10.3 Types of Chemical Reactions

Transcription

1 10.3 Types of Chemical Reactions Chemical equations follow certain patterns Combination (synthesis ) The number of possible chemical is very large. However, there are a limited number of structural patterns that every chemical reaction may fall into. By learning these patterns, we will be able to better understand these chemical. These patterns can be best seen by considering the following four classifications of chemical : 1. combination (synthesis ), 2. decomposition, 3. single displacement, and 4. double displacement (metathesis ). We will now describe the structure of each one of these types of. A reaction in which two substances combine to form a third substance is called a combination or synthesis reaction. The general form of a combination reaction is shown here: Synthesis of compounds from elements Synthesis of compounds from other compounds This symbolic form shows substances A and B combining to form a new substance AB. The substances A and B can be two compounds, two elements, or an element and a compound. The formation of water from hydrogen and oxygen is a synthesis reaction that combines two pure elements to form a compound: 2H 2 (g) + O 2 (g) 2H 2 O(g) Another example of a synthesis reaction combines magnesium oxide (MgO) with water to make magnesium hydroxide, Mg(OH) 2, also known an milk of magnesia. In this example, two compounds combine to form a third compound. Synthesis combine different substances. Therefore the number of products is less than the number of reactants. A NATURAL APPROACH TO CHEMISTRY 305

2 Section 10.3 Types of Chemical Reactions Decomposition and single displacement Decomposition is the opposite of synthesis A decomposition reaction is the opposite of a synthesis reaction. In a decomposition reaction, a single substance breaks apart (decomposes) to form two or more new substances. The new substances may be elements or compounds. Decomposition of limestone Decomposition usually requires energy Single displacement Calcium oxide (CaO), also called lime, has many uses, including water treatment, glass manufacturing, food preservation, and cement manufacturing. Lime is produced from the decomposition of calcium carbonate (CaCO 3 ) in limestone. This reaction occurs when limestone is heated to 825 C: CaCO 3 (s) CaO(s) + CO 2 (g) Another important reaction is the decomposition of water, which happens when electrical current passes through it: 2H 2 O(l) 2H 2 (g) + O 2 (g) Most decomposition require the addition of some type of external energy to proceed. The energy can be in the form of heat or electrical current, as in the previous examples. Energy can also be in the form of light, as in the case of the destruction of ozone (O 3 ) in the upper atmosphere: O 3 + light O 2 + O A single displacement reaction happens when an element reacts with a compound, replacing an element inside of it. The general form of a displacement reaction is shown here: Examples of displacement The element A replaces the B part of compound BD. It could also replace the D part of compound BD. A single displacement reaction occurs when zinc (Zn) or iron (Fe) is immersed in a solution of copper sulfate (CuSO 4 ): Zn(s) + CuSO 4 (aq) Cu(s) + ZnSO 4 (aq) Fe(s) + CuSO 4 (aq) Cu(s) + FeSO 4 (aq) 306 A NATURAL APPROACH TO CHEMISTRY

3 Double displacement (precipitate ) Ions trade partners in double displacement Precipitate often involve the exchange of parts of the reactants. The general form of this type of reaction is shown here: Precipitate reaction Here we see that parts B and X, which represent the negative ions of the compounds AB and DX, exchange places. This gives us the new compounds AX and DB. Basically the positive and negative ions trade partners. Double displacement or precipitate are one very common type of reaction that takes place in aqueous solutions. Recall that a precipitate is an insoluble solid that forms when certain aqueous solutions are mixed. The precipitate separates from the solution where it can be easily observed. A good example is the reaction between lead nitrate [Pb(NO 3 ) 2 ] and potassium iodide (KI). The pictures show a 0.1 M solution of lead nitrate and a 0.1 M solution of potassium iodide. Look carefully at their colors. Notice that the solutions are both essentially clear and colorless. When Pb(NO 3 ) 2 is added, notice the brilliant yellow color! The yellow comes from an insoluble precipitate (PbI 2 ), which is a product of the reaction. The chemical equation for the reaction The chemical reaction shows that the lead ion displaces the potassium ion in KI. The potassium ion in turn displaces the lead ion in Pb(NO 3 ) 2. A NATURAL APPROACH TO CHEMISTRY 307

4 Section 10.3 Types of Chemical Reactions Precipitates and solubility The nitrate anion When a precipitate forms Before we find out why a precipitate forms, consider the polyatomic nitrate ion (NO 3 ). The nitrate ion acts as a unit, so the nitrogen atom and three oxygen atoms remain covalently bonded together. The negative charge is the charge of the whole nitrate ion. A helpful comparison is to treat nitrate ions as if they were monatomic Cl ions. A precipitate forms when one of the products of a reaction is insoluble in water. To be able to predict which ionic compound will be the precipitate, we need to refer to a set of solubility rules. The rules summarize which compounds are soluble and which are not. TABLE Solubility rules for common ionic compounds in water Soluble compounds Group 1 metal ions: Li +, Na +, K +, Rb +, Cs + Ammonium (NH 4 + ) Acetate (C 2 H 3 O 2 ) Nitrate (NO 3 ) Sulfate (SO 4 2 ) Chlorides, bromides, and iodides, except with Cu, Pb, Ag, and Hg Insoluble compounds (except when with group 1 metal ions and NH + 4 ) Carbonates (CO 2 3 ) Hydroxides (OH ) (except for Ba 2+ ) Chlorides of Cu, Pb, Ag and Hg Bromides of Cu, Pb, Ag and Hg Iodides of Cu, Pb, Ag,and Hg Sulfides (S 2 ) Phosphates (PO 3 4 ) Using the solubility table The balanced chemical equation for the reaction between lead nitrate and potassium iodide is given below. Note from the table that nitrates are typically soluble. Therefore we expect potassium nitrate to be soluble, and it is. The table also says that iodides are generally soluble except for compounds with copper (Cu), lead (Pb), silver (Ag), and mercury (Hg). Potassium iodide is soluble. Lead iodide is insoluble and forms a precipitate. 308 A NATURAL APPROACH TO CHEMISTRY

5 Precipitate reaction uses and analysis Hard water Water softening Gas-forming Precipitate are important to our environment and our drinking water supply. Minerals such as Mg 2+ and Ca 2+ leach into our water supply and cause a phenomenon known as water hardness. You have experienced water hardness if you ever been in the shower and the soap did not lather well. Hard water leaves your skin feeling sort of sticky and filmy, because soaps and oils do not come off as easily. Iron (Fe 2+ ) and manganese (Mn 2+ ) also cause some other problems, such as staining clothes and increased bacterial growth. To condition (or soften) the water in areas where there are large amounts of dissolved minerals, water treatment plants add special ions that precipitate out some of the Mg 2+ and Ca 2+ ions. Hydroxides (OH ) and carbonates (CO 3 2 ) are added to the water supply to precipitate out large amounts of these contaminating ions. For example, Mg 2+ and OH form Mg(OH) 2 (s), and this solid settles to the bottom where it is easily removed. Some double displacement involve the production of a gas. For example, here is what happens when magnesium carbonate (MgCO 3 ) and hydrochloric acid (HCl) mix: MgCO 3 (s) + 2HCl(aq) MgCl 2 (aq) + H 2 O(l) + CO 2 (g) In this reaction, Mg 2+ and H + ions exchange places, resulting in magnesium chloride (MgCl 2 ) and H 2 O. Carbon dioxide (CO 2 ) is also evolved. Predict what happens when silver nitrate (AgNO 3 ) is mixed with sodium chloride (NaCl). Follow the steps and write the complete balanced reaction. Asked: Given: The reaction that represents the mixing of AgNO 3 and NaCl Formulas for both reactant solutions Relationships: Solubility rules tell us that group 1 metal ions (here Na + ) and nitrates are soluble, but chlorides of silver are insoluble. Solve: AgNO 3 (aq) + NaCl(aq) AgCl(s) + NaNO 3 (aq) According to the solubility table, sodium nitrate is soluble and dissociates into Na + (aq) and NO 3 (aq). Silver chloride is insoluble and forms a precipitate. Discussion: We know the precipitate is AgCl because it does not have a group 1 metal ion or nitrate. All ions are balanced. A NATURAL APPROACH TO CHEMISTRY 309

6 Section 10.3 Types of Chemical Reactions Polymerization Polymers are long chains of repeated units Many important molecules in both technology and biology are polymers. The word provides a clue as to how a polymer is made. The prefix poly means many, and the Greek word polymeres means of many parts. A polymer is a molecule made of many identical parts called monomers. A good example of a naturally occurring polymer is starch. Starch is a long-chain molecule, built from the repeated addition of sugar molecules. Polymerization Dehydration synthesis Starch is a staple food Photosynthesis in plants produces glucose. However, glucose is also rapidly used to release energy, even by plants. To save chemical energy for long-term use, plants convert glucose into starch by polymerization. Polymerization is a repeated addition reaction that assembles a polymer from smaller molecules. Anyone who likes fresh corn has tasted the difference between sugar and starch. Fresh-picked ripe corn has a high sugar content. Over a period of a day or two, enzyme in the corn convert the sugar to starch. Starch does not taste sweet, and the polymerization reaction is responsible for the old corn taste. The polymerization reaction that produces starch is called dehydration synthesis. Look carefully and notice that glucose has the chemical formula C 6 H 12 O 6 but the monomer for starch is C 6 H 10 O 5. The difference is one water molecule. To dehydrate means to take away water and to synthesize means to put together. Dehydration synthesis builds up the starch molecule by removing an oxygen and two hydrogens from each glucose molecule that gets added to the chain. The bond between adjacent monomers is made through the oxygen. Starch is the staple carbohydrate of many animal diets, including humans! Bread, pasta, and all grains are mostly starches. The body reverses the synthesis process and digests starch back into glucose. polymer: a molecule built from many repeating units of a smaller molecular fragment. polymerization: a reaction that assembles a polymer through repeated additions of smaller molecular fragments. 310 A NATURAL APPROACH TO CHEMISTRY