1. Zumdahl • Zumdahl • DeCoste
World of
CHEMISTRY

2. Reactions in Aqueous Solutions
Chapter 8
Reactions in Aqueous Solutions

3. Reactions that keep us alive:
An aqueous solution is a solution in which water is the solvent (the dissolving compound).
Reactions that keep us alive:
Oxygen dissolves in blood, associates with hemoglobin
Oxygen reacts with fuel (food) to provide energy
Hydrochloric Acid reacts with food
Copyright © Houghton Mifflin Company

4. Factors that cause reactions to occur
Goals of Chapter 8
Factors that cause reactions to occur
Identify solid formed in precipitation reaction
Describe reactions in solutions by writing molecular and ionic equations
Characteristics of reactions between strong acids and strong bases
Characteristics of reactions between metals and nonmetals
Electron transfer – driving force for chemical reaction
Classification schemes for reactions
Copyright © Houghton Mifflin Company

5. Most common driving forces:
Why does a chemical reaction occur? What causes reactants to “want” to form products?
Most common driving forces:
Formation of solid
Formation of water
Transfer of electrons
Formation of a gas
Copyright © Houghton Mifflin Company

6. Driving force: Formation of solid Reactions in Which a Solid Forms
Precipitation: process where solid is formed
Precipitation reaction: chemical reaction where solid forms
Precipitate: solid that forms
Copyright © Houghton Mifflin Company

7. What happens when ionic compound dissolves in water?
Almost always, the ions separate and move around independently
Ions of solid dissociate when solid dissolves in water
Strong electrolyte: every molecule separates into ions when dissolved in water
How do we know this really happens?
Copyright © Houghton Mifflin Company

8. Figure 8.2: Pure water does not conduct an electric current.
Copyright © Houghton Mifflin Company

9. Figure 8.2: When an ionic compound is dissolved in water, current flows.
Copyright © Houghton Mifflin Company

10. Deciding what products form
Not easy question to answer
Experienced chemists do not know what will happen in new reaction
Think of possibilities → consider likelihood → make prediction → determine experimentally
Knowledge of facts
Knowledge of concepts
Copyright © Houghton Mifflin Company

11. Solubility Table: based on observations
Solubility Rules
Soluble solid: readily dissolves in water; solid “disappears” as ions dispersed in water
Insoluble solid (slightly soluble solid): tiny amount dissolves in water, undetectable to human eye
Solubility Table: based on observations
Copyright © Houghton Mifflin Company

12. Table 8.1
Copyright © Houghton Mifflin Company

13. Figure 8.3: Solubilities of common compounds.
Copyright © Houghton Mifflin Company

14. KCl(aq) + AgNO3 (aq)→ KNO3(aq) + AgCl(s)
Solid AgCl Precipitates
Copyright © Houghton Mifflin Company

15. KCl & AgNO3 dissociate AgCl is insoluble in water, forms precipitate
Copyright © Houghton Mifflin Company

16. How can you predict precipitates when solutions of two ionic compounds are mixed?
Copyright © Houghton Mifflin Company

17. Types of Equations for Reactions in Aqueous Solutions
Molecular Equation: shows overall reaction, not necessarily actual forms of reactants and products in solution
Complete Ionic Equation: shows all reactants & products that are strong electrolytes as ions. Includes all reactants & products.
Net Ionic Equation: includes only components that undergo change. Spectator ions not included.
Copyright © Houghton Mifflin Company

18. Molecular Equation
Aqueous potassium chromate is mixed with aqueous barium nitrate, a reaction occurs to form solid barium chromate and dissolved potassium nitrate.
K2CrO4(aq) + Ba(NO3)2(aq) → BaCrO4(s) + 2KNO3(aq)
Equation shows complete formulas of all reactants and products.
Doesn’t give clear picture of what occurs in solution

19. Complete Ionic Equation
2K+(aq) + CrO42-(aq) + Ba2+(aq) + 2NO3-(aq) →
BaCrO4(s) + 2K+(aq) + 2NO3-(aq)
Better representation of reactants & products in solution
All strong electrolytes represented as ions
BaCrO4 present as solid – not dissolved
K+ and NO3- on both sides of equation = spectator ions

20. Net Ionic Equation Ba2+(aq) + CrO42-(aq) → BaCrO4(s)
Only includes ions that participate in reaction
Spectator ions do not participate directly in reaction in solution – eliminated from equation
Only includes species that undergoes a change

21. Acids First associated with sour taste of citrus fruits
From Latin acidus meaning “sour”
Vinegar – acetic acid; citric acid – lemons
Substance that produces H+ ions (protons) when it is dissolved in water
When HCl, HNO3, and H2SO4 are placed in water, virtually every molecule dissociates to give ions = strong acids

22. Bases (alkalis) Bitter taste and slippery feel (like wet soap)
Drano (other commercial products for unclogging drains)
Substance that produces hydroxide (OH-) ions in water
Sodium hydroxide (NaOH) is most common base used in chemical laboratory
When NaOH and KOH are placed in water they completely dissociate = strong bases

23. Reaction That Form Water Acids & Bases
When strong acids and strong bases mix: H+ ions react with OH- ions to form water
Water very stable (large abundance on earth is evidence), so when substances that can form water are mixed there is a strong tendency for the reaction to occur
Copyright © Houghton Mifflin Company

24. H+ (aq) + OH- (aq) → H2O (l)
Hydroxide ion has strong affinity for H+ ion to produce water

25. Strong Acid – Strong Base Reactions
Water is always a product
Second product is ionic compound which may precipitate or remain dissolved depending on solubility (called a salt – do not confuse with NaCl) – can obtain salt be evaporating the water
Copyright © Houghton Mifflin Company

26. Reactions Involving Transfer of Electrons aka Reactions of Metals with Nonmetals aka Oxidation-Reduction Reactions
Example: Sodium Chloride is formed from reaction of sodium metal and chlorine gas
2Na (s) + Cl2 (g) → 2NaCl (s)

27. Sodium metal – net charge of zero
Chlorine molecule – net charge of zero
Sodium chloride is made up of Na+ and Cl-
How is this possible?
One electron is transferred from each sodium atom to each chlorine atom
(see page 231)
Copyright © Houghton Mifflin Company

28. Characteristics of Oxidation-Reduction Reactions
Metal-Nonmetal Reaction: Metal becomes Cation, Non-metal becomes Anion
Two non-metals can undergo oxidation-reduction reaction, we will discuss later. Compound formed in nonionic. O2 present as reactant or product.
Copyright © Houghton Mifflin Company

29. Classifying Reactions
Copyright © Houghton Mifflin Company

30. Precipitation Reactions
Formation of solid when 2 solutions are mixed
Double-displacement reaction:
AB + CD → AD + CB
A,B,C,D are ions
Example: reaction of potassium chromate with barium nitrate
Copyright © Houghton Mifflin Company

31. Involves H+ ion that ends up in the product water
Acid-Base Reactions
Involves H+ ion that ends up in the product water
Net ionic equations similar (see slide 24)
Example: Hydrochloric Acid reacting with Potassium Hydroxide
Copyright © Houghton Mifflin Company

32. Formation of a Gas
Usually classified as acid-base, oxidation-reduction, etc.
Have to look at chemical reaction to determine
Copyright © Houghton Mifflin Company

33. Single-Replacement Reactions
A + BC → B + AC
Two types of anion are exchanged
See example on page 239
Copyright © Houghton Mifflin Company

34. Oxidation-Reduction Reactions (see page 238)
Copyright © Houghton Mifflin Company

35. Figure 8.8: The space shuttle Discovery.
Copyright © Houghton Mifflin Company

36. Produces heat so rapidly, flame results
Combustion Reactions
Oxygen involved
Produces heat so rapidly, flame results
Special class of oxidation-reduction
See examples pp
Most provide heat or electricity for homes/businesses or energy for transportation
Copyright © Houghton Mifflin Company

37. Compound is formed from simpler materials Plastic, polyester, aspirin
Synthesis Reactions
Compound is formed from simpler materials
Plastic, polyester, aspirin
Subclass of oxidation-reduction reactions
Example: Synthesis of water
2H2 (g) + O2 (g) → 2 H2O (l)
Copyright © Houghton Mifflin Company

38. Decomposition Reactions
Compound broken down to simpler compounds or to component elements
Heating
Application of electric current
The opposite of synthesis
Example: Decomposition of water
2H2O (l) → 2H2 (g) + O2 (g)
Subclass of oxidation-reduction
Copyright © Houghton Mifflin Company

39. Figure 8.11: Classes of reactions.
Copyright © Houghton Mifflin Company

40. Figure 8.12: Summary of classes of reactions.
Copyright © Houghton Mifflin Company

41. What’s in your water?
Obtain various samples of water (bottled, tap, distilled, drinking fountain, etc.)
Add a few drops of silver nitrate solution (0.1 M)
Write down observations, compare with classmates.
Write net ionic equation(s).