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Acids And Bases: Titrations, Reactions, And Standardizations

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Acids & Bases:

Reactions, Standardizations, & Titrations

Experiments 21 & 22

Experimental Overview:

The procedure for this experiment was carried out as instructed in the laboratory manual, Experiments in General Chemistry, 4th ed., S.L. Murov, Experiment 21, Acids and Bases: Reactions and Standardizations, and Experiment 22, Acids and Bases: Analysis. There were modifications made by the instructor to dilute the 6M NaOH to 0.1M in 300mls instead of 500mls in Part B of Experiment 21.

One objective for performing these two experiments was to observe qualitatively the reactions between common acids, bases, and the indicators phenolphthalein, methyl orange, bromothymol blue, and red cabbage extract. Another focus of this experiment was to perform a standardization of sodium hydroxide solution using the titration technique with potassium hydrogen phthalate. By titrating the standardized NaOH, determine the percent by mass of acetic acid in an unknown vinegar solution as well as determine the molecular mass of an unknown acid.

Background Information:

When an acid such as HCl and base such as NaOH are mixed with each other they neutralize and produce salts and react as follows:

HCl + NaOH „Ñ- NaCl + H2O + Heat

Heat can be quantitatively measured using a thermometer as a result of the reaction forming water.

Other times, combining an acid such as HCl with a base such as Na2CO3 will result in the following:

2HCl(aq) + Na2CO3 (aq)„Ñ- 2 NaCl + CO2 (g) + H2O (l)

Another unique property of acids and bases are their high reactivity with certain metals. Often their reactions will often be violent.

NaOH + Al(s) + „Ñ- AlO (s) + H2 (g) + Na+ (aq)

Chemical indicators of acids or bases work because their molecules change shape in the presence of ions. The following indicators were used in this lab:

Methyl Orange is often used as an indicator for strong acids. In the presence of H+ ions the indicator turns a bright red, but in a pH of about 4 it is orange, and in more basic solutions it turns yellow.

Bromothymol blue is a more diverse indicator because it turns yellow below pH 6, green at neutral 7, and blue at 8 and above.

Red cabbage juice also turns different colors in different pH. It turns red in an acid and green in a base.

Phenolphthalein is an indicator for bases that turns from clear to pink in the presence of OH- ions. This is due to fact that phenolphthalein molecules are clear, but when they ionize in solution, their H+ ions that make them a weak acid, are pink.

The titration procedure is a quantitative analysis which is used to determine the concentration of a known reactant. A calibrated burette is used to add a reagent to determine the exact amount. The titration is stopped when the solution slightly changes due to the indicator. Indicators such as phenolphthalein are useful in determining the molarities of bases such as NaOH. Through titration, NaOH is carefully added into a specific amount of diluted KHP (potassium hydrogen phthalate 204.22 g/mol) with a few drops of phenolphthalein. When the system can no longer equilibrate the NaOH being added to it because all the KHP, will be used up, there will be an excess of NaOH which the phenolphthalein will indicate by turning pink. By knowing the amount of NaOH titrated, we can calculate the average molarity of OH- reacted because it will react in a one to one ratio with the monoprotic KHP. Titration is a necessary method because it is inaccurate to calculate the amount of OH- ions using the molarity of NaOH, because each dilution of the solution contains a different amount of ions. This process of determining the concentration of a solution is called standardization.

Vinegar is produced by the fermentation of sugar to ethanol and then bacteria catalyze the oxidation of the ethanol to acetic acid. The legal minimum is 4% by mass acetic acid in vinegar. Using the standardized NaOH, it is also possible to determine the percent by mass of acetic acid contained in vinegar as well as the molar mass of and unknown monoprotic acid. Like KHP, acetic acid, HC2H3O2, is an example of a monoprotic acid because in solution, it only releases one H+ ion, becoming H+(aq) and C2H3O2-(aq).

Results and Calculations:

Table 1 summarizes the experimental results of experiment 21 part A. 1 through 4 and includes the reagents added for each step. The reagents which were added are listed on the left, as well as their balanced equations. Observations are displayed on the right hand side. This table summarizes the common reactions of acids and bases as well as the reaction time for an indicator to work.

Table 1. Results of Reactions with Acids and Bases.

Reagents added Observations

2mls of 3M HCl + 2mls of 3M NaOH

HCl + NaOH „Ñ- NaCl + H2O + Heat The initial temperature of the 3M HCl was measured at 24 C . When 3M NaOH was added and stirred, the temperature was found to be 28 C.

1mL of 3M HCl + 1mL of 1M Na2CO3 in a large test tube

2HCl + Na2CO3 „Ñ-2 NaCl + CO2 + H2O Slight fizzing when the solutions were combined. When a lighted splint is inserted into the tube, the flame get smaller and goes out.

3mLs of 3M NaOH + wad of aluminum foil

2NaOH + 2Al + „Ñ- 2AlO (s) + H2(g) + 2Na+ (aq)

A lot of loud fizzing, gas can be seen forming. When lighted splint is inserted, the flame immediately is sucked in and a loud pop noise is made.

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