1 Goals 1. How are chemical reactions written? Given the identities of reactants and products, be able to write a balanced chemical equation or net ionic equation. 2. How are chemical reactions of ionic compounds classified? Be able to recognize precipitation, acid base neutralization, and redox reactions. 3. What are oxidation numbers, and how are they used? Be able to assign oxidation numbers to atoms in compounds and identify the substances oxidized and reduced in a given reaction. 4. What is a net ionic equation? Be able to recognize spectator ions and write the net ionic equation for reactions involving ionic compounds.
2 5.1 Chemical Equations Chemical reactions can be thought of as recipes. Starting materials and final products are listed, with chemical names replaced with formulas. A chemical equation is an expression in which symbols and formulas are used to represent a chemical reaction.
3 5.1 Chemical Equations A reactant is a substance that undergoes change in a chemical reaction and is written on the left side of the reaction arrow in a chemical equation. A product is a substance that is formed in a chemical reaction and is written on the right side of the reaction arrow in a chemical equation. Necessary conditions are written above the arrow.
4 5.1 Chemical Equations A coefficient is a number placed in front of a formula to balance a chemical equation. Coefficients multiply all the atoms in a formula. Be sure to consider coefficients. Substances that take part in chemical reactions may be solids, liquids, gases, or dissolved. This information is added to an equation by placing symbols after the formulas. (s) = solid (l) = liquid (g) = gas (aq) = dissolved in aqueous solution
5 5.2 Balancing Chemical Equations STEP 1: Write an unbalanced equation, using the correct formulas for all reactants and products. For example, hydrogen and oxygen must be written as H 2 and O 2 rather than as H and O, since both elements exist as diatomic molecules. Subscripts in chemical formulas cannot be changed because doing so would change the identity of the substances in the reaction.
6 5.2 Balancing Chemical Equations STEP 2: Add appropriate coefficients to balance the numbers of atoms of each element. Begin with elements that appear in only one compound or formula on each side of the equation. Leave elements that exist in elemental forms, such as oxygen and hydrogen, until last.
7 5.2 Balancing Chemical Equations If a polyatomic ion appears on both sides of an equation, it is treated as a single unit.
8 5.2 Balancing Chemical Equations STEP 3: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are the same. STEP 4: Make sure the coefficients are reduced to their lowest whole-number values.
9 5.3 Classes of Chemical Reactions Acid base neutralization reactions are processes in which an acid reacts with a base to yield water plus an ionic compound called a salt. A neutralization reaction removes H + and OH ions from solution and yields neutral H 2 O. Any ionic compound produced in an acid base reaction is called a salt.
10 5.3 Classes of Chemical Reactions Oxidation reduction reactions or redox reactions, are processes in which one or more electrons are transferred. As a result of this transfer, the number of electrons assigned to individual atoms in the various reactants change. Reactions involving covalent compounds are redox reactions since electrons are rearranged as bonds are broken and new bonds are formed.
11 5.4 Precipitation Reactions and Solubility Guidelines General Rules on Solubility 1. A compound is probably soluble if it contains one of the following cations. Group 1A cation: Li +, Na +, K +, Rb +, Cs + Ammonium cation: NH A compound is probably soluble if it contains one of the following anions. Halide: Cl, Br, I, except Ag +, Hg 2 2+, and Pb 2+ compounds Nitrate (NO 3 ), perchlorate (ClO 4 ), acetate (CH 3 CO 2 ), sulfate (SO 4 2 ), except Ba 2+, Hg 2 2+, and Pb 2+ sulfates
12 5.4 Precipitation Reactions and Solubility Guidelines
13 5.6 Redox Reactions This category of reactions is more complex. Historically, the word oxidation referred to the combination of an element with oxygen. Reduction referred to the removal of oxygen from an oxide to yield the element.
14 5.6 Redox Reactions Today the words have taken on a much broader meaning. Oxidation is the loss of one or more electrons. Reduction is the gain of one or more electrons. An oxidation reduction reaction, or redox reaction, is one in which electrons are transferred from one atom to another.
15 5.6 Redox Reactions
16 5.6 Redox Reactions An iodine ion gives an electron to bromine, forming iodine and bromide ions. An iodide ion is oxidized as its charge increases from 1 to 0. Bromine is reduced as its charge decreases from 0 to 1.
17 5.6 Redox Reactions Oxidation and reduction always occur together. When one substance loses an electron (is oxidized), another substance must gain that electron (be reduced). The substance that gives up an electron and causes the reduction is a reducing agent. The substance that gains an electron and causes the oxidation is an oxidizing agent. The charge on the reducing agent increases. The charge on the oxidizing agent decreases.
18 5.6 Redox Reactions A reducing agent loses one or more electrons. Causes reduction Undergoes oxidation Becomes more positive (or less negative) May gain oxygen atoms An oxidizing agent gains one or more electrons. Causes oxidation Undergoes reduction Becomes more negative (or less positive) May lose oxygen atoms
19 5.6 Redox Reactions Generalizations about redox behavior: 1. In reactions involving metals and nonmetals, metals tend to lose electrons while nonmetals tend to gain electrons. The number of electrons lost or gained can often be predicted based on the position of the element in the periodic table. 2. In reactions involving nonmetals, the more metallic element (farther down and/or to the left in the periodic table) tends to lose electrons, and the less metallic element (up and/or to the right) tends to gain electrons.
20 5.6 Redox Reactions Combustion is the burning of a fuel by rapid oxidation with oxygen in air. Gasoline, fuel oil, natural gas, wood, paper, and other organic substances of carbon and hydrogen are the most common fuels that burn in air. Some metals will burn in air, including magnesium and calcium.
21 5.7 Recognizing Redox Reactions A formal system has been devised for keeping track of changes in electron sharing, and determining whether atoms are oxidized or reduced in reactions. A value called an oxidation number (or oxidation state), indicates whether the atom is neutral, electronrich, or electron-poor. By comparing the oxidation number of an atom before and after a reaction, we can tell whether the atom has gained or lost shares in electrons. Oxidation numbers do not necessarily imply ionic charges. They are simply a convenient device for keeping track of electrons in redox reactions.
22 5.7 Recognizing Redox Reactions The rules for assigning oxidation numbers are straightforward. An atom in its elemental state has an oxidation number of 0. A monatomic ion has an oxidation number equal to its charge. In a molecular compound, an atom usually has the same oxidation number it would have if it were a monatomic ion. The sum of the oxidation numbers in a neutral compound is 0. In a polyatomic ion, the sum of the charges is the charge on the ion.
23 5.8 Net Ionic Equations In reactions involving ions, it is more accurate to write the reaction as an ionic equation. An ionic equation is one in which ions are explicitly shown. Some ions undergo no change during the reaction. They appear on both sides of the reaction but play no role. A spectator ion is an ion that appears unchanged on both sides of a reaction arrow.
24 5.8 Net Ionic Equations The actual reaction can be described more simply by writing a net ionic equation, which includes only the ions that undergo change. A net ionic equation is an equation that does not include spectator ions. A net ionic equation must be balanced both for atoms and for charge, with all coefficients reduced to their lowest whole numbers. All insoluble and molecular compounds are represented by their full formulas.
25 5.8 Net Ionic Equations Applied to neutralization reactions, KOH (aq) + HNO 3 (aq) H 2 O (l) + KNO 3 (aq) can be rewritten as an ionic equation, K + (aq) + OH (aq) + H + (aq) + NO 3 (aq) H 2 O (l) + K + (aq) + NO 3 (aq). Eliminating the spectator ions (K + and NO 3 ) yields OH (aq) + H + (aq) H 2 O (l). This net ionic equation confirms a neutralization reaction.
26 5.8 Net Ionic Equations Applied to redox reactions, Cu (s) + 2 AgNO 3 (aq) 2 Ag (s) + Cu(NO 3 ) 2 (aq) can be rewritten as an ionic equation, Cu (s) + 2 Ag + (aq) + 2 NO 3 (aq) 2 Ag (s) + Cu 2+ (aq) + 2 NO 3 (aq). Eliminating the spectator ion (NO 3 ) yields Cu (s) + 2 Ag + (aq) 2 Ag (s) + Cu 2+ (aq). This net ionic equation confirms that copper is oxidized and silver is reduced.
27 Chapter Summary 1. How are chemical reactions written? Chemical equations must be balanced; that is, the numbers and kinds of atoms must be the same in both the reactants and the products. To balance an equation, coefficients are placed before formulas, but the formulas themselves cannot be changed.
28 Chapter Summary, Continued 2. How are chemical reactions of ionic compounds classified? Precipitation reactions are processes in which an insoluble solid called a precipitate is formed. Most precipitations take place when the anions and cations of two ionic compounds change partners. Acid base neutralization reactions are processes in which an acid reacts with a base to yield water plus an ionic compound called a salt. Since acids produce H + ions and bases produce OH ions when dissolved in water, a neutralization reaction removes H + and OH ions from solution and yields water.
29 Chapter Summary 2. How are chemical reactions of ionic compounds classified? (Continued) Oxidation reduction (redox) reactions are processes in which one or more electrons are transferred between reaction partners. An oxidation is defined as the loss of one or more electrons by an atom. A reduction is the gain of one or more electrons. An oxidizing agent causes the oxidation of another reactant by accepting electrons. A reducing agent causes the reduction of another reactant by donating electrons.
30 Chapter Summary, Continued 3. What are oxidation numbers, and how are they used? Oxidation numbers are assigned to atoms in reactants and products to provide a measure of whether an atom is neutral, electron-rich, or electron-poor. By comparing the oxidation number of an atom before and after a reaction, we can tell whether the atom has gained or lost shares in electrons and thus, whether a redox reaction has occurred.
31 Chapter Summary, Continued 4. What is a net ionic equation? The net ionic equation only includes those ions that are directly involved in the ionic reaction. These ions are found in different phases or compounds on the reactant and product sides of the chemical equation. The net ionic equation does not include spectator ions, which appear in the same state on both sides of the equation.