Chapter 7Chemical Reactions

Tro's "Introductory Chemistry", Chapter 7

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Evidence of Chemical Change

Color Change

Formation of Solid PrecipitateFormation of a Gas

Emission of LightRelease or Absorption of Heat

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How Do We Represent A Chemical Reaction?

• Reactions are represented by Chemical Equations

Chemical Equations• Short-hand way of describing a reaction.• Provides information about the reaction.

Formulas of reactants and products.States of reactants and products.Relative numbers of reactant and product molecules

that are required.Can be used to determine masses of reactants used and

products that can be made.

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Symbols Used in Equations• Symbols used to indicate state after chemical.

(g) = gas; (l) = liquid; (s) = solid.(aq) = aqueous = dissolved in water.

C(s) + O2(g) ® CO2(g)

reactants products• Energy symbols used are:

D = heat. hn = light.

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Conservation of Mass• Matter cannot be created or destroyed.

Therefore, the total mass cannot change.And the total mass of the reactants will be the

same as the total mass of the products.

• In a chemical reaction, all the atoms present at the beginning are still present at the end.If all the atoms are still there, then the mass will

not change.

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Balancing Equations activity

Rules for balancing equations:

- Balance only one element at a time

- Always balance free elements last

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Aqueous Solutions• Many times, the chemicals we are reacting

together are dissolved in water.Mixtures of a chemical dissolved in water are

called aqueous solutions.• Dissolving the chemicals in water helps them

to react together faster.The water separates the chemicals into individual

molecules or ions.The separate, free-floating particles come in

contact more frequently so the reaction speeds up.

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Dissociation• When ionic compounds dissolve

in water, the anions and cations are separated from each other. This is called dissociation.However, not all ionic compounds

are soluble in water!

• When compounds containing polyatomic ions dissociate, the polyatomic group stays together as one ion.

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Dissociation, Continued• Potassium iodide dissociates in water into

potassium cations and iodide anions.

KI(aq) → K+1(aq) + I-1(aq)

• Copper(II) sulfate dissociates in water into copper(II) cations and sulfate anions.

CuSO4(aq) → Cu+2(aq) + SO4-2(aq)

K+1 I-1K I

Cu+2 SO4-2Cu SO4

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Dissociation, Continued

• Potassium sulfate dissociates in water into potassium cations and sulfate anions.

K2SO4(aq) → 2 K+1(aq) + SO4-2(aq)

K+1

SO4-2

K+1

KK SO4

Tro's "Introductory Chemistry", Chapter 7

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Electrolytes• Electrolytes---

substances whose water solution is a conductor of electricity.

• All electrolytes have ions dissolved in water.

Tro's "Introductory Chemistry", Chapter 7

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Electrolytes, Continued• In strong electrolytes, all the

electrolyte molecules or formula units are separated into ions.

• In weak electrolytes, a small percentage of the molecules are separated into ions

• In nonelectrolytes, none of the molecules are separated into ions.

Tro's "Introductory Chemistry", Chapter 7

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Types of Electrolytes• Salts = Water soluble ionic compounds.

Mostly strong electrolytes.

• Acids = Form H+ ions and anions in water solution.Sour taste.React and dissolve many metals.

• Bases = Water-soluble metal hydroxides.Form OH- ions in water solution.Bitter taste, slippery (soapy) feeling solutions.

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When Will a Salt Dissolve?• A compound is soluble in a

liquid if it dissolves in that liquid.NaCl is soluble in water, but

AgCl is not.• A compound is insoluble if a

significant amount does not dissolve in that liquid.AgCl is insoluble in water.

Though there is a very small amount dissolved, but not enough to be significant.

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Compounds containing the following ions are generally soluble

Exceptions(when combined with ions on the left the compound is insoluble)

Li+, Na+, K+, NH4+ none

NO3–, C2H3O2

– none

Cl–, Br–, I– Ag+, Hg22+, Pb2+

SO42– Ca2+, Sr2+, Ba2+, Pb2+

Solubility Rules:Compounds that Are Generally Soluble in Water

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Compounds containing the following ions are generally insoluble

Exceptions(when combined with ions on the left the compound is soluble or slightly soluble)

OH– Li+, Na+, K+, NH4+, H+

Ca2+, Sr2+, Ba2+

S2– Li+, Na+, K+, NH4+, H+

Ca2+, Sr2+, Ba2+

CO32–, PO4

3– Li+, Na+, K+, NH4+, H+

Solubility Rules:Compounds that Are Generally Insoluble

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Using the Solubility Rules to Predict an Ionic Compound’s Solubility in Water

• First check the cation: If it is Li+, Na+, K+, or NH4+,

then the compound will be soluble in water.Regardless of the anion.

• If the cation is not Li+, Na+, K+, or NH4+, then

follow the rule for the anion.• If a rule says the compounds are mostly soluble,

then the exceptions are insoluble.• If a rule says the compounds are mostly insoluble,

then the exceptions are soluble.Note: slightly soluble insoluble.

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Examples of Soluble and Insoluble Compounds

• KOH Soluble, because the cation is K+.• AgBr Insoluble, even though most compounds

with Br− are soluble, this is an exception. • CaCl2 Soluble, most compounds with Cl− are

soluble.• Pb(NO3)2 Soluble, because the anion is NO3

−.

• PbSO4 Insoluble, even though most compounds with SO4

2− are soluble, this is an exception.

Types of Reactions (Part A)

• Precipitation Reactions

• Acid-Base Reactions

• Gas Evolution Reactions

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Reaction Type 1 A: Precipitation Reactions

• A reaction that takes place from the exchange of cations and anions

• This results in the formation of a solid known as a precipitate

• Note: The reactants can be in any state other than the solid state

• .

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Precipitation Reactions, Continued2 KI(aq) + Pb(NO3)2(aq) 2 KNO3(aq) + PbI2(s)

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No Precipitate Formation = No Reaction

KI(aq) + NaCl(aq) KCl(aq) + NaI(aq)All ions still present, no reaction.

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Process for Predicting the Products ofa Precipitation Reaction

1. Write the formula for the reactants and Determine what ions each aqueous reactant has.

2. Exchange ions. (+) ion from one reactant with (-) ion from the other.

3. Balance charges of combined ions to get formula of each product.

4. Balance the equation. Count atoms.

5. Determine solubility of each product in water. Use the solubility rules. If product is insoluble or slightly soluble, it will precipitate. If neither product will precipitate, no reaction.

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Example 7.7—Write a balanced chemical equation for the reaction between an Aqueous Solution of Sodium

Carbonate and an Aqueous Solution of Copper(II) Chloride

1. Write the form the reactants and Determine the ions present when each reactant dissociates.

Na2CO3(aq) + CuCl2(aq)

(Na+ + CO32-) + (Cu+2 + Cl-)

2. Exchange the ions.(Na+ + CO3

2-) + (Cu+2 + Cl-) (Na+ + Cl-) + (Cu+2 + CO32-)

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3. Write the formulas of the products. Cross charges and reduce.

Na2CO3(aq) + CuCl2(aq) NaCl + CuCO3

4. Balance the equation.Na2CO3(aq) + CuCl2(aq) 2 NaCl + CuCO3

Example 7.7—Write a balanced chemical equation for the reaction between an Aqueous

Solution of Sodium Carbonate and an Aqueous Solution of Copper(II) Chloride

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5. Determine the solubility of each product. Write an (s) after the insoluble products and a (aq) after the soluble products

NaCl is soluble.

CuCO3 is insoluble.

Na2CO3(aq) + CuCl2(aq) 2 NaCl(aq) + CuCO3(s)

Example 7.7—Write a balanced chemical equation for the reaction between an Aqueous

Solution of Sodium Carbonate and an Aqueous Solution of Copper(II) Chloride

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Practice–Predict the Products and Balance the Equation

• KCl(aq) + AgNO3(aq) ®

• Na2S(aq) + CaCl2(aq) ®

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Reaction Type 2 A: Acid–Base Reactions

• Also called neutralization reactions

• Acid contains H+ and base contains OH-

• In all acid-base reactions, the H+ and the OH- combine to

make H2O.

acid + base salt + water

2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l)

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Predicting the products of an acid-base reactions:

Example 7.11—Write the Molecular Equation for the Reaction of Aqueous Nitric Acid with Aqueous

Calcium Hydroxide.

1. Write the formulas of the reactants.

HNO3(aq) + Ca(OH)2(aq)

2. Since this is an acid-base reaction, one of the products must be water! H2O(l).

HNO3(aq) + Ca(OH)2(aq) H2O(l) + other product

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3. The other product will be formed by combining the ions other than H+ and OH-. i.e. NO3

− and Ca2+

HNO3(aq) + Ca(OH)2(aq) H2O(l) + Ca(NO3)2

4. Balance the equation.

2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2 + 2 H2O(l)

Example 7.11—Write the Molecular Equation for the Reaction of Aqueous Nitric Acid with Aqueous

Calcium Hydroxide, Continued.

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5. Determine the solubility of the salt.Ca(NO3)2 is soluble.

a. Write an (s) after an insoluble salt and an (aq) after a soluble salt.

2 HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2 H2O(l)

Example 7.11—Write the Molecular, Ionic, and Net-Ionic Equation for the Reaction of Aqueous Nitric

Acid with Aqueous Calcium Hydroxide, Continued.

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Practice—Complete and Balance These Acid–Base Reactions.

NH4OH(aq) + H2SO4(aq) ®

Al(OH)3(aq) + H2SO3(aq) ®

Ba(OH)2(aq) + HCl(aq) ®

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Reaction type 3A:Gas Evolution Reactions

• Reactions that produce a gas. Note: the reactants can be in any state other than a gas.

• To identify a gas evolution reaction one of the reactant is usually an acid or a base but not both.

K2S(aq) + H2SO4(aq) K2SO4(aq) + H2S(g)

• Note that if an acid and a base are present as reactants then the reaction will be an acid-base reaction.

• Other reactions form a gas by the decomposition of one of their products into a gas and water.

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Predicting the Products ofGas Evolving Reactions (Table 7.4)

Reactanttype

Reactingwith

Ion exchangeproduct

Decom-pose?

Gasformed

Example

MetalnS,

metal HS

Acid H2S No H2S K2S(aq) + 2HCl(aq) 2KCl(aq) + H2S(g)

MetalnCO3,

metal HCO3

Acid H2CO3 Yes CO2 K2CO3(aq) + 2HCl(aq) 2KCl(aq) + CO2(g) + H2O(l)

MetalnSO3

metal HSO3

Acid H2SO3 Yes SO2 K2SO3(aq) + 2HCl(aq) 2KCl(aq) + SO2(g) + H2O(l)

(NH4)nanion Base NH4OH Yes NH3 KOH(aq) + NH4Cl(aq) KCl(aq) + NH3(g) + H2O(l)

Decomposition productsH2CO3(aq) CO2(g) + H2O(l)H2SO3(aq) SO2(g) + H2O(l)NH4OH(aq) NH3(g) + H2O(l)

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Process for Predicting the Products ofa Gas-Evolving Reaction

1. Write the formula of the reactants2. Use table 7.4 to predict the ion exchange product i.e.:

H2S, H2CO3, H2SO3, or NH4OH3. Form the second product from the remaining cation

and anion4. Check to see if either product decomposes. If so,

rewrite as H2O(l) and a gas. See Table 7.4

5. Balance the equation.6. Determine solubility of other product in water.

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Example—Write a molecular equation for the reaction between an Aqueous Solution of Sodium Sulfite and

an Aqueous Solution of Nitric Acid.

1. Write the formulas of the reactants.

Na2SO3(aq) + HNO3(aq)

2. Use table 7.4 to predict the ion exchange product i.e.: H2S, H2CO3, H2SO3, or NH4OH

Na2SO3(aq) + HNO3(aq) H2SO3(aq) + other product

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3. Form the second product from the remaining cation and anion

Na2SO3(aq) + HNO3(aq) NaNO3 + H2SO3

4. Check to see if either product decomposes. If so, rewrite as H2O(l) and a gas. See Table 7.4

Na2SO3(aq) + HNO3(aq) NaNO3 + SO2(g) + H2O(l)

Example—Write a molecular equation for the reaction between an Aqueous Solution of

Sodium Sulfite and an Aqueous Solution of Nitric Acid.

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6. Balance the equation.Na2SO3(aq) + 2 HNO3(aq) 2 NaNO3 + SO2(g) + H2O(l)

7. Determine the solubility of other product.NaNO3 is soluble.

a. Write an (s) after the insoluble products and an (aq) after the soluble products.

Na2SO3(aq) + 2 HNO3(aq) 2 NaNO3(aq) + SO2(g) + H2O(l)

Example—When an Aqueous Solution of Sodium Sulfite Is Added to an Aqueous Solution of Nitric

Acid, a Gas Evolves, Continued.

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Practice—Complete the Following Reactions.

PbS(s) + H2SO4(aq)

HNO3(aq) + NaHCO3(aq)

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Ionic Equations

• Equations that describe the chemicals put into the water and the product molecules are called molecular equations.

2 KOH(aq) + Mg(NO3)2(aq) ® 2 KNO3(aq) + Mg(OH)2(s)

• Equations that describe the chemicals in terms of ions are called complete ionic equations. Aqueous electrolytes are written as ions.

Soluble salts, strong acids, strong bases.

Insoluble substances and nonelectrolytes written in molecule form.Solids, liquids, and gases are not dissolved, therefore, molecule form.

2K+1(aq) + 2OH-1

(aq) + Mg+2(aq) + 2NO3

-1(aq) ® 2K+1

(aq) + 2NO3-1

(aq) + Mg(OH)2(s)

Ionic Equations

• Precipitation, acid-base, and gas evolution reactions can all be written as ionic equations.

• When Writing Ionic Equations

Only substances in an aqueous state can be written as ions

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Writing Ionic Equations• Only aqueous substances can be written as ions

2 KOH(aq) + Mg(NO3)2(aq) ® 2 KNO3(aq) + Mg(OH)2(s)

2K+1(aq) + 2OH-1

(aq) + Mg+2(aq) + 2NO3

-1(aq) ® 2K+1

(aq) + 2NO3-1

(aq) + Mg(OH)2(s)

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Net Ionic Equations• Ions that are both reactants and products are called

spectator ions.

2K+1(aq) + 2OH-1

(aq) + Mg+2(aq) + 2NO3

-1(aq) ® 2K+1

(aq) + 2NO3-1

(aq) + Mg(OH)2(s)

• An ionic equation in which the spectator ions are removed is called a net ionic equation.

2OH-1(aq) + Mg+2

(aq) ® Mg(OH)2(s)

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**Example**–Write the Ionic and Net Ionic Equation.

K2SO4(aq) + Ba(NO3)2(aq) ® 2 KNO3(aq) + BaSO4(s)

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Other Reaction Types

• Oxidation-reduction reactions• Combustion reactions• Decomposition• Synthesis or combination reactions• Single Displacement reactions• Double Displacement reactions

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Reduction-Oxidation Reactions• Also known as RedOx

• Involves transferring electrons from one atom to another

• Can be easily identified asReactions in which either one or more of the reactants or

propducts are free elements.Reactions that involve a change in oxidation state from

reactant to product

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Oxidation–Reduction Reactions• Oxidation: An element is oxidized when:

it loses electrons OR when there is an increase in the oxidation state

• Reduction: An element is reduced when: it gains electrons OR when there is an decrease in the oxidation state

• Whenever you think of Oxidation-

Reduction reactions, think of OIL-RIG

• Oxidation• Is• Loss

• Reduction• Is• Gain

What is “OIL-RIG”?

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Electron Loss/Gain as Redox

• In the following reaction:

Mg(s) + S(s) MgS(s)• The magnesium atoms are oxidized.

Mg0 Mg2+ + 2 e

• The oxygen atoms are reduced.

S0 + 2 e S2

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Practice—Decide Whether Each of the Following Reactions Is a Redox Reaction, Continued.

2 Al(s) + 3 Br2(l) 2 AlBr3(s)

CaSO3(s) + 2 HCl(aq) CaCl2(aq) + SO2(g) + H2O(l)

Fe2O3(s) + C(s) 2 Fe(s) + 3 CO(g)

SO2(g) + O2(g) + H2O(l) H2SO4(aq)

—Yes, metal + nonmetal.

—No, this is a gas evolving reaction.

—Yes

—Yes, O2 reactant.

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Combustion Reactions• Reactions in which O2(g) is a

reactant are called combustion reactions.

• Combustion reactions release lots of energy. They are exothermic.

• Combustion reactions are a subclass of oxidation–reduction reactions.

2 C8H18(g) + 25 O2(g) 16 CO2(g) + 18 H2O(g)

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• To predict the products of a combustion reaction, combine each element in the other reactant with oxygen.

Reactant Combustion product

Contains C CO2(g)

Contains H H2O(g)

Contains S SO2(g)

Contains N NO(g) or NO2(g)

Contains metal M2On(s)

Products of Combustion

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Example—Write the Equation for Each Reaction

• Combustion of the anesthetic cyclopropane, C3H6(g).

2 C3H6(g) + 9 O2(g) 6 CO2(g) + 6 H2O(g)

• Combustion of the non-toxic antifreeze propylene glycol, C3H8O2(l).

2 C3H6O2(l) + 7 O2(g) 6 CO2(g) + 6 H2O(g)

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Classifying Reactions, Continued• Another scheme classifies reactions by what

the atoms do.Type of reaction General equation

Synthesis A + B AB

Decomposition AB A + B

Single Displacement A + BC AC + B

Double displacement AB + CD AD + CB

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Combination Reactions• Also known as composition or synthesis

reactions.• Two (or more) reactants combine together to

make one product.Simpler substances combining together.

2 CO + O2 ® 2 CO2

2 Mg + O2 ® 2 MgO

HgI2 + 2 KI ® K2HgI4

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Decomposition Reactions

• A large molecule is broken apart into smaller molecules or its elements.Caused by addition of energy into the molecule.

• Have only one reactant, makes 2 or more products.

2h

3

2

223

O 3 O 2

O Hg 2 HgO 2

Cl FeCl 2 FeCl 2

)(

)()()(

g(l)(s)

gls elec

Tro's "Introductory Chemistry", Chapter 7

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Single Displacement Reactions• Reactions that involve one atom displacing

another and replacing it in a compound.• In the reaction Zn(s) + 2 HCl(aq)

ZnCl2(aq) + H2(g), the atom Zn displaces H from the compound.

• Other examples of displacement reactions are:

Fe2O3(s) + Al(s) Fe(s) + Al2O3(s)

2 Na(s) + 2 H2O(aq) 2 NaOH(aq) + H2(g)

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Double Displacement Reactions

• Two ionic compounds exchange ions.• May be followed by decomposition of one

of the products to make a gas.• X ÅYq (aq) + A ÅBq (aq) ® XB + AY• Precipitation, acid–base, and gas evolving

reactions are also double displacement reactions.

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Examples of Double Displacement

CaCl2(aq) + Na2CO3(aq) CaCO3(s) + 2 NaCl(aq)

Ba(OH)2(s) + 2 HNO3(aq) Ba(NO3)2(aq) + 2 H2O(l)

Li2CO3(aq) + 2 HCl(aq) 2 LiCl(aq) + CO2(g) + H2O(l)

Tro's "Introductory Chemistry", Chapter 7

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Example—Classify the Following Reactions as Synthesis, Decomposition, Single Displacement, or

Double Displacement, Continued.

3 Mg(s) + 2 FeCl3(aq) 3 MgCl2(aq) + 2 Fe(s)

CO2(g) + H2O(l) H2CO3(aq)

3 KOH(aq) + H3PO4(aq) K3PO4(aq) + 3 H2O(l)

)(CO )CaO( )(CaCO 23 gss heat

Single displacement.

Double displacement.

Decomposition.

Synthesis.

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Recommended Study Problems Chapter 7NB: Study problems are used to check the student’s understanding

of the lecture material. Students are EXPECTED TO BE ABLE TO SOLVE ALL THE SUGGESTED STUDY PROBLEMS.

If you encounter any problems, please talk to your professor or seek help at the HACC-Gettysburg learning center.

Questions from text book Chapter 7, p 2325, 7, 29, 37, 53, 55, 61, 65, 67, 75, 83, 85, 87, 89, 93, 115

ANSWERS-The answers to the odd-numbered study problems are found at the back of your textbook