16.2 Reactions of Acids and Bases
Learning Objectives
By the end of this section, you will be able to:
- Summarize general reactions of acids and bases
In this section, the more common types of acid-base reactions that take place in aqueous solutions will be summarized. In continuation with the context learned earlier in this chapter, a strong acid is a substance that will dissolve in water to yield hydronium ions, H3O+. Whereas, a strong base is a substance that will dissolve in water to yield hydroxide ions, OH−. We will learn other definitions of acids and bases in the following sections. The most common bases are ionic compounds composed of alkali or alkaline earth metal cations (groups 1 and 2) combined with the hydroxide ion. For example, NaOH and Ca(OH)2. When these compounds dissolve in water, hydroxide ions are released directly into the solution. These ions are responsible for the characteristic behaviour of acids and bases in aqueous solution.
Neutralization Reactions
The reaction between an acid and a base is called an acid-base reaction or a neutralization reaction. Later in this chapter, a section is dedicated to discussing neutralization reactions and introducing calculations relating to neutralization reactions. However, a brief introduction into this type of reaction is provided. In reactions where the acid is a hydrogen-ion-containing compound and the base is a hydroxide-ion-containing compound, the two react chemically to form water and a salt. The general form for a neutralization reaction is
acid + base → water + salt
where the term salt is used to define any ionic compound (soluble or insoluble) that is formed from a reaction between an acid and a base, as outlined in chapter 15. In chemistry, the word salt refers to more than just table salt. For example, the balanced chemical equation for the reaction between HCl(aq) and KOH(aq) is
HCl(aq) + KOH(aq) → H2O(l) + KCl(aq)
where in this instance, the salt is soluble potassium chloride, KCl. By counting the number of atoms of each element, we find that only one water molecule is formed as a product. However, in the reaction between HCl(aq) and Mg(OH)2(aq), additional molecules of HCl and H2O are required to balance the chemical equation:
2 HCl(aq) + Mg(OH)2(aq) → 2 H2O(l) + MgCl2(aq)
Here, the soluble salt is MgCl2. (This is one of several reactions that take place when a type of antacid – a base – is used to treat stomach acid.)
There are acid-base reactions that do not follow the “general acid-base” equation given above. For example, the balanced chemical equation for the reaction between HCl(aq) and NH3(aq) is
HCl(aq) + NH3(aq) → NH4Cl(aq)
Neutralization reactions are one type of chemical reaction that proceeds even if one reactant is not in the aqueous phase. For example, the chemical reaction between HCl(aq) and Fe(OH)3(s) still proceeds according to the equation
3 HCl(aq) + Fe(OH)3(s) → 3 H2O(l) + FeCl3(aq)
even though Fe(OH)3 is not soluble. When one realizes that Fe(OH)3(s) is a component of rust, this explains why some cleaning solutions for rust stains contain acids – the neutralization reaction produces products that are soluble and wash away. Washing with acids like HCl is one way to remove rust and rust stains, but HCl must be used with caution!
Gas-Forming Acid-Base Reactions
A driving force for certain acid-base reactions is the formation of a gas. Common gases formed are hydrogen (H2), oxygen (O2), and carbon dioxide (CO2).
Acid Reactions with Carbonates
Many acids react with carbonates to produce a salt, carbon dioxide gas and water.
acid + carbonate → salt + CO2(g) + H2O(l)
Consider the following reaction between hydrochloric acid and sodium carbonate (which is considered a base):
2 HCl(aq) + Na2CO3(aq) → H2CO3(aq) + 2 NaCl(aq) → CO2(g) + H2O(l) + 2 NaCl(aq)
The above example can be viewed as an acid-base reaction followed by a decomposition. The driving force in this case is the gas formation. The decomposition of H2CO3 into CO2 and H2O is a very common reaction. Both sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) mixed with acid result in a gas-forming acid-base reaction.
HCl(aq) + NaHCO3(aq) → H2CO3(aq) + NaCl(aq) → CO2(g) + H2O(l) + NaCl(aq)
More Reactions Involving Acids and Bases
There are many common reactions of acids chemically combining with carbonate, to form a salt and carbonic acid as demonstrated above. Due to the chemical disposition of carbonic acid as weak acid, it readily decomposes to water and carbon dioxide gas. The final products of chemical reactions are shown for examples 1 through 3 below, but remember that the formation of carbonic acid occurs as an intermediate step before decomposing to the final products.
- Example 1: Nitric acid reacts with sodium carbonate to form sodium nitrate, carbon dioxide, and water.
2 HNO3(aq) + Na2CO3(aq) → 2 NaNO3(aq) + CO2(g) + H2O(l)
- Example 2: Hydrochloric acid reacts with calcium carbonate to form calcium chloride, carbon dioxide, and water:
2 HCl(aq) + CaCO3(aq) → CaCl2(aq) + CO2(g) + H2O(I)
- Example 3: Sulfuric acid reacts with calcium carbonate to form calcium sulfate, carbon dioxide, and water:
H2SO4(aq) + CaCO3(aq) → CaSO4(aq) + CO2(g) + H2O(l)
Watch Acid + Metal Carbonate | Acids, Bases & Alkalis | Chemistry | FuseSchool (3min 0s).
Video Source: FuseSchool – Global Education. Acid + metal carbonate | Acids, bases & alkalis | Chemistry [Video]. YouTube.
Acid Reactions with Metals
The oxidation of metals in acidic solutions is another chemical process that generates a gas. This reaction will yield a metal salt and hydrogen gas and has the general formula
acid + metal → hydrogen + salt
Consider the following single displacement reaction between hydrochloric acid and solid zinc metal:
2 HCl(aq) + Zn(s) → ZnCl2(aq) + H2(g)
Here, hydrochloric acid reacts with zinc to produce an aqueous metal salt and hydrogen gas bubbles.
Base Reactions with Metals
Some bases can also react with metals to form a gaseous product. For instance, strong bases, such as sodium hydroxide and potassium hydroxide, react with amphoteric metals to produce hydrogen gas and a salt:
Base + metal + water → hydrogen + salt
Consider the following example of sodium hydroxide reacting with Zinc in water to form sodium aluminate and hydrogen gas:
2 NaOH(aq) + Zn(s) → Na2Zn(OH)4(aq) + H2(g)
Reactions with Metal Oxides
The oxides of metals are basic and when a metal oxide reacts with an acid, it forms a salt with water.
acid + metal oxide → salt + H2O(l)
An example of this type of reaction is mixing a small amount of the black, insoluble salt, copper(II) oxide, with dilute sulfuric acid. Upon mixing of the reactants, the colour of the solution becomes blue and the copper oxide dissolves. The blue colour of the solution indicates the formation of the soluble salt, copper(II) sulfate and water.
This chemical reaction can be written as the following:
CuO(s) + H2SO4 (aq) → CuSO4(aq) + H2O(l)
Source: Except where otherwise noted, “More Reactions Involving Acids & Bases” is created by Jackie MacDonald, CC BY-NC-SA 4.0
Food and Drink: Acids in Foods
Many foods and beverages contain acids. Acids impart a sour note to the taste of foods, which may add some pleasantness to the food. For example, orange juice contains citric acid, H3C6H5O7. Note how this formula shows hydrogen atoms in two places; the first hydrogen atoms written are the hydrogen atoms that can form H+ ions, while the second hydrogen atoms written are part of the citrate ion, C6H5O73−. Lemons and limes contain much more citric acid – about 60 times as much – which accounts for these citrus fruits being more sour than most oranges. Vinegar is essentially a ~5% solution of acetic acid (HC2H3O2) in water. Apples contain malic acid (H2C4H4O5; the name malic acid comes from the apple’s botanical genus name, malus), while lactic acid (HC3H5O3) is found in wine and sour milk products, such as yogurt and some cottage cheeses.
Table 16.2a lists some acids found in foods, either naturally or as an additive. Frequently, the salts of acid anions are used as additives, such as monosodium glutamate (MSG), which is the sodium salt derived from glutamic acid. As you read the list, you should come to the inescapable conclusion that it is impossible to avoid acids in food and beverages.
| Acid Name | Acid Formula | Use and Appearance |
|---|---|---|
| acetic acid | HC2H3O2 | flavouring; found in vinegar |
| adipic acid | H2C6H8O4 | flavouring; found in processed foods and some antacids |
| alginic acid | various | thickener; found in drinks, ice cream, and weight loss products |
| ascorbic acid | HC6H7O6 | antioxidant, also known as vitamin C; found in fruits and vegetables |
| benzoic acid | HC6H5CO2 | preservative; found in processed foods |
| citric acid | H3C6H5O7 | flavouring; found in citrus fruits |
| dehydroacetic acid | HC8H7O4 | preservative, especially for strawberries and squash |
| erythrobic acid | HC6H7O6 | antioxidant; found in processed foods |
| fatty acids | various | thickener and emulsifier; found in processed foods |
| fumaric acid | H2C4H2O4 | flavouring; acid reactant in some baking powders |
| glutamic acid | H2C5H7NO4 | flavouring; found in processed foods and in tomatoes, some cheeses, and soy products |
| lactic acid | HC3H5O3 | flavouring; found in wine, yogurt, cottage cheese, and other sour milk products |
| malic acid | H2C4H4O5 | flavouring; found in apples and unripe fruit |
| phosphoric acid | H3PO4 | flavouring; found in some colas |
| propionic acid | HC3H5O2 | preservative; found in baked goods |
| sorbic acid | HC6H7O2 | preservative; found in processed foods |
| stearic acid | HC18H35O2 | anticaking agent; found in hard candies |
| succinic acid | H2C4H4O4 | flavouring; found in wine and beer |
| tartaric acid | H2C4H4O6 | flavouring; found in grapes, bananas, and tamarinds |
Attribution & References
reaction between an acid and a base to produce salt and water
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