Chemical Change

Chapter 9

Section 9.1

Chemical Change

Electrolytes and Solution Conductivity

Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.


Goal 1

Electrical Conductivity

Distinguish among strong electrolytes, weak electrolytes, and nonelectrolytes.



Strong Electrolyte: A substance that, when dissolved in water, conducts electricity strongly

Weak Electrolyte: A substance that, when dissolved in water, conducts electricity, but does so poorly



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Nonelectrolyte: A substance that, when dissolved in water, does not conduct electricity



Conducting solutions contain ions

The extent of conductivity is related to the concentration of ions in the solution

Section 9.2 Solutions of Ionic Compounds



Goal 2

When an ionic compound dissolves in water, its solution consists of water molecules and ions The ions are identified by separating the compound into its ions

Given the formula of an ionic compound (or its name), write the formula of the ions present when it is dissolved in water.


NaCl(s)

Na+(aq) + Cl–(aq)


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NaCl(s)

Na+(aq) + Cl–(aq)

Polyatomic ions are treated in the same way in writing the formulas of ions in solution

K2SO4(s)


2 K+(aq) + SO42–(aq)


Goal 3 Section 9.3 Strong and Weak Acids


Goal 4

Explain why the solution of an acid may be a good conductor or a poor conductor of electricity.


Acid: Hydrogen-bearing molecular compound made up of nonmetal elements that reacts with water to form a hydrated hydrogen ion and an anion H2O + HX

+ > H3O + X–

Given the formula of a soluble acid (or its name), write the major and minor species present when it is dissolved in water.



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Acid solutions can be strong or weak electrolytes, depending on the extent to which the acid molecule ionizes in solution Strong acid: Ionizes almost completely HSt(aq) H+(aq) + St–(aq)

Weak acid: Ionizes very slightly HWe(aq) H+(aq) + We–(aq)

Major species: molecules

Major species: ions

Minor species: ions

Minor species: molecules Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.


Strong and weak acids at the macroscopic and particulate levels



Seven Common Strong Acids

Two WellKnown Acids

Three Group 7A/17 Acids

Two Chlorine Oxyacids

HNO3

HCl

HClO3

H2SO4

HBr

HClO4

HI Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.


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Identifying the Major Species in a Solution Ions are the major species in the solutions of two kinds of substances: All soluble ionic compounds The seven strong acids Neutral molecules are the major species in solutions of everything else, primarily: Weak acids Weak bases Water Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.

Consider the reaction of solutions of silver nitrate and sodium chloride What is the particulate-level composition of each solution before the reaction occurs? Silver nitrate solution: Silver ions, Ag+(aq) Nitrate ions, NO3–(aq) Water molecules

Section 9.4 Net Ionic Equations: What They Are and How to Write Them Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.

How are these represented in a conventional equation? AgNO3(aq) + NaCl(aq) The problem with this conventional representation is that it disguises the true composition of the solutions

Sodium chloride solution: Sodium ions, Na+(aq) Chloride ions, Cl–(aq) Water molecules Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.


To avoid this misrepresentation, we can write the formulas of the reactants as they actually occur in solution

We can perform a similar analysis with the products of the reaction Silver chloride precipitate: Solid silver chloride, AgCl(s)

Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) Sodium nitrate solution: Sodium ions, Na+(aq) Nitrate ions, NO3–(aq) Water molecules



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Conventional representation of products:

Combining reactants and products:

AgCl(s) + NaNO3(aq) AgNO3(aq) + NaCl(aq) Products as they actually occur: AgCl(s) + Na+(aq) + NO3–(aq)

AgCl(s) + NaNO3(aq) This is the conventional equation It is good for stoichiometry problems but lacks in revealing what really happens in solution



Writing actual reactants and products:

Note that some species appear as both reactant and product in the total ionic equation:

Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) AgCl(s) + Na+(aq) + NO3–(aq) This is the total ionic equation It represents each species as it actually occurs in solution

Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) AgCl(s) + Na+(aq) + NO3–(aq) NO3–(aq) is both reactant and product; Na+(aq) is both reactant and product Neither ion undergoes a chemical change



If we eliminate these non-reacting spectator ions,

A net ionic equation describes what happens in a chemical change

Ag+(aq) + NO3–(aq) + Na+(aq) + Cl–(aq) AgCl(s) + Na+(aq) + NO3–(aq) What remains is the net ionic equation: Ag+(aq) + Cl–(aq)

AgCl(s)


Aqueous silver ion reacts with aqueous chloride ion to yield solid silver chloride A net ionic equation ignores non-reacting species: Sodium ion Nitrate ion Water molecules Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.

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Procedure: How to write a net ionic equation 1. Write the conventional equation. Include state symbols.

Section 9.7

2. Write the total ionic equation. Solids, liquids, and gases do not ionize. Aqueous compounds may ionize: A. Ionic compounds and strong acids ionize B. Weak acids do not ionize

Double-Replacement Precipitation Reactions

3. Write the net ionic equation. Eliminate the spectators. Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.


Goal 7

Goal 8

Predict whether a precipitate will form when known solutions are combined; if a precipitate forms, write the net ionic equation. (Reference to a solubility table or solubility rules may or may not be allowed.)


Given the product of a precipitation reaction, write the net ionic equation.


Double-Replacement Precipitation Reactions

Reactants: Reaction type: Equation type:

Solutions of two ionic compounds or a solution of an ionic compound and an acid solution

Double-replacement precipitation AX + BY

AY + BX

Products: Precipitate of ionic compound and second ionic compound or acid Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.


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Example: Calcium nitrate and potassium sulfate solutions are combined. Write the net ionic equation. Ca(NO3)2(aq)

+

Ca(NO3)2(aq)

+

K2SO4(aq) + 2 KNO3(aq)

CaSO4(s) Total ionic equation:

K2SO4(aq) CaSO4( )

Conventional equation:

2+

+ 2KNO3( )

Ca (aq) + 2 NO3–(aq) + 2 K+(aq) + SO42–(aq) CaSO4(s) + 2 K+(aq) + 2 NO3–(aq) Net ionic equation: 2+

Ca (aq) + SO42–(aq) Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.

CaSO4(s)


Goal 9 Section 9.8 Double-Replacement Molecule-Formation Reactions Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.

Given reactants that yield a molecular product, write the net ionic equation.


Double-Replacement Neutralization Reactions

Ion Combinations That Form Molecules

The reaction of an acid often leads to an ion combination that yields a molecular product, typically water or a weak acid

Reactants: Reaction type: Equation type:

Acid (HX) and a hydroxide base (MOH) Double-replacement neutralization HX + MOH

H2O + HX

Products: Water and a salt (MX) (an ionic compound) Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.


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Example: Write the net ionic equation for the reaction of solutions of hydrobromic acid and sodium benzoate (benzoate ion, C7H5O2–). Conventional equation: HBr(aq) + NaC7H5O2(aq)

Conventional equation: HBr(aq) + NaC7H5O2(aq) HC7H5O2(aq) + NaBr(aq) Total ionic equation: H+(aq) + Br–(aq) + Na+(aq) + C7H5O2–(aq) HC7H5O2(aq) + Na+(aq) + Br–(aq)

HC7H5O2(aq) + NaBr(aq) HC7H5O2 is a weak acid NaBr: Group 1A salts are soluble Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.

Net ionic equation: +

H (aq) + C7H5O2–(aq)

HC7H5O2(aq)


Goal 10 Section 9.9 Double-Replacement Reactions That Form Unstable Products Presentation Slides to Accompany Cracolice/Peters Introductory Chemistry: An Active Learning Approach, Third Edition Copyright © 2007 Brooks/Cole, a part of the Thomson Corporation.

Three common unstable products: H2CO3(aq)

H2O(l) + CO2(g)

H2SO3(aq)

H2O(l) + SO2(aq)

“NH4OH”

NH3(aq) + H2O(l)

Given reactants that form H2CO3, H2SO3, or “NH4OH” by ion combination, write the net ionic equation for the reaction.


Example: Write the net ionic equation for the reaction that occurs when hydrochloric acid and sodium sulfite solutions are combined. Conventional equation: 2 HCl(aq) + Na2SO3(aq)

H2SO3( ) +2 NaCl(aq) H2O(l) + SO2(aq)

Group 1A salts are soluble



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Conventional equation: 2 HCl(aq) + Na2SO3(aq) H2O(l) + SO2(aq) + 2 NaCl(aq) Total ionic equation: 2 H (aq) + 2 Cl (aq) + 2 Na+(aq) + SO32–(aq) +

–

H2O(l) + SO2(aq) + 2 Na+(aq) + 2 Cl–(aq) Net ionic equation: 2 H+(aq) + SO32–(aq)

H2O(l) + SO2(aq)


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