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Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

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Page 1: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Chemistry: Atoms FirstJulia Burdge & Jason Overby

Chapter 8

Chemical Reactions

Kent L. McCorkle

Cosumnes River College

Sacramento, CA

Page 2: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Stoichiometry: Ratios of CombinationStoichiometry: Ratios of Combination8

8.1 Chemical EquationsInterpreting and Writing Chemical EquationsBalancing Chemical EquationsPatterns of Chemical Reactivity

8.2 Combustion AnalysisDetermination of Empirical Formula

8.3 Calculations with Balanced Chemical EquationsMoles of Reactants and ProductsMass of Reactants and Products

8.4 Limiting ReactantsDetermining the Limiting ReactantReaction Yield

8.5 Periodic Trends in Reactivity of the Main Group ElementsGeneral Trends in ReactivityReactions of the Active MetalsReactions of the Other Main Group ElementsComparison of Group 1A and Group 1B Elements

Page 3: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Chemical EquationsChemical Equations

A chemical equation uses chemical symbols to denote what occurs in a chemical reaction.

NH3 + HCl → NH4Cl

Ammonia and hydrogen chloride react to produce ammonium chloride.

Each chemical species that appears to the left of the arrow is called a reactant.

NH3 + HCl → NH4Cl

Each species that appears to the right of the arrow is called a product.

NH3 + HCl → NH4Cl

8.1

Page 4: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Interpreting and Writing Chemical EquationsInterpreting and Writing Chemical Equations

Labels are used to indicate the physical state:

(g) gas

(l) liquid

(s) solid

(aq) aqueous [dissolved in water]

NH3(g) + HCl(g) → NH4Cl(s)

SO3(g) + H2O(l) → H2SO4(aq)

Page 5: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Chemical equations must be balanced so that the law of conservation of mass is obeyed.

Balancing is achieved by writing stoichiometric coefficients to the left of the chemical formulas.

Balancing Chemical EquationsBalancing Chemical Equations

Page 6: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Balancing Chemical EquationsBalancing Chemical Equations

Generally, it will facilitate the balancing process if you do the following:

1)Change the coefficients of compounds before changing the coefficients of elements.

1)Treat polyatomic ions that appear on both sides of the equation as units.

1)Count atoms and/or polyatomic ions carefully, and track their numbers each time you change a coefficient.

Page 7: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Balancing Chemical EquationsBalancing Chemical Equations

Write the balanced chemical equation that represents the combustion of propane.

Solution: Step 1: Write the unbalanced equation:

C3H8(g) + O2(g) → CO2(g) + H2O(l)

Step 2: Leaving O2 until the end, balance each of the atoms:

C3H8(g) + 5O2(g) → 3CO2(g) + 4H2O(l)

Step 3: Double check to make sure there are equal numbers of each type on atom on both sides of the equation.

Page 8: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.1

Strategy The reactants are Ba(OH)2 and HClO4, and the products are Ba(ClO4)2 and H2O. Because the reaction is aqueous, all species except H2O will be labeled (aq) in the equation. Being a liquid, H2O will be labeled (l). Adjust the coefficients to ensure that there are identical numbers of each type of atom on both sides of the reaction arrow.

Write and balance the chemical equation for the aqueous reaction of barium hydroxide and perchloric acid to produce aqueous barium perchlorate and water.

Solution The chemical statement “barium hydroxide and perchloric acid react to produce barium perchlorate and water” can be represented with the following unbalanced equation:

Ba(OH)2(aq) + HClO4(aq) → Ba(ClO4)2(aq) + H2O(l)

Perchlorate ions (ClO4-) appear on both sides of the equation, so count them as

units, rather than count the individual atoms they contain. Thus, the tally of atoms and polyatomic ions is

Page 9: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.1 (cont.)

Solution (cont.)Ba(OH)2(aq) + HClO4(aq) → Ba(ClO4)2(aq) + H2O(l)

1 – Ba – 12 – O – 13 – H – 2

1 – ClO4- – 2

The barium atoms are already balanced, and placing a coefficient of 2 in front of HClO4(aq) balances the number of perchlorate ions.

Ba(OH)2(aq) + 2HClO4(aq) → Ba(ClO4)2(aq) + H2O(l)

1 – Ba – 12 – O – 14 – H – 2

2 – ClO4- – 2

(not including O atoms in ClO4- ions)

(not including O atoms in ClO4- ions)

Page 10: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.1 (cont.)

Solution (cont.)Placing a 2 in front of H2O(l) balances both the O and H atoms, giving us the final balanced equation:

Ba(OH)2(aq) + 2HClO4(aq) → Ba(ClO4)2(aq) + 2H2O(l)

1 – Ba – 12 – O – 24 – H – 4

2 – ClO4- – 2

(not including O atoms in ClO4- ions)

Think About It Check to be sure the equation is balanced by counting all the atoms individually.

1 – Ba – 110 – O – 10

4 – H – 42 – Cl – 2

Page 11: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.2

Strategy Begin by writing an unbalanced equation to represent the combination of reactants and formation of products as stated in the problem, and then balance the equation.

Butyric acid (also known as butanoic acid, C4H8O2) is one of many compounds

found in milk fat. First isolated from rancid butter in 1869, burtyic acid has received a great deal of attention in recent years as a potential anticancer agent. Write a balanced equation for the metabolism of butyric acid. Assume that the overall process of metabolism and combustion are the same (i.e., reaction with oxygen to produce carbon dioxide and water).

Page 12: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.2 (cont.)

Solution C4H8O2(aq) + O2(g) → CO2(g) + H2O(l)

Balance the number of C atoms by changing the coefficient for CO2 from 1 to 4.

C4H8O2(aq) + O2(g) → 4CO2(g) + H2O(l)

Balance the number of H atoms by changing the coefficient for H2O from 1 to 4.

C4H8O2(aq) + O2(g) → 4CO2(g) + 4H2O(l)

Finally, balance the number of O atoms by changing the coefficient for O2 from 1 to 5.

C4H8O2(aq) + 5O2(g) → 4CO2(g) + 4H2O(l)

Think About It Count the number of each type of atom on each side of the reaction arrow to verify that the equation is properly balanced. The are 4 C, 8 H, and 12 O in the reactants and in the products, so the equations is balanced.

Page 13: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Patterns of Chemical ReactivityPatterns of Chemical Reactivity

Three of the most commonly encountered reaction types are combination, decomposition, and combustion.

Combination – two or more reactants combine to form a single product

NH3(g) + HCl(g) → NH4Cl(s)

Decomposition – two or more products form from a single reactant

CaCO3(s) → CaO(s) + CO2(g)

Combustion – a substance burns in the presence of oxygen. Combustion of a compound that contains C and H (or C, H, and O) produces carbon dioxide gas and water.

CH2O(l) + O2(g) → CO2(g) + H2O(l)

Δ

Page 14: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.3

Strategy The equation in part (a) depicts two reactants and one product. The equation in part (b) represents a combination of a compound containing C, H, and O–with O2–to produce CO2 and H2O. The equation in part (c) represents two products being formed from a single reactant.

Determine whether each of the following equations represents a combination reaction, a decomposition reaction, or a combustion reaction:

(a) H2(g) + Br2(g) → 2HBr(g), (b) 2HCO2H(l) + O2(g) → 2CO2(g) + 2H2O(l),

(c) 2KClO3(s) → 2KCl(s) + 3O2(g).

Solution These equations represent (a) a combination reaction, (b) a combustion reaction, and (c) a decomposition reaction.

Think About It Make sure that a reaction identified as combination has only one product [as in part (a)], a reaction identified as combustion consumes O2 and produces CO2 and H2O [as in part (b)], and a reaction identified as a decomposition has only one reactant [as in part (c)].

Page 15: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Combustion AnalysisCombustion Analysis

The experimental determination of an empirical formula is carried out by combustion analysis.

8.2

Page 16: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Combustion AnalysisCombustion Analysis

In the combustion of 18.8 g of glucose, 27.6 g of CO2 and 11.3 g of H2O are produced.

It is possible to determine the mass of carbon and hydrogen in the original sample as follows:

The remaining mass is oxygen:18.8 g glucose – (7.53 g C + 1.26 g H) = 10.0 g O

22

2

1 mol CO 1 mol C 12.01 g Cmass of C = 27.6 g CO 7.53 g C

44.01 g C 1 mol CO 1 mol C

22

2

1 mol H O 2 mol H 1.008 g Hmass of H = 11.3 g H O 1.26 g H

18.01 g C 1 mol H O 1 mol H

Page 17: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Combustion AnalysisCombustion Analysis

It is now possible to calculatethe empirical formula.

Step 1:Determine the number of moles of each element.

Step 2:Write the empirical formula and divide by the smallest subscript to find the whole number ratio.

C0.627H1.25O0.626 simplifies to CH2O

1 mol Cmoles of C = 7.53 g C 0.627 moles C

12.01 g C

1 mol Hmoles of H = 1.26 g H 1.25 moles H

1.008 g H

1 mol Omoles of O = 10.0 g O 0.626 moles O

16.00 g O

0.626 0.626 0.626

Page 18: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Combustion AnalysisCombustion Analysis

The molecular formula may be determined from the empirical formula if the approximatemolecular mass is known.

To determine the molecular formula, divide the molar mass by the empirical formula mass.

For glucose:Empirical formula: CH2O

Empirical formula mass: [12.01 g/mol + 2(1.008 g/mol) + 16.00 g/mol] ≈ 30 g/mol

Molecular mass: 180 g/mol

Molecular mass/Empirical mass: 180/30 = 6

Molecular formula = [CH2O] x 6 = C6H12O6

Page 19: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.4

Strategy Determine the mass of C and H in the 5.50-g sample of benzene. Sum these masses and subtract from the original sample mass to find the mass of O. Convert the mass of each sample to moles, and use the results as subscripts in a chemical formula. Convert the subscripts to whole numbers by dividing each number by the smallest subscript to obtain the empirical formula. To calculate the molecular formula, first divide the molar mass given in the problem by the empirical formula mass. Then, multiply the subscripts in the empirical formula by the resulting number to obtain the subscripts in the molecular formula.

Combustion of a 5.50-g sample of benzene produces 18.59 g CO2 and 3.81 g

H2O. Determine the empirical formula and the molecular formula of benzene,

given that its molar mass is approximately 78 g/mol.

Page 20: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.4 (cont.)

Solution We calculate the mass of carbon and the mass of hydrogen in the products (and therefore the original 5.50-g sample) as follows:

The total mass of products is 5.073 g + 0.426 g = 5.499 g. Because the combined masses of C and H account for the entire mass of the original sample (5.499 ≈ 5.50 g), this compound must not contain O.

mass of C = 18.59 g CO2 ×1 mol CO2

44.01 g CO2

1 mol C1 mol CO2

× ×12.01 g C1 mol C = 5.073 g C

mass of H = 3.81 g H2O ×1 mol H2O

44.01 g H2O1 mol H

1 mol H2O× ×

1.008 g H1 mol H = 0.426 g C

Page 21: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.4 (cont.)

Solution Converting mass to moles for each element present in the compound,

gives the formula C0.4244H0.423. Converting the subscripts to whole numbers (0.4224/0.4224 ≈ 1; 0.423/0.4224 ≈ 1) gives the empirical formula CH.

Finally, dividing the approximate molar mass (78 g/mol) by the empirical molar mass (12.01 g/mol + 1.008 g/mol = 13.02 g/mol) gives 78/13.02 ≈ 6. Then, multiplying both subscripts in the empirical formula by 6 gives the molecular formula C6H6.

moles of C = 5.073 g C ×1 mol C

12.01 g C = 0.4224 mol C

moles of H = 3.81 g H ×1 mol H

12.01 g H = 0.423 mol H

Think About It Use the molecular formula to determine the molar mass and make sure that the result agrees with the molar mass given in the problem. For C6H6, the molar mass is 6(12.01 g/mol) + 6(1.008 g/mol) = 78.11 g/mol, which agrees with the 78 g/mol given in the problem statement.

Page 22: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Calculations with Balanced Chemical EquationsCalculations with Balanced Chemical Equations

Balanced chemical equations are used to predict how much product will form from a given amount of reactant.

2 moles of CO combine with 1 mole of O2 to produce 2 moles of CO2.

2 moles of CO is stoichiometrically equivalent to 2 moles of CO2.

8.3

Page 23: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Calculations with Balanced Chemical EquationsCalculations with Balanced Chemical Equations

Consider the complete reaction of 3.82 moles of CO to form CO2. Calculate the number of moles of CO2 produced.

22 2

2 mol COmoles CO produced = 3.82 mol CO = 3.82 mol CO

2 mol CO

Page 24: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Calculations with Balanced Chemical EquationsCalculations with Balanced Chemical Equations

Consider the complete reaction of 3.82 moles of CO to form CO2. Calculate the number of moles of O2 needed.

22 2

1 mol Omoles O needed = 3.82 mol CO = 1.91 mol O

2 mol CO

Page 25: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.5

Strategy Use the balanced chemical equation to determine the correct stoichiometric conversion factors, and then multiply by the number of moles of ammonia given.

Urea [(NH2)2CO] is a by-product of protein metabolism. This waste product is

formed in the liver and then filtered from the blood and excreted in the urine by the kidneys. Urea can be synthesized in the laboratory by the combination of ammonia and carbon dioxide according to the equation

(a) Calculate the amount of urea that will be produced by the complete reaction of 5.25 moles of ammonia. (b) Determine the stoichiometric amount of carbon dioxide required to react with 5.25 moles of ammonia.

Page 26: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.5 (cont.)

Solution

(a) moles (NH2)2CO produced = 5.25 mol NH3 ×

(b) moles CO2 produced = 5.25 mol NH3 ×

1 mol (NH2)2CO 2 mol NH3

= 2.63 mol (NH2)2CO

1 mol CO2

2 mol NH3

= 2.63 mol CO2

Think About It As always, check to be sure that units cancel properly in the calculation. Also, the balanced equation indicates that there will be fewer moles of urea produced than ammonia consumed. Therefore, your calculated number of moles of urea (2.63) should be smaller than the number of moles given in the problem (5.25). Similarly, the stoichiometric coefficients in the balanced equation are the same for carbon dioxide and urea, so your answers to this problem should also be the same for both species.

Page 27: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.6

Strategy For part (a), use the molar mass of nitrous oxide to convert the given mass of nitrous oxide to moles, use the appropriate stoichiometric conversion factor to convert to moles of ammonium nitrate, and then use the molar mass of ammonium nitrate to convert to grams of ammonium nitrate. For part (b), use the molar mass of nitrous oxide to convert the given mass of nitrous oxide to moles, use the stoichiometric conversion factor to convert from moles of nitrous oxide to moles of water, and then use the molar mass of water to convert to grams of water.

Dinitrogen monoxide (N2O), also known as nitrous oxide or “laughing gas,” is

used as an anesthetic in dentistry. It is manufactured by heating ammonium nitrate. The balanced equation is

NH4NO3(s) → N2O(g) + 2H2O(l)

(a) Calculate the mass of ammonium nitrate that must be heated in order to produce 10.0 g of nitrous oxide. (b) Determine the corresponding mass of water produced in the reaction.

Δ

Page 28: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.6 (cont.)

Solution (a) 10.0 g N2O ×

0.227 mol N2O ×

0.227 mol NH4NO3 ×

Thus, 18.2 g of ammonium nitrate must be heated in order to produce 10.0 g of nitrous oxide.

(b) Starting with the number of moles of nitrous oxide determined in the first step of (a), 0.227 mol N2O ×

0.454 mol H2O ×

1 mol N2O44.02 g N2O

= 0.227 mol N2O

1 mol NH4NO3

1 mol N2O= 0.227 mol NH4NO3

80.05 g NH4NO3

1 mol NH4NO3

= 18.2 g NH4NO3

2 mol H2O1 mol N2O

= 0.454 mol H2O

18.02 g H2O1 mol H2O

= 8.18 g H2O

Think About It Use the law of conservation of mass to check your answers. Make sure that the combined mass of both products is equal to the mass of reactant you determined in part (a). In this case (rounded to the appropriate number of significant figures), 10.0 g + 8.18 g = 18.2 g. Remember that small differences may arise as the result of rounding.

Page 29: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

The reactant used up first in a reaction is called the limiting reactant.

Excess reactants are those present in quantities greater than necessary to react with the quantity of the limiting reactant.

CO(g) + 2H2(g) → CH3OH(l)

Limiting ReactantsLimiting Reactants8.4

Page 30: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Limiting ReactantsLimiting Reactants

Consider the reaction between 5 moles of CO and 8 moles of H2 to produce methanol.

How many moles of H2 are necessary in order for all the CO to react?

How many moles of CO are necessary in order for all of the H2 to react?

10 moles of H2 required; 8 moles of H2 available; limiting reactant.

4 moles of CO required; 5 moles of CO available; excess reactant.

22 2

2 mol Hmoles of H = 5 mol CO = 10 mol H

1 mol CO

CO(g) + 2H2(g) → CH3OH(l)

22

1 mol COmoles of CO = 8 mol H = 4 mol CO

2 mol H

Page 31: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.7

Strategy Convert each of the reactant masses to moles. Use the balanced equation to write the stoichiometric conversion factor and determine which reactant is limiting. Next, determine the number of moles of excess reactant remaining and the number of moles of CO2 produced. Finally, use the appropriate molar masses to convert moles of excess reactant and moles of CO2 to grams.

Alka-Seltzer tablets contain aspirin, sodium bicarbonate, and citric acid. When

they come into contact with water, the sodium bicarbonate (NaHCO3) and citric

acid (H3C6H5O7) react to form carbon dioxide gas, among other products.

3NaHCO3(aq) + H3C6H5O7(aq) → 3CO2(g) + 3H2O(l) + Na3C6H5O7(aq)

The formation of CO2 causes the trademark fizzing when the tablets are dropped

into a glass of water. An Alka-Seltzer tablet contains 1.700 g of sodium bicarbonate and 1.000 g citric acid. Determine, for a single tablet dissolved in water, (a) which ingredient is the limiting reactant, (b) what mass of the excess

reactant is left over when the reaction is complete, and (c) what mass of CO2

forms.

Page 32: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.7 (cont.)

Solution 1.700 g NaHCO3 ×

1.000 g H3C6H5O7 ×

(a) To determine which reactant is limiting, calculate the amount of citric acid necessary to react completely with 0.02024 mol sodium bicarbonate.

0.02024 mol NaHCO3×

The amount of H3C6H5O7 required to react with 0.02024 mol of NaHCO3 is more than a tablet contains. Therefore, citric acid is the limiting reactant and sodium bicarbonate is the excess reactant.

1 mol NaHCO3

84.01 g NaHCO3

= 0.02024 mol NaHCO3

1 mol H3C6H5O7

192.12 g H3C6H5O7

= 0.005205 mol H3C6H5O7

1 mol H3C6H5O7

3 mol NaHCO3

= 0.006745 mol H3C6H5O7

Page 33: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.7 (cont.)

Solution (b) To determine the mass of excess reactant (NaHCO3) left over, first calculate the amount of NaHCO3 that will react:

0.005205 mol H3C6H5O7 ×

Thus, 0.01562 mole of NaHCO3 will be consumed, leaving 0.00462 mole unreacted. Convert the unreacted amount to grams as follows:

0.00462 mol NaHCO3 ×84.01 g NaHCO3

1 mol NaHCO3

= 0.388 g NaHCO3

3 mol NaHCO3

1 mol H3C6H5O7

= 0.01562 mol NaHCO3

Page 34: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.7 (cont.)

Solution (c) To determine the mass of CO2 produced, first calculate the moles of CO2 produced from the number of moles of limiting reactant (H3C6H5O7) consumed.

0.005205 mol H3C6H5O7 ×

Convert this amount to grams as follows:

0.01562 mol CO2×

To summarize the results: (a) citric acid is the limiting reactant, (b) 0.388 g sodium bicarbonate remains unreacted, and (c) 0.6874 g carbon dioxide is produced.

44.01 g CO2

1 mol CO2

= 0.6874 g CO2

3 mol CO2

1 mol H3C6H5O7

= 0.01562 mol CO2Think About It In a problem such as this, it is a good idea to check your work by calculating the amounts of the other products in the reaction. According to the law of conservation of mass, the combined starting mass of the two reactants (1.700 g + 1.000 g = 2.700 g) should equal the sum of the masses of products and leftover excess reactant. In this case, the masses of H2O and Na3C6H5O7 produced are 0.2815 g and 1.343 g, respectively. The mass of CO2 produced is 0.6874 g [from part (c)] and the amount of excess NaHCO3 is 0.388 g [from part (b)]. The total, 0.2815 g + 1.343 g + 0.6874 g + 0.388 g, is 2.700 g, identical to the total mass of reactants.

Page 35: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Limiting ReactantsLimiting Reactants

The theoretical yield is the amount of product that forms when all the limiting reactant reacts to form the desired product.

The actual yield is the amount of product actually obtained from a reaction.

The percent yield tells what percentage the actual yield is of the theoretical yield.

actual yield% yield = 100%

theoretical yield

Page 36: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.8

Strategy Convert reactant grams to moles, and determine which is the limiting reactant. Use the balanced equation to determine the moles of aspirin that can be produced and convert to grams for the theoretical yield. Use this and the actual yield given to calculate the percent yield.

Aspirin, acetylsalicylic acid (C9H8O4), is the most commonly used pain reliever in

the world. It is produced by the reaction of salicylic acid (C7H6O3) and acetic

anhydride (C4H6O3) according to the following equation:

In a certain aspirin synthesis, 104.8 g of salicylic acid and 110.9 g of acetic anhydride are combined. Calculate the percent yield if 105.6 g of aspirin are produced.

C7H6O3 salicylic acid

C4H6O3

acetic anhydride C9H8O4

acetylsalicylic acid HC2H3O2

acetic acid+ +→

Page 37: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.8 (cont.)

Solution

104.8 g C7H6O3×

110.9 g C4H6O3×

Because the two reactants combine in a 1:1 mole ratio, the reactant present in the smallest number of moles (in this case, salicylic acid) is the limiting reactant. According to the balanced equation, one mole of aspirin is produced for every mole of salicylic acid consumed.

Therefore, the theoretical yield of aspirin is 0.7588 mol. We convert this to grams using the molar mass of aspirin:

0.7588 mol C9H8O4×

1 mol C7H6O3

138.12 g C7H6O3

= 0.7588 mol C7H6O3

1 mol C4H6O3

102.09 g C4H6O3

= 1.086 mol C4H6O3

180.15 g C9H8O4

1 mol C9H8O4

= 136.7 g C9H8O4

Page 38: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Worked Example 8.8 (cont.)

Solution Thus, the theoretical yield is 136.7 g. If the actual yield is 105.6, the percent yield is

% yield = 105.6 g136.7 g ×100% = 77.25%

Think About It Make sure you have used the proper molar masses and remember that percent yield can never exceed 100 percent.

Page 39: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Periodic Trends in Chemical Properties of the Main Periodic Trends in Chemical Properties of the Main Group ElementsGroup Elements

Ionization Energy and Electron Affinity enable us to understand types of reactions that elements undergo and the types of compounds formed.

8.5

Page 40: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

General Trends in Reactivity

Hydrogen (1s1) Grouped by itself

Forms a cation with a +1 charge (H+)

Forms an anion with a -1 charge (H-)

Hydrides react with water to produce hydrogen gas and a base.

CaH2(s) + H2O(l) → Ca(OH)2(aq) + H2(g)

Page 41: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Reactions of the Active MetalsReactions of the Active Metals

Group 1A Elements (ns1, n ≥ 2) Low IE

Never found in nature in pure elemental state

React with oxygen to form metal oxides

Alkali metals reacting with water

sodium potassium cesium

Page 42: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Reactions of the Active MetalsReactions of the Active Metals

Group 2A Elements (ns2, n ≥ 2) Less reactive than 1A

Some react with H2O to produce H2

Some react with acid to produce H2

Barium reacting with water

Page 43: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Reactions of Other Main Group ElementsReactions of Other Main Group Elements

Group 3A elements (ns2np1, n ≥ 2) Metalloid (B) and metals (all others)

Al forms Al2O3 with oxygen

Al forms +3 ions in acid

Others form +1 and +3

Finely-divided aluminum sprinkled into a flame to form Al2O3

Page 44: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Reactions of Other Main Group ElementsReactions of Other Main Group Elements

Group 4A elements (ns2np2, n ≥ 2) Nonmetal (C); metalloids (Si, Ge) and others metals

Form +2 and +4 oxidation states

Sn, Pb react with acid to produce H2

Page 45: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Reactions of Other Main Group ElementsReactions of Other Main Group Elements

Group 5A elements (ns2np3, n ≥ 2) Nonmetal (N2, P), metalloid (As, Sb), and metal (Bi)

Nitrogen, N2, forms variety of oxides

Phosphorus, P4

As, Sb, Bi (crystalline)

HNO3 and H3PO4 important industrially

Page 46: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Reactions of Other Main Group ElementsReactions of Other Main Group Elements

Group 6A elements (ns2np4, n ≥ 2) Nonmetals (O, S, Se)

Oxygen, O2

Sulfur, S8

Selenium, Se8

Metalloids (Te, Po) Te, Po (crystalline)

SO2, SO3, H2S, H2SO4

Nonmetal oxides added to water produce an acid

Forest damaged by acid rain

Page 47: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Reactions of Other Main Group ElementsReactions of Other Main Group Elements

Group 7A elements (ns2np5, n ≥ 2) All diatomic

Do not exist in elemental form in nature

Form ionic “salts”

Form molecular compounds

with each other

React with hydrogen to form

hydrogen halides

Page 48: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Reactions of Other Main Group ElementsReactions of Other Main Group Elements

Group 8A elements (ns2np6, n ≥ 2) All monatomic

Filled valence shells

Considered “inert” until 1963 when Xe and Kr were used to form compounds

No major commercial use

Page 49: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Comparison of Group 1A and Group 1B ElementsComparison of Group 1A and Group 1B Elements

Have single valence electron

Properties differ

Group 1B much less reactive than 1A

High IE of 1B - incomplete shielding of nucleus by inner “d” -outer “s” electron of 1B strongly attracted to nucleus

1B metals often found elemental in nature (coinage metals)

Page 50: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Chapter Summary: Key PointsChapter Summary: Key Points8

Chemical EquationsInterpreting and Writing Chemical EquationsBalancing Chemical EquationsPatterns of Chemical Reactivity

Combustion AnalysisDetermination of Empirical Formula

Calculations with Balanced Chemical EquationsMoles of Reactants and ProductsMass of Reactants and Products

Limiting ReactantsDetermining the Limiting ReactantReaction Yield

Periodic Trends in Reactivity of the Main Group ElementsGeneral Trends in ReactivityReactions of the Active MetalsReactions of the Other Main Group ElementsComparison of Group 1A and Group 1B Elements

Page 51: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

• Balance the following equations:• __Fe2(SO4)3 + __K3PO4 __K2SO4 +

__FePO4

• __Hg(NO3)2 + __ KI __HgI2 + __ KNO3

• __Li2O(s) + __ H2O __LiOH

• __HBr + __Ba(OH)2 __BaBr2 + __H2O

• __K2PtCl4 + __NH3 __Pt(NH3)2Cl2 + __KCl

Group Quiz #14

Page 52: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

Group Quiz #15

• How many atoms are in 12.987 g of iron?

• How many H atoms are in a 3.4215 gram sample of H2O?

Page 53: Chemistry: Atoms First Julia Burdge & Jason Overby Chapter 8 Chemical Reactions Kent L. McCorkle Cosumnes River College Sacramento, CA

• Complete the equation below (with phases) and balance it.

• K2S(aq) + AgNO3(g)

• If you combine 10.21 mL of 0.152 M K2S with 1.0092 g AgNO3, how much solid product can be formed?

• If 0.2744 g of product were actually formed, what is the percent yield?

Group Quiz #16