CHM 145 Lab Exam 1
exothermic
-decrease in temp -reactant=products+heat
endothermic (heat absorbing)
-increase in temp -increases the rate constant -reactant+heat=products
Buffers are...
...made by partially titrating the buffer acid with a strong base, partially titrating the buffer base with a strong acid, or by combining the weak buffer acid and a salt of the buffer acid.
net absorbance
= (absorbance) - (absorbance of the blank solution)
conjugate base
A buffer is an aqueous solution of a weak acid and its _______________ ___________.
0.00016 M, M1V1=M2V2
After you prepare all of the solutions for the quantitative portion of the experiment #3, you'll need to calculate their concentration after mixing. What is the final concentration of SCN- for calibration solution #4 (after mixing)? Enter your answer in decimal form (not scientific notation) and in the correct number of significant figures. What formula will be used to find this concentration?
y-intercept
As part of your lab report for this experiment #4, you will need to include a plot of pH (y-axis) vs. log ([HPO42-]/[H2PO4-]) (x-axis) from your part I data. This plot is based on the Henderson-Hasselbalch equation (see introduction of Expt. #4 for formula). Using the linear least-squares fit of the data, how would you determine the pKa of the acid?
pKa for H2PO4- in part 1 is 6.68 Ka = [HPO42-][H3O+] / [H2PO4-] Ka = 10^(-pKa) = 10^(-6.68) = 2.1 x 10^7
Based on reported pKa for H2PO4-, calculate Ka
2 H2O2 (aq) --> 2 H2O (l) + O2 (g)
Decomposition reaction of H2O2
slowly
Does hydrogen peroxide decompose slowly or fast over a period of years?
acid dissociation constant
For an acid-base equilibrium, the equilibrium constant, Keq, is known as Ka, the _______ ______________ _________.
slope, rate
For the data analysis of Expt #2, you will make a plot of pressure vs. time. The ________ of that plot will measure the ______ of the reaction.
pH > pKa
If [HA] < [A-], _________________ because the log of any number greater than 1 is positive
pH = pKa
If [HA] = [A-], _________________ because the log 1 = 0
pH < pKa
If [HA] > [A-], _________________ because the log of a decimal is negative
The concentration of sugar or salt would increase. Concentration is found by using molality which is measured in moles and liters. Decreasing the volume would increase the concentration. The number of moles of solute would stay the same but the boiling point would increase.
If the solution boiled too vigorously and the volume of water is reduced what will happen to the concentration of the salt or sugar solutions and measured boiling point of these solutions in experiment #1?
pressure sensor
In Expt #2 you will be using a __________________ to monitor the rate.
pressure
In Expt #2, the kinetics of the decomposition of hydrogen peroxide (H2O2) will be monitored in a closed vessel by measurement of the change in ___________ of oxygen.
HCl, 3.0
In Part 1 of the experiment #5, 1.0 M _______ is added to the phosphate buffer solution until the pH reaches ~ ______.
0.38
In Part 2 of the experiment #5, a student was assigned the task of preparing an acetic acid buffer with a pH = 4.32. The ratio of conjugate base to acid (i.e. [CH3COO-]/[CH3COOH] or [A-]/[HA]), for this students buffer must equal _____. Hint: you need to calculate this ratio. The formula along with the pKa of acetic acid can be found in the introduction to the experiment in the lab manual.
0.3
In Part 2 of the experiment #5, once calculations are complete, you will make your "target" buffer. The pH of your acetate buffer should be within ± ____ of the "target" that was assigned to you. This will count for accuracy points on your lab report grade.
0.05, 0.1, 82.03
In Part 2 of the experiment #5, pairs will be assigned to make an acetate buffer of a particular pH. This will involve some calculations on your part. The total concentration of the "target" buffer is _____ M. The buffer will be made from acetic acid (______ M), sodium acetate (______ g/mol), and water. You will need to set up a system of two equations and two unknowns to solve. Remember, you are ultimately solving for a volume of acetic acid and a mass of sodium acetate.
1.0
In both Parts 1 and 2 of the experiment #5, you need to use a small graduated cylinder to determine and record the number of drops of HCl and NaOH solution that constitutes ____ mL. This is necessary because your graphs will report "mL" and not "drops".
delta Tb = Tb (solution) - Tb (solvent)
In experiment #1 we calculated the average temperature at the two plateaus to boiling point elevation using what equation?
Y, X
In experiment #1 we had to create a plot of the temperature of water on the _____axis as a function of time _____axis
delta Tb, molality of particles
In experiment #1, we made a plot of ________ on the y-axis vs. ______________ _____ _______________ on the x-axis for the class data for NaCl.
y=mx+b
In experiment #1, we used which equation that describes the relationship between the boiling point elevation and the molality of particles from the class data for NaCl only?
KI has a lower activation energy and it is broken down and regenergated
In experiment #2 how KI can increase the rate of decomposition of hydrogen peroxide without undergoing a net change in its own chemical composition?
room temperature, ~20 degrees Celsius
In experiment #2 the kinetic order with respect to H2O2 and the kinetic order with respect to KI was conducted at _________________________
Kinetics of the Decomposition of Hydrogen Peroxide
In experiment #2, ___________________________________, our objectives were to monitor the rate of a chemical reaction, determine the kinetic order of a reaction from dependence of the rate on reactant concentrations, understand how the molecular components of the rate limiting step of the reaction are determined by the kinetic order of the reaction, and determine the energy of activation of the reaction from the temperature dependence of the reaction rate.
ln k (y-axis), 1/T (x-axis)
In experiment #2, for the 4 trials performed under the same conditions at different temperatures, the energy of activation was determined by plotting ___________ vs ___________. The slope of the line is equal to -Ea/R.
pressure vs time
In experiment #2, initial rates of reaction were measured by plotting ________________ ___ ______________
3
In experiment #2, the energy of activation was conducted at _______ different temperatures in addition to the room temp trial.
4, 1
In experiment #2, we needed ___ different trials to determine kinetic order for each reactant, changing only ___ condition at a time for each individual trial.
pressure (kPa), time (s)
In experiment #2, we plotted _______________ as a function of ____________ for each of the 10 trials and determined the slope by linear least-squares fit analysis.
M1V1 = M2V2
In experiment #2, what equation was used to solve for M2, the final molarity of either H2O2 or KI reagent?
hydrogen peroxide, H2O2
In experiment #2, what is the important oxidizing agent?
Fe3+ (aq) + SCN-(aq) <---> FeSCN2+ (aq)
In experiment #3 the reaction to generate the complex ion, FeSCN2+
colorless
In experiment #3, Fe³+ and SCN- are _______(color)
Determination of an Equilibrium Constant
In experiment #3, ______________________, our objectives were to observe a system under chemical equilibrium, to observe changes in the state of equilibrium caused by changes in reactant concentrations and temperature, to apply Le Chatelier's Principle to understand those changes, and to measure an equilibrium constant
darker
In experiment #3, as the concentration of Fe3+ and SCN- increased, the color got ____________
calibration curve, equilibrium constant
In experiment #3, part one measured the absorbance of the standard solutions, the _________________________ ___________________, and part two measured the _______________________ ___________________.
calibration curve
In experiment #3, the ______________________ is made by using solutions with at least 250 fold excess of Fe³+ so that all the SCN- is converted into the complex ion
red rust
In experiment #3, the complex ion, FeSCN2+, has a _______ color
450-470nm
In experiment #3, the complex ions absorption max and the visible range of the electromagnetic spectrum fall between _____________.
light peach, orange
In experiment #3, the standard solutions needed for the calibration curve, ranged in color from ____________ to _____________
absorbance, concentration of the complex ion
In experiment #3, we plotted _________________(y-axis) and ___________________(x-axis)
12
In experiment #4, _____ large and medium test tubes were used.
Ionization Constants of Weak Acids
In experiment #4, __________________________, our objectives were to understand the concept of the ionization constant Ka and to measure Ka for weak acids using pH measurements and absorbance measurements.
HIn
In experiment #4, at pH < pKa, the solution containing the indicator will be the color of the acid, _________
In-
In experiment #4, at pH > pKa, the solution will be the color of the conjugate base, _______
pH trend
In experiment #4, the ________________ observed during part one of the experiment showed that the initial buffer solution (test tube #1) was the most acidic, going in sequential order, to test tube #12 being the most basic. According to Le Châtelier's Principle, if more reactant is added the forward reaction at equilibrium, the reaction will happen at a larger rate until equilibrium is established again with a smaller equilibrium concentration of the product, HPO42-. When this occurs the acid, H2PO4-, donates a proton and the conjugate base, HPO42-, accepts the proton from the acid as described in the balanced equation. This principle applied to test tubes #1-6. Test tube #7 was ~neutral. Test tubes #8-12 had a reverse reaction greater than the forward reaction. More product was added so the reverse reaction took place at a larger rate and re-established a larger equilibrium concentration of the reactant, H2PO4-.
pH of solutions
In experiment #4, the ____________________ is governed by the Henderson-Hasselbalch equation:
estimated pKa
In experiment #4, the __________________________ of the indicator solution is test tube #6. Test tube #6 had a color that looked to be an even blend of blue and green-yellow, composing of equal parts Bromothymol blue and pH solutions. Test tube #6 had a pH of 6.68, positioning itself in the middle range of the data, considering the calibration was at a pH of 7.02.
H2PO4- (aq) + H2O (l) <---> HPO4 2- (aq) + H3O+
In experiment #4, the weak acid, H2PO4- ionizes according to what equilibrium equation?
pH, log
In experiment #4, we plotted the ________ of each buffer solution concentrations on the y-axis vs. the ______ ([HPO42]/[H2PO4-]) on the x-axis.
H2PO4-
In part I of the experiment #4, students will determine the pKa of a weak acid. Which acid will be used?
pH, absorbance
In part I of this experiment #4, students will use the LabQuest to measure the _________ of solutions. In part II, the LabQuest will be used to measure the _________ of solutions
blue, yellow
In part II of the experiment #4, students will be assigned an indicator dye (either Bromothymol blue or Phenol red). In basic solution, the indicator dye Bromothymol blue is _______. In acidic solution, the indicator dye Phenol red is __________.
7
In part II of this experiment #4, you will need to calibrate the spectrometer for your LabQuest station. To do this, insert a cuvette containing solution from Erlenmeyer flask # ___ from the first week of the experiment.
0.200 mol/L
In this experiment #3, there are two different concentrations of both Fe3+ and SCN- solutions. Please make sure you are using the correct solutions in all cases or you may need to repeat the experiment. To generate the calibration curve in the quantitative part of the experiment, the concentration of Fe(NO3)3 is:
2, 4
In this experiment #3, there is a qualitative portion and a quantitative portion. Students will work in groups of ____ for the qualitative portion and groups of ____ for the quantitative portion.
weak acid
Ka<1 and HA is incompletely ionized; both the acid and its conjugate base (A-) will be present at equilibrium
strong acid
Ka>>1 and the acid is completely ionized in water to generate H3O+ and A-
ideal gas law
M = n/V; M = P/(RT)
pH probe
Students will use a _____ ____________ attached to the Vernier LabQuest for measurements in this experiment #5.
4.5
The Henderson-Hasselbalch equation is derived from the equilibrium expression for an aqueous solution of a weak acid. It reveals that the pH of an aqueous solution is governed by the pKa of the acid and the log of [A-]/[HA] where [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid. If a solution of a weak acid has a pH of 4.50 and [A-]/[HA] = 1.0, what is the pKa of the acid? (the log of 1 is equal to 0, the pH = pKa)
energy of activation
The __________ ___ _______________ is interpreted as the minimum energy required to bring about the reaction. You will determine this quantity from rate constant measurements at a series of different temperatures.
reaction orders
The ___________ ____________ (typically small whole numbers) will be determined in this experiment by graphical analysis of your data.
slope
The ___________ of a plot of pressure vs time will measure the rate of the reaction.
lowers
The addition of a non-volatile solute _____________ the vapor pressure of the solution.
O2
The decomposition of H2O2 generates ______ as a product
n = c x V n = .30m x 0.05L n = 0.015 mol m = n x MM m = (0.015mol)(166g/mol) m = 2.5 g
The decomposition of hydrogen peroxide (H2O2) is sped up by the catalyst, potassium iodide (KI). How many grams of KI would be needed to prepare 50. mL of a 0.30 M aqueous solution? Be sure to report your answer in the correct number of significant figures.
1.0
The effective range of a buffer is governed by the Henderson-Hasselbalch equation. Buffers are effective over a range of pH = pKa ± _____.
8.314
The ideal gas law will be used in this experiment #2 to convert the rate of each of your reactions into units of molarity/second (M/s). The value of R used in this experiment is _________ L·kPa/mol·K.
change in pressure
The kinetics of the reaction can be monitored in a closed vessel by measurement of _____________________________ above the solution in which the decomposition occurs as a function of time
individually
The lab report you turn in for this experiment #3 is a typed summary report with the sub-headings "Qualitative Data" and "Quantitative Data". You will need to prepare a large table for the quantitative part of your report. Guidelines are in the lab manual in the section titled "The Report". This lab report should be completed ____________, just like all of your lab reports in CHM 144 & CHM 145.
lowest
The leveling off occurs in the forward reaction due to the concentration volume of the conjugate base, HPO42- being at its ___________. Therefore, there is not much of that aqueous solution to react with the acid/reactant, H2PO4-. This is also true for the reverse reaction.
lower
The magnitude of Ka indicates the tendency of the acid to ionize in water. Therefore, a large value of Ka would indicate a strong acid. Just as pH = -log [H3O+], pKa = -log Ka. Hence, the stronger the acid, the _____________ the pKa of the acid.
First Derivative
The method that gave results closer to the true value was the ______________________ method performed in LoggerPro.
Colligative Properties
The objectives of experiment #1 were to understand _______________ _________________ by measuring the boiling point elevation of water caused by the dissolved compounds table salt (NaCl) and table sugar (C12H22O11).
closer
The pKa of the indicator estimated by visual inspection compared to the pKa determined from the absorbance measurements, shows that there is a slight variance between the values. The estimated pKa at 6.68, is lower than the pKa values determined at 7.08 and 7.23. The value of 7.08 would be the one that I trust more. I think this because it is ___________ to the neutral pH value on the pH scale.
Rate = k[H2O2][KI]
The rate law for experiment #2 is _________________ and depends on the concentrations of reactants.
unit of time
The rate of a chemical reaction is defined as the change in the concentration of reactants (or products) per _____ ___ ______.
colorless, colorless, rust-red
What color is the aqueous solution of Fe(NO3)3? What color is NH4SCN? What about the complex ion FeSCN2+?
As time increased, the temperature increased.
What did the temperature of water as a function of time plot tell us in experiment #1?
spectrometer, absorbance
What is the name of the LabQuest instrument that is used for the quantitative part of experiment #3? What is being measured?
To maintain a relatively constant pH in a solution
What is the purpose of an acid-base buffer?
As the dependent variable increases, the independent variable decreases. The molality of sugar did not increase above 0.80 mol/kg and the molality of salt did not increase greater than 8.0 mol/kg. The boiling point elevation for sugar peaked at 2.1 degrees Celsius and for salt it peaked at 5.8 degrees Celsius.
What trends occurred between the boiling point elevation and molality in experiment #1? Were there any notable differences between the results for salt and sugar?
Le Chatelier's
When a system at equilibrium is disturbed by a change in temperature or a change in the concentration of one of the components, it will re-attain equilibrium by undergoing a net reaction that reduces the effect of the disturbance. This is known as _____________________ principle.
S
When the data from part II of the experiment #4 is graphed, the plot should resemble an ____-shaped curve.
KCl, because it forms an ionic compound. Intermolecular forces tell us that KCl has a high boiling point because of the strong bond holding the compound together. Stronger bonds need more energy to dissolve or break apart in water.
Which solution did you expect to have a higher boiling point in experiment #1?
graduated cylinder, this improves precision
Which volume-measuring device will be used to prepare all of the solutions for the quantitative portion of the experiment #3? Why?
provides insight into how a reaction takes place
Why is it important to know the order of a reaction?
procedure
With respect to CHM 145 notebook rules (see syllabus), before coming to lab, students should write out the _______________ for each experiment from the directions given in the lab manual.
catalysts
___________ increase the rate of a reaction without undergoing a net change in their own structures.
equilibrium
_______________ occurs in a chemical reaction when the rates of the forward and reverse reactions are equal
reaction mechanism
a sequence of possible single reaction steps that sum to the overall reaction
proton donor (H+)
acid
chemical kinetics
area of chemistry that is concerned with the speeds, or rates, of reactions
proton acceptor
base
series of different temperatures
by measuring the rate constant at a ________________________________, the activation energy can be discovered
Arrhenius Theory
defined an acid as a substance with H in its formula that dissociates in water to yield H3O+ and a base as a substance with OH in its formula that dissociates in water to yield OH-
Lewis Theory
defined an acid as any species that accepts an electron pair to form a bond and a base as any species that donates an electron pair to form a bond
Bronsted and Lowery Theory
defined an acid as any species that donates a H+ ion, a proton donor, and a base as any species that accepts a H+ ion, a proton acceptor
k = rate (m/s)/[H2O2]a[KI]b
formula used to calculate the rate constant for all 10 trials in experiment #2
rate
how rapidly a chemical reaction occurs
reverse reaction
if additional product is added the _________________________ occurs at a larger rate -system reacts to consume excess product (so there will be more reactants-shfts towards reactants)
forward reaction
if additional reactant is added the ________________________ occurs at a larger rate -system reacts to consume the excess reactant (so there will be more products-shifts towards products)
elevation
lowering the vapor pressure leads to an _____________ in the boiling point of the solvent
acid
molecule that dissociates in water to yield H₃O+
base
molecule that dissociates in water to yield OH-
molality
moles of solute particles/kg of solvent
colligative properties
properties that depend on the collective effect of the number of solute particles
k
rate constant
rises
rates of most chemical reactions increase as the temperature ____________
exponents a and b
reaction orders; typically small whole numbers that are derived experimentally; normally 0, 1, or 2
reactant concentrations
reaction rates depend on ________________ ________________
non-volatile solutes
substances with no measurable vapor pressure
straight line
the Arrhenius equation has the form y = mx + b which is shown in the form of a ________________ on a plot
H2PO4- (aq) + H2O (l) ⇌ HPO42- (aq) + H3O+ (aq)
the balanced equation that shows how H2PO4- reacts with water
higher
the boiling point of a solution will be ____________ than the boiling point of the pure solvent
reaction rate
the change in the concentration of reactants or products per unit of time
system at equilibrium
the concentrations of the reactants and products have reached constant values.
boiling point elevation
the difference between the boiling point of the solution and the boiling point of the pure solvent
HA (aq) + H2O (l) <---> H3P+ (aq) + A- (aq)
the equilibrium that describes the ionization of an acid in water
[H3O+] = Ka ([HA]/[A-])
the ionization equilibrium expression solving for H3O+
temperature, temperature
the magnitude of k changes with __________________ and therefore determines how ___________________ affects the rate.
Keq, equilibrium constant
the ratio of the forward and reverse rate constants
graphically
the reaction orders were determined _________________ in experiment #2
kPa/s
the slope in the reaction rate is expressed as ________
normal boiling point
the temperature at which the vapor pressure equals 1 atm
V2
total volume of the two combined solutions
S curve, henderson-hasselbach
two ways to determine the pKa 1. find the derivative of the __________________ of (plot of absorbance vs pH) 2.the ____________________________ equation
Le Chatelier's Principle
when a chemical system at equilibrium is distrurbed by a change in temperature, concentration, or pressure, it re-attains equilibrium by undergoing a net reaction that reduces the effect of the disturbance