Basic Principles of Chemistry Lecture Notes #11 Types of Chemical Reactions

Transcription

1 Basic Principles of Chemistry Lecture Notes #11 Types of Chemical Reactions Types of Chemical Reactions Part 1 Precipitation Reactions In a precipitation reaction, a solid substance is formed in a solution where there was no solid previously. This solid is sometimes referred to as a precipitate, hence the name. The precipitation reaction is part of a broader class of chemical reactions called doubledisplacement reactions. In a double-displacement reaction, two elements or groups of elements in two different compounds exchange places to form two new compounds. To understand a precipitation reaction, we first need to know a little about what makes a compound become a solid in water and what makes a compound stay dissolved in water. When we sprinkle table salt into water, the salt granules disappear quickly, leaving a homogeneous mixture that looks just like water. Unless we tasted it, we might think it WAS just water. This mixture is called a solution an aqueous solution to be exact. Water is acting as a SOLVENT in this process. There are lots of solvents in the world. For example, acetone is a good solvent that dissolves fingernail polish. Turpentine is a good solvent that dissolves paint. Water is by far the most common solvent in the world. The oceans and living things are all made up of mainly water, and most of the chemicals of life are dissolved in that water as aqueous solutions. Water is therefore sometimes called the universal solvent. Back to the salt solution Water is the solvent, salt is the SOLUTE, and the saltwater mixture is called a solution. Sodium chloride dissolves in water very easily to make a solution. Some other salts do not dissolve so easily in water. Still others do not dissolve in water at all. What does it mean for a substance to dissolve in water? When we place a substance in water, the forces of attraction that hold the molecules or ions of the substance together with each other are in competition with the forces of attraction between the molecules or ions and the water molecules. What kind of attractive force is there between the ions in sodium chloride (NaCl; table salt)? What kind of attractive forces are there between sodium ions (Na + ) and water? Between chloride ions (Cl - ) and water? Think about this: Which force is stronger, the attractive force between sodium ions and chloride ions or the attractive forces between sodium ions and water and chloride ions and water? So, if we understand the general principle behind the solubility of compounds in water, how can we predict which compounds will be soluble in water and which compounds will not be soluble in water?

2 The short answer is we can t at least not in this class. The factors that affect the solubility of a compound turn out to be pretty complicated, so we won t begin to get into them in this class. Luckily for us, though, scientists have done lots of experiments to determine the solubility of different compounds in water. In fact, not only have they tested compounds to tell whether they are soluble in water or not, they have even calculated how much of a certain compound will dissolve in a certain amount of water. We will talk about these measurements in chapter 13, but for now we will just concern ourselves with the question of whether a compound will dissolve in water or not. Again, scientists have done lots of experiments to figure out which compounds are soluble in water and which compounds are not soluble in water. These results have been summarized for us in the form of SOLUBILITY RULES (page 200, Table 7.2). A table like this that summarizes actual experimental results is known as empirical data. The opposite of empirical data would be theoretical or hypothetical data. This latter type of data would be estimated based upon our understanding of the theory behind solubility. Since we have already said that the theory behind solubility is very complicated, we need to use empirical solubility information to predict whether a compound will dissolve in water or not. What are the advantages of using theoretical data and information to predict a property? What are the disadvantages? What are the advantages of using empirical data and information to predict a property? What are the disadvantages? The solubility rules table gives you two columns of information. In the left-hand column, you have a general rule for salts containing particular ions. In the right-hand column, you have a listing of exceptions to that general rule. Let s use the solubility rules to predict whether the following compounds will be soluble in water or not: silver nitrate (AgNO 3 ) sodium chloride (NaCl) silver chloride (AgCl) ammonium nitrate (NH 4 NO 3 ) calcium chloride (CaCl 2 ) calcium carbonate (CaCO 3 ) In a precipitation reaction we begin with two compounds that are soluble in water. When we mix aqueous solutions of these two compounds together, the cations and anions in the two compounds exchange places (a double displacement) to form two new compounds. If one of those new compounds is insoluble in water, a precipitation reaction will occur. What if neither of these two new compounds is soluble in water? AgNO 3 (aq) + NaCl(aq) AgCl(?) + NaNO 3 (?) 2 NH 4 NO 3 (aq) + CaCl 2 (aq) 2 NH 4 Cl(?) + Ca(NO 3 ) 2 (?) CaCl 2 (aq) + Na 2 CO 3 (aq) CaCO 3 (?) + 2 NaCl(?)

3 Other ways of writing precipitation reaction equations: AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) Ag + (aq) + NO 3 - (aq) + Na + (aq) + Cl - (aq) AgCl(s) + Na + (aq) + NO 3 - (aq) Notice what we did here. If a compound was soluble in water, we wrote it in its DISSOCIATED form, as separate ions. If a compound is not soluble in water, such as AgCl above, we cannot write it in its dissociated form. Why not? The top equation above is known as a molecular equation. It shows the combined neutral formulas of each compound in the reaction. The bottom equation is known as a total ionic equation. It shows the reactants and products as they actually are in the solution. If they are aqueous, the ions are separated from each other by water molecules. If they are solids, the ions are combined together, rather than separate. Notice the left side and the right side of the total ionic equation. Does anything strike you? Ag + (aq) + Cl - (aq) AgCl(s) net ionic equation Who cares? Are there any precipitation reactions that matter to you? Kidney stones are formed when calcium ions (from dietary sources) combine with certain anions to form insoluble compounds. Which ions from the solubility rules might form insoluble calcium salts? What might be a good practical measure to prevent kidney stones? Hard water is a problem caused by calcium (Ca 2+ ) and magnesium (Mg 2+ ) cations dissolved in household water. Adding salts containing anions that form insoluble calcium and magnesium salts causes the calcium and magnesium ions to precipitate out of the water, thus softening the water. From the solubility rules, name a few compounds that might be added to water to soften it. A common way of seeing fingerprints left at a crime scene is by spraying them with a solution of silver nitrate (AgNO 3 ) in water. Considering that human perspiration is made up of mostly aqueous sodium chloride (NaCl), what precipitation reaction is responsible for our ability to see these fingerprints? Plumbing in households with hard water can often become clogged with a substance called scale. Scale is primarily composed of calcium carbonate (CaCO 3 ), which the solubility rules tell us is insoluble in water. Interestingly, the same chemical reaction in caves forms stalactites and stalagmites, and in oysters it forms pearls. Next time we will learn about another type of chemical reaction that allows us to break up this kind of clog in pipes. Types of Chemical Reactions Part 2 Acid-Base Neutralization Reactions HCl(aq) + NaOH(aq) H 2 O(l) + NaCl HC 2 H 3 O 2 (aq) + NaHCO 3 (aq) H 2 O(l) + NaC 2 H 3 O 2 (aq) + CO 2 (g)

4 In general, these reactions form water as one product and salt as a second product. Some acid-base neutralization reactions also form a gas, such as carbon dioxide above. These are sometimes called Gas Evolution reactions. What is the net ionic equation for the reaction of hydrochloric acid with sodium hydroxide above? Acid-Base Neutralization reactions are also classified as reactions. Some important acid-base reactions in our lives Antacids, like TUMS, Rolaids, and Pepto Bismol contain bases that react with excess acids in our stomachs to relieve heartburn. For example, TUMS neutralizes stomach acid in the following way: HCl(aq) + CaCO 3 (aq) H 2 O(l) + CaCl 2 (aq) + CO 2 (g) Acid rain contains enough acid to destroy marble statues and buildings over time. Marble is made up of calcium carbonate, and the same reaction that neutralizes our stomach acid also destroys our marble statues. After we eat a meal, bacteria in our mouths have a meal also. They eat up the excess food that is in our teeth. As part of the metabolic process of the bacteria eating the food, they generate a product called lactic acid. Our teeth are protected by a hard enamel layer. When the lactic acid reacts with the enamel, an acid-base neutralization reaction occurs, causing our protective enamel layer to break down, leading to cavities. HC 3 H 5 O 4 (aq) + Ca 5 (PO 4 ) 3 OH(s) H 2 O(l) + Ca 5 (PO 4 ) 3 C 3 H 5 O 4 (aq) The hard scale build-up found in some plumbing systems is caused by a precipitation reaction of calcium cations and carbonate anions. When this precipitate is treated with hydrochloric acid, however, it is neutralized to form water, a salt, and carbon dioxide gas. This is an acid-base neutralization reaction. Oxidation-Reduction Reactions CaCO 3 (s) + 2HCl(aq) CaCl 2 + H 2 O(l) + CO 2 (g) We learned about this type of reaction in laboratory last week, in addition to the acid-base neutralization reaction. When an atom or a molecule LOSES electrons, we say that it has become oxidized. Its charge becomes more positive. When an atom or molecule GAINS extra electrons, we say that it has become reduced. Its charge becomes more negative. CuSO 4 (aq) + Zn(s) ZnSO 4 (aq) + Cu(s) Write down the charge on each metal atom above. Which atom is being oxidized? Which atom is being reduced? Which atoms are not changed at all?

5 Write a net equation for this reaction: + + Another example of an oxidation-reduction reaction is shown below: Al(s) + 6HCl(aq) 2AlCl 3 (aq) + 3H 2 (g) This reaction and the one above it is not only an oxidation-reduction reaction, but it may also be classified as a single displacement reaction. The reaction also fits into the category of a gas evolution reaction. Combustion Reactions In a combustion reaction, a substance combines with oxygen and usually gives off energy. A Bunsen burner flame is a combustion of methane gas: CH 4 (g) + 2O 2 (g) CO 2 (g) + 2H 2 O(g) An automobile engine runs on the combustion of gasoline: Your body runs on the combustion of glucose: 2C 6 H 6 (l) + 15O 2 (g) 12CO 2 (g) + 6H 2 O(g) C 6 H 12 O 6 (aq) + 6O 2 (g) 6CO 2 (g) + 6H 2 O(l) The space shuttle launches with the help of a couple of different combustion reactions. The big orange liquid fuel tank contains hydrogen and oxygen, which mix to make a powerful combustion reaction: 2H 2 (g) + O 2 (g) 2H 2 O(g) Besides these specific categories, reactions can be grouped into more general categories: Type of Reaction combination reaction decomposition reaction single displacement reaction double displacement reaction Generic Equation A + B AB AB A + B A + BC AC + B AB + CD AD + CB