in
if the solution is more basic than its pka it will be in basic form, if the solution is more acidic than the pka it will be in its acidic form
what happens to entropy in any spontaneous reaction
in any spontaneous reaction the entropy of the universe increases so delta S is positive
calculate standard cell potential
it is the reduction potentials from the chart, one flipped, add together to get standard reduction potential
what happens if base is added to a buffer
it reacts with the HA and the reaction goes forward to make more A- and water
what happens if acid is added to a buffer
it will react with the A- to produce more HA
if the pKa of phenolphthalein is 9 then what color would the indicator be at pH 3
it would be clear since it is in its acidic form below pH 8. If it was above pH 10 it would be dark pink since that is its basic form and if it is between 8.1-9.9 it would be the mix of clear and dark pink which is light pink
what happens if you add and acid to solution with M2 and OH
it would increase the solubility because the H+ ions would react with the OH and the solution would move to the right to fill in for the lost OH ions
how do we find the volume of titrant added at equivalence point
know amount of moles in the weak acid or weak base, so then using molarity of the titrant and those moles find the volume of titrant needed to have that number of moles in it
what plot gives straight line for first order reaction
ln[A] vs. t
when calculating E that is not at standard temperature what is important to pay attention to in the reactions
make sure the number of electrons are the same in each half reaction, and multiply the same number as you do to the electrons to the entire equation
electroplating questions
make sure to always convert time to seconds, since A is C/second, C from Faradays constant will be on the bottom, and then balance mol of the element with the number of moles of electrons per mole of the element
what is entropy
measure of disorder, so increases as the number of arrangements that are available to a system increases
pka
measure of protein binding affinity
what affects reaction rate
nature of the reactants- gas-liquids-solids concentration of the reactants- higher concentration increases the frequency of collisions between reactant particles, higher pressure of gases increases concentration temperature- higher temp presence of catalyst- affect speed of reaction without being consumed
does first order half life depend on concentration
no
can you compare Ksp values
no, unlike Ka values you cannot compare Ksp values, unless the number of ions is equal
catalyst
not used up in the reaction, appears as a reactant and THEN as a product too
precipitation
occurs when concentration of the ions exceed the solubility of the ionic compound
anode and cathode in electrolytic cell
oxidation at anode and reduction at cathode- but + terminal is the anode and - terminal is the cathode, electrons are forced toward the cathode
Henderson-Hasselbalch equation
pH = pKa + log [A-]/[HA], only use for buffer solutions
what is the effective pH range of a buffer
pKa +/- 1
how do you find pka
pKa=-logKa
how to slow corrosion of metals
paint, sacrificial anode that is a metal that is more reactive and will be oxidized firstghhhh
how to solve what is the minimum [OH] necessary to begin precipitation of Mg2+ with [.059] with seawater
precipitating occurs when Q=Ksp, so Q=[Mg][OH], look up value for Ksp and just solve for OH
reaction mechanism
proposed series of elementary steps that add up to overall equation
how do you write the equation for just a buffer solution
put the acid on the left + H2O and the conjugate base on the right side + H3O +
general rate law equation
rate law= k[A]^y [B]^x, where x and y are determined experimentally and ARE NOT the coefficients, if A is one then if the reaction rate doubles A concentration doubles
what is the arrhenius equation used for
relationship between temperature and rate constant, k
what do you need to remember for RICE chart after the Bdelta A chart
remember to convert moles to MOLARITY- need to include the total amount of liquid so add the mLs together, also remember to pay attention to the equation, if it is an acid that is produced then in the RICE chart that is the starting species and it produces conjugate base and H3O +
which order has slope= to + k unlike the others
second order
what is the plot of lnk vs. 1/T
slope is -Ea/R
Ksp
solubility product constant
Q>Ksp
solution is above saturation, salt above will precipitation
Q=Ksp
solution is saturated, so no precipitation
Q<Ksp
solution is unsaturated, no precipitation
what are most buffers made of
solution of weak acid with solution of soluble salt containing the conjugate base anion- so basically a weak acid + its conjugate base in the form of a salt
what are buffers
solutions that resist changes in pH when acid or base is added by neutralizing the added acid or base
How do you solve a problem, how many grams of hydroxyapatite will dissolve in 1L of H2O, given molar mass and Ksp
solve for the Molar solubility using Ksp and the coefficients with S, then multiply the value of S by molar mass to find grams, since S is the amount of moles that dissolve
what is an intermediate in a reaction
species that is formed and consumed in the steps so it is not in the overall reaction- first is a product and then is a reactant
how do you solve for the units for k for any general reaction
start with k being m/s in 0 order then figure out what k would be with addition of new concentrations with those coefficients
Faraday's law
the amount of metal deposited during electrolysis is directly proportional to the charge on the cation, the current, and the length of time the cell runs
first law of thermodynamics
the energy of the universe is constant, delta E is 0
third law of thermodynamics
the entropy of a perfect crystal at 0K is 0, s is 0 when T=0
what are the exponents in the rate law when doing reaction mechanisms
the exponents are equal to the coefficients in front of the species in the equation
how do you know what reaction will happen in an electrolytic cell
the half reaction that leads to the most positive or least negative E cell will be the one that occurs EXCEPT- Cl- oxidizes to Cl2 before water oxidizes to O2
half reactions
the individual reduction or oxidation reactions that are combined in the overall reaction
what is true about moles at the equivalence point
the number of moles added from the known is equal to the number of moles of the acid or base that was present in the solution
molar solubility
the number of moles of solute that will dissolve in a liter of solution
what is pKa
the pH where the concentrations of A- and HA- are equal
what can we tell from the halfway point on a titration curve
the pH- and we know the halfway point from the equivalence point- if the equivalence point is at 10mL then the halfway point is at 5 mL
what does it mean delta E is a state function
the path does not affect the outcome, there are not 2 states that would affect the change in E
instantaneous rate- how to find
the rate at one instant in time using concentration vs time plot, use slop of the line tangent to the curve at that time, so if you want to find instantaneous rate at 100 s then draw a straight line that only intersects 100 seconds and find the slope of that line- substitute into the rate equation and then do 1/the coefficient
what is comparative rates
the rate of one species relative to another, depends on the coefficients in that equation and which side of the arrow it is on
what happens when you change the concentration of a species at equilibrium
the reaction will shift but the K remains the same
Ksp expression
the same as K expression but we do not include solids in our expressions so it will just be the products, and remember to raise them to the power of their coefficients
what is the rate determining step
the slowest step
which step has the largest activation energy
the slowest step
how do the buffers work
the weak acid reacts with added acid and the conjugate base reacts with added acid- eventually the necessary component of the buffer will run out and the solution pH will change significantly, this works by le chateliers principle- if you add H+ to the system it will react with the conjugate base to make HA and keep the concentration of H3O constant
electroplating
the work piece is the cathode, cations are reduced at the cathode and plate to the surface of the work piece
what concentrations are true when pH < pKa
there is a higher concentration in the acidic form
what concentrations are true when pH> pKa
there is a higher concentration of basic form
what must happen for collisions to be effective
they must occur with proper orientation and sufficient energy
where do the salt bridge anions move
toward the anode, since oxidation occurs here the + charge builds up as electrons leave resulting in the anions being attracted to it
where do salt bridge cations move
toward the cathode
how do you calculate initial pH on a titration curve
use a RICE chart to find the pH initially because it is just the pH of the weak acid or the weak base
how do you calculate what ratio of NaCHO2:HCHO2 would be required to make buffer with pH of 4.25
use equation pH=pKa + log([A-]/[HA] , the ratio equals 3.24 so need 3.24 as many moles of CHO2 as HCHO2
what species exist at the equivalence point
water and a salt - NO acid or base
how do we select an indicator
we find the pH at which our solution has the equivalence point, and then find the indicator with the pKa equal to that pH
what are indicators generally made of
weak acid
what characteristics allow a buffer to have largest capacity
when HA- to A- ratio is 1 and have a reasonable molarity
how can you predict the color of indicators
when there is 50/50 of the acid and base components of the indicator the pH that will give you that mix is equal to the pka of the indicator and that is where there will be a color change- the acidic form of the indicator is one color, the basic form is another color, and where there is 50/50 acid and base it is the mixed color of the two
how to write a chart for electrolysis
write down all the possible reactive species given your element (for ex: given MgI2, then you have Mg2+ and I-, and also H2O since it is (aq) write down the possible reactions that could occur- if a reduction could occur or an oxidation- look at the chart and see what half reactions you can find with the species you are looking for, then choose the redox rxn with the most positive reduction potential and the oxidation rxn with the most positive oxidation potential, then using the 2 you selected write the combined equation
how do you calculate the molar solubility of lead (II) chloride in water given Ksp
write expression (always the solid on the left), write out a RICE chart with the initial amounts of both species on the right being 0, then +S for both, but use +2S when the coefficient is greater than 1, fill in the expression using the S terms and set equal to Ksp and solve for S, the value of S is the molar solubility
How do you find the Ksp of Lead (II) bromide if its molar solubility in water is given
write out the equation and RICE chart, start with 0 again as initial amounts, then fill in the change column with the value for the molar solubility- remember to multiply the value by the coefficients, then Ksp is equal to those values as you multiply them out according to Ksp expression
how do you calculate the molar solubility of calcium fluoride in .1M sodium fluoride
write out the expression like normal with RICE chart, in the initial column however, write down the molarity of what is already present, so F- ions will already be present at .1M, then add S and 2S accordingly since there is a coefficient, and F- will be .1 + 2S, write out the Ksp using your values, S= the molar solubility
how do you calculate the rate law experimentally
write the rate law out with variables so that you can enter in values from experimental data, choose two sets of data where only one of the concentrations of reactants changes, solve for one of the exponent variables, then use another set of experiments plug into that and solve for y
does 0 order half life depend on concentration
yes
does second order half life depend on concentration
yes
what happens in a reaction where the concentrations of 2 of the reactants is much larger than the third reactant
you can write a new rate law that is k'[concentration of lowest reactant A] to find k then k'=k[reactant B][reactant C] solve for k using k' and the reactants of B and C to solve for the actual rate constant k
how do you calculate the pH before the equivalence point
you need to know the volume at equivalence point