Learning Objective Write acid-base neutralization reactions. What happens when an acid such as HCl is mixed with a base such as NaOH: \[\ce{HCl (aq) + NaOH (aq) → NaCl (aq) + H_2O (l)} \nonumber \] When an acid and a base are combined, water and a salt are the products . Double displacement reactions of this type are called neutralization reactions . We can write an expanded version of this equation, with aqueous substances written in their longer form: \[\ce{H^{+} (aq) + Cl^{-} (aq) + Na^{+} (aq) + OH^{-} (aq) → Na^{+} (aq) + Cl^{-} (aq) + H_2O (l)} \nonumber \] Removing the spectator ions we get the net ionic equation: \[\ce{H^{+} (aq) + OH^{-} (aq) → H_2O (l)} \nonumber \] When a strong acid and a strong base are combined in the proper amounts - when \([\ce{H^{+}}]\) equals \([\ce{OH^{-}}\)]\) - a neutral solution results in which pH = 7. The acid and base have neutralized each other, and the acidic and basic properties are no longer present. Salt solutions do not always have a pH of 7, however. Through a process known as hydrolysis , the ions produced when an acid and base combine may react with the water molecules to produce a solution that is slightly acidic or basic. We will not go into details here, but generally if a strong acid is mixed with a weak base there the resulting solution will be slightly acidic; if a strong base is mixed with a weak acid the solution will be slightly basic. Neutralization Video \(\PageIndex{1}\) Acid-base neutralization reaction. Example \(\PageIndex{1}\): propionic acid + calcium hydroxide Calcium propionate is used to inhibit the growth of molds in foods, tobacco, and some medicines. Write a balanced chemical equation for the reaction of aqueous propionic acid (CH 3 CH 2 CO 2 H) with aqueous calcium hydroxide [Ca(OH) 2 ]. Solution Steps Reaction Write the unbalanced equation. This is a double displacement reaction, so the cations and anions swap to create the water and the salt. CH3CH2CO2H(aq) + Ca(OH)2(aq)→(CH3CH2CO2)2Ca(aq) + H2O(l) Balance the equation. Because there are two OH− ions in the formula for Ca(OH)2, we need two moles of propionic acid, CH3CH2CO2H to provide H+ ions. 2CH3CH2CO2H(aq) + Ca(OH)2(aq)→(CH3CH2CO2)2Ca(aq) +2H2O(l) Exercise \(\PageIndex{1}\) Write a balanced chemical equation for the reaction of solid barium hydroxide with dilute acetic acid. Answer \[\ce{Ba(OH)2(s) + 2CH3CO2H (aq)→Ba(CH3CO2)2 (aq) + 2H2O(l)} \nonumber \] Applications In analytical laboratories, chemical methods are used for determining the amounts of acids or bases in foods and household and industrial chemicals. In , chemical neutralization methods are often applied to reduce the damage that an effluent may cause upon release to the environment. There are many uses of neutralization reactions that are acid-alkali reactions. A very common use is antacid tablets (section 7.8). In chemical synthesis of nanomaterials, the heat of neutralization reaction can be used to facilitate the chemical reduction of metal precursors. Also in the digestive tract, neutralization reactions are used when food is moved from the stomach to the intestines. In order for the nutrients to be absorbed through the intestinal wall, an alkaline environment is needed, so the pancreas produce an antacid bicarbonate to cause this transformation to occur. Another common use, though perhaps not as widely known, is in fertilizers and control of soil pH. Slaked lime (calcium hydroxide) or limestone (calcium carbonate) may be worked into soil that is too acidic for plant growth. Fertilizers that improve plant growth are made by neutralizing sulfuric acid (H 2 SO 4 ) or nitric acid (HNO 3 ) with ammonia gas (NH 3 ), making or . These are salts utilized in the fertilizer. Industrially, a by-product of the burning of coal, gas, may combine with water vapor in the air to eventually produce sulfuric acid, which falls as acid rain. To prevent the sulfur dioxide from being released, a device known as a scrubber gleans the gas from smoke stacks. This device first blows calcium carbonate into the combustion chamber where it decomposes into calcium oxide (lime) and carbon dioxide. This lime then reacts with the sulfur dioxide produced forming calcium sulfite. A suspension of lime is then injected into the mixture to produce a slurry, which removes the and any remaining unreacted sulfur dioxide. Fluffy Cakes Q: How do acid-base reactions make cake fluffy? A: Usually cakes include an acidic ingredient (this varies) and sodium bicarbonate, a base. When they react, the proton from the acid is transferred to the bicarbonate, making the weak acid carbonic acid. Carbonic acid is the product of an acid anhydride reaction between carbon dioxide and water. This reaction can be reversed, or carbonic acid can decompose into water and carbon dioxide. Especially at the high temperatures inside a baking cake, this decomposition will happen, and produce carbon dioxide gas. The pressure of the hot gas will form bubbles inside the cake, making it fluffy. Summary Acids and bases combine and produce salt and water.