Classifying Chemical Reactions Quiz

PDF page 358: A precipitation reaction is one in which dissolved substances react to form one (or more) solid products. Many reactions of this type involve the exchange of ions between ionic compounds in aqueous solution and are sometimes referred to as double displacement, double replacement, or metathesis reactions. These reactions are common in nature and are responsible for the formation of coral reefs in ocean waters and kidney stones in animals. They are used widely in industry for production of a number of commodity and specialty chemicals. Precipitation reactions also play a central role in many chemical analysis techniques, including spot tests used to identify metal ions and gravimetric methods for determining the composition of matter (see the last module of this chapter). The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be soluble. A substance will precipitate when solution conditions are

PDF page 358: A precipitation reaction is one in which dissolved substances react to form one (or more) solid products. Many reactions of this type involve the exchange of ions between ionic compounds in aqueous solution and are sometimes referred to as double displacement, double replacement, or metathesis reactions. These reactions are common in nature and are responsible for the formation of coral reefs in ocean waters and kidney stones in animals. They are used widely in industry for production of a number of commodity and specialty chemicals. Precipitation reactions also play a central role in many chemical analysis techniques, including spot tests used to identify metal ions and gravimetric methods for determining the composition of matter (see the last module of this chapter). The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be soluble. A substance will precipitate when solution conditions are

PDF page 359: such that its concentration exceeds its solubility. Substances with relatively low solubilities are said to be insoluble, and these are the substances that readily precipitate from solution. More information on these important concepts is provided in the text chapter on solutions. For purposes of predicting the identities of solids formed by precipitation reactions, one may simply refer to patterns of solubility that have been observed for many ionic compounds (Table 7.1). Solubilities of Common Ionic Compounds in Water Soluble compounds contain + + + + +

PDF page 358: A precipitation reaction is one in which dissolved substances react to form one (or more) solid products. Many reactions of this type involve the exchange of ions between ionic compounds in aqueous solution and are sometimes referred to as double displacement, double replacement, or metathesis reactions. These reactions are common in nature and are responsible for the formation of coral reefs in ocean waters and kidney stones in animals. They are used widely in industry for production of a number of commodity and specialty chemicals. Precipitation reactions also play a central role in many chemical analysis techniques, including spot tests used to identify metal ions and gravimetric methods for determining the composition of matter (see the last module of this chapter). The extent to which a substance may be dissolved in water, or any solvent, is quantitatively expressed as its solubility, defined as the maximum concentration of a substance that can be achieved under specified conditions. Substances with relatively large solubilities are said to be soluble. A substance will precipitate when solution conditions are

PDF page 359: A vivid example of precipitation is observed when solutions of potassium iodide and lead nitrate are mixed, resulting in the formation of solid lead iodide: 2KI(aq) + Pb(NO ) (aq) ⟶ PbI (s) + 2KNO (aq) 3 2 2 3 This observation is consistent with the solubility guidelines: The only insoluble compound among all those involved is lead iodide, one of the exceptions to the general solubility of iodide salts. The net ionic equation representing this reaction is: 2+ − Pb (aq) + 2I (aq) ⟶ PbI (s) 2 Lead iodide is a bright yellow solid that was formerly used as an artist’s pigment known as iodine yellow (Figure 7.4). The properties of pure PbI2 crystals make them useful for fabrication of X-ray and gamma ray detectors.

PDF page 361: Which solution could be used to precipitate the barium ion, Ba , in a water sample: sodium chloride, sodium hydroxide, or sodium sulfate? What is the formula for the expected precipitate? Answer: sodium sulfate, BaSO4

PDF page 360: (a) The two possible products for this combination are KNO3 and BaSO4. The solubility guidelines indicate

PDF page 360: BaSO4 is insoluble, and so a precipitation reaction is expected. The net ionic equation for this reaction,

PDF page 361: An acid-base reaction is one in which a hydrogen ion, H , is transferred from one chemical species to another. Such reactions are of central importance to numerous natural and technological processes, ranging from the chemical transformations that take place within cells and the lakes and oceans, to the industrial-scale production of fertilizers, pharmaceuticals, and other substances essential to society. The subject of acid-base chemistry, therefore, is worthy of thorough discussion, and a full chapter is devoted to this topic later in the text. For purposes of this brief introduction, we will consider only the more common types of acid-base reactions that take place in aqueous solutions. In this context, an acid is a substance that will dissolve in water to yield hydronium ions,

PDF page 362: The nature of HCl is such that its reaction with water as just described is essentially 100% efficient: Virtually every HCl molecule that dissolves in water will undergo this reaction. Acids that completely react in this fashion are called strong acids, and HCl is one among just a handful of common acid compounds that are classified as strong (Table 7.2). A far greater number of compounds behave as weak acids and only partially react with water, leaving a large majority of dissolved molecules in their original form and generating a relatively small amount of hydronium ions. Weak acids are commonly encountered in nature, being the substances partly responsible for the tangy taste of citrus fruits, the stinging sensation of insect bites, and the unpleasant smells associated with body odor. A familiar example of a weak acid is acetic acid, the main ingredient in food vinegars: − + CH CO H(aq) + H O(l) ⇌ CH CO (aq) + H O (aq) 3 2 2 3 2 3 When dissolved in water under typical conditions, only about 1% of acetic acid molecules are present in the ionized − form, CH CO (Figure 7.6). (The use of a double-arrow in the equation above denotes the partial reaction aspect 3 2 of this process, a concept addressed fully in the chapters on chemical equilibrium.)

PDF page 363: Common Strong Acids Compound Formula Name in Aqueous Solution HBr hydrobromic acid HCl hydrochloric acid HI hydroiodic acid nitric acid HNO3 perchloric acid HClO4 sulfuric acid H2SO4

PDF page 363: A base is a substance that will dissolve in water to yield hydroxide ions, OH . 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

PDF page 364: compounds are also abundant in nature and important commodities in various technologies. For example, global production of the weak base ammonia is typically well over 100 metric tons annually, being widely used as an agricultural fertilizer, a raw material for chemical synthesis of other compounds, and an active ingredient in household cleaners (Figure 7.7). When dissolved in water, ammonia reacts partially to yield hydroxide ions, as shown here: + − NH (aq) + H O(l) ⇌ NH (aq) + OH (aq) 3 2 4 +

PDF page 364: This is, by definition, an acid-base reaction, in this case involving the transfer of H ions from water molecules to + ammonia molecules. Under typical conditions, only about 1% of the dissolved ammonia is present as NH ions. 4

PDF page 364: The chemical reactions described in which acids and bases dissolved in water produce hydronium and hydroxide ions, respectively, are, by definition, acid-base reactions. In these reactions, water serves as both a solvent and a reactant. A neutralization reaction is a specific type of acid-base reaction in which the reactants are an acid and a base, the products are often a salt and water, and neither reactant is the water itself: acid + base ⟶ salt + water To illustrate a neutralization reaction, consider what happens when a typical antacid such as milk of magnesia (an aqueous suspension of solid Mg(OH)2) is ingested to ease symptoms associated with excess stomach acid (HCl): Mg(OH) (s) + 2HCl(aq) ⟶ MgCl (aq) + 2H O(l). 2 2 2 Note that in addition to water, this reaction produces a salt, magnesium chloride.