CHAPTER 4 Chemical Reactions 75 Objectives You will be able to do the following. 1. Write a description of the information given by a chemical equation. 2. Write or identify the symbols used in chemical equations to describe solid, liquid, gas and aqueous. 3. Balance chemical equations. 4. Given a description of a solution of two components, identify the solute and the solvent. 5. Write a description of the process for dissolving an ionic compound in water. Your description should include mention of the nature of the particles in solution and the attractions between the particles in the solution. 6. Given the formula for an ionic compound, predict whether it is soluble in water or not. 7. Given formulas for two ionic compounds; a. predict whether a precipitate will form when the water solutions of the two are mixed, b. if there is a reaction, predict the products of the reaction and write their formulas, c. and if there is a reaction, write the complete, complete ionic, and net ionic equations that describe the reaction. 8. Describe precipitation reactions. Your descriptions should include mention of the nature of the particles in the system before and after the reaction, a description of the cause of the reaction, and a description of the attractions between the particles before and after the reaction. 9. Write a description of the process for the solution of a strong acid in water. 10. Write a description of the process for the solution of sulfuric acid in water. 11. Write a description of the process for the solution of a weak acid in water. 12. Identify the following as strong monoprotic acids: HCl, HBr, HI, HNO 3 , and HClO 4 . 13. Identify sulfuric acid, H 2 SO 4 , as a strong diprotic acid. 14. Given a formula for any acid, identify it as a strong or weak acid. 15. Write a description of the process for the solution of ammonia, NH 3 , in water. 16. Given the formula or name of a compound, classify it as a strong electrolyte, weak electrolyte, or nonelectrolyte. 17. Given a formula for an ionic compound, identify it as representing a weak base, a strong base, or a substance that is neutral in the Arrhenius acid-base sense. 18. Identify the following as anions that are neutral in the acid-base sense: Cl − , Br − , I − , NO 3 − , and ClO 4 − . 19. Identify the following anions as weak acids: hydrogen sulfate ion, HSO 4 − , and dihydrogen phosphate ion, H 2 PO 4 − .
Transcript
CHAPTER 4Chemical Reactions
75
Objectives
You will be able to do the following.
1. Write a description of the information given by a chemical equation.2. Write or identify the symbols used in chemical equations to describe solid, liquid,
gas and aqueous. 3. Balance chemical equations.4. Given a description of a solution of two components, identify the solute and the
solvent.5. Write a description of the process for dissolving an ionic compound in water.
Your description should include mention of the nature of the particles in solution and the attractions between the particles in the solution.
6. Given the formula for an ionic compound, predict whether it is soluble in water or not.
7. Given formulas for two ionic compounds; a. predict whether a precipitate will form when the water solutions of the two
are mixed,b. if there is a reaction, predict the products of the reaction and write their
formulas,c. and if there is a reaction, write the complete, complete ionic, and net ionic
equations that describe the reaction.8. Describe precipitation reactions. Your descriptions should include mention of the
nature of the particles in the system before and after the reaction, a description of the cause of the reaction, and a description of the attractions between the particles before and after the reaction.
9. Write a description of the process for the solution of a strong acid in water.10. Write a description of the process for the solution of sulfuric acid in water.11. Write a description of the process for the solution of a weak acid in water.12. Identify the following as strong monoprotic acids: HCl, HBr, HI, HNO3, and
HClO4.13. Identify sulfuric acid, H2SO4, as a strong diprotic acid.14. Given a formula for any acid, identify it as a strong or weak acid.15. Write a description of the process for the solution of ammonia, NH3, in
water.16. Given the formula or name of a compound, classify it as a strong electrolyte,
weak electrolyte, or nonelectrolyte. 17. Given a formula for an ionic compound, identify it as representing a weak base, a
strong base, or a substance that is neutral in the Arrhenius acid-base sense. 18. Identify the following as anions that are neutral in the acid-base sense: Cl−, Br−,
I−, NO3−, and ClO4
−.19. Identify the following anions as weak acids: hydrogen sulfate ion, HSO4
−, and dihydrogen phosphate ion, H2PO4
−.
76 Chapter 4 Chemical Reactions
20. Identify ammonia as an uncharged, weak base.21. Identify ionic compounds containing hydroxide ions as strong bases.22. Given the pH of a solution, identify the solution as acidic, basic, or neutral. 23. Given the pH of two acidic solutions, identify which solution is more acidic. 24. Given the pH of two basic solutions, identify which solution is more basic.25. Describe how litmus paper can be used in the laboratory to identify whether a
solution is acidic or basic.26. Write a description of a neutralization reaction in terms of the Arrhenius
theory.27. Write a description of the process that takes place at the molecular level when
a strong acid reacts with an aqueous strong base. Your descriptions should include mention of the nature of the particles in the system before and after the reaction, a description of the cause of the reaction, and a description of the attractions between the particles before and after the reaction.
28. Given the formulas for an acid and a base: a. predict the products that would form from the reaction between them.b. write the balanced complete, complete ionic, and net ionic equations that
describe the reaction.29. Identify H2O(l) and CO2(g) as the products of the reaction of an acid with
carbonate, CO32−, or hydrogen carbonate, HCO3
−.30. Write an explanation for why a substance can be a Brønsted-Lowry acid in
one reaction and a Brønsted-Lowry base in a different reaction. Give an example to illustrate your explanation.
31. Write an explanation for why the Arrhenius definitions for acid and base and not the Brønsted-Lowry definitions are used to describe whether an isolated substance is an acid or base.
32. Write an explanation for why it is useful to have two sets of definitions for acids and bases (the Arrhenius definitions and the Brønsted-Lowry definitions).
33. Given a Brønsted-Lowry neutralization equation, identify the Brønsted-Lowry acid and Brønsted-Lowry base.
34. Write a description of the difference between the redox reactions that form binary ionic compounds like zinc oxide from their elements and the similar redox reactions that form molecular compounds like sulfur dioxide from their elements. (Your description should include mention of the degree to which the electrons are transferred in the reactions.)
35. Given an equation for a chemical reaction, identify whether the equation represents a redox reaction or not.
36. Given an equation for a redox reaction, identify the substance that is oxidized and the substance that is reduced.
37. Given an equation for a redox reaction, identify the substance that is the reducing agent and the substance that is the oxidizing agent.
38. Write or identify descriptions of combination, decomposition, combustion, displacement (or single displacement), and exchange (or double displacement) reactions.
39. Write chemical equations for combustion reactions of compounds that contain carbon, hydrogen, sulfur, and/or oxygen.
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40. Given a chemical equation, identify it as representing a combination, decomposition, combustion, displacement (or single displacement), and exchange (or double displacement) reaction.
41. Convert between the definition and the term for the following words or phrases.
Chapter 4 Glossary
Chemical reaction or chemical change e conversion of one or more pure substances into one or more different pure substances.
Reactants e substances that change in a chemical reaction. eir formulas are on the left side of the arrow in a chemical equation.
Products e substances that form in a chemical reaction. eir formulas are on the right side of the arrow in a chemical equation.
Coefficients e numbers in front of chemical formulas in a balanced chemical equation.
Solution A mixture whose particles are so evenly distributed that the relative concentrations of the components are the same throughout. Solutions can also be called homogeneous mixtures.
Aqueous solution A solution in which water is the solvent. Solute e gas in a solution of a gas in a liquid. e solid in a solution of a solid in a
liquid. e minor component in other solutions.Solvent e liquid in a solution of a gas in a liquid. e liquid in a solution of a solid
in a liquid. e major component in other solutions. Hydrated Bound to one or more water molecules. Hydration e binding of one or more water molecules to an ion or molecule.Electrolyte A substance that ionizes or dissociates in water to form an electrically
conducting solution. Strong electrolyte A substance that ionizes or dissociates completely in an aqueous
solution.Nonelectrolyte A substance that ionizes or dissociates incompletely in an aqueous
solution.Ionize To form ions (often as a substance dissolves in water).Dissociate e separation of ions (often as a substance dissolves in water).Double-displacement reaction A chemical reaction that has the following form
AB + CD → AD + CBPrecipitation reaction A reaction in which one of the products is insoluble in water
and comes out of solution as a solid. Precipitate A solid that comes out of solution.Precipitation e process of forming a solid in a solution. Crystals Solid particles whose component atoms, ions, or molecules are arranged in
an organized, repeating pattern.Complete ionic equation A chemical equation that describes the actual form for
each substance in solution. For example, ionic compounds that are dissolved in water are described as separate ions.
Spectator ions Ions that play a role in delivering other ions into solution to react but that do not actively participate in the reaction themselves.
78 Chapter 4 Chemical Reactions
Complete equation or molecular equation A chemical equation that includes uncharged formulas for all of the reactants and products. e formulas include the spectator ions, if any.
Net ionic equation A chemical equation for which the spectator ions have been eliminated, leaving only the substances actively involved in the reaction.
Solubility e maximum amount of solute that can be dissolved in a given amount of solvent.
Hydronium ion H3O+ Arrhenius acid According to the Arrhenius theory, any substance that generates
hydronium ions, H3O+, when added to water. Monoprotic acid An acid that donates one hydrogen ion per molecule in a
reaction. Polyprotic acid An acid that can donate more than one hydrogen ion per molecule
in a reaction.Diprotic acid An acid that can donate two hydrogen ions per molecule in a
reaction.Triprotic acid An acid that can donate three hydrogen ions per molecule in a
reaction. Strong acid An acid that donates its H+ ions to water in a reaction that goes
completely to products. Such a compound produces close to one H3O+ ion in solution for each acid molecule dissolved in water.
Completion reaction A reaction that shifts completely to products, that is, a reaction that is not significantly reversible.
Reversible reaction A reaction in which the reactants are constantly forming products and, at the same time, the products are reforming the reactants.
Weak acid A substance that is incompletely ionized in water due to the reversibility of the reaction that forms hydronium ions, H3O+, in water. Weak acids yield significantly less than one H3O+ ion in solution for each acid molecule dissolved in water.
Strong monoprotic acids HCl(aq), HBr(aq), HI(aq), HNO3, HClO4. Strong diprotic acids H2SO4. Arrhenius base A substance that produces hydroxide ions, OH−, when added to
water.Strong base A substance that generates at least one hydroxide ion in solution for
every unit of substance added to water. Weak base A substance that produces fewer hydroxide ions in water solution than
particles of the substance added. Miscible Able to be mixed in any proportion, that is, infinitely soluble.Acidic solution A solution with a significant concentration of hydronium ions,
H3O+.Basic solution A solution with a significant concentration of hydroxide ions, OH−. Neutralization reaction A chemical reaction between an acid and a base. Brønsted-Lowry acid-base reaction A chemical reaction in which a proton, H+, is
transferred. Brønsted-Lowry Acid A substance that donates protons, H+, in a Brønsted-Lowry
acid-base reaction.
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Brønsted-Lowry Base A substance that accepts protons, H+, in a Brønsted-Lowry acid-base reaction.
Amphoteric substance A substance that can act as either a Brønsted-Lowry acid and a Brønsted-Lowry base, depending on the circumstances.
Oxidation Any chemical change in which at least one element loses electrons, either completely or partially.
Reduction Any chemical change in which at least one element gains electrons, either completely or partially.
Oxidation-reduction reactions e chemical reactions in which there is a complete or partial transfer of electrons, resulting in oxidation and reduction. ese reactions are also called redox reactions.
Half-reactions Separate oxidation and reduction reaction equations in which electrons are shown as a reactant or product.
Reducing agent A substance that loses electrons, making it possible for another substance to gain electrons and be reduced.
Oxidizing agent A substance that gains electrons, making it possible for another substance to lose electrons and be oxidized.
Oxidation number A tool for keeping track of the flow of electrons in redox reactions (also called oxidation state).
Combination or synthesis reaction e joining of two or more elements or compounds into one product.
Decomposition reaction e conversion of one compound into two or more simpler substances.
Combustion reaction Rapid oxidation accompanied by heat and usually light. Single-displacement reaction Chemical change in which atoms of one element
displace (or replace) atoms of another element in a compound.
Sample Study Sheet 4.1 Balancing Chemical Equations
80 Chapter 4 Chemical Reactions
T- You are asked to balance a chemical equation. G S
• Consider the first element listed in the first formula in the equation. If this element is mentioned in two or more formulas on the same side of the arrow, skip it until after the other elements are balanced. (See Example 4.2.)If this element is mentioned in one formula on each side of the arrow, balance it by placing coefficients in front of one or both of these formulas.
• Moving from left to right, repeat the process for each element.• When you place a number in front of a formula that contains an element you tried to balance previously, recheck that element and put its atoms back in balance. (See Examples 4.2 and 4.3.)• Continue this process until the number of atoms of each element is balanced.
e following strategies can be helpful for balancing certain equations. STRATEGY 1 Often, an element can be balanced by using the subscript for this element on the left side of the arrow as the coefficient in front of the formula containing this element on the right side of the arrow and vice versa (using the subscript of this element on the right side of the arrow as the coefficient in front of the formula containing this element on the left side). (See Example 4.3.)STRATEGY 2 It is sometimes easiest, as a temporary measure, to balance the pure nonmetallic elements (H2, O2, N2, F2, Cl2, Br2, I2, S8, Se8, and P4 ) with a fractional coefficient (1/2, 3/2, 5/2, etc.). If you do use a fraction during the balancing process, you can eliminate it later by multiplying each coefficient in the equation by the fraction’s denominator (usually the number 2). (See Example 4.4.)STRATEGY 3 If polyatomic ions do not change in the reaction, and therefore appear in the same form on both sides of the chemical equation, they can be balanced as if they were single atoms. (See Example 4.5.)STRATEGY 4 If you find an element difficult to balance, leave it for later.
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EXERCISE 4.1 - Balancing Chemical Equations
Balance the following chemical equations.a. P4(s) + Cl2(g) → PCl3(l)
b. PbO(s) + NH3(g) → Pb(s) + N2(g) + H2O(l)
c. P4O10(s) + H2O(l) → H3PO4(aq)
d. Mn(s) + CrCl3(aq) → MnCl2(aq) + Cr(s)
e. C2H2(g) + O2(g) → CO2(g) + H2O(l)
f. Fe(NO3)3(aq) + Na2CO3(aq) → Fe2(CO3)3(s) + NaNO3(aq)
g. CH3NH2(g) + O2(g) → CO2(g) + H2O(l) + N2(g)
Figure 4.1 Sodium Chloride Dissolving in Water This shows the mixture immediately after sodium chloride has been added to water. Certain water molecules are highlighted to draw attention to their role in the process.
82 Chapter 4 Chemical Reactions
Figure 4.2 Aqueous Sodium Chloride This image shows a portion of the solution that forms when sodium chloride dissolves in water. Certain water molecules are highlighted to draw attention to their role in the process.
Figure 4.3 Aqueous Calcium Nitrate Notice that there are twice as many −1 nitrate ions as +2 calcium ions.
You can find an animation that shows the process by which sodium chloride dissolves at the following web address:http://www.mpcfaculty.net/mark_bishop/NaCl_dissolves.htm
Sample Study Sheet 4.2 Predicting Precipitation Reactions and Writing Precipitation Equations
T- You are asked to predict whether a precipitation reaction will take place between two aqueous solutions of ionic compounds, and if the answer is yes, to write the complete equation for the reaction.
G S STEP 1 Determine the formulas for the possible products using the general double-displacement equation. (Remember to consider ion charges when writing your formulas.)
AB + CD → AD + CB
STEP 2 Predict whether either of the possible products is water insoluble. If either possible product is insoluble, a precipitation reaction takes place, and you may continue with Step 3. If neither is insoluble, write “No reaction”.
STEP 3 Follow these steps to write the complete equation.
• Write the formulas for the reactants separated by a “+”.
• Separate the formulas for the reactants and products with a single arrow.
• Write the formulas for the products separated by a “+”.
• Write the physical state for each formula.
e insoluble product will be followed by (s).
Water-soluble ionic compounds will be followed by (aq).
• Balance the equation.
EXERCISE 4.3 - Precipitation Reactions
Predict whether a precipitate will form when each of the following pairs of water
solutions are mixed. If there is a precipitation reaction, write the complete, complete
ionic, and net ionic equations that describe the reaction.
a. Li2CO3(aq) + Al(NO3)3(aq)
b. KOH(aq) + Fe(NO3)3(aq)
c. NaC2H3O2(aq) + CaS(aq)
d. K2SO4(aq) + Pb(NO3)2(aq)
86 Chapter 4 Chemical Reactions
You can find a description of the procedure for writing complete and net ionic equations at the following web address:
Are you having trouble with this chapter? People often do. To successfully complete each of the tasks in this chapter, you need to have mastered the skills from previous sections. If you missed anything along the way, you will have trouble. Here is a list of the tasks you need to be able to do in order to work the problems in this chapter. You should go through the list in order and be sure you have mastered each skill before you go on to the next one.
1. Convert between names and symbols for the common elements.
2. Identify whether an element is a metal or a nonmetal.
3. Determine the charges on many of the monatomic ions.
4. Convert between the name and formula for polyatomic ions.
5. Recognize a name or formula as ionic or molecular.
6. Recognize a name or formula as representing a binary acid or an oxyacid.
7. Convert between the name and formula for ionic compounds, binary acids and oxyacids.
8. Balance chemical equations.
9. Predict the products of double displacement reactions.
10. Predict ionic solubility.
11. Predict the states of ionic compounds and H2O.
You can recognize strong and weak acids by using the following criteria. ♦ You can recognize acids either from their formula or their name.
∗ Acid formulas can be either HX(aq) like HCl(aq), H2S(aq), or HaXbOc like HNO2.
∗ Acid names end in acid like hydrochloric acid, hydrosulfuric acid and nitrous acid.
♦ e strong monoprotic acids you should know are HCl, HBr, HI, HNO3 and HClO4.
♦ Sulfuric acid, H2SO4, is a strong diprotic acid.
You will recognize a substance as a weak acid if it is not on the list of strong acids.
Figure 4.8 Ammonia in Water
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Table 4.2 Classifying Arrhenius acids
Classification of Arrhenius Acids
Weak or Strong
Characteristics How to Recognize
MonoproticStrong Completely
ionized in waterMemorize HCl(aq), HBr(aq), HI(aq), HNO3, and HClO4
Weak Incompletely ionized in water
General formula of HX(aq) or HXbOc; not on the list of strong acids*
DiproticStrong First H+ ion
lost completely: second incompletely
H2SO4
Weak Both H+ ions lost incompletely
H2S(aq) or General formula of H2XbOc (not H2SO4)
Triprotic Weak All three H+ ions lost incompletely
General formula H3XbOc
*Some acids have formulas that do not fit our general formulas. Acetic acid, HC2H3O2,
is the only example that you need to recognize at this point.
Table 4.3 Classifying Arrhenius bases
Arrhenius Bases Strong or Weak
Characteristics How recognize?
Anions strong Each unit added leads to at least one OH− ion in solution
Soluble Metal hydroxides*
(in ionic compounds)
weak Reversible reaction with water to yield fewer OH− ions than units added
Anions in ionic compounds except with OH−, Cl−, Br−, I−, NO3
−, ClO4
−, HSO4−, and H2PO4
−
Some uncharged molecules
weak Reversible reaction with water to yield fewer OH− ions than molecules added
NH3
* Note: Since a metal hydroxide must be soluble in water to yield a significant
concentration of hydroxide in solution, it is common to consider only water soluble
metal hydroxides like NaOH to be strong bases. Insoluble metal hydroxides like
aluminum hydroxide, Al(OH)3, react like the soluble hydroxides in neutralization
reactions, so for some purposes they can be considered strong bases.
90 Chapter 4 Chemical Reactions
Sample Study Sheet 4.3 Identification of Strong And Weak Acids and Bases
T- You can use the following steps to identify a name or chemical formula as representing either (1) an Arrhenius strong acid, (2) an Arrhenius weak acid, (3) an Arrhenius strong base, (4) an Arrhenius weak base, or (5) not acidic or basic in the Arrhenius sense (neutral).
G S
STEP 1 Identify the substance as an acid, a base, or neither. ♦ You can identify acids in the following ways. • If you are given a name:
a. e names of the uncharged acids end in acid. For example, hydrochloric acid is an acid.
b. e names for the only ionic compounds you are expected to recognize as acidic end in hydrogen sulfate or dihydrogen phosphate.
Sodium hydrogen sulfate is acidic. • If you are given a molecular formula:
a. Remember that acids have one of these forms: HX(aq) or HaXbOc.HCl(aq) and H2SO4 are acids. b. Acidic ionic compounds have formulas that include HSO4
− or H2PO4
−. NaH2PO4 is acidic.
♦ You can identify bases in the following ways. • We expect ionic compounds to be basic, except those containing the
following anions are basic. a. Cl−, Br−, I−, NO3
−, ClO4− - neutral
b. HSO4−, H2PO4
− - acidicNaF is basic, NaCl is neutral, and NaHSO4 is acidic.
• Ammonia, NH3, is a base. STEP 2 If you have an acid or base, determine whether it is strong or weak.
♦ We will consider all acids except HCl(aq), HBr(aq), HI(aq), HNO3, HClO4, and H2SO4 to be weak.
HF is a weak acid. ♦ We will consider all bases except metal hydroxides to be weak.
NaF is a weak base.
EXERCISE 4.4 - Acid and Base Classification
Identify HNO2, lithium hydroxide, NaCN, sodium iodide, NaHSO4, nitric acid, CH3OH, hydrofluoric acid, and KC2H3O2 as either (1) an Arrhenius strong acid, (2) an Arrhenius weak acid, (3) an Arrhenius strong base, (4) an Arrhenius weak base, or (5) not acidic or basic in the Arrhenius sense (neutral).
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e following are characteristics of acids.1. Acids have a sour taste.
2. Acids turn litmus from blue to red.
3. Acids react with bases. (When the base includes carbonate or hydrogen
carbonate ions, carbon dioxide gas is released.)
e following are characteristics of bases.1. Bases have a bitter taste.
Nonelectrolytes Alcohol CaHbOH name ends in -anol or alcohol
methanol or methyl alcohol
Sugar C6H12O6 or C12H22O11
name ends in -ose glucose or sucrose
M=symbol of metal, A=symbol of nonmetal, X=some element other than H or O, and a, b, c & d indicate subscripts
EXERCISE 4.5 - Strong, Weak, and Nonelectrolytes
Identify Al(NO3)3, acetic acid, NH3, ammonium acetate, HCl, glucose, CH3OH, barium chloride, K2Cr2O7, nitric acid, HBrO2, 2–propanol (or isopropyl alcohol) as either a (1) strong electrolyte, (2) weak electrolyte, or (3) nonelectrolyte, and in parentheses write the type of compound each name or formula represents. Each dissolves in water.
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Figure 4.10 Aqueous Nitric Acid
Figure 4.11 Water Solution of Nitric Acid and Sodium Hydroxide before Reaction
94 Chapter 4 Chemical Reactions
Figure 4.12 After Reaction of Nitric Acid and Sodium Hydroxide
Table 4.6 Prediction of whether neutralization reactions are completion reactions or reversible reactions
Strong Acid Weak Acid
Strong Base Completion Completion
Weak Base Completion Reversible or Completion*
*When you need to know whether a neutralization reaction is a completion reaction or reversible, you will be told.
Sample Study Sheet 4.4
Steps for Writing Neutralization Equations
T- You will be given formulas or names of two substances and asked to predict whether a neutralization reaction will take place between them. If it does, you will be asked to write the complete and net ionic equations for the reaction. e following steps allow you to do this.G S
STEP 1 Ask, “Do you have an acid and a base?” If yes, go to Step 2. If no, say “No reaction”. See Previous Study Sheets.
Any one of the following conditions will lead you to predict “no reaction”. 1) One of the substances is neutral in the Arrhenius acid-base sense.2) Both substances given are acids. 3) Both substances given are bases.
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STEP 2 Describe a completion reaction with a single arrow, (→). Describe a reversible reaction with a double arrow, ( ). Determine whether the reaction is reversible or goes to completion by applying the following guidelines.
a. When one or both of the acid and base are strong, the reaction is a completion reaction. See Previous Study Sheets.
b. If you have a weak acid and a weak base, you can assume that the reaction is reversible.
STEP 3 Write the formulas and states of the products.a. You must first decide on the correct form of the equation.
1) Most neutralization reactions that you will see in this chapter will be double displacement. If the base is ammonia, NH3, the reaction will have the second form.
Double Displacement AB + CD → AD + CB
NH3(aq) + HX(aq) → NH4X(aq)
2) For double displacement reactions, follow these steps.
a. Identify the A, B, C and D.
(1) For acids the positive component is H+.
(2) Split ionic compounds into cation and anion.
b. Write the AD and CB formulas. Be careful to balance charges.
c. Remember that carbonic acid, H2CO3(aq), decomposes to form CO2(g) and H2O(l).
d. We will assume that all of the H+ ions react for polyprotic acids, e.g. H2SO3 loses two H+ ions, and H3PO4 loses three H+ ions.
b. Write the states of reactants and products.
1. e states are usually given for the original reactants.
2. For the ionic products:
1). Most ionic products of neutralization reactions are water soluble and described with (aq).
2). If they are insoluble, describe them with (s).
3. Describe acids as aqueous, (aq), unless stated otherwise.
4. Ammonia is aqueous, (aq), when in water and gaseous, (g), when pure.
5. Carbon dioxide is insoluble in water and described as a gas, (g).
6. Water is a liquid, (l).
STEP 4 Balance the complete equation.
96 Chapter 4 Chemical Reactions
STEP 5 Write the net ionic equation.
a. Describe each reactant and product as ions or with a complete (not ionized)
formula.
1. Strong acids and ionic(aq) are strong electrolytes and completely ionized.
a. For this task, describe strong acids, HA(aq), as H+(aq) and A−(aq).
(e A−(aq) represents an anion)
b. Even though describing strong acids, HA(aq), as H3O+(aq) and
A−(aq) is a better description of the nature of the ions in solution, it
is probably more common to use H+(aq) and A−(aq) for the strong
acids.
2. Everything else is described with a complete (not ionized) formula.
b. Eliminate spectator ions. Remember that these are the ions that are in an
identical form on both sides of the equation.
c. Rewrite what is left and balance.
d. You can check that you have written the net ionic equation correctly by
making sure the following are true.
1. Strong monoprotic acids are described as H+(aq) in a net ionic equation.
(Even though H3O+(aq) provides a better description of the ions in
solution, it is probably more common to use H+(aq).)
2. e strong diprotic acid sulfuric acid, H2SO4, is described as H+ and
HSO4−.
3. Weak, uncharged acids, like acetic acid, HC2H3O2(aq), are described
with complete formulas.
a. A very common mistake is to describe the weak acids like the strong
acids, as H+ in the net ionic equation.
b. Ionic formulas that contain the anions that are weak acids are
described as ions. For instance, NaHSO4(aq) is described as
Na+(aq) and HSO4− (aq).
4. Any ions in strong electrolytes on both sides of the equation are
spectator ions and should be eliminated in a net ionic equation.
5. Formulas with (g), (l), and (s) are described with complete formulas.
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EXERCISE 4.6 - Writing Neutralization Equations
For each of the following pairs of possible reactants, predict whether a neutralization reaction will take place between them. If there is no reaction, write, “No Reaction”. If there is a reaction, write complete, complete ionic, and net ionic equations for the reaction. (e reactions between weak acids and weak bases given here are reversible reactions. If an acid has more than one acidic hydrogen, assume that there is enough base to remove all of them. Assume that there is enough acid to add as many protons to the base as possible)
For each of the following equations, identify the Brønsted-Lowry acid and base.
a. HNO2(aq) + NaBrO(aq) → HBrO(aq) + NaNO2(aq)
b. H2PO4−(aq) + HNO2(aq) H3PO4(aq) + NO2
−(aq)
c. H2PO4−(aq) + 2OH−(aq) → PO4
3−(aq) + 2H2O(l)
d. H2SO3(aq) + 2NaOH(aq) → Na2SO3(aq) + 2H2O(l)
Table 4.7 Key definitions for Redox Reactions
Term Definition
oxidation-reduction reaction
An electron transfer reaction
oxidation loss of electrons
reduction gain of electrons
reducing agent e substance that loses electrons (is oxidized) and makes it possible for something else to gain electrons (be reduced)
oxidizing agent e substance that gains electrons (is reduced) and makes it possible for something else to lose electrons (be oxidized)
half-reaction one-half of a redox reaction; just the oxidation or just the reduction
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Sample Study Sheet 4.5
Assignment of Oxidation Numbers
T- You are asked to determine the oxidation number of an atom, or you need to assign oxidation numbers to atoms to determine whether a reaction is a redox reaction, and if it is, to identify which element is oxidized, which is reduced, what the oxidizing agent is, and what the reducing agent is.
G S Use the following guidelines to assign oxidation numbers to as many atoms as you can. (e following table provides a summary of these guidelines with examples.)
• e oxidation number for each atom in a pure element is zero. • e oxidation number of a monatomic ion is equal to its charge. • When fluorine atoms are combined with atoms of other elements, their
oxidation number is −1. • When oxygen atoms are combined with atoms of other elements, their
oxidation number is −2, except in peroxides, like hydrogen peroxide, H2O2, where their oxidation number is −1.
• e oxidation number for each hydrogen atom in a molecular compound or a polyatomic ion is +1.
If a compound’s formula contains one element for which you cannot assign an oxidation number using the guidelines listed above, calculate the oxidation number according to the following rules.
• e sum of the oxidation numbers for the atoms in an uncharged formula is equal to zero.
• e sum of the oxidation numbers for the atoms in a polyatomic ion is equal to the overall charge on the ion.
Table 4.8 Oxidation Numbers for Some Elements
Oxidation Number
Examples Exceptions
Pure, uncharged element
0 Each atom is 0 in Zn, H2, P4, and Cl2
None
Monatomic ions charge on ion
Cd in CdCl2 is +2.Cl in CdCl2 is −1.
H in LiH is −1.
None
Fluorine in the combined form
−1 F in AlF3 is −1.F in CF4 is −1.
None
Oxygen in the combined form
−2 O in ZnO is −2. O in H2O is −2.
O is −1 in peroxides like H2O2 and O2
2−.
Hydrogen in the combined form
+1 H in H2O is +1. H is −1 when combined with a metal.
100 Chapter 4 Chemical Reactions
Equations for redox reactions can be difficult to balance, but your ability to determine oxidation numbers can help. You can find a description of the process for balancing redox equations at the following web address.
Identify whether the following equations describe redox reactions or not. For each of the redox reactions, identify what is oxidized, what is reduced, what the reducing agent is, and what the oxidizing agent is.
Sample Study Sheet 4.6 Writing Equations for Combustion
T- You are asked to write an equation for the complete combustion of a substance composed of one or more of the elements carbon, hydrogen, oxygen, and sulfur. G S
STEP 1 Write the formula for the substance combusted.
STEP 2 Write O2(g) for the second reactant. STEP 3 Predict the products using the following guidelines.
If the compound contains carbon, one product will be CO2(g).If the compound contains hydrogen, one product will be H2O(l).
(Even though water may be gaseous when it is first formed in a combustion reaction, we usually describe it as a liquid in the equation. By convention, we describe the state of each reactant and product as their state at room temperature and pressure. When water returns to room temperature, it is a liquid.)
If the compound contains sulfur, one product will be SO2(g).Any oxygen in the combusted substance would be distributed between the products already mentioned.
STEP 4 Balance the equation.
EXERCISE 4.10 - Writing Combustion Equations
Write balanced equations for the complete combustion of (a) C4H10(g),
(b) C3H7OH(l), and (c) C4H9SH(l).
102 Chapter 4 Chemical Reactions
Figure 4.14 Single-Displacement Reaction Between Copper(II) Sulfate and Solid Zinc
EXERCISE 4.11 - Classification of Chemical Reactions
Classify each of these reactions with respect to the following categories: combination reaction, decomposition reaction, combustion reaction, and single-displacement reaction.
