Neutralization Reaction

In a neutralization reaction, an acid reacts with a base to form a neutral solution containing a salt and water (equation 1). When a neutralization is performed in aqueous solution, the essential feature of the reaction is the combination of hydronium ions and hydroxide ions to form water (equation 2).

HCl (aq) + NaOH (aq) ----> NaCl (aq) + H2O (aq)             (equation 1)

H3O+ (aq) + OH- (aq) ----> 2 H2O (aq)                                 (equation 2)

By adding an indicator, a chemical which changes color when the solution pH moves from an acidic to a basic range, it is possible to approach the exact volume of base needed to completely neutralize the acid but not to add more.

If you can calculate the number of moles of one of the reactants when the indicator changes color, you can calculate the number of moles of the other.

 

Standardization

A standard solution is a reagent whose concentration has been very accurately determined. The concentration of a standard solution is usually determined by titration against a primary standard. In today's lab you will standardize a sodium hydroxide solution by titrating it against an acidic primary standard, potassium hydrogen phthalate. The sodium hydroxide standard solution will then be used in another titration to determine the molecular weight of an acid. The accuracy with which you determine the concentration of the standard NaOH solution sets a limit on the accuracy of all the molecular weights derived from a titration.

The concentration of a solution is a measure of the amount of dissolved substance per unit volume. The concentration unit of Molarity (moles per Liter of solution) will be used in this experiment. A one molar solution (1M) is made by dissolving one mole of a substance in one liter of solution.

A primary standard is a highly purified chemical used to determine the concentration of a standard solution. Potassium hydrogen phthalate, C8H5O4K, ("KHP" for short) will be the primary standard in today's lab. It has a molecular weight of 204.2 g/mole and has one replaceable hydrogen (Figure 1).

Figure 1. Potassium Hydrogen Phthalate

Molecular Diagram: Potassium Hydrogen Phthalate

 

Protic acids are acids with acidic hydrogens. When there is only one acidic hydrogen per molecule, the acid is called monoprotic. When there are two acidic hydrogens per molecule, the acid is called diprotic. KHP is a monoprotic acid with the acidic hydrogen bonded to oxygen.

The end point in a titration is the point at which the neutralization reaction is judged complete. A small amount of an indicator solution is added to the titration to allow you to visually determine the end point. The indicator used in this lab changes from colorless to pink at the end point.

 

Buret

A buret is a glass tube that has a stopcock at one end and very accurate volume markings along its length. The burets used in this lab are 50.0 mL burets with marks every 0.1 mL. The space between the 0.1 mL marks can be estimated to the nearest 0.02 mL. If a volume of at least 10 mL is always used, then the volume can be accurately determined to three significant figures.

A meniscus is the curvature that occurs at the surface of a liquid when it is confined in a narrow glass tube such as a pipet or buret (See Figure 2). The volume is read at the bottom of the meniscus.

Figure 2. Example of a meniscus.

meniscus

 

 

 

Titration

Titration is a technique used to determine the amount of acid or base in a solution. For example, a dilute NaOH solution , approximately 0.1 M, can be prepared and titrated against three weighed samples of the primary standard acid, "KHP". You can use a buret to accurately measure the volume of NaOH needed to reach the endpoint of the reaction. At the endpoint, the number of moles of NaOH used in the titration is equal to the number of moles of KHP weighed out:

moles KHP = moles NaOH

At the end point of the titration, you assume that the moles of added NaOH equals the moles of acid. Thus, the solution has been neutralized.

Knowing the volume of base from the buret and number of moles of acid, you can calculate the concentration of the NaOH solution using the following expression:

MacidVacid = MbaseVbase

Macid = molarity (moles/L) of acid

Vacid = volume (L) of acid

Mbase = molarity (moles/L) of base

Vbase = volume (L) of base

In this example, 

concentration of NaOH = moles KHP  /  volume NaOH

-Example Calculation.

If 32.15 mL of NaOH solution is used to reach the endpoint in a titration of 0.985 g of KHP, what is the concentration of the NaOH solution used to titrate the KHP? Remember, at the endpoint, the number of moles of NaOH equals the number of moles of KHP.

Molarity Equation

 

Endpoint of the Titration

Since all of the solutions in these neutralization reactions are colorless, an indicator is added to help you see the endpoint of the titration. The indicator is a chemical which undergoes a physical change at the endpoint. The indicator used in this experiment is phenolphthalein, a chemical that is colorless in acidic solutions and pink in basic solutions. In each of the titrations you will do today, a NaOH (basic solution) is added to an acidic solution until the solution turns and remains after stirring, a very pale pink. This very slight color change will indicate the endpoint of the titration. At this point, the number of moles of acid and base are equal. If too much base is added, the solution turns darker pink. This indicates that you have gone past the endpoint and you will not be able to use the titration!

When the solution has just turned light pink, the end point is reached. Going past this point produces error which will invalidate your results.

Advanced Concept: titration error. In a titration of an acid with a base, the solution is said to have reached the endpoint when a color change has taken place. Actually, the color change is caused by excess base and thus, at the end point a small amount of excess base exists. However, the assumption is used that at the endpoint the moles of added base equals the moles of acid. This assumption is usually acceptable if the titration only goes slightly beyond the end point. If correct titration techniques are used, titration error can be held below 1%.

Primary Standard Acid, "KHP"

A primary standard is a highly purified chemical used to determine the concentration of a standard solution. Potassium hydrogen phthalate, C8H5O4K, ("KHP" for short) will be the primary standard in today's lab. It has a molecular weight of 204.2 g/mole and has one replaceable hydrogen (Figure 1).

Figure 1. Potassium Hydrogen Phthalate

Molecular Diagram: Potassium Hydrogen Phthalate

 

Protic acids are acids with acidic hydrogens. When there is only one acidic hydrogen per molecule, the acid is called monoprotic. When there are two acidic hydrogens per molecule, the acid is called diprotic. KHP is a monoprotic acid with the acidic hydrogen bonded to oxygen.

 

Determine the Molecular Weight of an Acid

After you have standardized your NaOH solution, you will use the technique of titration to determine the molecular weight of an monoprotic acid. The same procedure will be followed as for the standardization. That is, you will carefully weigh out an amount of an acid. You will do a titration to determine the volume of NaOH solution required to exactly neutralize the acid (reach the end point). This time by knowing the concentration of the NaOH solution, the volume of NaOH, and the mass of the acid, you will be able to determine the molecular weight of the acid.

- Example Calculation.

2.134 g of a monoprotic acid was weighed out and dissolved in 50.00 mL of water. This sample was then titrated with 35.45 mL of a 0.1500 M NaOH solution. What is the molecular weight of the acid? Remember, at the end point, the number of moles of NaOH equals the number of moles of monoprotic acid.

moles of acid