1. Chemical Reactions: Classification and Mass Relationships
Fundamentals of General, Organic, and Biological Chemistry
5th Edition
Chapter Six
Chemical Reactions:
Classification and Mass Relationships
James E. Mayhugh
Oklahoma City University
2007 Prentice Hall, Inc.

2. Outline 6.1 Chemical Equations 6.2 Balancing Chemical Equations
6.3 Avogadro’s Number and the Mole
6.4 Gram–Mole Conversions
6.5 Mole Relationships and Chemical Equations
6.6 Mass Relationships and Chemical Equations
6.7 Percent Yield
6.8 Classes of Chemical Reactions
6.9 Precipitation Reactions and Solubility Guidelines
6.10 Acids, Bases, and Neutralization Reactions
6.11 Redox Reactions
6.12 Recognizing Redox Reactions
6.13 Net Ionic Equations
Prentice Hall © 2007
Chapter Six

3. 6.1 Chemical Equations
Chemical equation: An expression in which symbols are used to represent a chemical reaction.
Reactant: A substance that undergoes change in a chemical reaction and is written on the left side of the reaction arrow in a chemical equation.
Product: A substance that is formed in a chemical reaction and is written on the right side of the reaction arrow in a chemical equation.
Prentice Hall © 2007
Chapter Six

4. Solid=(s) Liquid=(l) Gas=(g) Aqueous solution=(aq)
The numbers and kinds of atoms must be the same on both sides of the reaction arrow.
Numbers in front of formulas are called coefficients; they multiply all the atoms in a formula.
The symbol 2 NaHCO3 indicates two units of sodium bicarbonate, which contains 2 Na, 2 H, 2 C, and 6 O.
Substances involved in chemical reactions may be solids, liquids, gases, or they may be in solution.
This information is added to an equation by placing the appropriate symbols after the formulas:
Solid=(s) Liquid=(l) Gas=(g) Aqueous solution=(aq)
Prentice Hall © 2007
Chapter Six

5. 6.2 Balancing Chemical Equations
Balancing chemical equations can be done using four basic steps:
STEP 1: Write an unbalanced equation, using the correct formulas for all reactants and products.
STEP 2: Add appropriate coefficients to balance the numbers of atoms of each element.
Prentice Hall © 2007
Chapter Six

6. A polyatomic ion appearing on both sides of an equation can be treated as a single unit.
STEP 3: Check the equation to make sure the numbers and kinds of atoms on both sides of the equation are the same.
Prentice Hall © 2007
Chapter Six

7. is balanced, but can be simplified by dividing all coefficients by 2:
STEP 4: Make sure the coefficients are reduced to their lowest whole-number values.
The equation:
2 H2SO4 + 4 NaOH  2 Na2SO4 + 4 H2O
is balanced, but can be simplified by dividing all coefficients by 2:
H2SO4 + 2 NaOH  Na2SO4 + 2 H2O
Hint: If an equation contains a pure element as a product or reactant it helps to assign that element’s coefficient last.
Prentice Hall © 2007
Chapter Six

8. Law of Conservation of Matter
In a chemical reaction, matter can neither be created nor destroyed.
In other words… The mass of the reactants must equal the mass of the products.
See questions
Prentice Hall © 2007
Chapter Six

9. Mass of Reactants = Mass of Product
Prentice Hall © 2007
Chapter Six

10. Balance the Reaction
Prentice Hall © 2007
Chapter Six

11. First, balance the Calcium
Prentice Hall © 2007
Chapter Six

12. Then, Balance the Phosphate
Prentice Hall © 2007
Chapter Six

13. Finally, Balance the H and O with H2O
Prentice Hall © 2007
Chapter Six

14. Balance the equations
Prentice Hall © 2007
Chapter Six

15. 6.3 Molecular Weight
Molecular weight: The sum of atomic weights of all atoms in a molecule. (covalent compound)
Formula weight: The sum of atomic weights of all atoms in one formula unit of any ionic compound.
Molecular weight of H2O
2(1.01) = amu
Formula weight of MgCl2
(35.45) = amu
Prentice Hall © 2007
Chapter Six

16. 6.3 The Mole
Mole: One mole of any substance is the amount whose mass in grams (molar mass) is numerically equal to its molecular or formula weight.
a mole of carbon weighs g
a mole of H2O weighs g
Prentice Hall © 2007
Chapter Six

17. 6.3 Avogadro’s Number
Avogadro’s number: The number of molecules or formula units in a mole. NA = x 1023
A mole of carbon weighs g and has 6.02 x 1023 atoms.
A mole of H2O weighs g and has 6.02 x 1023 molecules.
A mole is an incredibly large number because atoms are so incredibly tiny.
Prentice Hall © 2007
Chapter Six

18. Analogies Avogadro's Number compared to the Population of the Earth.
We will take the population of the earth to be six billion (6 x 109 people). We can compare to Avogadro's Number like this:
6.022 x 1023 divided by 6 x 109 = approx. 1 x 1014
In other words, it would take about 100 trillion Earth populations to add up to Avogadro's number.
Prentice Hall © 2007
Chapter Six

19. Or…
A mole of marbles would cover the United States to a depth of 4 meters.
If you spent 1 million dollars every second, it would take you 1.9 x 1010 (19 billion) years to spend $6.02 x 1023
Or… If you started spending $4 million per second at the time of the big bang, you would still not have spent Avogadro’s number of dollars (although you would be close).
Prentice Hall © 2007
Chapter Six

20. Froot Loops
If you know that the average mass of a Froot Loop is 0.20 grams, and that the average box contains 425 g of cereal; Approximately how many Froot Loops are in a box? What is the fastest way to measure 1,000,000 Froot Loops? Should we count them?? Should we weigh them??
Prentice Hall © 2007
Chapter Six

21. Prentice Hall © 2007
Chapter Six

22. 6.4 Gram – Mole Conversions
Molar mass = Mass of 1 mole of a substance.
= Mass of x 1023 molecules of a substance.
= Molecular (formula) weight of substance in grams.
Molar mass serves as a conversion factor between numbers of moles and mass. If you know how many moles you have, you can calculate their mass; if you know the mass of a sample, you can calculate the number of moles.
Prentice Hall © 2007
Chapter Six

23. The molar mass of water is 18. 0 g
The molar mass of water is 18.0 g. The conversion factor between moles of water and mass of water is 18.0 g H2O = 1 mol H2O
Prentice Hall © 2007
Chapter Six

24. Conversion factor is 1 mol = 6.02 x 1023
Problem
How many molecules are there in 0.25 mol of H2O?
Conversion factor is 1 mol = 6.02 x 1023
0.25 mol H2O x x 1023 molecules/mol H2O
known conversion factor
= 1.51 x 1023 molecules H2O
Prentice Hall © 2007
Chapter Six

25. One Nickel…
…weighs 4.94 g
1) How many moles if Ni are there in 4.94 g of Ni?
4.94 g Ni x 1 mol Ni / g
= mol Ni
2) How many atoms of Ni in a nickel?
mol Ni x x 1023 atoms / mol Ni
= x 1022 atoms Ni
Prentice Hall © 2007
Chapter Six

26. 6.5 Mole Relationships and Chemical Equations
The coefficients in a balanced chemical equation tell how many molecules, and thus how many moles, of each reactant are needed and how many molecules, and thus moles, of each product are formed. See the example below:
Prentice Hall © 2007
Chapter Six

27. The coefficients can be put in the form of mole ratios, which act as conversion factors when setting up dimensional analysis calculations.
In the ammonia synthesis, the mole ratio of H2 to N2 is 3:1, the mole ratio of H2 to NH3 is 3:2, and the mole ratio of N2 to NH3 is 1:2 leading to the following conversion factors:
(3 mol H2)/(1 mol N2)
(3 mol H2)/(2 mol NH3)
(1 mol N2)/(2 mol NH3)
Prentice Hall © 2007
Chapter Six

28. 6.6 Mass Relationships and Chemical Equations
Mole to mole conversions are carried out using mole ratios as conversion factors.
Prentice Hall © 2007
Chapter Six

29. Balanced Reaction: Mole to Mole Conversions
In the reaction below, how many moles of Ag will form from 6.0 moles of Cu?
How many moles of Cu are needed to react with 3.5 mol of AgNO3?
Prentice Hall © 2007
Chapter Six

30. Mole to mass and mass to mole conversions are carried out using molar mass as a conversion factor.
Mass to mass conversions are frequently needed, but cannot be carried out directly.
Overall, there are four steps for determining mass relationships among reactants and products.
Prentice Hall © 2007
Chapter Six

31. Mass to mass conversions:
STEP 1: Write the balanced chemical equation.
STEP 2: Choose molar masses and mole ratios to convert known information into needed information.
STEP 3: Set up the factor-label expression, and calculate the answer.
STEP 4: Estimate or check the answer using a ballpark solution.
Prentice Hall © 2007
Chapter Six

32. Turning Rust into Iron
Prentice Hall © 2007
Chapter Six

33. Turning Rust into Iron
Prentice Hall © 2007
Chapter Six

34. Mole to Mole Mole to Gram
How many moles of Fe will form from 6.0 moles of Fe2O3?
How many grams of Fe will form from 6.0 moles Fe2O3?
Prentice Hall © 2007
Chapter Six

35. Gram to Gram conversions
How many grams of Carbon are needed to react with 26.3 grams of Fe2O3?
Prentice Hall © 2007
Chapter Six

36. Gram to Gram conversions
How many grams of Carbon are needed to react with 26.3 grams of Fe2O3?
Prentice Hall © 2007
Chapter Six

37. Gram to Gram conversions
Prentice Hall © 2007
Chapter Six

38. 6.7 Percent Yield
The amount of product actually formed in a chemical reaction is often somewhat less than the amount predicted by theory.
Unwanted side reactions and loss of product during handling prevent one from obtaining a perfect conversion of all the reactants to desired products.
The amount of product actually obtained in a chemical reaction is usually expressed as a percent yield.
Prentice Hall © 2007
Chapter Six

39. Percent yield is defined as: (Actual yield ÷ Theoretical yield) x 100%
The actual yield is found by weighing the product obtained.
The theoretical yield is found by a mass-to-mass calculation.
Prentice Hall © 2007
Chapter Six

40. How many grams of H2O will be produced in the combustion of 225 g of methane (CH4)?
Prentice Hall © 2007
Chapter Six

41. Set the reaction up and balance it.
How many grams of H2O will be produced in the combustion of 225 g of methane (CH4)?
Set the reaction up and balance it.
Prentice Hall © 2007
Chapter Six

42. Set the reaction up and balance it.
How many grams of H2O will be produced in the combustion of 225 g of methane (CH4)?
Set the reaction up and balance it.
Prentice Hall © 2007
Chapter Six

43. Cannot convert grams to grams directly
How many grams of H2O will be produced in the combustion of 225 g of methane (CH4)?
Set the reaction and balance it.
Prentice Hall © 2007
Chapter Six

44. Prentice Hall © 2007
Chapter Six

45. 6.8 Classes of Chemical Reactions
When learning about chemical reactions it is helpful to group the reactions of ionic compounds into three general classes: precipitation reactions, acid–base neutralization reactions, and oxidation–reduction reactions.
Precipitation reactions are processes in which an insoluble solid (s) called a precipitate forms when reactants are combined in aqueous solution.
Prentice Hall © 2007
Chapter Six

46. Acid–base neutralization reactions are 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. The “salt” produced need not be common table salt. Any ionic compound produced in an acid–base reaction is called a salt.
Oxidation–reduction reactions, or redox reactions, are 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.
Prentice Hall © 2007
Chapter Six

47. 6.9 Precipitation Reactions and Solubility Guidelines
Reaction of aqueous Pb(NO3)2 with aqueous KI gives a yellow precipitate of PbI2.
To predict whether a precipitation reaction will occur on mixing aqueous solutions of two ionic compounds, you must know the solubility of the potential products.
Prentice Hall © 2007
Chapter Six

48. If a potential product does not contain at least one of the ions listed below, it is probably not soluble and will precipitate from solution when formed.
Prentice Hall © 2007
Chapter Six

49. Precipitation Reactions
Prentice Hall © 2007
Chapter Six

50. 6.10 Acids, Bases, and Neutralization Reactions
When acids and bases are mixed together in correct proportion, acidic and basic properties disappear.
A neutralization reaction produces water and a salt. HA(aq) + MOH(aq)  H2O(l) + MA(aq)
acid base  water + salt
The reaction of hydrochloric acid with potassium hydroxide to produce potassium chloride is an example:
HCl(aq) + KOH(aq)  H2O(l) + KCl(aq)
Prentice Hall © 2007
Chapter Six

51. Acid + Base = Salt + H20
Prentice Hall © 2007
Chapter Six

52. 6.11 Redox Reactions
Oxidation–reduction (redox) 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.
Prentice Hall © 2007
Chapter Six

53. Oxidation and reduction always occur together.
A substance that is oxidized gives up an electron, causes reduction, and is called a reducing agent.
A substance that is reduced gains an electron, causes oxidation, and is called an oxidizing agent.
The charge on the reducing agent increases during the reaction, and the charge on the oxidizing agent decreases.
Prentice Hall © 2007
Chapter Six

54. Loses one or more electrons Causes reduction Undergoes oxidation
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)
Prentice Hall © 2007
Chapter Six

55. 6.12 Recognizing Redox Reactions
One 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.
By comparing the oxidation state of an atom before and after reaction, we can tell whether the atom has gained or lost electrons.
Prentice Hall © 2007
Chapter Six

56. Rules for assigning oxidation numbers:
An atom in its elemental state has an oxidation number of zero.
A monatomic ion has an oxidation number equal to its charge.
Prentice Hall © 2007
Chapter Six

57. In a molecular compound, an atom usually has the same oxidation number it would have if it were a monatomic ion.
Examples: H often has an oxidation number of +1, oxygen often has an oxidation number of -2, halogens often have an oxidation number of -1.
Prentice Hall © 2007
Chapter Six

58. 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.
Prentice Hall © 2007
Chapter Six

59. What is oxidized; what is reduced?
Prentice Hall © 2007
Chapter Six

60. Oxidized species loses e- Reduced species gains e-
Prentice Hall © 2007
Chapter Six

61. 6.13 Net Ionic Equations
Ionic equation: An equation in which ions are explicitly shown.
Spectator ion: An ion that appears unchanged on both sides of a reaction arrow.
Net ionic equation: An equation that does not include spectator ions.
Prentice Hall © 2007
Chapter Six

62. Ionic Equation
Prentice Hall © 2007
Chapter Six

63. Write net ionic equations for…
Prentice Hall © 2007
Chapter Six

64. 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.
A mole refers to Avogadro’s number of formula units of a substance. One mole of any substance has a mass equal to its formula weight in grams.
Molar masses act as conversion factors between numbers of molecules and masses in grams.
Prentice Hall © 2007
Chapter Six

65. Chapter Summary Cont.
The coefficients in a balanced chemical equation represent the numbers of moles of reactants and products in a reaction.
Mole ratios relate amounts of reactants and/or products. Using molar masses and mole ratios in factor-label calculations relates unknown masses to known masses or molar amounts.
The yield is the amount of product obtained.
The percent yield is the amount of product obtained divided by the amount theoretically possible and multiplied by 100%.
Prentice Hall © 2007
Chapter Six

66. Chapter Summary Cont.
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.
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.
Prentice Hall © 2007
Chapter Six