Exam 1 Flashcards
what were the trends in boiling point elevation and molality in the salt and sugar in the colligative property lab? How did their values compare?
Boiling point increased as molality increased for both. Sugar had much lowers values and bp elevation values than salts
What would happen if you boiled the solutions and salt and sugar too vigorously in the colligative properties lab?
The volume of water would significantly reduce as a result of evaporation. Therefore, the concentrations and boiling points of the salt and sugar solutions would increase. The vapor pressure would decrease
non-volatile solute
substances with no measurable vapor pressure
Four main colligative properties
CHANGES IN osmotic pressure, freezing point, boiling point and vapor pressure
How does the addition of a nonvolatile solute affect a solution?
It lowers the vapor pressure
How do solute decrease vapor pressured of pure solvents?
They decrease the amount of solvent particles on the surface that can enter the gas phase
Kb(in terms of colligative properties, not acids/bases)d
boiling point elevation constant, magnitude of constant depends ONLY on identity of the solvent
What does boiling point elevation depend on?
moles of particles
What does reaction rate depend on?
reactant concentrations
catalysts in the kinetics experiment
KI
Hydrogen peroxide decomposition equation
2H2O2(aq)=2H2O(l) + O2(g)
catalysts
increase the rate of a reaction without undergoing a net change within their own structures
How is the rate of decomposition of hydrogen peroxide physically observed?
change in pressure above the solution caused by O2
How were initial reaction rates observed in the kinetics lab?
plotting pressure vs time
Describe the pressure change at the beginning of the decomposition of H2O2 and why
the pressure will change linearly with time. there is a short 70-100 second lag period because the initial O2 that is generated will dissolve in the aqueous solution until it reaches its saturation limit
How do we get the rate of the H2O2 decomposition reaction graphically?
the steepest slope, closest to the beginning of the reaction, but AFTER the initial buffer period
How do you know when the initial lag phase in the graph of the rate of decomposition of O2 is over?
the pressure will start to increase with time
How does temperature affect the rate law?
The magnitude of k in the rate law changes with temperature and therefore determines how temperature affects rate
Order of a reaction
the exponents to the individual concentrations in the rate law. They are typically small, whole numbers that are derived experimentally
What is the point in knowing the order of a reaction?
It provides insight into how a reaction takes place, such as which molecules collide with which molecules as bonds break, new bonds form and reactions are converted to products). It indicates which molecular components, and the number of those components, are present during the rate-limiting step of the reaction
How are initial rate measurements converted to a more useful form?
They start in pressure/time, and are converted to concentration time via the ideal gas law. M=P/(RT)
Why are initial pressures collected in pressure/time?
Pressure in the units pKa/s are the most convenient units to collect to collect data with on the labquests
What can be determined after the order of a reaction has been determined and how?
the magnitude of k can be determined as an average of the rate constants determined from the individual kinetic trials at a constant temperature
Relationship between temperature and rate
the rates of MOST chemical reactions rises as temperature rises
How can the temperature dependence of a reaction rate be expressed?
the Arrhenius equation
Energy of activation
Ea, the minimum energy required to make the reaction happen, or an energy barrier reactants must traverse
What is A in the Arrhenius equation?
A constant or frequency factor that is related to the frequency of reactant collisions
How do we physically discover the activation energy?
Measuring the rate constant at a series of different temperatures
How is the Arrhenius equation made graphable?
taking ln of both sides puts it into y-mx+b form where ln(K) is y, m is -(Ea/RT), x is 1/Tand ln(A) is b
What is determined in the kinetics experiment in order to determine the rate law?
- Order with respect to H2O2
- Order with respect to KI
- Energy of activation
What is required to physically determine the kinetic order?
5 different trials where one of the reagents is kept constant
Why are a total of 9 trials needed to determine the kinetic order of H2O2 and KI?
The data from trial 1 can be the first trial for both substances
What must the total volume in the reaction flask be in the kinetics experiment order trials?
3mL
What are the limits on concentrations of H2O2 and KI in the kinetics experiment?
0.22M-0.88M H2O2 and at least 0.13M KI
What temperature differences are required for determining activation energy?
10K differences
How do you find final concentrations of H2O2 and KI in the kinetics experiment?
Use M1V1=M2V2 with the initial concentration, initial volume, and final volume
How do you prepare the KI solution in a volumetric flask?
Add KI to flask, wash the neck with distilled water, fill halfway with distilled water and slowly swirl to mix, dilute to etched line, put on parafilm and invert to mix
What is the purpose of the water baths for determining the order of H2O2 and KI?
to keep temperature constant
What units do you convert kPa/s to?
M/s
Finding a and b graphically
Method 1: plot rate as a function of concentration and see which exponent makes it linear
Method 2: plot log(rate) vs log(concentration) and the slope=the order
Rates of reaction at equilibrium
Rates of forward and reverse reactions are equal
What happens when a reaction at equilibrium undergoes a change in temperature, concentration, or pressure?
it will reattain equilibrium by undergoing a net reaction that reduces the net effect of the disturbance
How does temperature change the rate of a reaction?
It changes the rate constant
In the equilibrium lab, why do the Fe3+ solutions contain 1 M HNO3? Why does this work?
To prevent precipitation of Fe3+ as Fe(OH)3. Since neither H+ nor NO3- are involved int he thiocyanate equilibrium, the addition of HNO3 has no effect on the reaction of interest
Colors of solutions where concentration of SCN- is varied and Fe3+ is constant
Forms and orange color, gets darker as concentration of SCN- increases
Colors of solutions where Fe3+ is varied and SCN- is constant
Higher Fe concentrations are darker, orange color
Effect of temperature baths on Fe3+ and SCN- solutions
Cold baths were darkest and warm baths were lightest
Why is each solution of Fe3+ and SCN- prepared with same graduated cylinder?
to improve precision
Why must absorbance measurements of Fe3+ and SCN- solutions be made within 2-4 minutes after they are prepared?
The color of the complex ion fades over time
Why do the Fe3+ and SCN- solutions where we measure of absorbance have a large excess of Fe3+?
So that all of the SCN- is converted into the complex ion FeSCN2=
How are the concentrations of FeSCN2+ and SCN- related?
The concentration of the complex ion can be assumed to be the same as the initial concentration as SCN-
Why might the absorbance solutions in the equilibrium constant lab be yellow?
Fe(NO3)3 is present
How do you know a cuvette is oriented correctly?
You cans see the light go through the clear sides of the cuvette
Why are equal concentrations of Fe(NO3)3 and SCN- used to find the equilibrium constant
the equilibrium constant is measured under conditions under which there is not a large excess of one reactant over another. there are sifnificant concentrations of all species present at equilibrium
What should be true of the absorbance values of the equilibrium constant trials?
they fall within the range of the calibration curve already prepared with excess Fe3+
How do you find the equilibrium concentrations of Fe3+ and SCN-?
subtract the equilibrium concentration of FeSCN2+ from their initial concentrations,
How do you determine the equilibrium concentration of FeSCN2+?
By interpolation from the calibration curve
Arrhenius definition of an acid and a base
An acid dissociates in water to yield H3O+ and a base dissociates in water to yield OH-
Bronsted-Lowry definition of an acid and a base
An acid is a proton donor and a base is a proton acceptor
Lewis definition of an acid an a base
An acid accepts an electron pair to form a bond and a base donates an electron pair to form a bond
Ionization of acid in water equation
HA(aq)+H2O(l)=H3O+(aq)+A-(aq)
Ka
an equilibrium constant called an acid-dissociation constant. It’s magnitude indicates the tendency of the acid to ionize in water. Larger Ka=stronger acid. Ka=[H3O+][A-]/[HA]
What is special about when [A-]=[HA]?
In the Henderson-Hasselbalch equation, you will get log([A-]/[HA])=log(1)=0 and pH will equal pKa
Graduated cylinder sizes
10mL, 100mL
Graph used in part 1 of ionization constant of weak acids and how it was read/useful
pH of each buffer solution(y-axis) vs log([HPO4(2-)]/[H2PO4-]) (x-axis) was graphed. pKa was derived from the y-intercept of this graph because at the y-intercept, log([HPO4^2-]/[H2PO4^-]) ont he x-axis=0. Therefore, in the Henderson-Hasselbalch equation, that leaves pH=pKa. m is supposed to equal 1 because in the middle because the volumes change by 5 mL in both solutions
In the first part of the ionization constants of weak acids lab, as you went through trials 1 through 12, explain the pH trend observed
As the experiment went on, [H2PO4^2-] decreased, so the reaction shifted to that side and away from H3O+. Therefore, [H3O+] decreased, increasing the pH and making the solution more basic
Why did the data points appear to level off at the extreme ends of the pH vs log([HPO4^-]/(H2PO4^2-])graph?
At the extreme end of the plot, the intervals of volume between different solutions were much smaller than the intervals of solutions toward the middle of the plot, therefore, the change in pH was much smaller. The volumes started/ended with volume differences of 0.5mL and in the middle the differences were 5mL
How was ºC/m obtained in the colligative properties la b?
it is the slope of the △Tb vs. molality plot
finding reaction rate graphically and its units
the reaction rate is the slope of the graph of pressure(kPa, y-axis) and time(s, x-axis). The units of the reaction rate from this graph are kPa/s
What was graphed in the Arrhenius plot and how was it used?
1/(temp in kelvin) on the x-axis vs ln(k) on the y-axis. The slope of this line is equal to -Ea/R
How were the initial concentrations of H2O2 and KI planned/determined
M1V1=M2V2 was used to determined how much water would be added to the given H2O2 and KI to get a certain molarity. The molarities covered the ranges specified in the lab manual
How is kPa/s converted into M/s?
The formula M=P/RT derived from PV=nRT. The temperature is in Kelvin and the pressure comes from the kPa/s value
What was the point of the calibration curve in equilibrium constant lab?
It was used to interpolate the concentration of the complex ion FeSCN^2+ at equilibrium by plugging in the measured absorbance of a solution
colligative properties
properties that are a function of the number of dissolved particles, or depend of the collective effect of the number of solute particles
Normal boiling point
The temperature at which the vapor pressure equals 1atm
🔼Tb
boiling point elevation, function of concentration of solute particles(specifically molality)
How are reaction orders determined in the kinetics experiment?
graphically
What precaution must be taken with the high temperature trial in the kinetics experiment?
Hold thumb on rubber stopper because the pressure may buildup a lot
adding reactant to an equilibrium reaction
forward reaction occurs at larger rate until equilibrium is reestablished with a larger equilibrium concentration of product and an equilibrium concentration of reactant smaller than its concentration immediately after the disturbance, system will consume excess reactant
adding product to an equilibrium reaction
reverse reaction occurs at larger rate until equilibrium is reestablished with a larger equilibrium concentration of reactant, system will consume excess product
increasing temperature
favor endothermic/heat absorbing reaction direction, increases rate constant for that reaction more than rate constant for the exothermic direction. Equilibrium constant will change to favor the endothermic reaction and the product of the endothermic reaction increases in concentration
Decreasing temperature
Favors product of exothermic reaction, increases exothermic reaction rate constant more than endothermic one, equilibrium constant will change to favor the exothermic reaction and the product of the exothermic reaction increases in concentration
If 🔼Hrxn=0, what happens if you add or take away heat from a reaction?
A temperature change will have no effect on the equilibrium concentrations of reactants or products. Temperature would have the same effect on both directions and therefore no equilibrium effect
Why do we monitor FeSCN^2+ in the equilibrium lab?
Fe^3+ and SCN^- are colorless while FeSCN^2+ is rust red and has a maximum wavelength absorption in the 450nm-470nm range
how is the slight yellow color caused by Fe(NO3)3 in the equilibrium constant experiment accounted for?
calibration of the spectrometer
When preparing the calibration curve, what is measured next and why?
the solution with the largest SCN^- concentration because you are measuring the absorbance of the maximum wavelength of that solution
How is a cuvette prepared for a wavelength measurement?
It is rinsed with distilled water at the end of the previous measurement, then filled with the next solution and dumped out 3 times before filling it again to measure the absorbance of that solution
What was varied in making the calibration curve in the equilibrium experiment?
Concentration of SCN^-
How do you calculate the equilibrium concentration of FeSCN^2+, SCN^- and Fe^3+ ?
[FeSCN^2+]eq is calculated by interpolation from the calibration curve
[Fe^3+]eq = [Fe^3+]initial - [FeSCN^2+]eq
[SCN^-]= [SCN^-]initial - [FeSCN^2+]eq
What happens to FeSCN^2+ formation as temperature decreases?
its concentration increases
How can the relationship between pH and pKa be described?
When [HA]=[A^-], pH=pKa. When |HA]>[A^-], pH
What was graphed in part 1 of the ionization constant of weak acids experiment and what did each part of the graph represent?
pH of each buffer solution(y-axis) and log([HPO4^2-]/[H2PO4^-]) (x-axis)
y-intercept=pKa
m=1
What are indicators?
weak acids and their conjugate bases that exhibit different colors depending on the pH of the solution
What determines which color will show when HIn is put in solution?
At pH>pKa, basic color will show, and vice versa
HIn in blue/red indicators
Bromothymol blue: HIn is yellow and In^- is blue
Phenol Red: yellow for HIn and pink/red for In^-
How do you qualitatively estimate the pKa of the indicator?
choose which of the 12 prepared solutions seems to have equal amounts of both color, the intermediate solution
in the second part of the ionization constant lab, in what order were solution absorbances measured?
The solution where pH=pKa, the the most basic solution with declining basicity afterwords
Methods in which pKa of indicator dye can be determined
- peak of first derivative of S-curve, or the pH at which absorbance is changing fastest with pH
- plot pH vs log ratio and get the pKa that way
How are [In^-] and absorbance related?
they are proportional
What is [HIn] proportional to?
absorbance of the indicator in basic form minus the absorbance at each buffer’s pH
What can be substituted for [In^-] and [HIn] in the Henderson-Hasselbalch equation?
replace [In^-] with Ai-Aa
replace [HIn] with Ab-Ai
Why is the peak of the first derivative graph the pKa?
because this point is where absorbance changes fastest with pH which is the point of color change from acidic to basic
