Classification and Balancing of Chemical Reactions Fundamentals of General, Organic, and Biological Chemistry 7th Edition Chapter Five Classification and Balancing of Chemical Reactions 2013 Pearson Education, Inc.

Chapter 5 Goals Topics and Goals for the Chapter: How are chemical reactions written? Be able to define the terms reactant and product. Be able to define the Law of Conservation of Mass. Be able to write and interpret a chemical equation. Be able to understand and apply the steps for balancing and balance simple chemical equations. Be able to write molecular, ionic and net ionic equations. How are chemical reactions of ionic compounds classified? Be able to understand and distinguish between the three main classes of chemical reactions: precipitation reactions, acid-base neutralization reactions and oxidation-reduction reactions. Chapter 5 2 2013 Pearson Education, Inc. Chapter 5

Chapter 5 Goals Cont. What are oxidation numbers, and how are they used? Be able to define oxidation, reduction and oxidation number. Be able to assign oxidation numbers to atoms in a chemical equation. Be able to identify the substances oxidized and reduced in a given reaction. What is a net ionic equation? Be able to recognize spectator ions and write the net ionic equation for reactions involving ionic compounds. 2013 Pearson Education, Inc. Chapter 5 3 Chapter 5

Components of Chemical Equations 5.1 Chemical Reactions Chemical equations are a representation of what items and what quantities we need to “react” to “produce” a substance or substances. Components of Chemical Equations Conditions necessary for rxn. Heat Coefficients CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (ℓ) Products Reactants States of matter Chemical change Chapter 5 4 2013 Pearson Education, Inc. Chapter 5

CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (ℓ) Components of Chemical Equations heat CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (ℓ) Reactants written in the on left, products written on the right Arrow = chemical change Conditions necessary for reaction – written above the arrow in equation. A chemical reaction must have an equal number of atoms of each element on each side of the reaction arrow. The chemical reaction must be balanced. Law of Conservation of Mass – Matter is neither created nor destroyed in chemical reactions. Coefficients – numbers before formulas. Tell us “how many” of each formula we have in a balanced reaction. Correct chemical formulas contain the states of matter: (g), (ℓ), (s), (aq) Chapter 5 5 2013 Pearson Education, Inc. Chapter 5

5.2 Balancing Chemical Equations CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (ℓ) Is the above reaction equation balanced? Do we have the same total number of each type of atom on each side of the equation? Total # of type of atom = Sum of (coefficient in equation for chemical formula) (subscript in formula for the atom) Atom Reactants Products C H O Chapter 5 6 2013 Pearson Education, Inc. Chapter 5

N2H4 (ℓ) + N2O4 (ℓ) → N2 (g) + H2O (g) Four Step Method for Writing and Balancing a Chemical Reaction Equation Suppose we are told that liquid hydrazine (N2H4) and liquid dinitrogen tetraoxide react to form gaseous nitrogen and water vapor. Write an unbalanced equation using the correct formulas for all reactants and products. Indicate the physical state of each reactant and product. N2H4 (ℓ) + N2O4 (ℓ) → N2 (g) + H2O (g) 2. Add appropriate coefficients to balance the numbers of atoms of each element. When balancing equations keep the following in mind: a. Always balance by adjusting coefficients of formulas, NEVER subscripts in formulas. Why? b. Start with an element that appears in only one species on each side of the equation, if possible. c. Treat polyatomic ions as a single unit. Chapter 5 7 2013 Pearson Education, Inc. Chapter 5

Four Step Method for Balancing a Chemical Reaction Equation Cont. __N2H4 (ℓ) + __N2O4 (ℓ) → __N2 (g) + __H2O (g) Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are the same. Make sure the coefficients are reduced to their lowest whole-number values. Chapter 5 8 2013 Pearson Education, Inc. Chapter 5

Four Step Method for Balancing a Chemical Reaction Equation Cont. Note: It is possible to balance an equation using fractions. (And sometimes easier). After the equation is balanced, just multiply through by a factor to convert into whole numbers. PbS (s) + 3/2 O2 (g) → PbO (s) + SO2 (g) (balanced) 2PbS (s) + 3 O2 (g) → 2PbO (s) + 2SO2 (g) (balanced) 2013 Pearson Education, Inc. Chapter 5 9 Chapter 5

Ex: Balance the following equations: __Pb(NO3)2 (aq) + __AlCl3 (aq) → __PbCl2 (s) + __Al(NO3)3 (aq) __NH3 (g) + __O2 (g) → __NO (g) + __H2O (g) Chapter 5 10 2013 Pearson Education, Inc. Chapter 5

5.3 Classes of Chemical Reactions Reactions involving ionic compounds are grouped into 3 general classes: Precipitation Reactions - Processes in which an insoluble solid called a precipitate forms when reactants are combined in aqueous solution. Acid–Base Neutralization Reactions - Processes in which H+ ions from an acid react with OH- ions from a base to yield water. An ionic compound called a salt is also produced. Oxidation–Reduction Reactions, or Redox reactions - Processes in which one or more electrons are transferred between reaction partners (atoms, molecules, or ions). As a result of this transfer, the charges on atoms in the various reactants change. Chapter 5 11 2013 Pearson Education, Inc. Chapter 5

5.4 Precipitation Reactions and Solubility Guidelines Precipitation reactions occur when an insoluble solid, a precipitate, forms when ionic reactants are combined in aqueous solution. The Big Question: When we mix two aqueous solutions of two ionic compounds will a precipitate from? To predict – Look at solubilities of potential products. Solubility – How much compound will dissolve in a given amount of solvent at a given temperature. Low solubility in H2O – Precipitate will form. High solubility in H2O – No precipitate will form. Chapter 5 12 2013 Pearson Education, Inc. Chapter 5

The table below provides solubility information for use in predicting precipitation Chapter 5 13 2013 Pearson Education, Inc. Chapter 5

Na2CO3 (aq) + CaCl2 (aq) → 2NaCl (aq or s?) + CaCO3 (aq or s?) Example: We can take any two ionic solutions and determine whether a precipitate will form if they are mixed. Na2CO3 (aq) + CaCl2 (aq) → 2NaCl (aq or s?) + CaCO3 (aq or s?) Products are mixtures of alternate cations and anions. Are these compounds soluble (aq)? Using a visual often helps. Visualize what the reaction looks like in the initial and final states. Chapter 5 14 2013 Pearson Education, Inc. Chapter 5

Another Example Problem: Will a precipitate form when the following are mixed? Pb(NO3)2 (aq) + Na2SO4 (aq) → Chapter 5 15 2013 Pearson Education, Inc. Chapter 5

} Chapter 4 5.5 Acids, Bases and Neutralization Reactions When we mix acids and bases in correct proportions we can get a “neutralization”. Acid = H+ donor in water Base = OH- donor in water } Chapter 4 Most common type of neutralization: Between an acid and a metal hydroxide. Ex: HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (ℓ) acid metal salt water hydroxide Chapter 5 16 2013 Pearson Education, Inc. Chapter 5

Another type of neutralization: Between acid and carbonate: Ex: 2HCl (aq) + Na2CO3 (aq) → 2NaCl (aq) + H2CO3 (aq) ↓ H2O (ℓ) + CO2 (g) Carbonates also act as bases because they yield OH- ions when dissolved in water. Na2CO3 (s) + H2O (ℓ) → 2Na+ (aq) + HCO3- (aq) + OH- (aq) Remember: Equations for reactions must be balanced! Chapter 5 17 2013 Pearson Education, Inc. Chapter 5

Complete (and balance) the following reaction: Problem: Complete (and balance) the following reaction: HCl (aq) + Ba(OH)2 (aq) → Chapter 5 18 2013 Pearson Education, Inc. Chapter 5

5.6 Redox Reactions Redox reactions are much more complex then precipitation or neutralization reactions. Redox = Oxidation-Reduction Reaction: A reaction in which electrons transfer from one atom to another. Oxidation: Loss of one or more electrons by an atom. Reduction: Gain of one or more electrons by an atom. Loss of Gain of Electrons Goes Electrons Oxidation Reduction Chapter 5 19 2013 Pearson Education, Inc. Chapter 5

Oxidation: Loss of one or more electrons by an atom. Reduction: Gain of one or more electrons by an atom. Chapter 5 20 2013 Pearson Education, Inc. Chapter 5

Demonstration of a redox reaction: Copper wire in AgNO3 (aq) Cu (s) + AgNO3 (aq) → silvery gray solid + blue solution Cu (s) + 2AgNO3 (aq) → 2Ag (s) + Cu(NO3)2 (aq) 2013 Pearson Education, Inc. Chapter 5 21 Chapter 5

Cu (s) + 2AgNO3 (aq) → 2Ag (s) + Cu(NO3)2 (aq) Let’s look at the above reaction more closely: The above reaction equation is the un-ionized reaction equation. The un-ionized equation simply lists the balanced ratio of reactants and products in their appropriate states. No dissociation accounted for. What does the reaction really look like at the beginning and end of the reaction? In solution ionic compounds completely dissociate into cations and anions. Cu (s) + 2Ag+ (aq) + 2NO3- (aq) → Cu2+(aq) + 2Ag (s) + 2NO3- (aq) The total ionic equation represents a more accurate description of the reaction reactants and products. Chapter 5 22 2013 Pearson Education, Inc. Chapter 5

Cu (s) + 2Ag+ (aq) + 2NO3- (aq) → Cu2+ (aq) + 2Ag (s) + 2NO3- (aq) Total Ionic Equation Look at the total ionic equation. Do you notice any reactants and products that are identical? Ions that appear identically (same charge, same atom or group of atoms and same phase) on both sides of the equation = “spectator ions” NO3- does not take part in the reaction – a “spectator ion”. Reaction equations never have identical chemicals on both sides of the reaction equation. Those substances are eliminated from the equation. Chapter 5 23 2013 Pearson Education, Inc. Chapter 5

Cu (s) + 2Ag+ (aq) + 2NO3- (aq) → Cu2+ (aq) + 2Ag (s) + 2NO3- (aq) Total Ionic Equation Eliminating spectator ions we simplify the equation to obtain the Net Ionic Equation Cu (s) + 2Ag+ (aq) → Cu2+ (aq) + 2Ag (s) The net ionic equation represents substances that are actually undergoing reaction. 2013 Pearson Education, Inc. Chapter 5 24 Chapter 5

Cu (s) + 2Ag+ (aq) → Cu2+ (aq) + 2Ag (s) Net Ionic Equation Look at the charges on each atom/ion: Cu (s) → Cu2+ (aq) 0 2+ 2Ag+ (aq) → 2Ag (s) + 0 Charges can only change through electrons being lost or gained. Where are the electrons in the above reactions? On the reactant or product side? Cu (s) + 2Ag+ (aq) → Cu2+ (aq) + 2Ag (s) The above reaction is really just a transfer of electrons. 2013 Pearson Education, Inc. Chapter 5 25 Chapter 5

Look at the charges on each atom/ion. Net Ionic Equation Look at the charges on each atom/ion. Two electrons are transferred from Cu (s) to 2Ag+ (aq). 2013 Pearson Education, Inc. Chapter 5 26 Chapter 5

Another Oxidation-Reduction Reaction 2KI (aq) + Br2 (aq) → I2 (aq) + 2KBr (aq) How do we look at this reaction? Write the total ionic equation – makes it easier to see changes. Eliminate any spectator ions. Write the simplified equation (net ionic equation). Look at charges on species. 1. 2K+ (aq) + 2I- (aq) + Br2 (aq) → I2 (aq) + 2K+ (aq) + 2Br- (aq) 2.-4. 2I- each lose 1 e- - oxidized Br2 gains 2e- - each atom gains 1e- - reduced Chapter 5 27 2013 Pearson Education, Inc. Chapter 5

Oxidation and reduction always occur together. Note: Oxidation and reduction always occur together. They have to. We need source of e- and a place for them to go. Other names associated with oxidation-reduction reactions: Reducing Agent: Loses one or more electrons Causes reduction Undergoes oxidation Becomes more positive (or less negative) Oxidizing Agent: Gains one or more electrons Causes oxidation Undergoes reduction Becomes more negative (or less positive) Chapter 5 28 2013 Pearson Education, Inc. Chapter 5

Redox Behavior of Metals and Nonmetals Simplest redox reactions are reaction of an element (usually a metal) with an aqueous cation to yield a different element and a cation. Element1 + Cation2 → Element2 + Cation1 Cu (s) + 2Ag+ (aq) → 2Ag (s) + Cu2+ (aq) Fe (s) + Cu2+ (aq) → Cu (s) + Fe2+ (aq) Mg (s) + 2H+ (aq) → H2 (g) + Mg2+ (aq) Chapter 5 29 2013 Pearson Education, Inc. Chapter 5

Redox Behavior of Metals and Nonmetals – Generalizations Alkali and alkali earth metals Very good reducing agents (good e- donors) 2. Transition metal Reducing agents but not as good as group 1 and group 2 metals 3. Metals near bottom right of periodic table (Pt, Au) Weak reducing agents Nonmetals at top of periodic table Good oxidizing agents (good e- acceptors) In reactions involving metals and nonmetals Metals tend to lose e- while nonmetals tend to gain e-. In reactions involving nonmetals Elements that are closer to metals in periodic table tend to lose e-. Elements that are closer to nonmetals in periodic table tend to gain e-. Chapter 5 30 2013 Pearson Education, Inc. Chapter 5

Examples of Redox Reactions Corrosion – Deterioration of a metal by oxidation Rusting of iron 4Fe (s) + 3O2 (g) → 2Fe2O3∙H2O (s) Combustion – Burning of fuel by O2 in air CH4 (g) + 2O2 (g) → CO2 (g) + 2H2O (ℓ) Carbon is gaining oxygen atoms = oxidation H2O Chapter 5 31 2013 Pearson Education, Inc. Chapter 5

Examples of Redox Reactions Cont. Respiration – Similar to combustion C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy Bleaching NaOCl + dye → oxidized dye Metallurgy – Extracting and purifying metals from ores Ex: Production of iron from Fe2O3 (hematite) Fe2O3 (s) + 3CO (g) → 2Fe (s) + 3CO2 (g) CO oxidized Fe3+ reduced to Fe Chapter 5 32 2013 Pearson Education, Inc. Chapter 5

5.7 Recognizing Redox Reactions We can determine whether atoms are oxidized or reduced in a reaction by keeping track of changes in electron sharing by the atoms. Each atom in a substance is assigned a value called an oxidation number or oxidation state. The oxidation number indicates whether the atom is neutral, electron-rich, or electron-poor. Oxidation numbers are a bookkeeping device only! We develop charges on atoms but they aren’t real charges. This is a way for us to determine if oxidation or reduction has occurred by comparing assigned oxidation numbers in reactants and products. Chapter 5 33 2013 Pearson Education, Inc. Chapter 5

Rules for Assigning Oxidation Numbers An atom in its elemental state always has an oxidation number of “0”. Ex: Na, O2, Cl2 Monatomic ions have an oxidation number = charge on ion. In a covalent compound, an atom usually has the same oxidation number it would have if it were a monatomic ion. Grp 1A = +1 Grp 2A = +2 Grp 5A = -3 Grp 6A = -2 Grp 7A = -1 Chapter 5 34 2013 Pearson Education, Inc. Chapter 5

Rules for Assigning Oxidation Numbers Cont. For compounds with more than one nonmetal element, such as SO2, NO, and CO2, the more electronegative element—oxygen in these examples—has its preferred negative oxidation number. The less electronegative element is assigned a positive oxidation number so that the sum of the oxidation numbers in a neutral compound is 0. S (s) + O2 (g) → SO2 (g) Ox. # 0 0 +4 -2 Chapter 5 35 2013 Pearson Education, Inc. Chapter 5

Rules for Assigning Oxidation Numbers Cont. The sum of the oxidation numbers in a neutral species is 0. In a polyatomic ion the sum is equal to the charge on the ion. Ex: HNO3 Chapter 5 36 2013 Pearson Education, Inc. Chapter 5

5.8 Net Ionic Equations Net ionic equations are derived from un-ionized equations. We actually went through this process when looking at Redox reactions. 3 step Process Ex: Un-Ionized Equation – Gives formulas Pb(NO3)2 (aq) + 2KI (aq) → 2KNO3 (aq) + PbI2 (s) But we know ionic compounds in aqueous solution exist as separate ions. Total Ionic Equation – Gives ions in solution More accurate representation of what is in solution. Pb2+ (aq) + 2NO3- (aq) + 2K+ (aq) + 2I- (aq) → 2K+ (aq) + 2 NO3- (aq) + PbI2 (s) Look at equation above. K+ and NO3- aren’t participating in any reaction. They are “spectator” ions We like to write reactions with just substances that take part in the reaction. We drop out spectator ions. 3. Net Ionic Equation – Gives only species involved in reaction Pb2+ (aq) + 2I- (aq) → PbI2 (s) Chapter 5 37 2013 Pearson Education, Inc. Chapter 5

Net ionic equations are used to represent several types of reactions: 1. Precipitation Reactions Pb(NO3)2 (aq) + 2KI (aq) → 2KNO3 (aq) + PbI2 (s) Pb2+ (aq) + 2NO3- (aq) + 2K+ (aq) + 2I- (aq) → 2K+ (aq) + 2NO3- (aq) + PbI2 (s) Pb2+ (aq) + 2I- (aq) → PbI2 (s) Acid-Base Neutralization HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (ℓ) H+ (aq) + Cl- (aq) + Na+ (aq) + OH- (aq) → Na+ (aq) + Cl- (aq) + H2O (ℓ) H+ (aq) + OH- (aq) → H2O (ℓ) Oxidation-Reduction Cu (s) + 2AgNO3 (aq) → 2Ag (s) + Cu(NO3)2 (aq) Cu(s) + 2Ag+(aq) + 2NO3-(aq) → Cu2+(aq) + 2Ag(s) + 2NO3-(aq) Cu(s) + 2Ag+(aq) → Cu2+(aq) + 2Ag(s) Notice – In every case only those cations and anions participating as reactants appear in the equation and the resulting products contain only those atoms. Chapter 5 38 2013 Pearson Education, Inc. Chapter 5

Here are some interactive sites which might be helpful. Interactive Student Tutorial http://genchem.chem.wisc.edu/sstutorial/Text11/NetIonic/NetIonicIndex.htm Practice Writing Net Ionic Equations http://www.files.chem.vt.edu/RVGS/ACT/notes/net_ionic_rxns/net_ionic_rxns.html Chapter 5 39 2013 Pearson Education, Inc. Chapter 5

Chapter Summary Chemical equations must be balanced; 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. Precipitation reactions are processes in which an insoluble solid called a precipitate is formed. In acid–base neutralization reactions an acid reacts with a base to yield water plus a salt. Chapter 5 40 2013 Pearson Education, Inc. Chapter 5

Chapter Summary Cont. Oxidation–reduction (redox) reactions are processes in which one or more electrons are transferred between reaction partners. Oxidation is the loss of electrons by an atom, and reduction is the gain of electrons by an atom. Oxidation numbers are assigned to provide a measure of whether an atom is neutral, electron-rich, or electron-poor. Chapter 5 41 2013 Pearson Education, Inc. Chapter 5