example 7.1

Copyright ©2009 by Pearson Education, Inc.Upper Saddle River, New Jersey 07458

All rights reserved.

Introductory Chemistry, Third EditionBy Nivaldo J. Tro

EXAMPLE 7.1 Evidence of a Chemical Reaction

Which of the following is a chemical reaction? Why?(a) butane burning in a butane lighter(b) butane evaporating out of a butane lighter(c) wood burning(d) dry ice subliming

SKILLBUILDER 7.1 Evidence of a Chemical Reaction

FOR MORE PRACTICE Example 7.16; Problems 29, 30, 31, 32, 33, 34.

Solution:(a) Not a chemical reaction; melting ice forms water, but both the ice and water are composed of water molecules.(b) Chemical reaction; water decomposes into hydrogen and oxygen, as evidenced by the bubbling.(c) Chemical reaction; iron changes into iron oxide, changing color in the process.(d) Not a chemical reaction; even though there is bubbling, it is just carbon dioxide coming out of the liquid.

Which of the following is a chemical reaction? Why?(a) ice melting upon warming(b) an electric current is passed through water, resulting in the formation of hydrogen and oxygen gas that appear as bubbles rising in the water(c) iron rusting(d) bubbles forming when a soda can is opened




2. If an element occurs in only one compound on both sides of the equation, balance it first. If there is more than one such element, balance metals before nonmetals

EXAMPLE 7.3Writing Balanced Chemical Equations

1. Write a skeletal equation by writing chemical formulas for each of the reactants and products. Review Chapter 5 for nomenclature rules. (If a skeletal equation is provided, skip this step and go to Step 2.)

EXAMPLE 7.2Write a balanced equation for the reaction between solid silicon dioxide and solid carbon to produce solid silicon carbide and carbon monoxide gas.

Write a balanced equation for the combustion of liquid octane (C8H18), a component of gasoline, in which it combines with gaseous oxygen to form gaseous carbon dioxide and gaseous water.

Solution: Solution:

Begin with Sl: Begin with C:




EXAMPLE 7.3Writing Balanced Chemical EquationsContinued2. continued

EXAMPLE 7.2

3. If an element occurs as a free element on either side of the chemical equation, balance it last. Always balance free elements by adjusting the coefficient on the free element.

Balance O next: Balance H next:

Balance C: Balance O:




EXAMPLE 7.3

This step is not necessary in this example. Proceed to Step 5.

Writing Balanced Chemical EquationsContinued4. If the balanced equation contains coefficient fractions, clear these by multiplying the entire equation by the appropriate factor

EXAMPLE 7.2

5. Check to make certain the equationis balanced by summing the total number of each type of atom on both sides of the equation.




EXAMPLE 7.3Writing Balanced Chemical EquationsContinued

EXAMPLE 7.2

SKILLBUILDER 7.2 SKILLBUILDER 7.3

Write a balanced equation for the reaction between solid chromium(III) oxide and solid carbon to produce solid chromium and carbon dioxidegas.

Write a balanced equation for the combustion of gaseous in which it combines with gaseous C4H10

oxygen to form gaseous carbon dioxide and gaseous water.FOR MORE PRACTICE

Example 7.17; Problems 37, 38, 39, 40, 41, 42.




Use your knowledge of chemical nomenclature from Chapter 5 to write a skeletal equation containing formulas for each of the reactants and products. The formulas for each compound MUST BE CORRECT before you begin to balance the equation.

EXAMPLE 7.4 Balancing Chemical EquationsWrite a balanced equation for the reaction of solid aluminum with aqueous sulfuric acid to form aqueous aluminum sulfate and hydrogen gas.

Since both aluminum and hydrogen occur aspure elements, balance those last. Sulfur and oxygen occur in only one compound on each side of the equation, so balance these first. Sulfur and oxygen are also part of a polyatomic ion that stays intact on both sides of the equation. Balance polyatomic ions such as these as a unit. There are 3 SO4

2– ions on the right side of the equation, so put a 3 in front of H2SO4.

Solution:




Balance Al next. Since there are 2 Al atoms on the right side of the equation, place a 2 in front of Al on the left side of the equation.

EXAMPLE 7.4 Balancing Chemical Equations

Continued

Balance H next. Since there are 6 H atoms on the left side, place a 3 in front of on the right side.Finally, sum the number of atoms on each side to make sure that the equation is balanced. The equation is balanced.





Continued

FOR MORE PRACTICE Problems 43, 44, 45, 46, 47, 48.

Write a balanced equation for the reaction of aqueous lead(II) acetate with aqueous potassium iodide to form solid lead(II) iodide and aqueous potassium acetate.

SKILLBUILDER 7.4 Balancing Chemical Equations




Since Cl occurs in only one compound on each side of the equation, balance it first. There is 1 Cl atom on the left side of the equation and 3 Cl atoms on the right side. Balance Cl by placing a 3 in front of HCl.


Since the equation now contains a coefficientfraction, clear it by multiplying the entire equation (both sides) by 2.

Solution:

Since H and Fe occur as free elements, balance them last. There is 1 Fe atom on the left side of the equation and 1 Fe atom on the right, so Fe is balanced. There are 3 H atoms on the left and 2 H atoms on the right. Balance H by placing a 3/2 in front of H2. (That way you don’t alter other elements that are already balanced.)

Balance the following chemical equation.





Continued

FOR MORE PRACTICE Problems 49, 50, 51, 52, 53, 54.

Finally, sum the number of atoms on each side to check that the equation is balanced.

Balance the following chemical equation.

SKILLBUILDER 7.5 Balancing Chemical Equations




EXAMPLE 7.6 Determining Whether a Compound Is Soluble

Determine whether each of the following compounds is soluble or insoluble.

(a) CuS(b) FeSO4

(c) PbCO3

(d) NH4Cl

SKILLBUILDER 7.6 Determining Whether a Compound Is Soluble

FOR MORE PRACTICE Example 7.18; Problems 61, 62, 63, 64, 65, 66.

Solution:(a) Insoluble; compounds containing Br– are normally soluble, but Ag+ is an exception.(b) Soluble; compounds containing Cl– are normally soluble, and Ca2– is not an exception.(c) Soluble; compounds containing NO3

– are always soluble.(d) Insoluble; compounds containing SO4

2– are normally soluble, but PB2+ is an exception.

Determine whether each of the following compounds is soluble or insoluble.(a) AgBr(b) CaCl2

(c) Pb(NO3)2

(d) PbSO4




2. Below the equation, write theformulas of the potentially insolubleproducts that could form from the reactants. Obtain these by combining the cation from one reactant with the anion from the other. Make sure to write correct formulas for these ionic compounds as described in Section 5.5.

EXAMPLE 7.8Writing Equations for Precipitation Reactions

1. Write the formulas of the two compounds being mixed as reactants in a chemical equation

EXAMPLE 7.7Write an equation for the precipitation reaction that occurs (if any) when solutions of sodium carbonate and copper(II) chloride are mixed.

Write an equation for the precipitation reaction that occurs (if any) when solutions of lithium nitrate and sodium sulfate are mixed.

Potentially Insoluble Products:NaCl CuCO3

Potentially Insoluble Products:NaNO3 Li2SO4

Solution: Solution:




EXAMPLE 7.8

NaCl is soluble (compounds containing Cl– are usually soluble and Na+ is not an exception). CuCO3 is insoluble (compounds containing CO3

2– are usually insoluble and Cu2+ is not an exception).

Writing Equations for Precipitation ReactionsContinued3. Use the solubility rules to determine whether any of the new products are indeed insoluble.

EXAMPLE 7.7

4. If all of the potentially insolubleproducts are soluble, there will be no precipitate. Write NO REACTION next to the arrow.

NaNO3 is soluble (compounds containing NO3

– are soluble and Na+ is not an exception). Li2SO4 is soluble (compounds containing SO4

2– are soluble and Li+ is not an exception).

Since this example has an insoluble product, we proceed to the next step.

5. If one or both of the potentially insoluble products are indeed insoluble, write their formula(s) as the product(s) of the reaction, using (s) to indicate solid. Write any soluble products with (aq) to indicate aqueous.




EXAMPLE 7.8Writing Equations for Precipitation ReactionsContinued

EXAMPLE 7.7

SKILLBUILDER 7.7Write an equation for the precipitation reaction that occurs (if any) when solutions of potassium hydroxide and nickel(II) bromide are mixed.

SKILLBUILDER 7.8Write an equation for the precipitation reaction that occurs (if any) when solutions of ammonium chloride and iron(III) nitrate are mixed.FOR MORE PRACTICEExample 7.17; Problems 37, 38, 39, 40, 41, 42.

6. Balance the equation. Remember to adjust only coefficients, not subscripts




1. Write the formulas of the two compoundsbeing mixed as reactants in a chemical equation.

EXAMPLE 7.9 Predicting and Writing Equations for Precipitation ReactionsWrite an equation for the precipitation reaction (if any) that occurs when solutions of lead(II) acetate and sodium sulfate are mixed. If no reaction occurs, write NO REACTION.

2. Below the equation, write the formulas of the potentially insoluble products that could form from the reactants. These are obtained by combining the cation from one reactantwith the anion from the other. Make sure to adjust the subscripts so that all formulas are charge-neutral.

3. Use the solubility rules to determine whether any of the potentially insoluble products are indeed insoluble.

NaC2H3O2 is soluble (compounds containing Na+ are always soluble). PbSO4 is insoluble (compounds containing SO4

2– are normally soluble, but Pb2+ is an exception).4. If all of the potentially

insoluble products are soluble, there will be no precipitate. Write NO REACTION next to the arrow.

Potentially insoluble productsNaC2H3O2 PbSO4

Solution:

Since we have an insoluble product, we proceed to the next step.




EXAMPLE 7.9 Predicting and Writing Equations for Precipitation Reactions

Continued

FOR MORE PRACTICE Problems 71, 72.

Write an equation for the precipitation reaction (if any) that occurs when solutions of potassium sulfate and strontium nitrate are mixed. If no reaction occurs, write NO REACTION.

SKILLBUILDER 7.9 Predicting and Writing Equations for Precipitation Reactions

5. If one or both of the potentially insolubleproducts are indeed insoluble, write their formula(s) as the product(s) of the reaction, using (s) to indicate solid. Write any soluble products with (aq) to indicate aqueous.6. Balance the equation.




EXAMPLE 7.10 Writing Complete Ionic and Net Ionic Equations

FOR MORE PRACTICE Example 7.20; Problems 73, 74, 75, 76.

Write the complete ionic equation by separating aqueous ionic compounds into their constituent ions. The PbCl2(s) remains as one unit

Write the net ionic equation by eliminating the spectator ions, those that do not change during the reaction.

Consider the following precipitation reaction occurring in aqueous solution.

Write a complete ionic equation and a net ionic equation for this reaction.

Solution:Complete ionic equation:

Net ionic equation:

Consider the following reaction occurring in aqueous solution.

Write a complete ionic equation and net ionic equation for this reaction.

SKILLBUILDER 7.10 Writing Complete Ionic and Net Ionic Equations




EXAMPLE 7.11 Writing Equations for Acid–Base Reactions

Write a molecular and net ionic equation for the reaction between aqueous HNO3 and aqueous Ca(OH)2.


Write a molecular and net ionic equation for the reaction that occurs between aqueous H2SO4 and aqueous KOH.

SKILLBUILDER 7.11 Writing Equations for Acid–Base Reactions

You must recognize these substances as an acid and a base. First write the skeletal reaction following the general pattern of acid plus base goes to water plus salt.Next, balance the equation.

Write the net ionic equation by eliminating those ions that remain the same on both sides of the equation.

Solution:




EXAMPLE 7.12 Writing Equations for Gas Evolution ReactionsWrite a molecular equation for the gas evolution reaction that occurs when you mix aqueous nitric acid and aqueous sodium carbonate.

FOR MORE PRACTICE Example 7.22; Problems 85, 86.

Write a molecular equation for the gas evolution reaction that occurs when you mix aqueous hydrobromic acid andaqueous potassium sulfite.

SKILLBUILDER 7.12 Writing Equations for Gas Evolution Reactions

Begin by writing a skeletal equation that includes the reactants and products that form when the cation of each reactant combines with the anion of the other.You must recognize that H2CO3(aq) decomposes into H2O(l) and CO2(g) and write the corresponding equation.Finally, balance the equation.

Write a net ionic equation for the previous reaction.

SKILLBUILDER PLUS

Solution:




EXAMPLE 7.13 Identifying Redox Reactions

Solution:(a) Redox reaction; Mg reacts with elemental oxygen.(b) Not a redox reaction; it is an acid–base reaction.(c) Redox reaction; a metal reacts with a nonmetal.(d) Redox reaction; Zn transfers two electrons to Fe2+.

Which of the following is a redox reaction?


SKILLBUILDER 7.13 Identifying Redox Reactions





EXAMPLE 7.14 Writing Combustion ReactionsWrite a balanced equation for the combustion of liquid methyl alcohol (CH3OH).


Write a balanced equation for the combustion of liquid pentane (C5H12), a component of gasoline.

SKILLBUILDER 7.14 Writing Combustion Reactions

Begin by writing a skeletal equation showing the reaction of CH3OH with O2 to form CO2 and H2O.Balance the skeletal equation using therules in Section 7.4

Write a balanced equation for the combustion of liquid propanol (C3H7OH).

SKILLBUILDER PLUS

Solution:




EXAMPLE 7.15 Classifying Chemical Reactions According to What Atoms Do

Solution:(a) Synthesis; a more complex substance forms from two simpler ones.(b) Double-displacement; Ba and K switch places to form two new compounds.(c) Single-displacement; Al displaces Fe in Fe2O3

(d) Decomposition; a complex substance decomposes into simpler ones.(e) Synthesis; a more complex substance forms from two simpler ones.

Classify each of the following reactions as a synthesis, decomposition, single-displacement, or double-displacement reaction.

Classify each of the following reactions as a synthesis, decomposition, single-displacement, or double-displacement reaction.

SKILLBUILDER 7.15 Classifying Chemical Reactions According to What Atoms Do




EXAMPLE 7.15 Classifying Chemical Reactions According to What Atoms Do


Continued




EXAMPLE 7.16 Identifying a Chemical Reaction

Solution:(a) Chemical reaction, as evidenced by the color change.(b) Chemical reaction, as evidenced by the evolution of a gas.(c) Not a chemical reaction; solid ice is still water.(d) Chemical reaction, as evidenced by the evolution of a gas and by a color change.

Which of the following is a chemical reaction?(a) Copper turns green on exposure to air.(b) When sodium bicarbonate is combined with hydrochloric acid, bubbling is observed.(c) Liquid water freezes to form solid ice.(d) A pure copper penny forms bubbles of a dark brown gas when dropped into nitric acid. The nitric acid solution turns blue.




EXAMPLE 7.17 Writing Balanced Chemical Equations

Write a balanced chemical equation for the reaction of solid vanadium(V) oxide with hydrogen gas to form solid vanadium(III) oxide and liquid water.

Solution:Vanadium occurs in only one compound on both sides of the equation. However, it is balanced, so we proceed to balance oxygen by placing a 2 in front of H2O on the right side.




EXAMPLE 7.18 Determining Whether a Compound Is Soluble

Solution:(a) Insoluble; compounds containing CO3

2– are insoluble, and Cu2+ is not an exception.(b) Insoluble; compounds containing SO4

2– are usually soluble, but Ba2+ is an exception.(c) Soluble; all compounds containing NO3

– are soluble.

Determine whether each of the following compounds is soluble(a) CuCO3

(b) BaSO4

(c) Fe(NO3)3




EXAMPLE 7.19 Predicting Precipitation Reactions

Write an equation for the precipitation reaction that occurs, if any, when solutions of sodium phosphate and cobalt(II) chloride are mixed.

Solution:

Potentially Insoluble Products:NaCl Co3(PO4)2

NaCl is solubleCo3(PO4)2 is insoluble.




EXAMPLE 7.20 Writing Complete Ionic and Net Ionic Equations

Write a complete ionic and net ionic equation for the following reaction.

Solution:Complete ionic equation:

Net ionic equation:




EXAMPLE 7.21 Writing Equations for Acid–Base Reactions

Write an equation for the reaction that occurs when aqueous hydroiodic acid is mixed with aqueous barium hydroxide.

Solution:




EXAMPLE 7.22 Writing Equations for Gas Evolution Reactions

Write an equation for the reaction that occurs when aqueous hydrobromic acid is mixed with aqueous potassium bisulfite.

Solution:




EXAMPLE 7.23 Identifying Redox Reactions

Solution:Only (a) is a redox reaction.





EXAMPLE 7.24 Writing Equations for Combustion Reactions

Write a balanced equation for the combustion of gaseous ethane (C2H6), a minority component of natural gas.Solution:




EXAMPLE 7.25 Classifying Chemical Reactions

Solution:(a) Synthesis; KBr, a more complex substance, is formed from simpler substances.(b) Single-displacement; Fe displaces Ag in AgNO3.(c) Decomposition; CaSO3 decomposes into simpler substances.(d) Double-displacement; Ca and Li switch places to form new compounds.

Classify each of the following chemical reactions as a synthesis, decomposition, single-displacement, or double- displacement reaction.

example 7.1

Documents