Chapter 6 Chemical Reactions: An Introduction

Chapter 6 Table of Contents Copyright © Cengage Learning. All rights reserved Evidence for a Chemical Reaction 6.2 Chemical Equations 6.3 Balancing Chemical Equations

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 3 Chemical reactions often give a visual signal. But reactions are not always visible. What are the clues that a chemical change has taken place?

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 4 Some Clues That a Chemical Reaction Has Occurred

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 5 Exercise What is a clue that a chemical reaction has occurred? a)The color changes. b)A solid forms. c)Bubbles are present. d)A flame is produced.

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 6 Exercise What is a clue that a chemical reaction has occurred? “Colorless hydrochloric acid is added to a red solution of cobalt(II) nitrate, turning the solution blue.” a)The color changes. b)A solid forms. c)Bubbles are present. d)A flame is produced.

Section 6.1 Evidence for a Chemical Reaction Return to TOC Copyright © Cengage Learning. All rights reserved 7 Exercise What is a clue that a chemical reaction has occurred? “A solid forms when a solution of sodium dichromate is added to a solution of lead nitrate.” a)A gas forms. b)A solid forms. c)Bubbles are present. d)A flame is produced.

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 8 Chemical reactions involve a rearrangement of the ways atoms are grouped together. A chemical equation represents a chemical reaction.  Reactants are shown to the left of the arrow.  Products are shown to the right of the arrow.

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 9 In a chemical reaction atoms are not created or destroyed. All atoms present in the reactants must be accounted for in the products.  Same number of each type of atom on both sides of the arrow.

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 10 Unbalanced Equation: Balancing the Equation: The balanced equation: CH 4 + 2O 2  CO 2 + 2H 2 O Balancing a Chemical Equation

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 11 Physical states of compounds are often given in a chemical equation. Physical States

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 12 Example

Section 6.2 Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 13 Exercise When blue light shines on a mixture of hydrogen and chlorine gas, the elements react explosively to form gaseous hydrochloric acid. What is the unbalanced equation for this process? a)H 2 (g) + CH 4 (g) HCl(g) b)HCl(g) H(g) + Cl(g) c)H(g) + Cl(g) HCl(g) d)H 2 (g) + Cl 2 (g) HCl(g)

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 14 The principle that lies at the heart of the balancing process is that atoms are conserved in a chemical reaction. Atoms are neither created nor destroyed. The same number of each type of atom is found among the reactants and among the products.

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 15 Chemists determine the identity of the reactants and products of a reaction by experimental observation. The identities (formulas) of the compounds must never be changed in balancing a chemical equation.

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 16 1.Read the description of the chemical reaction. What are the reactants, the products, and their states? Write the appropriate formulas. Hydrogen gas (H 2 ) and oxygen gas (O 2 ) combine to form liquid water (H 2 O). 2.Write the unbalanced equation that summarizes the information from step 1. H 2 (g) + O 2 (g)  H 2 O(l) How to Write and Balance Equations

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 17 3.Balance the equation by inspection, starting with the most complicated molecule. Equation is unbalanced by counting the atoms on both sides of the arrow. How to Write and Balance Equations

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 18 3.Balance the equation by inspection, starting with the most complicated molecule. We must balance the equation by adding more molecules of reactants and/or products. How to Write and Balance Equations

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 19 How to Write and Balance Equations 4.Check to see that the coefficients used give the same number of each type of atom on both sides of the arrow. Also check to see that the coefficients used are the smallest integers that give the balanced equation. The balanced equation is: 2H 2 (g) + O 2 (g)  2H 2 O(l) or could be: 4H 2 (g) + 2O 2 (g)  4H 2 O(l) preferred

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 20 Exercise Balance the following equation in standard form (lowest multiple integers) and determine the sum of the coefficients? FeO(s) + O 2 (g)  Fe 2 O 3 (s) a)3 b)4 c)7 d)14

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 21 Exercise Which of the following correctly balances the chemical equation given below? There may be more than one correct balanced equation. If a balanced equation is incorrect, explain what is incorrect about it. CaO + C  CaC 2 + CO 2 I. CaO 2 + 3C  CaC 2 + CO 2 II. 2CaO + 5C  2CaC 2 + CO 2 III. CaO + (2.5)C  CaC 2 + (0.5)CO 2 IV. 4CaO + 10C  4CaC 2 + 2CO 2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 22 Exercise Of the three that are correct, which one is preferred most (the most accepted convention)? Why? CaO + C  CaC 2 + CO 2 I. CaO 2 + 3C  CaC 2 + CO 2 II. 2CaO + 5C  2CaC 2 + CO 2 III. CaO + (2.5)C  CaC 2 + (0.5)CO 2 IV. 4CaO + 10C  4CaC 2 + 2CO 2

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 23 Concept Check When balancing a chemical equation, which of the following statements is false? a)Subscripts in the reactants must be conserved in the products. b)Coefficients are used to balance the atoms on both sides. c)When one coefficient is doubled, the rest of the coefficients in the balanced equation must also be doubled. d)Phases are often shown for each compound but are not critical to balancing an equation.

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 24 The number of atoms of each type of element must be the same on both sides of a balanced equation. Subscripts must not be changed to balance an equation. A balanced equation tells us the ratio of the number of molecules which react and are produced in a chemical reaction. Coefficients can be fractions, although they are usually given as lowest integer multiples. Notice

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 25 Formation of a solid Formation of water Transfer of electrons Formation of a gas Driving Forces for a Reaction

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 26 Synthesis Reaction (Combination) two or more simple substances combine to form a new, more complex substance the reactants are generally either 2 elements or 2 compounds the product is always a compound example: 4Fe(s) + 3O 2 (g)  2Fe 2 O 3 (s)

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 27 Decomposition Reaction a complex substance is broken down into 2 or more simpler substances the products can be any combination of elements and compounds most reactions require energy in the form of heat, light, or electricity EXs: (NH 4 ) 2 Cr 2 O 7  Cr 2 O 3 + 4H 2 O + N 2 2NaClO 3 (s)  2NaCl(s) + 3O 2 (g)

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 28 Single-Replacement Reaction an uncombined element replaces an element that is part of a compound whether one metal will displace another metal from a compound can be determined by the relative reactivities of the 2 metals. The activity series of metals lists metals in order of decreasing reactivity. A reactive metal will replace any metal listed below it in the activity series. the product will be a different element and a new compound EX: Fe 2 O 3 (s) + 2Al(s)  Al 2 O 3 (s) + 2Fe(s)

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 29 Activity Series of Metals - A reactive metal will replace any metal listed below it in the activity series Decreasing Reactivity Lithium Potassium Barium Calcium Sodium Magnesium Aluminum Manganese Zinc Chromium Iron Cobalt Nickel Tin Lead Hydrogen Copper Silver Mercury Platinum Gold *metals from lithium to sodium will replace hydrogen from acids and water; from magnesium to lead they will replace hydrogen from acids only*

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 30 Double-Replacement Reaction involve an exchange of positive ions between 2 reacting compounds generally involve the production of a precipitate and another compound EX: Pb(NO 3 ) 2 (aq) + 2KI(aq)  PbI 2 (s) + 2KNO 3 (aq)

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 31 Using Solubility Rules

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 32 Using Solubility Rules Predicting Precipitates  Soluble solid  Insoluble solid  Slightly soluble solid

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 33 1.Write the reactants as they actually exist before any reaction occurs. Remember that when a salt dissolves, its ions separate. 2.Consider the various solids that could form. To do this, simply exchange the anions of the added salts. 3.Use the solubility rules to decide whether a solid forms and, if so, to predict the identity of the solid. How to Predict Precipitates When Solutions of Two Ionic Compounds Are Mixed

Section 6.3 Balancing Chemical Equations Return to TOC Copyright © Cengage Learning. All rights reserved 34 Combustion Reaction a chemical change in which oxygen reacts with another substance, often producing energy in the form of heat and light products will be carbon dioxide and water or a new compound may also be a synthesis (combination) reaction EXs: C 12 H 22 O 11 (s) + 3O 2 (g)  9C(s) + 3CO 2 (g) + H 2 O(g) kJ 2Mg(s) + O 2 (g)  2MgO(s)