CHEM 104 Quiz: Acids/Bases Quiz

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violetoyster294

30 Oct 2014

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The following acid/base table may be useful in answering the question below. When an acid is added to water, it will donate a hydrogen ion (proton) to the water resulting in the production of the conjugate base of the acid and a hydronium ion. You should review sections 16. 1 and 16. 8 from your text. For more help, you may want to go through the molecular structure and behavior module of the online acids and bases tutorial. Consider the stoichiometric reaction between equal volumes of 0. 10 m solutions of the following acid and base: Will any of the products affect the ph of the aqueous solution, and if so, will it be acidic or basic. Li+(aq) + oh-(aq) + h2o(l): yes, basic ph. The reaction between a strong acid and a strong base results in the production of water and a salt that forms a neutral solution.

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Â¥Write a brief (less than 150 words, 3 or 4 sentences) of the aim of the experiment

Â¥Address question of what was done, how it was done, and what compounds were used.

Â¥Include Chemical Reaction where relevant.

See text below. Please provide Purpose following the steps above. Thank you.

Alcohols R-O-H are characterized by the presence of the hydroxyl group

Because of intermolecular co-association, alcohols show higher boiling points

than aldehydes, ethers, or ketones of similar mass. Among alcohols, boiling

points increase with increased surface area. Increased surface area occurs as

more carbons are added to the alkyl portion or with decreased branching of th

alkyl group. Thus, 1-propanol has a higher boiling point than ethanol

(increased C#). 1-propanol also has a higher boiling point than 2-propanol

(decreased branching)

The solubility of alcohols in water decreases as the surface area of the

alkyl (nonpolar) portion increases. Thus, alcohols become less soluble in water

as the number of carbons in the alkyl group increases. When comparing

alcohols that contain the same number of carbons, branching of the alkyl group

decreases its surface area and thus increases the water solubility of the alcohol.

1-Propanol is slightly soluble in water while 2-propanol is quite soluble

The limit of water solubility is reached at about 4 or 5 carbons in the

alkyl group. However, it is possible to "salt out" or separate a water-soluble

alcohol from the water it is mixed with by the addition of certain salts such as

K2CO3. If an alcohol-water mixture is saturated with such a salt, the anion

selectively forms strong ion-dipole bonds to water, leaving the alcohol as a

separate layer. Thus, water is separated from the alcohol because the ionic salt

bonds more strongly with water than with the alcohol. There are twice as many

ion-dipole bonds possible with water than with the alcohol.

Alcohols are relatively acidic organic molecules, being weaker acids than

water, yet stronger than acetylene.

Acidity: water > alcohol > acetylene > ammonia > alkanes

Alcohols may be converted to sodium alkoxides by the action of metallic sodium

under anhydrous conditions. Primary alcohols react more readily tharn

secondary or tertiary alcohols due to the relative stability of the alkoxide ions in

the solvent alcohol.

R-o:-

Alcohols undergo nucleophilic substitution reactions with ease under

acidic conditions. The relative ease of such substitution for primary, secondary

and tertiary alcohols serves as a means of classification. Upon treatment with

concentrated HC1, tertiary alcohols are rapidly converted to alkyl chlorides

Secondary alcohols require warming in a boiling water bath for 15 minutes

Primary alcohols do not react at all under these conditions. The alkyl chlorides

formed by this reaction are insoluble in the concentrated HCl and form an

easily detectable second layer

R-O-H HC1 >

Ð-Ð-Ð

Complications arise from the fact that benzylic and allylic alcohols also

react immediately, but yield products that are soluble in the concentrated HCI

Alcohols are easily oxidized by any of these strong oxidizing agents:

dilute KMnO4, NajCr.07 in acid, or CrO3 (chromic anhydride)

acidified sodium dichromate is somewhat helpful in the classification of

alcohols. Prim

ary and secondary alcohols are oxidized rapidly within several

minutes, while tertiary alcohols do not react under these conditions

Oxidation occurs more easily when the carbon bearing the -OH group

bonded to at least one atom of hydrogen. The complex mechanism of

involves the transfer of a hydride ion H:- as a critical step.

hydrogen atom is absent (as in tertiary alcohols), no oxidation can occur

directly. Tertiary alcohols may be oxidized indirectly through the formation of

Alcohols which contain the partial structure C- CH3 give positive

Ð¾Ð½

results in the iodofora reaction. When the alcohol is dissolved in water and

KOH, the addition of a solution of 12 in KI yields a yellow precipitate of

iodoform, CHI3, along with the oxidized portion of the alcohol

Hypoiodite ion is produced by the reaction between molecular iodine and

hydroxide ion:

I2 + 4(OH) -

The hypoiodite ion in turn oxidizes an alcohol with the proper structure

Once this oxidation has occurred, additional hypoiodite ion continues to

react with the aldehyde or ketone produced:

CH,-C

3 (0I)-IC-c3 (OH)-

resulting in the final substitution reaction to give iodoform and the anion of a

carboxylic acid containing one carbon atom less than the original alcohol:

128

lodoform precipitates as fine yellow crystals. This precipitate, a result of a

specific oxidation followed by a substitution, is an indication of the partial

structure of the original alcohol.

Simple alcohols of up to ten carbon atoms may be detected by the

ammonium hexanitrocerrata(lV) reagent, (NH4Ce(NO,). The hexanitro-

cerrate(IV) ion has a distinctive orange-red color. When an alcohol, ROH is

added, one or more OR- groups replace the nitro groups as ligands in the

complex ion. The new complex ion formed has a distinctive color. The general

equation for this reaction is:

This test is positive for aliphatic alcohols up to approximately ten carbons in

length. Phenols or larger alcohols yield insoluble mixtures

Alcohols react with acids or with acid derivatives to form esters. This

characteristic reaction is useful in the formation of specific esters such as ethyl

acetate, acetylsalicylic acid (aspirin) and methyl salicylate (oil of wintergreen). In

this experiment, methyl salicylate will be prepared by reacting methyl alcohol

with salicylic acid in the presence of a trace of sulfuric acid as the catalyst

Lower molecular weight alcohols such as ethanol, methanol, and

isopropyi alcohol absorb water on exposure to moist air or are prepared as

azeotropic mixtures containing water. The presence of water in alcohols may be

detected in several ways. If a small lump of CaC2 is added to any alcohol that

contains a trace of water, a reaction occurs that produces immediate bubbles of

acetylene gas:

Thus, evolution of acetylene gas upon the addition of calcium carbide indicates

the presence of water in the alcohol. Another way to test for the presence of

water in alcohols involves the use of anhydrous cupric sulfate powder (CuSO4)

Anhydrous cupric sulfate is a very pale, almost white powder. In the presence of

water, deep blue crystals of cupric sulfate pentahydrate,

CuSO4-5H20, are formed. Thus, anhydrous cupric sulfate indicates the

presence of water in an alcohol by a simple color change.

Commercial ethanol is produced as an azeotropic mixture that contains

95% ethanol and 5% water. Absolute alcohol contains no water. Many reagent

grade alcohols are composed of a nonpotable mixture of 95% ethanol and 5%

isopropyl alcohol. Thus, absolute ethanol and reagent alcohols are both water

free and give negative results when tested for the presence of water

Titration is a technique used to find the concentration of a solution of known composition. For example, you may have a solution of hydrochloric acid whose concentration is known approximately and you want to determine the exact concentration. Or you may want to test a vitamin C supplement to see whether it really contains the stated amount of Vitamin C. Titration is a volumetric technique. A volumetric technique means that the concentration is determined by comparing the volume of a solution of known concentration to the volume of the unknown solution. The two solutions are combined until a known condition is achieved, such as the neutralization is an acid by a base. This is known as the equivalence point. The volumes of the two solutions with the known concentration of the base gives the concentration of the acid. Titration Materials There are three substances in every titration: â¢ the analyte â the solution in the reaction vessel â¢ the titrant â the solution that is being delivered into the reaction vessel â¢ the indicator â a Bronsted-Lowry conjugate acid-base pair, where the acid form of the indicator is a different color than the base. The indicator changes color at the end point. A typical indicator is phenolphthalein, which is colorless in the presence of an acid and pink in the presence of a base. During the titration, the indicator reacts with the other chemical compounds, so it is important to minimize the amount of indicator added. Titration Procedure For the titration of NaOH (a strong base) with HCl (a strong acid), phenolphthalein is a good indicator because it changes color at a pH that is close to the equivalence point. In a range of acidic to neutral solution (pH < 7), phenolphthalein is colorless, but as the solution becomes basic, at around a pH of 8.2, phenolphthalein turns pink. The glass container used to deliver precise quantities of titrant into the analyte is called a burette. It is a long glass tube that has finely marked graduations every 0.1 mL and a stopcock valve at the bottom. A 50 mL burette is about 1 meter long and can be read to an accuracy of 0.01 mL. The burette is filled to the top with a solution of known concentration, and a flask is filled with a solution of unknown concentration. A small amount of an indicator is also added to the flask to visualize the equivalence point. The titration is performed by progressively adding the reactant in the burette to a known volume of the other reactant in the flask. The reaction vessel with an indicator is placed beneath the burette. The stopcock is turned to allow the titrant to flow slowly into the flask. Titrant is added in increments, and after each increment the flask is swirled and checked for a color change. Here is where you have a decision to make. On the one hand, you want to be as exact as possible in order to get good results. On the other hand, it would take hours to add titrant one drop at a time from the beginning until the end point. The accepted technique, therefore, is to perform two titrations. The first is a coarse titration performed by adding relatively large increments of titrant. For example, you could add titrant (from the buret) in 2 mL increments. You will not get exact results, but you will quickly determine a narrow range in which the end point occurs. Suppose that with the phenolphthalein indicator, the color in the flask is colorless after addition of 20 mL of titrant, but it turns pink after the next 2 mL of titrant is added. You can conclude that the end point must lie between 20 and 22 mL. A second, fine titration is performed by refilling the burette to the titrant and using a new flask containing your second reactant. You already know that the end point occurs after 20 mL of titrant is added, so the first step is to go ahead and add nearly the whole 20 mL at once. Then you start adding titrant in much smaller quantities, say 0.2 mL at a time. You may even switch to adding single drops. This way, you can catch the end point of the titration to an accuracy of one or two drops. Be sure to swirl the flask after each addition, or even while you are adding the titrant. It is also possible to use a pH meter to identify the equivalence point. In this case, you would still perform two titrations, one coarse and one fine. The difference from using an indicator is that you would record all of the pH values and construct a graph of the pH vs. volume of added titrant. The point of inflection of the curve between the acid and base regions indicates the equivalence point. At the equivalence point, the analyte from the burette has neutralized the reactant in the flask. In an acid-base reaction, this means that the flask contains the same number of moles of both acid and base as long as the acid is monoprotic (an acid that gives up only one proton when it dissociates in water). Determining the Concentration of an Unknown You will use several equations to determine the concentration of the unknown using the measurements you will make in the lab. M(known) x V(known,L) = moles known moles known x (stoichiometric conversion, mols to mols step) = moles unknown (moles unknown) / V(unknown,L) = M unknown Procedure: Procedures 1. Move your mouse cursor over the hydrochloric acid (HCl) and sodium hydroxide(NaOH) on the Materials shelf. You will see that the concentration of the sodium hydroxide is known to be 0.1 M while the concentration of hydrochloric acid is unknown. 2. Take a burette from the Containers shelf and place it on the workbench. 3. Add 50 mL of sodium hydroxide (NaOH) titrant. (50 mL is the capacity of a burette). Notice that it doesn't fill completely to the top mark. This is because there is space in the burette below the bottom mark. In addition, this is NOT the volume in the burette but rather this is your INITIAL READING. In the wet lab, you would need to let some of the solution flow through the stopcock in order to assure that the NaOH solution completely filled the volume of the burette down to the tip. You don't have to do that here. 4. Note the buret reading as your initial NaOH volume (mL). You can double click on the buret and select show close up to view the volume more closely. 5. Take a clean Erlenmeyer flask from the Containers shelf and place it on the workbench. 6. Add 10 mL of hydrochloric acid(HCl), unknown concentration, to the Erlenmeyer flask. Record the volume of HCL (mL) in the data table. 7. Add 10 mL of water to the Erlenmeyer flask. This increases the total volume in the flask, making it easier to see the color change, but note that the value to use for the volume of hydrochloric acid HCl when calculating its concentration is still 10 mL. 8. Add 2 drops of phenolphthalein solution to the Erlenmeyer flask. 9. Move the Erlenmeyer flask anywhere on the base of the burette. The Erlenmeyer flask is connected to the burette so that liquid will drip from the burette into the Erlenmeyer Flask. 10. Flow of the titrant from the burette is controlled by the black knob at the bottom of the burette glass tube. You can deliver one drop of titrant with each short click of the black knob, and you can deliver a stream of titrant from the burette by clicking-and-holding the black knob - the longer you hold the knob, the more titrant will be delivered all at once. Here is what the setup on your workbench should look like: 11. You are now ready to start the first coarse titration. Try to find the length of time required to click and hold the burette knob so that you deliver approximately 2 mL of sodium hydroxide from the burette to the Erlenmeyer flask. After each click of the knob, move the mouse cursor over the burette â this will enable you to see its current volume. To determine the amount of titrant delivered from the beginning until now, subtract the amount of liquid currently in the burette from the starting volume. 12. Continue to add the sodium hydroxide titrant in 2 mL increments. Each time, note the burette volume level. Note when the end point is passed (when the color of your solution changes). You now know between which two readings the endpoint occurred. For example, if you recorded 32 mL before the end point, but 34 mL was past the endpoint, record the 32 mL as your FINAL NaOH (mL). Calculate the volume of titrant that was added when 32 mL were delivered (subtract your INITIAL NaOH (mL) reading). You know that the FINE TITRATION can begin after approximately that amount of titrant. 13. Remove the Erlenmeyer flask from the burette and place them both in the Recycle Bin. 14. Take a clean Erlenmeyer flask from the Containers shelf and place it on the workbench. 15. Add 10 mL of hydrochloric acid, 10 mL of Water and 2 drops of phenolphthalein to the Erlenmeyer flask. 16. Place the Erlenmeyer flask at the base of a new burette. 17. Add 50 mL of sodium hydroxide titrant to the burette. 18. Add the initial large quantity of titrant, determined performing the coarse titration, (which in the example in #10, was 16 mL) so that you can begin with the fine titration. The solution in the Erlenmeyer flask should still be colorless. 19. Add sodium hydroxide from the burette drop-wise. This means adding a single drop at time which is done with single, short clicks on the black knob. You may decide to click and hold for short times, but in doing so you may miss the exact endpoint of the titration. 20. When the solution in the Erlenmeyer flask changes color, stop adding titrant. Record FINAL NaOH (mL) reading in the data table. Calculate the NaOH Delivered (mL) by subtracting the initial reading from the final reading. Record both values in the data table. 21. In theory, we would repeat at least two more FINE TITRATIONS with fresh samples of HCl and full burets. However, since this is just a tutorial, exit the lab once you have a recorded the data in the data table. Data: This includes all observations and questions posed to you 1. For your most exact titration, record the following: a Volume of NaOH solution in the burette at the start (mL): 50.00ml b Volume of NaOH solution in the burette at the end (mL): 33.65ml c Volume of NaOH solution delivered to the flask (mL): 16.35ml d Volume of HCl solution in the flask (mL) 10.0ml 2. Calculate the molarity of the HCl. this is the value I come up with i need help calculate the molarity of acid

CHEM 104 Quiz: Acids/Bases Quiz

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