READ: Neutralization Reactions

READ: Neutralization Reactions

Neutralization Reactions

Neutralization (a reaction between an acid and a base that produces water and a salt) occurs whenever acids and bases are mixed. The general reaction is shown below:

acid+base→salt+water

Acids are a combination of a hydrogen cation and a nonmetal anion. Examples include HCl, HNO₃, and HC₂H₃O₂. Many bases are a combination of metal cations and nonmetal anions. Examples include NaOH, KOH, and Mg(OH)₂. According to the Arrhenius definitions of acids and bases, the acid will contribute an H⁺ ion that will react to neutralize the OH^- ion contributed by the base, producing neutral water molecules.

All acid-base reactions produce salts. The anion from the acid will combine with the cation from the base to form the ionic salt. Examples are shown below.

HClO4(aq)+NaOH(aq)→NaClO4(aq)+HOH(l)H2SO4(aq)+2KOH(aq)→K2SO4(aq)+2HOH(l)(Note:HOH=H2O)

No matter what the acid or the base may be, the products of this type of reaction will always be a salt and water. Aside from the fact that the H⁺ ion will neutralize the OH^- ion to form water.

Titrations

The typical laboratory procedure for determining the concentration of acid and/or base in a solution is to complete a titration. As we go through this lesson, we will take apply the knowledge we have obtained about acids and bases, chemical reactions, and molarity calculations to the concept of titrations.

The Titration Process

One of the properties of acids and bases is that they neutralize each other. In the laboratory setting, a titration - an experimental procedure where an acid is neutralized by a base (or vice versa) - is commonly performed to find the concentration of an acid or base. Titration is the addition of a known concentration of base (or acid), to a solution of acid (or base) of unknown concentration. Since both volumes of the acid and base are known, the concentration of the unknown solution is then mathematically determined.

When doing a titration, you need to have a few pieces of equipment. A burette like the one shown below is used to accurately dispense the volume of the solution of known concentration. An Erlenmeyer flask is used to hold a known volume of the solution whose concentration is unknown. A few drops of the indicator are added to the flask before you begin the titration. The endpoint is the point where the indicator changes color, which tells us that the acid is neutralized by the base. Theequivalence point (the point where the number of moles of acid exactly equals the number of moles of base) will be detected when the indicator changes color (if the appropriate indicator was chosen). The equivalence point can be used to calculate the original concentration of the acid or base.

The Mathematics of Titration

For the calculations involved here, we will only use our acid and base examples where the stoichiometric ratio of H⁺ and OH^- is 1:1. To determine the volume required to neutralize an acid or a base, or in other words, to reach the equivalence point, we will use a formula similar to the dilution formula:Ma×Va=Mb×Vb

Where Ma is the molarity of the acid, Va is the volume of the acid, Mb is the molarity of the base, and Vb is the volume of the base. Note that if the acid and base do not neutralize each other in a 1:1 ratio, this equation does not hold true.

Example 2

When 10.0 mL of a 0.125 mol/L solution of hydrochloric acid, HCl, is titrated with a0.100 mol/L solution of potassium hydroxide, KOH, what is the volume of the hydroxide solution required to neutralize the acid?

Solution:

Ma×VaMaVaMbVb=====Mb×Vb0.125 mol/L10.0 mL0.100 mol/L?

This video shows the technique for performing a titration using an indicator:

Summary

• An indicator is a substance that changes color at a specific pH and is used to indicate the pH of the solution relative to that point.
• Universal indicator is a mixture of indicators that produces a different color for each pH from 0 – 14.
• A neutralization reaction between an acid and a base will produce a salt and water.
• In a neutralization reaction, the acid will produce H⁺ ions that react to neutralize the OH^- ions produced by the base, forming neutral water. The other product will be an ionic salt.
• A titration is the addition of a known concentration of base (or acid) to a solution of acid (or base) of unknown concentration.
• The equivalence point is the point when the amount of acid equals the amount of base.
• A standard solution is a solution whose concentration is known exactly and is used to find the exact concentration of the titrant.
• For titrations where the stoichiometric ratio of mol H⁺ to mol OH^- is 1:1, the concentrations or volumes for the unknown acid or base can be calculated with the formula: Ma×Va=Mb×Vb.

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