1 Chemical Reactions Chapter 4 Stoichiometry. 2 Chemical Equations æ A chemical reaction shows the...

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ChemicalChemicalReactionsReactions

Chapter 4

Stoichiometry

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Balancing Chemical EquationsBalancing Chemical Equations

““Matter is conserved inMatter is conserved in

chemical change”chemical change”

Antoine Lavoisier, 1789Antoine Lavoisier, 1789

An An equation must be balanced:equation must be balanced:

It must have the same It must have the same numbernumber

of atoms of each kind on both sidesof atoms of each kind on both sides

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Combustion ReactionsCombustion Reactions

In combustion, a hydrocarbon or C–H–O fuel combines with O2 to form CO2 and H2O

__ CH4 + __ O2 __ CO2 + __ H2O

1 CH4 + 2 O2 1 CO2 + 2 H2O

Balanced equation shows 1 C, 4 H, and 4 O on each side If N or S are in the formula for the fuel, assume it

is oxidized to NO2 or SO2

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Example 4-2Example 4-2

Write a balanced equation for the complete combustion of glycerol, C3H8O3

Write a balanced equation for the complete combustion of thiosalicylic acid, C7H6O2S

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StoichiometryStoichiometry

Stoichiometry is chemical accounting The heart of stoichiometry is the mole ratio given

by the coefficients of the balanced equation

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StoichiometryStoichiometry

Stoichiometry is chemical accounting The heart of stoichiometry is the mole ratio given

by the coefficients of the balanced equation

moles A moles Bmole ratiomoles Bmoles A

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Example 4-3BExample 4-3B

How many moles of Ag are produced in the decomposition of 1.00 kg of silver (I) oxide:2 Ag2O (s) 4 Ag (s) + O2 (g)

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Example 4-6BExample 4-6B The model problem describes

an Al-Cu alloy composed of 93.7% Al and 6.3% Cu by mass, with a density of 2.85 g/cm3. The Al (but not the Cu) reacts with HCl:

2 Al (s) + 6 HCl (aq) 2 AlCl3 (aq) + 3 H2 (g)

How many grams of Cu are present in a sample of alloy that yields 1.31 g H2 when it reacts with HCl?

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Example 4-7BExample 4-7B

A vinegar contains 4.0% HC2H3O2 by mass and has a density of 1.01 g/mL. It reacts with sodium hydrogen carbonate:HC2H3O2 (aq) + NaHCO3 (s)

NaC2H3O2 (aq) + H2O (l) + CO2 (g)

How many grams of CO2 are produced by the reaction of 5.00 mL of this vinegar with NaHCO3?

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Chemical Reactions in SolutionChemical Reactions in Solution

Most reactions occur in aqueous solution SOLUTE is the substance to be dissolved in solution SOLVENT is the substance (often a liquid) the solute

dissolves in The concentration of the solution is

Molarity (M) = moles solute L solution

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Example 4-8BExample 4-8B

15.0 mL of concentrated acetic acid, HC2H3O2 (d = 1.048 g/mL), are dissolved in enough water to produce 500.0 mL of solution. What is the concentration of the solution?

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Example 4-9BExample 4-9B

How many grams of Na2SO4 • 10 H2O are needed to prepare 355 mL of 0.445 M Na2SO4?

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Dilution problemsDilution problems

It is common to prepare a solution by diluting a more concentrated solution (the stock solution).

The moles of solute taken from the stock solution are given by moles solute = volume x molarity

All the solute taken from the stock appears in the diluted solution, so moles solute are constant:

VstockMstock = VdiluteMdilute

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Example 4-10AExample 4-10A

15.00 mL of 0.450 M K2CrO4 solution are diluted to 100.00 mL. What is the concentration of the dilute solution?

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Example 4-10BExample 4-10B

After being left out in an open beaker, 275 mL of 0.105 M NaCl has evaporated to only 237 mL. What is the concentration of the solution after evaporation?

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Stoichiometry in SolutionStoichiometry in Solution

Stoichiometry in solution is just the same as for mass problems, except the conversion into or out of moles uses molarity instead of molar mass:

grams A

mL A

moles A moles B

grams B

mL B

mole ratiomoles Bmoles A

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Example 4-11BExample 4-11B

K2CrO4 (aq) + 2 AgNO3 (aq) Ag2CrO4 (s) + 2 KNO3 (aq)

How many mL of 0.150 M AgNO3 must react with excess K2CrO4 to produce exactly 1.00 g Ag2CrO4?

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Limiting reactantLimiting reactant

In a given reaction, often there is not enough of In a given reaction, often there is not enough of one reactant to use up the other reactant one reactant to use up the other reactant completelycompletely

The reactant in short supply The reactant in short supply LIMITSLIMITS the quantity the quantity of product that can be formedof product that can be formed

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Goldilocks ChemistryGoldilocks Chemistry

Imagine reacting different amounts of Zn with 0.100 mol HCl:Zn (s) + 2 HCl (aq) ZnCl2 (aq) + H2 (g)

Rxn 1 Rxn 2 Rxn 3Mass Zn 6.54 g 3.27 g 1.31 gMoles Zn 0.100 mol 0.0500 mol 0.0200 molMoles HCl 0.100 mol 0.100 mol 0.100 molRatio mol HCl 1.00 2.00 5.00

mol Zn

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Limiting reactant problemsLimiting reactant problems

The easiest way to do these is to do two stoichiometry calculations Find the amount of product possible from each reactant

The smaller answer is the amount of product you can actually make (you just ran out of one reactant)

The reactant on which that answer was based is the limiting reactant

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Example 4-13AExample 4-13A

When 215 g P4 react with 725 g Cl2

P4 (s) + 6 Cl2 (g) 4 PCl3 (l) (example 4-12A)

which reactant is in excess and what mass of that reactant remains after the reaction is finished?

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Example 4-13BExample 4-13B

12.2 g H2 and 154 g O2 are allowed to react. Identify the limiting reactant, which gas remains after the reaction, and what mass of it is left over.

2 H2 (g) + O2 (g) 2 H2O (l)

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Percent YieldPercent Yield

In real experiments we often do not get the amount of product we calculate we should, because the reactants may participate in other reactions (side

reactions) that produce other products (by-products) The reaction often does not go to completion.

Percent yield tells the ratio of actual to theoretical amount formed.

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Percent YieldPercent Yield

Suppose you calculate that a reaction will produce 50.0 g of product. This is the theoretical yield.

The reaction actually produces only 45.0 g of product . This is the actual yield.

Percent yield = 45.0 g (actual) x 100 = 90.0% 50.0 g (theoretical)

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Example 4-14BExample 4-14B

What is the percent yield if 25.0 g P4 reacts with 91.5 g Cl2 to produce 104 g PCl3:P4 (s) + 6 Cl2 (g) 4 PCl3 (l)

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Example 4-15BExample 4-15B

What mass of C6H11OH should you start with to produce 45.0 g C6H10 if the reaction has 86.2% yield and the C6H11OH is 92.3% pure: C6H11OH (l) C6H10 + H2O (l)

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Exercise 26Exercise 26

Balance these equations by inspection (NH4)2Cr2O7 (s) Cr2O3 (s) + N2 (g) + H2O (g)

NO2 (g) + H2O (l) HNO3 (aq) + NO (g)

H2S (g) + SO2 (g) S (g) + H2O (g)

SO2Cl2 + HI H2S + H2O + HCl + I2

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Exercise 30Exercise 30

Write balanced equations for these reactions: Sulfur dioxide gas with oxygen gas to produce sulfur

trioxide gas Solid calcium carbonate with water and dissolved

carbon dioxide to produce aqueous calcium hydrogen carbonate

Ammonia gas and nitrogen monoxide gas to produce nitrogen gas and water vapor

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Exercise 32Exercise 32

3 Fe (s) + 4 H2O (g) Fe3O4 (s) + H2 (g) How many moles of H2 can be produced from 42.7 g Fe

and excess steam? How many grams of H2O are consumed in the

conversion of 63.5 g Fe to Fe3O4?

If 7.36 mol H2 are produced, how many grams of Fe3O4 must also be produced?

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Exercise 36Exercise 36

Silver oxide decomposes above 300 °C to yield metallic silver and oxygen gas. 3.13 g impure silver oxide yields 0.187 g O2. Assuming there is no other source of O2, what is the % Ag2O by mass in the original sample?

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Exercise 42Exercise 42

How many grams of CO2 are produced in the complete combustion of 406 g of a bottled gas that consists of 72.7% C3H8 (propane) and 27.3% C4H10 (butane), by mass?

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Exercise 45Exercise 45

What are the molarities of these solutes? 150.0 g sucrose (C12H22O11) in 250.0 mL aqueous

solution 98.3 mg of 97.9% pure urea, CO(NH2)2, in 5.00 mL

aqueous solution 12.5.0 mL methanol (CH3OH, density = 0.792 g/mL) in

15.0 L aqueous solution

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Exercise 52Exercise 52

After 25.0 mL of aqueous HCl solution is diluted to 500.0 mL, the concentration of the diluted solution is found to be 0.085 M HCl. What was the concentration of the original HCl solution?

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Exercise 56Exercise 56

Ca(OH)2 (s) + 2 HCl (aq) CaCl2 (aq) + 2 H2O (l) How many grams of Ca(OH)2 will react completely with 415 mL

of 0.477 M HCl? How many kilograms of Ca(OH)2 will react with 324 L of an

HCl solution that is 24.28% HCl by mass, density = 1.12 g/mL?

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Exercise 63Exercise 63

0.3126 g oxalic acid, H2C2O4, is exactly neutralized by 26.21 mL of a NaOH solution. What is the concentration of the NaOH solution?

H2C2O4 + 2 NaOH Na2C2O4 + 2 H2O

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Exercise 70Exercise 70

Chlorine can be generated by heating calcium hypochlorite and hydrochloric acid to form chlorine gas, calcium chloride, and water. If 50.0 g Ca(OCl)2 and 275 mL 6.00 M HCl react, how many grams of Cl2 gas form? Which reactant is left over, and how much (in grams)?

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Exercise 72Exercise 72

2 C6H5NO2 + 4 C6H14O4 (C6H5N)2 + 4 C6H12O4 + 4 H2O

nitrobenzene triethylene azobenzene glycol

If 0.10 L nitrobenzene (d = 1.20 g/mL) react with 0.30 L triethylene glycol (d = 1.12 g/mL) to form 55 g azobenzene, find Theoretical yield Actual yield Percent yield