Chem 144 Final Exam Prep (Labs 5-10)
What are the sig figs for the buret reading for the base/titrant?
2 digits beyond decimal point
What pH range does phenolphthalein typically change in?
8.2-9.2
How can pH be directly measured in an acid-base titration?
A pH electrode that is sensitive to [H+]
How does the end point become apparent in the traditional method?
A persistent slight pink color will appear
What is an analyte and what was it in the fifth lab?
A solution of *unknown* concentration --> C6H8O7 in this lab
What is a titrant and what was it in the fifth lab?
A solution of known concentration --> NaOH in this lab
What is the purpose of citric acid?
Add tartness to otherwise sweet drinks, hence why it is one of the several acids that are commonly present in carbonated beverages to make them have a bitterness to them
What is significant about the strength of the protons in a polyprotic acid in terms of citric acid?
All 3 acidic protons of this acid are of similar strength, so they all react with NaOH to similar extent --> 3 reactions cannot be readily distinguished from each other
What is the waste disposal for the fifth lab?
All solutions may be poured down the laboratory sink drains and rinsed with tap water after use because they are simple acid-base reactions and only produce water/salt as a result!
What is a polyprotic acid?
An acid that has multiple acidic protons available to react with a base
What is a monoprotic acid?
An acid that has only one proton available to react the base --> HCl is an example
How can pH be indirectly measured in an acid-base titration?
An indicator dye that changes color at the desired pH
What is a spectator ion?
An ion NOT involved in the reaction therefore will not appear in the net ionic equation
Why is citric acid in much higher concentrations than other acids in carbonated beverages?
Because of its purpose to make a drink bitter
Why can you determine the amount of citric acid based on an acid-base titration?
Because of the large differences in concentrations between the acids
Why does the pH of the titration with citric acid fit more with the basic side of the pH scale?
Because the acid is not strong enough to make the solution entirely acidic/neutral so the solution ends up more basic
What is the only other acid than citric acid is NOT in a higher concentration to in comparison?
Carbonic acid (H2CO3)
How was it possible to determine only the citric acid in the beverages NOT the carbonic acid?
Carbonic acid was removed through boiling process and citric acid is in a high concentration
What is the color change with phenolphthalein?
Changes color from colorless in acid solution to pink in basic solution
What is the waste disposal for the sixth lab?
DO NOT flush any of these materials down sink and dispose in containers of fume hood
How do polyprotic acids react with bases?
Depends on number of H atoms attached to the acid --> also impacts stoichiometry of the base on reactants side (therefore impacting number of water molecules)
What is the purpose of calculating and showing the first derivative in the titration curve?
Displays inflection point that is critical for understanding change in concavity during titration AKA equivalence pt --> also determines amount of NaOH at this point
Why is a magnetic stir plate with a small magnetic stir bar used in the modern titration method?
Ensures thorough mixing of reactants
What is the summarized goal of the sixth lab?
Establish the relative chemical reactivity of several metals and hydrogen as reducing agents by utilizing two types of single displacement reactions
What is reduction?
Gain of electrons
What is the formula of removing carbonic acid to make a soda go flat?
H2CO3 (aq) --> H2O (l) + CO2 (g)
What is the balanced reaction for the acid-base reaction seen in the traditional method (also applies to modern)?
H3C6H5O7 (aq) + 3NaOH (aq) --> Na3C6H5O7 (aq) + 3H2O (l)
What is the formula for the acid-base reaction occurring in this lab with citric acid and the base NaOH?
H3C6H5O7 (aq) + 3OH- (aq) --> 3H2O (l) + C6H5O7 3- (aq)
Why can the logarithmic scale be used when discussing hydrogen ion concentrations in water?
Hydrogen ion concentrations in water can range from around 1.0 M (mol/L) in 1.0 M HCl (or a similar strong acid) to around 1E-14 M in 1.0 M NaOH --> aka it has a WIDE range!!!
What will provide the volume of standard NaOH solution necessary to titrate the acid in the flask?
Initial volume reading of the buret - the volume reading of the NaOH solution measured at the end point
Why does the modern titration method work for highly colored solutions?
It does NOT require the observation of a visual end point
What does it mean if a substance is oxidized?
Loses electrons and causes a reduction aka is the *reducing agent*
What is oxidation?
Loss of electrons
What is another method to determine the end point of a titration?
Measuring the pH of a solution with a pH electrode
Which method gives results that align more closely to true equivalence point?
Modern b/c quantitative data supporting where equivalence pt occurred and traditional was mostly qualitative w/ student observation (aka human error)
Why is a pH electrode used in the modern titration method?
Monitors pH of solution as titrant is added from the buret into known volume of acid solution
What type of acid is acetic acid?
Monoprotic so it reacts with NaOH with a 1/1 stoichiometry
What is significant about the setup for the traditional method?
Must be possible to see the color change from colorless to light pink at the end point of the titration
Where do the end points occur when there is a titration of a weak acid with a strong base?
NOT at pH 7.0
Why can the traditional method of finding the total volume of NaOH not be used to calculate the amount in the modern titration method?
NaOH was added past equivalence point until a pH of 10.5 so using total volume would account for more than what is neccessary to determine equivalence point
What are displacement reactions?
Number of substances remains same during reaction but atoms/ions exchange places *Main focus of fifth lab*
What is a single-displacement reaction?
One element displaces another element from a compound *Main focus of fifth lab*
What are decomposition reactions?
One reactant decomposes into two or more products
What occurs in any titration?
One solution of known concentration is used to determine the concentration of another solution through a monitored reaction of known stoichiometry that goes to completion
What does the measured end point of a titration with indicator dye do?
Overestimate the equivalence point slightly therefore the end point does not equal the equivalence point
Lab 6
Oxidation-Reduction Reactions - The Activity Series of Metals
What are all single-displacement reactions?
Oxidation-reduction processes
What does removing carbonic acid allow for scientists to do?
Quantitatively determine the amount of citric acid through an acid-base titration
What will make a soda go flat?
Removing carbonic acid
What is a molecular equation?
Shows all reactants and products as if they were intact, undissociated compounds
What is a total ionic equation?
Shows all soluble ionic substances dissociated into their component ions
What is the name for the estimate of the equivalence point with the indicator dye?
The end point
If the indicator has been chosen well and the experimenter has been careful not to overshoot the end point, what happens?
The end point will coincide with the equivalence point
What happens when a strong acid is titrated by a strong base?
The end points is reached at a pH of 7.0 ([H+] = 1.0E-7 M)
How is it that the buret is read to two decimal places?
The last digit is estimated by interpolation
What is an equivalence point?
The point in the titration when the number of moles of the base added (titrant) is stoichiometrically equal to the number of moles fo acid originally present in the analyte
When can the equivalence point be detected/estimated in the use of the LabQuest device in the modern titration method?
The point of most rapid pH change as a function of titrant volume in a plot of pH (y-axis) vs. titrant volume (x-axis)
What occurs in an acid-base titration?
The reaction that is utilized is an acid-base reaction in which a hydrogen ion (H+, proton) is transferred from an acid to a base
What is the summarized goal of the fifth lab?
Titrate samples of 2 different sodas containing citric acid with NaOH solution of known concentration (standard solution) to determine the concentration of citric acid present in each soda
Why is it important to use colorless soda only for the traditional titration method?
To detect the color change with the dye
What is the significance of quantitatively transferring a solution?
To ensure all of the solution is together and nothing remains in a container (no loss of sample!)
How is NaOH added in the modern titration method?
To pH of 6.5 then drop-wise until pH of 10.5
What are the two different titration methods that will be used in the fifth lab?
Traditional and modern
What type of polyprotic acid is citric acid?
Triprotic so 3 moles of the base are needed to completely neutralize 1 mole of the acid
What are combination reactions?
Two or more reactants form one product
What is the recommended technique for reading the buret?
Using a 3x5 notecard behind the buret and placing a black mark in the middle of it to determine the meniscus
What is one way scientists determine the equivalence point?
Using an indicator dye that changes color to signal that the amount of titrant added is equal to the analyte in a sample
When does a chemical reaction occur?
When a substance is converted into a different substance
What are the units of molarity?
mol/L
What is the end point for citric acid?
pH 8.5
What is a way to represent the pH of an acid-base titration?
pH = -log[H+]
Why is it not a big problem that phenolphthalein overestimates the equivalence point of the titration?
pH changes rapidly near the equivalence point and the different in NaOH volume needed to make the solution pH 7.0 or 9.0 is very small/negligible
What is different in the setup of the modern titration method compared to the traditional method?
-No dye added -Use 250mL beaker instead of flask
How many observable end points are there for citric acid? Why?
1 because all 3 protons of the acid have similar strengths --> therefore enough base will be added to neutralize all three of the acidic protons in one shot
What are the factors that make it possible to calculate the concentration of citric acid in the traditional method?
1) Known concentration of NaOH solution 2) Stoichiometry of the reaction 3) Volumes of acid solution and of the NaOH solution necessary to titrate the acid
What are the two types of single displacement reactions used in the sixth lab?
1) Metal and an aq solution of strong acid 2) Metal displacing a second metal from a solution of the second metal's ions
What initially happens when adding in the phenolphthalein dye?
A pink color will appear but then disappear upon swirling
What is citric acid's formula?
C6H8O7
What does it mean if a substance is reduced?
Gains electrons and causes an oxidation aka is the *oxidizing agent*
What is the most common indicator used for titrations of strong acids by strong bases?
Phenolphthalein
Lab 5
The Quantitative Determination of an Acid in Carbonated Beverages
What does an increase in oxidation number for an atom during a reaction indicate?
The atom is *oxidized*
What does an decrease in oxidation number for an atom during a reaction indicate?
The atom is *reduced*
How do monoprotic acids react with bases?
With a 1/1 stoichiometric ratio
Could modern titration method be done with colorless soda?
Yes b/c there is no color change and color does not impact pH levels within soda
Where are the solutions being placed and why?
-Acid and dye in the Erlenmeyer flask so that the titrant can enter and create the reaction in order to determine the equivalence pt -Base in the buret so that it can be added to the flask and determine end pt
What is the difference between the equivalence point and the end point of a titration?
-Equivalence point occurs when the moles of the titrant (base in this lab) equal the moles of the analyte (acid in this lab) -End point determined by an indicator in traditional method and color/pH change distinguishes end point
Why are oxidation numbers useful?
-To determine which species = reduced/oxidized in redox reaction -Helps balancing redox reactions
What are the objectives of the sixth lab?
-To explore and compare the relative reactivity as reducing agents of several metals and hydrogen (H2) -To use microscale techniques to evaluate a series of possible reactions btwn solid metals and dissolved cations in aq solution -To develop a limited activity series for a group of metals based on experimental data -To write balanced molecular, total ionic, and net ionic equations for displacement reactions
What are the objectives of the fifth lab?
-To understand the difference between polyprotic and monoprotic acids -To learn the basics of the pH scale -To learn how to perform an acid-base titration -To compare and contrast 2 methods (traditional indicator dye vs. modern pH electrode) for detection of the estimated equivalence pt of an acid-base titration -To calculate the molarity (mol/L) of citric acid in two sodas
What is the oxidation number?
-Way of keeping track of which atoms gain/lose electrons in redox reactions -Charge each atom would have if all shared electrons were transferred to the atom that more strongly attracts the electrons aka more EN atom -Sign is written BEFORE the number
What are the safety concerns with the fifth lab?
-Wear goggles in case of glass breaking -Treat sodas as laboratory chemicals and do not consume them -NaOH = caustic aka able to burn or corrode organic tissue (skin!) so wash affected area/clothing thoroughly if needed -Clean up any spills of NaOH as needed
What are the safety concerns of the sixth lab?
-Wear goggles/gloves at all times while messing with these acids that are harmful to the eyes -HCl = toxic if swallowed/inhaled --> also corrosive to skin/eyes -SnCl2 = toxic if swallowed/inhaled -CuSO4 = toxic if swallowed -AgNO3 = toxic and cause burns/stains to skin in brown color so avoid contact with eyes/skin
What are some sources of error associated with the use of a buret?
1) Air bubbles may be trapped in the stopcock or buret tip resulting in error of volume dispersed 2) Not reading the buret volume (meniscus) at eye level results in parallax error 3) Over/underestimating end point color results in error of volume dispersed
What are the steps to balance a redox reaction?
1) Determine O.N. for all elements in reactant/product side 2) Identify the oxidized/reduced species and add electrons to balance charges 3) Balance number of electrons gained/lost 4) Add the two half reactions together to cancel electrons 5) Add additional atoms for charge/mass balance and add states of matter
What are the oxidation rules?
1) Elemental form = 0 2) Monatomic ion = charge on ion 3) Neutral = 0 4) Polyatomic ion = charge on ion 5) F = -1 6) Group 1A = +1, Group 2A = +2 and Al = +3 7) O = -1 in peroxides and -2 in everything else (except when its with F aka OF2 so +2) 8) H = -1 w/ metals or boron and +1 with other nonmetals 9) Halogens = -1
What are the three types of equations used to represent aq ionic reactions?
1) Molecular 2) Total ionic 3) Net ionic
What are the three most important chemical reactions?
1) Precipitation (solid, or precipitate, is formed from materials = originally dissolved in a solvent) 2) Acid-base reactions 3) Redox reactions (net transfer of electrons from one reactant to another and involve elements that are classified as combination, decomposition, and displacement)
What calculations were performed in the traditional method?
1) Volume of NaOH used to reach end point (volume of NaOH in trad = volume of final NaOH - volume of initial NaOH) 2) Moles of NaOH (using volume (L) and molarity (mol/L) so multiply these two together) 3) Moles of citric acid (divide moles of NaOH by 3) 4) Mean/standard deviation for each trial
What are the factors that make it possible to calculate the concentration of citric acid in the modern method?
1) Volume of standard NaOH solution required to reach estimated equivalence point 2) Known stoichiometry 3) Known volume of acid solution
