7.02 Acid Base Reactions

If 23.45 milliliters of Ca(OH)2 with an unknown concentration is neutralized by 25.0 milliliters of 0.123 molar HCl, what is the concentration of the Ca(OH)2?

Ca(OH)2 + 2 HCl --> 2 H2O + CaCl2 Given: Molarity of acid: 0.123 M HCl Volume of acid: 24.5 mL HCl Volume of base: 23.45 mL Ca(OH)2 Solving for: Molarity of base: ? M Ca(OH)2 Moles of solute in solution: 25.00 mL HCl x 1 L HCl / 1000 mL HCl x 0.123 mol HCl / 1 L HCl x 1 mol Ca(OH)2 / 2 mol HCl = 0.00154 mol Ca(OH)2 in the solution Volume of solution: 23.45 mL Ca(OH)2 x 1 L Ca(OH)2 / 1000 mL Ca(OH)2 = 0.02345 L Ca(OH)2 Concentration of solution: 0.00154 mol Ca(OH)2 / 0.02345 L Ca(OH)2 = *0.0657 M Ca(OH)2*

If it took 24.5 milliliters of 0.35 molar NaOH to neutralize 15.0 milliliters of HNO3, what is the concentration (molarity) of the HNO3?

First, we need to write a balanced equation for this reaction so that we know the mole ration between the acid and the base. NaOH + HNO3 --> NaNO3 + H2O Next, we identify the given information and what we are trying to solve for in this problem. Given: Molarity of base: 0.35 M NaOH Volume of base: 24.5 mL NaOH Volume of acid: 15.0 mL HNO3 Solving for: Molarity of acid: ? M HNO3 Set up a stoichometry problem, starting with the volume and molarity of the known solution, to solve for moles of the solution with the unknown molarity. 24.5 mL NaOH x 1 L NaOH / 1000 mL NaOH x 0.35 mol NaOH / 1 L NaOH x 1 mol HNO3 / 1 mol NaOH = 0.00858 mol HNO3 in the given amount Once you determine the amount of moles present in the solution with the unknown molarity, you can divide by the volume of that same solution (in liters) to determine the molarity of the solution. You now know that 0.00858 moles of HNO3 are present in the 15.0 milliliters of HNO3 solution. Convert volume from milliliters to liters: 15.0 mL HNO3 x 1 L NNO3 / 1000 mL HNO3 = 0.0150 L HNO3 Divide moles of solute by liters of solution to determine the molarity of the solution: 0.00858 mol NNO3 / 0.0150 L HNO3 = *0.572 M HNO3*

If 40.0 milliliters of 0.15 molar HCl are required to completely neutralize 25.0 milliliters of NH3, what is the concentration of the NH3 solution?

HCl + NH3 --> NH4Cl Given: Molarity of acid: 0.15 M HCl Volume of acid: 40.0 mL HCl Volume of base: 25.0 mL NH3 Solving for: Molaity of base: ? M NH3 Moles of solute in solution: 40.0 mL HCl x 1 L HCl / 1000 mL HCl x 0.15 mol HCl / 1 L HCl x 1 mol NH3 / 1 mol HCl = 0.0060 mol NH3 in the solution Volume of solution: 25.0 mL NH3 x 1 L NH3 / 1000 mL NH3 = 0.0250 L NH3 Concentration of solution: 0.0060 mol NH3 / 0.0250 L NH3 = *0.24 M NH3*

Antacids

Neutralization reactions are also important outside of the laboratory. People often take antacids, medications that contain a weak base, to help neutralize some of the hydrochloric acid in their stomachs.

Stomach

Some plants in your garden may be sensitive to the acidity or basicity of the soil, so it may be necessary to add limestone to help neutralize acidic soil or sulfur to help neutralize basic soil.

Acid Spill

This is an important concept when dealing with spills in the laboratory. If an acid is spilled in a laboratory, the first thing to do is add a weak base (such as baking soda) to the acid to help decrease its acidic properties. This makes it safer to clean up the spill.

Determining Concentration of an Acid or Base

We have already seen that the coefficients in balanced equations can be used in calculations, called stoichiometry, to predict the amount of product that will be formed by a reaction or to determine the amount of a reactant needed. This is true with acid-base neutralization reactions, as well. Another useful application of stoichiometry is the ability to use an acid or base with a known concentration to determine the concentration of a base or acid that it neutralizes. Laboratory techniques that would be used to determine the concentration of an acid or base involve equipment that allows you to add small amounts of a known acid or base until the reaction just reaches completion. This process requires an indicator or pH meter to determine when the neutralization reaction is complete. An acid-base indicator is a weak acid (or base) that changes color as the amount of hydrogen or hydroxide ions present in the solution changes. A pH meter is a tool used in many labs that can measure the acidity of a solution, allowing you to observe when the acid or base has been neutralized.

Neutralization Reactions

When an acid and a base come together, they react to neutralize, or reduce, the acid and base properties of the reactants. When an Arrhenius acid reacts with an Arrhenius base, the neutralization is a double-replacement reaction that produces water and a salt. HCl + KOH --> H2O + KCl HCl is an Arrhenius acid because it gives off hydrogen ions in solution, while KOH is an Arrhenius base because it gives off hydroxide ions in solution. When the hydrogen ion and hydroxide ion come together, they combine to form a water molecule. The combination of hydrogen and hydroxide ions to form water helps to neutralize the solution by decreasing the amount of hydrogen ions and hydroxide ions present. In the neutralization reaction of HCl and KOH, the chloride and potassium ions combine together to form the salt, or ionic compound, potassium chloride (KCl). A salt is any ionic compound that can be formed from an acid-base neutralization reaction. In everyday life, we refer to sodium chloride (NaCl) as table salt, but this word salt actually describes a variety of ionic compounds in chemistry. The Arrhenius definition of acids and bases is very useful in explaining the fundamentals of acids and bases, but these definitions cannot describe every type of neutralization reaction. For example, some substances, such as baking soda (NaHCO3), can act as a base and neutralize an acid even though they do not contain any hydroxide ions. Scientists also use other definitions of acids and bases. The Brønsted-Lowry and Lewis definitions broaden the descriptions of acids and bases. Brønsted-Lowry neutralization reactions still involve an acid and a base reacting together to produce a salt and minimize the acidic and basic properties of the reactants, but water is not always produced by these reactions. HCl (acid) + NaHCO3 (base) --> H2CO3 + NaCl (salt) Acid-base reactions are often called neutralization reactions because when the acid and base react together, the acidic and basic properties of the solution are both decreased. This does not mean that the resulting solution or products are completely neutral, but the acidic or basic properties of each reactant have been reduced as a result of the reaction.