1 Chemical Equations Chapter 8 Hein and Arena Eugene Passer Chemistry Department Bronx Community College © John Wiley and Sons, Inc Version 1.1

2 Chapter Outline 8.1 The Chemical EquationThe Chemical Equation 8.2 Writing and Balancing EquationsWriting and Balancing Equations 8.3 What Information Does an Equation Tell UsWhat Information Does an Equation Tell Us 8.4 Types of Chemical EquationsTypes of Chemical Equations 8.5 Heat in Chemical ReactionsHeat in Chemical Reactions

3 Chemists use chemical equations to describe reactions they observe in the laboratory or in nature. Chemical equations provide us with the means to 1.summarize the reaction 2.display the substances that are reacting 3.show the products 4.indicate the amounts of all component substances in a reaction.

4 Chemical reactions always involve change. Atoms, molecules or ions rearrange to form new substances. The substances entering the reaction are called reactants. The substances formed in the reaction are called products. During reactions, chemical bonds are broken and new bonds are formed. The Chemical Equation

5 A chemical equation uses the chemical symbols and formulas of the reactants and products and other symbolic terms to represent a chemical reaction. A chemical equation is a shorthand expression for a chemical change or reaction.

6 Al + Fe 2 O 3  Fe + Al 2 O 3 reactantsproducts Al + Fe 2 O 3  Fe + Al 2 O 3 Chemical Equation iron oxygen bonds break aluminum oxygen bonds form

7 Coefficients (whole numbers) are placed in front of substances to balance the equation and to indicate the number of units (atoms, molecules, moles, or ions) of each substance that are reacting.

8 Al + Fe 2 O 3  Fe + Al 2 O 3 coefficient 2 2

9 Conditions required to carry out the reaction may be placed above or below the arrow.

10 Al + Fe 2 O 3  Fe + Al 2 O 3 coefficient 2 2   heat

11 The physical state of a substance is indicated by symbols such as (l) for liquid.

12 2Al(s) + Fe 2 O 3 (s)  2Fe(l) + Al 2 O 3 (s) All atoms present in the reactant must also be present in the products. In a chemical reaction atoms are neither created nor destroyed. (s)(s) (l)(l)(s)(s)(s)(s)

13 Symbols Used in Chemical Reactions

14 placed between substances + symbol plus meaning location

15  symbol yields meaning between reactants and products location

16 (s)(s) symbol solid meaning after formula location

17 (l)(l) symbol liquid meaning location after formula

18 (g)(g) symbol gas meaning location after formula

19 (aq) symbol aqueous meaning after formula location

20  symbol heat meaning written above  location

21 h symbol light energy meaning written above  location

22  symbol gas formation meaning after formula location

23 To balance an equation adjust the number of atoms of each element so that they are the same on each side of the equation. Never change a correct formula to balance an equation. Writing and Balancing Equations

24 Step 1 Identify the reaction. Write a description or word equation for the reaction. Mercury (II) oxide decomposes to form mercury and oxygen. mercury(II) oxide → mercury + oxygen Steps for Balancing Equations

25 HgO  Hg + O 2 –The formulas of the reactants and products must be correct. –The reactants are written to the left of the arrow and the products to the right of the arrow. Step 2 Write the unbalanced (skeleton) equation. The formulas of the reactants and products can never be changed.

26 Step 3a Balance the equation. –Count and compare the number of atoms of each element on both sides of the equation. –Determine the elements that require balancing.

27 HgO → Hg + O 2 Step 3a Balance the equation. –There is one mercury atom on the reactant side and one mercury atom on the product side. –Mercury is balanced. Element Reactant Side Product Side Hg 1 1

28 Element Reactant Side Product Side O 1 2 Step 3a Balance the equation. –There are two oxygen atoms on the product side and there is one oxygen atom on the reactant side. –Oxygen needs to be balanced. HgO  Hg + O 2

29 Step 3b Balance the equation. –Balance each element one at a time, by placing whole numbers (coefficients) in front of the formulas containing the unbalanced element. –A coefficient placed before a formula multiplies every atom in the formula by that coefficient.

30 Element Reactant Side Product Side O 1 2  Oxygen (O) is balanced. Step 3b Balance the equation. Place a 2 in front of HgO to balance O.  There are two oxygen atoms on the reactant side and there are two oxygen atoms on the product side. HgO  Hg + O 2 2 2

31 Step 3c Balance the equation. Check all other elements after each individual element is balanced to see whether, in balancing one element, another element became unbalanced.

32 Element Reactant Side Product Side Hg 2 1 Count and compare the number of mercury (Hg) atoms on both sides of the equation. Step 3c Balance the equation.  Mercury (Hg) is not balanced. 2HgO  Hg + O 2 There are two mercury atoms on the reactant side and there is one mercury atom on the product side.

33 2HgO  Hg + O 2 Step 3c Balance the equation. Place a 2 in front of Hg to balance mercury.  Mercury (Hg) is balanced.  There are two mercury atoms on the reactant side and there are two mercury atoms on the product side. Element Reactant Side Product Side Hg

34 2HgO  2Hg + O 2 Element Reactant Side Product Side Hg 2 2 O 2 2  THE EQUATION IS BALANCED 

35 sulfuric acid + sodium hydroxide → sodium sulfate + water Balance the Equation

36 There is one Na on the reactant side and there are two Na on the product side. Reactant Side Product Side SO 4 11 Na12 O1 1 H3 2 2 H 2 SO 4 (aq) + NaOH(aq) → Na 2 SO 4 (aq) + H 2 O(l) 2 Place a 2 in front of NaOH to balance Na. Balance the Equation 2 4

37 H 2 SO 4 (aq) + NaOH(aq) → Na 2 SO 4 (aq) + H 2 O(l) There are 4 H on the reactant side and two H on the product side. Reactant Side Product Side SO 4 11 Na22 O2 1 H4 2 2 Place a 2 in front of H 2 O to balance H  THE EQUATION IS BALANCED 

38 butane + oxygen → carbon dioxide + water Balance the Equation

39 C 4 H 10 (g) + O 2 (g) → CO 2 (g) + H 2 O(l) There are four C on the reactant side and there is one C on the product side. Reactant Side Product Side C 41 H102 O Place a 4 in front of CO 2 to balance C. 9 4 Balance the Equation

40 C 4 H 10 (g) + O 2 (g) → CO 2 (g) + H 2 O(l) There are 10 H on the reactant side and there are two H on the product side. Reactant Side Product Side C 44 H102 O 2 9 Place a 5 in front of H 2 O to balance H

41 C 4 H 10 (g) + O 2 (g) → CO 2 (g) + H 2 O(l) There is no whole number coefficient that can be placed in front of O 2 to balance O. Reactant Side Product Side C 4 4 H1010 O 2 13 To balance O double all of the coefficients

42 There are now 26 O on the product side. Reactant Side Product Side C 8 8 H2020 O Place a 13 in front of O 2 to balance O. 26  THE EQUATION IS BALANCED  C 4 H 10 (g) + O 2 (g) → CO 2 (g) + H 2 O(l) 28 10

43 The meaning of a formula is context dependent. The formula H 2 O can mean: 1.2H and 1 O atom 2.1 molecule of water 3.1 mol of water x molecules of water g of water What Information Does an Equation Tell Us

44 In an equation formulas can represent units of individual chemical entities or moles. H2H2 +Cl 2 2HCl→ 1 molecule H 2 1 molecule Cl 2 2 molecules HCl 1 mol H 2 1 mol Cl 2 2 mol HCl

45 Formulas Number of molecules Number of atoms Number of moles Molar masses

46 Combination Decomposition Single-Displacement Double-Displacement Types of Chemical Equations

47 A + B  AB Two reactants combine to form one product. Combination Reactions

48 2Ca(s) + O 2 (g)  2CaO(s) Metal + Oxygen → Metal Oxide 4Al(s) + 3O 2 (g)  2Al 2 O 3 (s)

49 S(s) + O 2 (g)  SO 2 (g) Nonmetal + Oxygen → Nonmetal Oxide N 2 (g) + O 2 (g)  2NO(g)

50 2K(s) + F 2 (g)  2KF(s) Metal + Nonmetal → Salt 2Al(s) + 3Cl 2 (g)  2AlCl 3 (s)

51 Na 2 O(s) + H 2 O(l)  2NaOH(aq) Metal Oxide + Water → Metal Hydroxide CaO(s) + 2H 2 O(l)  2Ca(OH) 2 (aq)

52 SO 3 (g) + H 2 O(l)  H 2 SO 4 (aq) Nonmetal Oxide + H 2 O(l) → Oxy-acid N 2 O 5 (g) + H 2 O(l)  2HNO 3 (aq)

53 AB  A + B A single substance breaks down to give two or more different substances. Decomposition Reactions

54 2Ag 2 O(s)  4Ag(s) + O 2 (g) Metal Oxide → Metal + Oxygen Metal Oxide → Metal Oxide + Oxygen 2PbO 2 (s)  2PbO(s) + O 2 (g)

55 Carbonate → CO 2 (g) CaCO 3 (s)  CaO(s) + CO 2 (g) 2NaHCO 3 (s)  Na 2 CO 3 (s) + H 2 O(g) + CO 2 (g) Hydrogen Carbonate → CO 2 (g)

56 Miscellaneous Reactions 2KClO 3 (s)  2KCl(s) + 3O 2 (g) 2NaNO 3 (s)  2NaNO 2 (s) + O 2 (g) 2H 2 O 2 (l)  2H 2 O(l) + O 2 (g)

57 A + BC  AC + B One element reacts with a compound to replace one the elements of that compound. Single Displacement Reactions

58 Mg(s) + 2HCl(aq)  H 2 (g) + MgCl 2 (aq) 2Al(s) + 3H 2 SO 4 (aq)  3H 2 (g) + Al 2 (SO 4 ) 3 (aq) salt Metal + Acid → Hydrogen + Salt salt

59 2Na(s) + 2H 2 O(l)  H 2 (g) + 2NaOH(aq) Ca(s) + 2H 2 O(l)  H 2 (g) + Ca(OH) 2 (aq) Metal + Water → Hydrogen + Metal Hydroxide metal hydroxide

60 Metal + Water → Hydrogen + Metal Oxide metal oxide 3Fe(s) + 4H 2 O(g)  4H 2 (g) + Fe 3 O 4 (s)

61 The Activity Series

62 Metals K Ca Na Mg Al Zn Fe Ni Sn Pb H Cu Ag Hg An atom of an element in the activity series will displace an atom of an element below it from one of its compounds. Sodium (Na) will displace an atom below it from one of its compounds. increasing activity

63 Examples Metal Activity Series

64 Mg(s) + PbS(s)  MgS(s) + Pb(s) Metal Higher in Activity Series Displacing Metal Below It Magnesium is above lead in the activity series. Metals Mg Al Zn Fe Ni Sn Pb

65 Ag(s) + CuCl 2 (s)  no reaction Metal Lower in Activity Cannot Displace Metal Above It Metals Pb H Cu Ag Hg Silver is below copper in the activity series.

66 Example Halogen Activity Series

67 Cl 2 (g) + CaBr 2 (s)  CaCl 2 (aq) + Br 2 (aq) Halogen Higher in Activity Series Displaces Halogen Below It Halogens F 2 Cl 2 Br 2 I 2 Chlorine is above bromine in the activity series.

68 AB + CD  AD + CB Two compounds exchange partners with each other to produce two different compounds. The reaction can be thought of as an exchange of positive and negative groups. A displaces C and combines with D B displaces D and combines with C Double Displacement Reactions

69 The Following Accompany Double Displacement Reactions formation of a precipitate release of gas bubbles release of heat formation of water

70 Acid Base Neutralization HCl(aq) + NaOH(aq)  NaCl(aq) + H 2 O(l) H 2 SO 4 (aq) + 2NaOH(aq)  Na 2 SO 4 (aq) + 2H 2 O(l) acid + base → salt + water

71 Formation of an Insoluble Precipitate AgNO 3 (aq) + NaCl(aq)  AgCl(s) + NaNO 3 (aq) Pb(NO 3 ) 2 (aq) + 2KI(aq)  PbI 2 (s) + 2KNO 3 (aq)

72 Metal Oxide + Acid CuO(s) + 2HNO 3 (aq)  Cu(NO 3 ) 2 (aq) + H 2 O(l) CaO(s) + 2HCl(aq)  CaCl 2 (s) + H 2 O(l) metal oxide + acid → salt + water

73 Formation of a Gas H 2 SO 4 (aq) + 2NaCN(aq)  Na 2 SO 4 (aq) + 2HCN(g) NH 4 Cl(aq) + NaOH(aq)  NaCl(aq) + NH 4 OH(aq) NH 4 OH(aq)  NH 3 (g) + H 2 O(l) indirect gas formation

74 Energy changes always accompany chemical reactions. One reason why reactions occur is that the product attains a lower energy state than the reactants. When this occurs energy is released to the surroundings. Heat in Chemical Reactions

75 H 2 (g) + Cl 2 (g) → 2HCl(g) kJ (exothermic) N 2 (g) + O 2 (g) kJ → 2NO(g) (exothermic) Exothermic reactions liberate heat. Endothermic reactions absorb heat. The amounts of substances are expressed in moles. 1 mol 2 mol 1 mol 2 mol

76 For life on Earth the sun is the major provider of energy. The energy for plant photosynthesis is derived from the sun. glucose 6CO 2 + 6H 2 O kJ → C 6 H 12 O 6 + 6O 2

77 A certain amount of energy is always required for a reaction to occur. The energy required to start a reaction is called the energy of activation. Energy of Activation

78 This reaction will not occur unless activation energy is supplied. The activation energy can take the form of a spark or a flame. CH 4 + 2O 2 → CO 2 + 2H 2 O kJ

80 Chapter Key concepts 8.1 The Chemical Equation – FormatThe Chemical Equation 8.2 Writing and Balancing EquationsWriting and Balancing Equations 8.3 What Information Does an Equation Tell UsWhat Information Does an Equation Tell Us 8.4 Types of Chemical EquationsTypes of Chemical Equations 8.5 Heat in Chemical ReactionsHeat in Chemical Reactions