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Unit 7 Reactions in Solution

Chem II ObjectivesDescribe the driving force for a chemical reaction.Use generalizations to predict the products of simple

reactions.Describe reactions in solutions by writing molecular,

complete ionic and net ionic equations.Identify the key characteristics of the reactions between

strong acids and strong basesIdentify the general characteristics of a reaction between a

metal and a nonmetal. Classify a reaction as one of five basic types: synthesis,

decomposition, single displacement, double displacement or combustion.

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Will a Reaction Occur?• What causes reactions to occur?

Reactants → Products

• Driving forces that make reactants go in the direction of the arrow include:– Formation of a solid– Formation of a gas– Formation of water– Transfer of electrons

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Reactions which form solids• One driving force for a chemical reaction is the

formation of a solid, a process called precipitation.

• The solid formed is called a precipitate• The reaction is called a precipitation reaction

Example:Potassium chromate + barium nitrate

↑ Write equation for products

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Precipitation Reactions

• To predict the identity of the precipitate, consider the possible products

• First look at the nature of each reactant in an aqueous solution

• Barium nitrate contains Ba2+ and NO3- ions

• Typically, when a solid containing ions dissolves in water, the ions separate, moving around independently

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Precipitation Reactions

• Ba(NO3) 2 (aq) does not contain Ba(NO3) 2 units

• The solution contains Ba2+ and NO3- ions

• There are two NO3- ions for every Ba2+ ion

• When each unit of a substance that dissolves in water produces separated ions, it is called a strong electrolyte

• Ba(NO3) 2 is a strong electrolyte as is K 2CrO 4

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Ionic Compounds• When ionic compounds dissolve, the resulting

solution contains ions.• We usually write these reactants as

Ba(NO3)2 (aq) + K2CrO4 (aq) → Products• It is more appropriately shown asBa2+ (aq) + 2NO3

- (aq) + 2K +(aq) + CrO42- (aq)

→ Products• The mixed solution contains four types of ions.

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What products form?

• A solid compound must have a net zero charge• Must contain cations and anions• K+ and Ba2+ could not combine to form the solid• Most ionic materials contain only two types of

ions – one cation and one anion• Possible compounds are

K2 CrO4 Ba(NO3)2 K(NO3) BaCrO4

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Predicting Products• Need more information to determine which of

these is more likely to be the precipitateK(NO3) BaCrO4

• Potassium nitrate is a white solid. The chromate ion is yellow. So, the likely precipitate is BaCrO4

and the unbalanced equation is

Ba(NO3)2 (aq) + K2CrO4 (aq) →KNO3 (aq) + BaCrO4(s)

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Solubility Vocabulary• A soluble solid readily dissolves in water.

• An insoluble solid, or slightly soluble solid, means only a small amount of the solid dissolves in water.

• A salt means an ionic compound.

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Solubility RulesTable 7.1, Z page 178

1. Most nitrate (NO3-) salts are soluble.

2. Most salts of Na+, K+ and NH4+ are soluble.

3. Most chloride salts are soluble. Notable exceptions are AgCl, PbCl2 and Hg2Cl2.

4. Most sulfate salts are soluble. Notable exceptions are BaSO4, PbSO4 and CaSO4.

5. Most hydroxide compounds are insoluble. Notable exceptions are NaOH, KOH, Ba(OH)2 and Ca(OH) 2 .

6. Most sulfide (S2-), carbonate (CO32-) and phosphate

(PO43-) salts are insoluble.

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Predicting Products - Z page 180

Step 1 – Write the reactants as they actually exist before any reaction occurs.

Step 2 – Consider the various solids that could form. To do this, simply exchange the anions of the added salts.

Step 3 – Use the solubility rules to decide whether a solid forms and to predict the identity of the solid.

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Predicting Products

Guided exampleWhen an aqueous solution of silver nitrate is added to an aqueous solution of potassium chloride, a white precipitate is formed. Identify the solid and write a balanced equation for the reaction.

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Predicting ProductsWhen an aqueous solution of silver nitrate is added to an aqueous solution of potassium chloride, a white precipitate is formed.

AgNO3 (aq) + KCl (aq) → AgCl (?) + KNO3 (?)

Solubility rule 1 – NO3- is soluble

Solubility rule 2 – K + is solubleSolubility rule 3 – AgCl is insoluble

AgNO3 (aq) + KCl (aq) → AgCl (s) + KNO3 (aq)

Equation is balanced

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Predicting Products

In class examples1. KNO3 (aq) and BaCl2 (aq)

2. Na2SO4 (aq) and Pb(NO3) 2 (aq)

3. KOH (aq) and Fe(NO3) 3 (aq)

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Predicting Products

HomeworkPage 202, problems 18 and 22

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Reactions in Aqueous Solutions

Returning to this equation, note that it is called a molecular equation.

Ba(NO3)2 (aq) + K2CrO4 (aq) →2KNO3 (aq) + BaCrO4(s)

It shows the complete formulas of all products and reactants.This does not give a clear picture of what occurs in solution.

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Reactions in Aqueous Solutions

• Aqueous solutions of barium nitrate, potassium chromate and potassium nitrate contain the individual ions, not the complete molecules implied by the molecular equation.

• The complete ionic equation better represents the actual forms of reactants and products in solution. Ba2+ (aq) + 2NO3

- (aq) + 2K +(aq) + CrO42- (aq) →

BaCrO4 (s) + 2K +(aq) + 2NO3- (aq)

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Reactions in Aqueous Solutions• The complete ionic equation reveals that only

some of the ions participate in the reaction.• Ions which do not participate in the reaction,

such as K + and NO3- , are called spectator ions.

• The ions that participate in the reaction are Ba2+ (aq) + CrO4

2- (aq) → BaCrO4 (s)• This equation, called the net ionic equation,

includes only those components that are directly involved on the reaction.

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Molecular, Complete Ionic and Net Ionic Equations

In classZ page 184 examples 7.3a and 7.3b

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Equations7.3a) Molecular equationNaCl (aq) + AgNO3 (aq) → AgCl (s) + NaNO3 (aq)

Complete ionic equationNa+ (aq) + Cl- (aq) + Ag + (aq) + NO3

- (aq) →

AgCl (s) + Na + (aq) + NO3- (aq)

Net ionic equationCl- (aq) + Ag + (aq) → AgCl (s)

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Equations7.3b ) Molecular equation3KOH(aq) +Fe(NO3) 3(aq) → Fe(OH) 3(s) + 3KNO3(aq)

Complete ionic equation3K + (aq) + 3OH- (aq) + Fe 3+ (aq) + NO3

- (aq) →

Fe(OH) 3(s) + 3K + (aq) + 3NO3- (aq)

Net ionic equation3OH- (aq) + Fe 3+ (aq) → Fe(OH) 3(s)

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Molecular, Complete Ionic and Net Ionic Equations

HomeworkPage 202 and 203, problem 26

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Reactions that Form Water• Acids – first associated with the sour taste of citrus

fruits. Dervied from Latin word acidus meaning sour

• Essential nature of acids discovered by Svante Arrhenius in the late 1800’s

• Arrhenius proposed that an acid is a substance that produces H+ ions when it is dissolved in water

• Studies show that when HCl, HNO3 and H2SO4 are placed in water nearly every molecule dissociates to give ions

• Because these substances are strong electrolytes that produce H + ions they are called strong acids

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Reactions that Form Water• Bases – characterized by bitter taste and

slippery feel• Arrhenius found that aqueous solutions that

exhibit basic behavior always contain hydroxide ions (OH-). He defined a base as a substance that produces OH- in water.

• Most common base in the lab is NaOH which dissolves in water to form Na+ and OH- ions

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Reactions that Form Water

• When strong acids and strong bases are mixed, the fundamental change that occurs is that H+ and OH- ions react to form water

H+ (aq) + OH- (aq) → H2O (l) • The tendency to form water is one of the

driving forces for chemical reactions

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Reactions that Form Water

The reaction between hydrochloric acid and sodium chloride shown as a molecular equation is:

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

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Reactions that Form Water

Because HCl and NaOH exist as completely separated ions in water, the complete ionic equation can be written as:

H+ (aq) + Cl- (aq) + Na + (aq) + OH- (aq) → Na + (aq) + Cl- (aq) + H2O (l)

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Reactions that Form Water

Notice that Cl- and Na + are spectator ions so the net ionic equation is:

H+ (aq) + OH- (aq) → H2O (l)

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Acid-Base Reactions

In class Z Page 187 – Example 7.4

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Acid-Base ReactionsIn class

Page 187 – Example 7.4HMolecular equation:

HNO3 (aq) + KOH (aq) → H2O (l) + KNO3 (aq)

Complete ionic equation:H+ (aq) + NO3

- (aq) + K + (aq) + OH- (aq) →

H2O (l) + NO3- (aq) + K + (aq)

Net ionic equation:H+ (aq) + OH- (aq) → H2O (l)

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Acid-Base ReactionsNote two things when looking at the reaction between HCl and NaOH and the reaction between HNO3 and KOH:

1. The net ionic equation is the sameH+ (aq) + OH- (aq) → H2O (l)

2. Besides water, the second product formed is an ionic compound called a salt. The salt may precipitate or remain in solution, depending on solubility. How could you recover the salt if it is soluble? If it is insoluble?

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Acid-Base Reactions

Homework

Page 203problems 39a, 39c, 39d, 40a, 40c, 40d

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Oxidation-Reduction Reactions• In a reaction between a metal and a non-

metal, electrons are transferred from the metal to the non-metal.

• Consider the reaction between sodium metal and chlorine gas

2Na (s) + Cl2(g) → 2NaCl (s)

• The product, sodium chloride, consists of Na+ ions and Cl- ions.

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Oxidation-Reduction Reactions• The sodium in the product has a positive

charge because it transferred an electron to the chlorine. The chlorine has a negative charge because it gained an electron from the sodium.

• A reaction in which electrons are transferred is called an oxidation-reduction reaction.

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Oxidation-Reduction Reactions• Consider the reaction between magnesium

metal and oxygen.2Mg(s) + O2 (g) → 2 MgO(s)

• Each magnesium atom loses 2 electrons.Mg → Mg2+ + 2e-

• Each oxygen atom gains 2 electronsO + 2e- → O2-

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Oxidation-Reduction Reactions

Page 190 - Example 7.5aShow how electrons are gained and lost.

2 Al (s) + 3 I2 (g) → 2AlI3 (s)

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Oxidation-Reduction Reactions

Page 190 - Example 7.5aShow how electrons are gained and lost.

2 Al (s) + 3 I2 (g) → 2AlI3 (s)

Al → Al3+ + 3e- I + e- → I-

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Oxidation-Reduction Reactions

Page 191 – Self check exercises 7.3a

2 Na (s) + Br2 (l) → 2 NaBr (s)

7.3b 2 Ca (s) + O2 (g) → 2 CaO (s)

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Oxidation-Reduction ReactionsPage 191 – Self check exercises 7.3a

2 Na (s) + Br2 (l) → 2 NaBr (s)

Na → Na+ + e- Br + e- → Br-

7.3b 2 Ca (s) + O2 (g) → 2 CaO (s)

Ca → Ca2+ + 2e- O + 2e- → O2-

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Oxidation-Reduction Reactions1. When a metal reacts with a nonmetal, an

ionic compound is formed. The ions are formed when the metal transfers one or more electrons to the nonmetal, the metal becoming a cation and the nonmetal becoming an anion. Therefore, a metal-nonmetal reaction can always be assumed to be an oxdiation-reduction reaction, which involves the transfer of electrons.

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Oxidation-Reduction Reactions

2. Two nonmetals can also undergo an oxidation-reduction reaction. At this point we can recognize these cases only by looking for O2 as a reactant or product. When two nonmetals react, the compound formed is not ionic.

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Classifying Reactions• Formation of a solid (precipitation reaction)• Formation of water (acid-base reaction)• Transfer of electrons (oxidation-reduction reaction)

-single replacement-double replacement

Other types of reactions• Combustion reaction• Synthesis or Combination reaction• Decomposition reaction