Copyright © McGraw-Hill Education. Permission required for reproduction or display. 5-1 Chapter 5: Chemical Reactions and Equations

Copyright © McGraw-Hill Education. Permission required for reproduction or display. 5-2 Questions for Consideration 1. What happens in a chemical reaction? 2. How do we know whether a chemical reaction takes place? 3. How do we represent a chemical reaction with a chemical equation? 4. How are chemical reactions classified? How can the products of different classes of chemical reactions be predicted? 5. How do we represent chemical reactions in aqueous solution?

Copyright © McGraw-Hill Education. Permission required for reproduction or display. 5-3 Chapter 5 Topics: 1. What is a Chemical Reaction? 2. How Do We Know a Chemical Reaction Occurs? 3. Writing Chemical Equations 4. Predicting Chemical Reactions 5. Representing Reactions in Aqueous Solution

Copyright © McGraw-Hill Education. Permission required for reproduction or display. 5-4Introduction Chemical reactions occur all around us. Chemical reactions occur all around us. How do we make sense of these changes? What patterns can we find? How do we make sense of these changes? What patterns can we find? Figure 5.1 Figure 5.29 Figure 5.8F

Copyright © McGraw-Hill Education. Permission required for reproduction or display What is a Chemical Reaction? A chemical reaction is a chemical change. A chemical reaction is a chemical change. A chemical reaction occurs when one or more substances is converted into one or more new substances. A chemical reaction occurs when one or more substances is converted into one or more new substances. Reactant – a substance that we start with that undergoes a change Reactant – a substance that we start with that undergoes a change Product – a new substance that forms during the reaction Product – a new substance that forms during the reaction Products differ from reactants only in the arrangement of their component atoms. Products differ from reactants only in the arrangement of their component atoms.

Copyright © McGraw-Hill Education. Permission required for reproduction or display How do We Know a Chemical Reaction Occurs? What observations might indicate that a chemical reaction has taken place? What observations might indicate that a chemical reaction has taken place? Figure 5.7

Copyright © McGraw-Hill Education. Permission required for reproduction or display. 5-7 What clues tell you a reaction is likely occurring? Figure 5.8

Copyright © McGraw-Hill Education. Permission required for reproduction or display. 5-8 Physical Clues of a Chemical Reaction Common observations that may accompany a chemical reaction are: Common observations that may accompany a chemical reaction are: change in color change in color production of light production of light formation of a solid (such as a precipitate in solution, or smoke in air, or a metal coating) formation of a solid (such as a precipitate in solution, or smoke in air, or a metal coating) formation of a gas (bubbles in solution or fumes in the gaseous state) formation of a gas (bubbles in solution or fumes in the gaseous state) absorption or release of heat (sometimes appearing as a flame) absorption or release of heat (sometimes appearing as a flame)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Writing Chemical Equations A chemical equation is a symbolic representation of a chemical reaction. A chemical equation is a symbolic representation of a chemical reaction. A chemical equation shows: A chemical equation shows: the formulas for reactants and products the formulas for reactants and products the physical states of each substance (s, l, g, aq) the physical states of each substance (s, l, g, aq) relative numbers of reactants that combine and products that form relative numbers of reactants that combine and products that form special conditions required for the reaction, such as constant heating. special conditions required for the reaction, such as constant heating.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Chemical Reactions When hydrogen gas is ignited in the presence of oxygen, an explosive reaction occurs producing gaseous water molecules. When hydrogen gas is ignited in the presence of oxygen, an explosive reaction occurs producing gaseous water molecules. Is mass conserved? Is mass conserved? Figure 5.5Figure 5.6 Replace with updated Figure 5.6

Copyright © McGraw-Hill Education. Permission required for reproduction or display Writing Chemical Equations Balanced equation Balanced equation The number of atoms of each element is the same in the products as in the reactants. The number of atoms of each element is the same in the products as in the reactants. Conservation of mass is always maintained. Conservation of mass is always maintained.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Writing Chemical Equations When a powdered mixture of aluminum metal and iron(III) oxide is heated, it reacts to form liquid iron metal and aluminum(III) oxide. When a powdered mixture of aluminum metal and iron(III) oxide is heated, it reacts to form liquid iron metal and aluminum(III) oxide. Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + Fe(l) Figure 5.9

Copyright © McGraw-Hill Education. Permission required for reproduction or display Chemical Equations Must be Balanced Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + Fe(l) Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + Fe(l) Is this equation balanced? Is this equation balanced? What has to change? What has to change? Figure from p. 177

Copyright © McGraw-Hill Education. Permission required for reproduction or display Balance Equations with Coefficients Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + Fe(l) Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + Fe(l) We balance the atoms in equations with coefficients. We balance the atoms in equations with coefficients. The aluminum atoms are not balanced so we place a coefficient of 2 in front of the Al reactant: The aluminum atoms are not balanced so we place a coefficient of 2 in front of the Al reactant: 2Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + Fe(l) The iron atoms are not balanced so we place a coefficient of 2 in front of the Fe product: The iron atoms are not balanced so we place a coefficient of 2 in front of the Fe product: 2Al(s) + Fe 2 O 3 (s)  Al 2 O 3 (s) + 2Fe(l) Figure from p. 177

Copyright © McGraw-Hill Education. Permission required for reproduction or display Writing Chemical Equations Methane + oxygen  carbon dioxide + water CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(g) The number of atoms of each element in the unbalanced equation is shown below. These numbers were obtained by multiplying the subscript to the right of the element’s symbol by the stoichiometric coefficient. # of atoms (reactants) # of atoms (products) 1 C 4 H 2 H 2 O 3 O

Copyright © McGraw-Hill Education. Permission required for reproduction or display Writing Chemical Equations CH 4 (g) + O 2 (g)  CO 2 (g) + H 2 O(g) First, we look at the carbon atoms. Since the number of carbon atoms on the reactant side is already equal to the number of carbon atoms on the product side, we don’t need to add coefficients. Next, we look at the hydrogen atoms. Currently, there are four hydrogen atoms on the reactant side and 2 hydrogen atoms on the product side. Thus, we need to add a coefficient of 2 in front of water to make the hydrogen atoms equal. CH 4 (g) + O 2 (g)  CO 2 (g) + 2H 2 O(g)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Writing Chemical Equations CH 4 (g) + O 2 (g)  CO 2 (g) + 2H 2 O(g) Finally, we look at the oxygen atoms. Currently, there are 2 oxygen atoms on the reactant side and 4 oxygen atoms (combined from carbon dioxide and water) on the product side. Thus, we add a coefficient of 2 in front of the oxygen gas. CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) Now the equation is balanced!

Copyright © McGraw-Hill Education. Permission required for reproduction or display A General Approach to Balancing Equations 1. Identify the reactants and products and write their correct formulas. 2. Write a skeletal equation including physical states. 3. Change coefficients one at a time until the atoms of each element are balanced. (Start with the elements that occur least often in the equation.) 4. Make a final check by counting the atoms of each element on both sides of the equation.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Balancing Equations Balance the following equations: Balance the following equations: 1. Potassium chlorate  potassium chloride + oxygen 2. Aluminum acetate reacts with potassium sulfate to form potassium acetate and aluminum sulfate 3. Hexane (C 6 H 14 ) reacts with oxygen gas to form carbon dioxide and water 4. Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Balancing Equations 1. Potassium chlorate  potassium chloride + oxygen First, translate the chemical names into chemical formulas: Potassium chlorate  potassium chloride + oxygen K + ClO 3 − K + Cl − O 2 (diatomic) K + ClO 3 − K + Cl − O 2 (diatomic) KClO 3 (s)  KCl(aq) + O 2 (g) Next, balance the chemical equation: 2KClO 3 (s)  2KCl(aq) + 3O 2 (g)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Balancing Equations 2. Aluminum acetate reacts with potassium sulfate to form potassium acetate and aluminum sulfate Aluminum acetate + potassium sulfate  Al(C 2 H 3 O 2 ) 3 + K 2 SO 4  potassium acetate + aluminum sulfate KC 2 H 3 O 2 + Al 2 (SO 4 ) 3 KC 2 H 3 O 2 + Al 2 (SO 4 ) 3 Al(C 2 H 3 O 2 ) 3 (aq) + K 2 SO 4 (aq)  KC 2 H 3 O 2 (aq) + Al 2 (SO 4 ) 3 (s) Balanced: 2Al(C 2 H 3 O 2 ) 3 (aq) + 3K 2 SO 4 (aq)  6KC 2 H 3 O 2 (aq) + Al 2 (SO 4 ) 3 (s)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Balancing Equations 3. Hexane (C 6 H 14 ) reacts with oxygen gas to form carbon dioxide and water Hexane was given. Oxygen gas is diatomic. Carbon dioxide has 1 carbon atom and 2 oxygen atoms (using the Greek prefixes) and water has 2 hydrogen atoms and 1 oxygen atom. C 6 H 14 (l) + O 2 (g)  CO 2 (g) + H 2 O(g) Balanced: C 6 H 14 (l) + 9.5O 2 (g)  6CO 2 (g) + 7H 2 O(g) Fractional coefficients are not acceptable, so we multiply all coefficients by 2: 2C 6 H 14 (l) + 19O 2 (g)  12CO 2 (g) + 14H 2 O(g)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solutions: Balancing Equations 4. Zinc reacts with hydrochloric acid to form zinc chloride and hydrogen gas Zinc + Hydrochloric acid  Zinc chloride + hydrogen Zn(s) + HCl(aq)  ZnCl 2 (aq) + H 2 (g) Balanced: Zn(s) + 2HCl(aq)  ZnCl 2 (aq) + H 2 (g)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Precipitation Reactions Jennifer mixes solutions of cadmium(II) nitrate, Cd(NO 3 ) 2, and sodium sulfide, Na 2 S. She obtains an orange precipitate that is the pigment known as cadmium orange. Identify the precipitate, and write a balanced chemical equation for the reaction she carried out. Jennifer mixes solutions of cadmium(II) nitrate, Cd(NO 3 ) 2, and sodium sulfide, Na 2 S. She obtains an orange precipitate that is the pigment known as cadmium orange. Identify the precipitate, and write a balanced chemical equation for the reaction she carried out. Cd(NO 3 ) 2 (aq) + Na 2 S(aq) 

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solution: Precipitation Reactions The starting solutions contain Cd 2+ (aq), NO 3 − (aq), Na + (aq), and S 2− (aq). The starting solutions contain Cd 2+ (aq), NO 3 − (aq), Na + (aq), and S 2− (aq). The solubility rules say that most compounds of sodium are soluble and all nitrates are soluble, so the precipitate must be formed between cadmium(II) ions and sulfide ions. The solubility rules say that most compounds of sodium are soluble and all nitrates are soluble, so the precipitate must be formed between cadmium(II) ions and sulfide ions. By matching the positive and negative charges to get electrical neutrality, we can determine the formula of the precipitate to be CdS(s). By matching the positive and negative charges to get electrical neutrality, we can determine the formula of the precipitate to be CdS(s).

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solution: Precipitation Reactions The solution contains Na + (aq) and NO 3 − (aq) ions in a 1:1 ratio, so we write NaNO 3 (aq). The solution contains Na + (aq) and NO 3 − (aq) ions in a 1:1 ratio, so we write NaNO 3 (aq). We can write the skeletal equation for this reaction as: We can write the skeletal equation for this reaction as: Cd(NO 3 ) 2 (aq) + Na 2 S(aq)  CdS(s) + NaNO 3 (aq) Balanced, the equation is: Balanced, the equation is: Cd(NO 3 ) 2 (aq) + Na 2 S(aq)  CdS(s) + 2NaNO 3 (aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Extra Activity: Precipitation Reactions Given that a precipitate forms in this reaction, predict the products and their physical states, and balance the equation. BaCl 2 (aq) + Na 2 CrO 4 (aq)  ? Figure from p. 205 BaCl 2 (aq) + Na 2 CrO 4 (aq)  BaCrO 4 (s) + 2NaCl(aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Acid Base Neutralizations Calcium oxide is the white powder, lime. When added to water, it makes slaked lime, which is a solution of the base calcium hydroxide. If sulfuric acid is added to slaked lime, what products form? Write the balanced chemical equation for this reaction. Calcium oxide is the white powder, lime. When added to water, it makes slaked lime, which is a solution of the base calcium hydroxide. If sulfuric acid is added to slaked lime, what products form? Write the balanced chemical equation for this reaction.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solution: Acid-Base Neutralizations The starting solutions contain H + (aq), SO 4 2− (aq), Ca 2+ (aq), and OH − (aq). The starting solutions contain H + (aq), SO 4 2− (aq), Ca 2+ (aq), and OH − (aq). The solubility rules say that most sulfate compounds are soluble, unless sulfate is combined with calcium. Thus, calcium sulfate is a precipitate. Hydrogen ions and hydroxide ions form water, H 2 O(l). The solubility rules say that most sulfate compounds are soluble, unless sulfate is combined with calcium. Thus, calcium sulfate is a precipitate. Hydrogen ions and hydroxide ions form water, H 2 O(l). By matching the positive and negative charges to get electrical neutrality, we can determine the formula of the precipitate to be CaSO 4 (s). By matching the positive and negative charges to get electrical neutrality, we can determine the formula of the precipitate to be CaSO 4 (s).

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solution: Acid-Base Neutralizations The solution contains no spectator ions. We can write the skeletal equation for this reaction as: The solution contains no spectator ions. We can write the skeletal equation for this reaction as: H 2 SO 4 (aq) + Ca(OH) 2 (aq)  CaSO 4 (s) + H 2 O(l) Balanced, the equation is: Balanced, the equation is: H 2 SO 4 (aq) + Ca(OH) 2 (aq)  CaSO 4 (s) + 2H 2 O(l)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Double-Displacement Reactions Acid-Base Neutralization Reactions Complete and balance the following equation: Complete and balance the following equation: HCl(aq) + Mg(OH) 2 (s)  ? Figure HCl(aq) + Mg(OH) 2 (s)  MgCl 2 (aq) + 2H 2 O(l)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Combustion Reactions A combustion reaction is a 5 th type of reaction that does not always fall into one of the earlier categories. A combustion reaction is a 5 th type of reaction that does not always fall into one of the earlier categories. In a combustion reaction, a substance reacts with oxygen in a reaction that burns to produce a flame. In a combustion reaction, a substance reacts with oxygen in a reaction that burns to produce a flame. CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g) Figure 5.31 Figure 5.8F

Copyright © McGraw-Hill Education. Permission required for reproduction or display Figure 5.30 Figure 5.8F CH 4 (g) + 2O 2 (g)  CO 2 (g) + 2H 2 O(g)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Combustion of Hydrocarbons The most common combustion reactions involve the burning of a hydrocarbon, or a hydrocarbon containing oxygen. The products are always carbon dioxide and water. The most common combustion reactions involve the burning of a hydrocarbon, or a hydrocarbon containing oxygen. The products are always carbon dioxide and water. 2C 8 H 18 (g) + 25O 2 (g)  16CO 2 (g) + 18H 2 O(g)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Representing Reactions in Aqueous Solution When reactions occur in water, soluble ionic compounds exist as separate ions, not grouped together with each other as might seem so from the equation, sometimes called a molecular equation: When reactions occur in water, soluble ionic compounds exist as separate ions, not grouped together with each other as might seem so from the equation, sometimes called a molecular equation: AgNO 3 (aq) + NaCl(aq)  AgCl(s) + NaNO 3 (aq) NaCl(aq) exists in solution as separate ions Figure 3.5

Copyright © McGraw-Hill Education. Permission required for reproduction or display Representing Reactions in Aqueous Solution An ionic equation shows soluble ionic compounds as separate ions: An ionic equation shows soluble ionic compounds as separate ions: Ag + (aq) + NO 3  (aq) + Na + (aq) + Cl  (aq)  AgCl(s) + Na + (aq) + NO 3  (aq) Insoluble compounds do not exist as ions. Insoluble compounds do not exist as ions. Spectator ions are ions that occur on both sides of the equation. Spectator ions are ions that occur on both sides of the equation.

Copyright © McGraw-Hill Education. Permission required for reproduction or display Ionic Equations and Spectator Ions What are the spectator ions in the ionic equation? What are the spectator ions in the ionic equation? Ag + (aq) + NO 3  (aq) + Na + (aq) + Cl  (aq)  AgCl(s) + Na + (aq) + NO 3  (aq) Since the spectator ions do not change, we can leave them out of the net ionic equation: Since the spectator ions do not change, we can leave them out of the net ionic equation: Ag + (aq) + Cl  (aq)  AgCl(s) NO 3 (aq) + Na + (aq) NO 3  (aq) + Na + (aq) Na + (aq) + NO 3 (aq) Na + (aq) + NO 3  (aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Representing Reactions in Aqueous Solution Barium sulfate, used in the white pigment lithopone, can be prepared by mixing solutions of barium chloride and sodium sulfate. Write balanced molecular, ionic, and net ionic equations for this reaction. Barium sulfate, used in the white pigment lithopone, can be prepared by mixing solutions of barium chloride and sodium sulfate. Write balanced molecular, ionic, and net ionic equations for this reaction. Figure 5.26

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solution: Representing Reactions in Aqueous Solution Both reactants are strong electrolytes, so they dissociate into the ions Ba 2+ (aq), Cl - (aq), Na + (aq) and SO 4 2- (aq) in solution. Both reactants are strong electrolytes, so they dissociate into the ions Ba 2+ (aq), Cl - (aq), Na + (aq) and SO 4 2- (aq) in solution. Possible formulas for the precipitate are BaCl 2, Na 2 SO 4, NaCl, or BaSO 4. Possible formulas for the precipitate are BaCl 2, Na 2 SO 4, NaCl, or BaSO 4. The first two choices can be eliminated because they are starting reactants. The first two choices can be eliminated because they are starting reactants. The solubility rules (Table 5.4) indicate that NaCl is soluble and BaSO 4 is insoluble. The solubility rules (Table 5.4) indicate that NaCl is soluble and BaSO 4 is insoluble. Thus, we can write the molecular equation: Thus, we can write the molecular equation: BaCl 2 (aq) + Na 2 SO 4 (aq)  BaSO 4 (s) + 2NaCl(aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solution: Representing Reactions in Aqueous Solution BaCl 2 (aq) + Na 2 SO 4 (aq)  BaSO 4 (s) + 2NaCl(aq) Since BaCl 2, Na 2 SO 4, and NaCl are all strong electrolytes in solution, we can write their formulas as separate ionic formulas, giving the following ionic equation: Ba 2+ (aq) + 2Cl − (aq) + 2Na + (aq) + SO 4 2− (aq)  BaSO 4 (s) + 2Na + (aq) + 2Cl − (aq)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity Solution: Representing Reactions in Aqueous Solution Ba 2+ (aq) + 2Cl − (aq) + 2Na + (aq) + SO 4 2− (aq)  BaSO 4 (s) + 2Na + (aq) + 2Cl − (aq) Finally, we note that Na + and Cl − occur on both sides of the equation, so they are spectator ions. They can be eliminated, giving us the following net ionic equation: Ba 2+ (aq) + SO 4 2− (aq)  BaSO 4 (s)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Spectator Ions and Net Ionic Equations Identify the spectator ions and write the net ionic equations from the following molecular equations: Identify the spectator ions and write the net ionic equations from the following molecular equations: 1) Cu(s) + 2AgNO 3 (aq)  Cu(NO 3 ) 2 (aq) + 2Ag(s) 2) 2Na(s) + 2H 2 O(l)  2NaOH(aq) + H 2 (g) Figure from p. 206 Cu(s) + 2Ag + (aq)  Cu 2+ (aq) + 2Ag(s) 2Na(s) + 2H 2 O(l)  2Na + (aq) + 2OH − (aq) + H 2 (g)

Copyright © McGraw-Hill Education. Permission required for reproduction or display Activity: Net Ionic Equations What is the net ionic equation for the reaction of KCl(aq) and NaNO 3 (aq)? What is the net ionic equation for the reaction of KCl(aq) and NaNO 3 (aq)? Figure 5.7 Ionic “equation” K + (aq) + Cl − (aq) + Na + (aq) + NO 3 − (aq)  K + (aq) + NO 3 − (aq) + Na + (aq) + Cl − (aq) No reaction occurs!