Ch 4. Chemical Quantities and Aqueous Reactions

Ch 4. Chemical Quantities and Aqueous Reactions

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)

1 mol 2 mol 1 mol 2 mol

Stoichiometry of the reaction

FIXED ratio for each reaction

Can calculate how much other chemicals are requiredor produced if the amount of one chemical is known.

CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)




5.22 mol 10.44 mol 5.22 mol 10.44 mol

4 NH3 (g) + 5 O2 (g) 4 NO (g) + 6 H2O (g)

5.02 mol of NH3 is used in the above reaction.

How many moles of O2 is required to react with all the NH3?

How many moles of H2O will be produced?

2NH3(g) + 3CuO(s) N2(g) + 3Cu(s) + 3H2O(g)

2 mol 3 mol 1 mol 3 mol 3 mol

5.02 mol x y z u

3

2

x

mol 5.02

mol 3

mol 2 mol 7.53x

y

mol 5.02

1

2 mol 2.51 y

z

mol 5.02

3

2 mol 7.53z

u

mol 5.02

3

2 mol 7.53u


2 mol 3 mol 1 mol 3 mol 3 mol

6.04 g

x y

x

mol 0.355

3

2 mol 0.533x

y

mol 0.355

1

2 mol 0.178 y

6.04 g ÷ (14.01 g/mol x 1 + 1.008 g/mol x 3) = 0.355 mol

0.355 mol

mass? mass?

Mass of CuO = 0.533 mol x 79.55 g/mol = 42.4 g

Mass of N2 = 0.178 mol x 28.02 g/mol = 4.99 g

Other Examples: page 131 ― 132

CH4 + 2O2 CO2 + 2H2O

0 mol 0 mol1 mol 2 molinitial:

0 mol 0 mol 1 mol 2 molfinal:

1 mol 1 mol 0 mol 0 molinitial:

? mol ? mol ? mol ? molfinal:

The actual amount of reactants consumed and actual amountof products generated agree with the stoichiometry.

CH4 + 2O2 CO2 + 2H2O


1 mol CH4 requires 2 mol O2, available O2 is 1 mol: limiting reagent.

(1 − 0.5) mol 0 mol = 0.5 mol = 1 mol

Result: 1 mol O2 will be consumed completely and CH4 will have leftover: excess reagent.

= 0.5 molconsumed: 1 molx

mol 1

x

2

1 x = 0.5 mol

final: y z

The reactant of which there are fewer moles than the stoichiometry requires is the limiting reagent.

The reactant of which there are more moles than thestoichiometry requires is the excess reagent.

Chemical reactions always occur according to thestoichiometry, therefore the limiting reagent is consumedand the excess reagent has leftover. The amount of productsis determined by the amounts of reagents that are actually consumed.

CH4 + 2O2 CO2 + 2H2O

limiting reagentexcess reagent


consumed: 0.5 mol 1 mol

(1 − 0.5) molfinal: 0 mol 0.5 mol 1 mol

0.5 : 1 : 0.5 : 1

1 : 2 : 1 : 2=

++

+++ +

+ +

2 slices of bread + 1 slice if ham 1 sandwich

4 slices of bread + 1 slice if ham

excess reagent excess reagentleftover

limiting reagent amount of product

1 sandwich + 2 slices of bread

For the following reaction, if a sample containing 18.1 g of NH3

is reacted with 90.4 g of CuO, which is the limiting reagent? How

many grams of N2 will be formed?

How many grams of excess reagent will be leftover?

If 6.63 g of N2 is actually produced, what is the percent yield?

% 100 yieldltheoretica

yieldactual eldpercent yi


1) Make sure the equation is balanced.2) Find the moles of each reactant:

moles = mass in gram / molar mass3) Pick up any reactant, say A, and use the stoichiometry to

calculate the required amount of the other reactant B.4) Compare the required amount of B with the available

amount of B.a) If required > available, then B is the limiting reagent and Ais the excess reagent.b) If required < available, then B is the excess reagent and Ais the limiting reagent.

5) Use the amount of the limiting reagent and the stoichiometryto calculate the amount of any product and the amount of theexcess reagent that has been consumed.

6) Leftover excess reagent = available − consumed7) If actual yield is given

percent yield = (actually yield / theoretical yield) x 100%

Procedure for limiting/excess reagent calculationsaA + bB cC + dD

68.5 g CO reacts with 8.60 g H2 in the following reaction.

What is the limiting reagent? How many grams of excess

reagent is leftover? What is the theoretical yield of CH3OH?

If 35.7 g CH3OH is actually produced, what is the percent

yield of CH3OH?

H2(g) + CO(g) CH3OH(g)

1) Make sure the equation is balanced.2) Find the moles of each reactant:

moles = mass in gram / molar mass3) Pick up any reactant, say A, and use the stoichiometry to

calculate the required amount of the other reactant B.4) Compare the required amount of B with the available

amount of B.a) If required > available, then B is the limiting reagent and Ais the excess reagent.b) If required < available, then B is the excess reagent and Ais the limiting reagent.

5) Use the amount of the limiting reagent and the stoichiometryto calculate the amount of any product and the amount of theexcess reagent that has been consumed.

6) Leftover excess reagent = available − consumed7) If actual yield is given

percent yield = (actually yield / theoretical yield) x 100%

Procedure for limiting/excess reagent calculationsaA + bB cC + dD

1.50 g of ammonia reacts with 2.75 g of oxygen gas to produce nitrogen monoxide and water.

NH3(g) + O2(g) NO(g) + H2O(g)

a) Balance the equation.b) What is the mass of O2 in grams required by NH3?c) Which reactant is the limiting reagent? d) How many grams of NO will be produced in theory?e) How many grams of H2O will be produced in theory?f) How many grams of the excess reagent remain unreacted?g) If only 1.80 g of NO are produced, what is the percent yield?

Classification of Matter

Matter

Elements

Compounds

Mixtures(multiple components)

Pure Substances(one component)

Homogeneous(visibly indistinguishable)

Heterogeneous (visibly distinguishable)

(Solutions)

Solute + Solvent Solution

Solvent = water, aqueous solution

Water can dissolve many substances

O

H H

H2O

C12H22O11

electrolytes

nonelectrolytes

Based on the electrical conductivity in aqueous solution

strong electrolytes

weak electrolytes

salts

strong acids

strong bases

weak acids

weak basessolutes

strong electrolytes: dissociate 100 % into ions

weak electrolytes: only a small fraction dissociate into ions

nonelectrolytes: no dissociation

salts: NaCl, Na2SO4, Fe(ClO4)3 ……

strong acids: HCl, HNO3, H2SO4, HClO4

strong bases: NaOH, KOH

Base: compounds that give OH− when dissolved in water.

weak acids: acetic acid: HC2H3O2

weak bases: ammonia: NH3

remember

remember

HCl is Completely Ionized

Aqueous Solution of NaOH

Reaction of NH3 in Water

concentrations

% 100 sample wholeof mass

component of mass percent mass

% 100 solution of mass

solute of mass percent mass

no unit

10. g of sugar is dissolved in 40. g of water.

What is the mass percent of sugar in this solution?

moles of soluteMolarity (M)

liters of solution

Unit: mol/L or M

0.50 mol of KBr is dissolved in water and forms a solution of 12 L. What is the molarity of the solution?

Example 4.5, page 141

25.5 g of KBr is dissolved in water and forms a solution of 1.75 L. What is the molarity of the solution?


How many liters of a 0.125 mol/L NaOH solution contains 0.255 mol of NaOH?

moles of soluteMolarity (M)

liters of solution

How to prepare 1.00 L of NaCl aqueous

solution with a molarity of 1.00 mol/L?

1.00 mol NaCl + 1.00 L of H2O = 1.00 mol/L NaCl (aq)

Solution Dilution

Concentrated solutions for storage, called stock solutions

stock solution + water desired solution

solution of liters

solute of moles (M)Molarity

moles of solute before dilution = moles of solute after dilution

M1V1 = M2V2

M1: molarity of concentrated solutionV1: volume of concentrated solutionM2: molarity of diluted solutionV2: volume of diluted solution

Example on page 143

A lab procedure calls for 3.00 L of a 0.500 mol/L CaCl2solution. How should we prepare it from a 10.0 mol/L stocksolution?


To what volume should you dilute 0.200 L of a 15.0 mol/LNaOH solution to obtain a 3.00 mol/L NaOH solution?

Types of reactions

Precipitation reactions

NaCl(aq) + AgNO3(aq) AgCl(s) + NaNO3(aq)

formula equation

Na+(aq) + Cl−(aq) + Ag+(aq) + NO3−(aq) AgCl(s) + Na+(aq) + NO3

−(aq)

complete ionic equation

Cl−(aq) + Ag+(aq) AgCl(s)

net ionic equation

Na+(aq) + Cl−(aq) + Ag+(aq) + NO3−(aq) AgCl(s) + Na+(aq) + NO3

−(aq)

spectator ions

EXAMPLE 4.9 Predicting whether an Ionic Compound Is Soluble

Predict whether each compound is soluble or insoluble.

(a) PbCl2 (b) CuCl2 (c)Ca(NO3)2 (d) BaSO4

BaCl2(aq) + K2SO4(aq)

BaCl2(aq) Ba2+(aq) + 2Cl−(aq)

K2SO4(aq) 2K+ (aq) + SO42− (aq)

BaSO4(s) + 2KCl(aq)

Ba2+(aq) + 2Cl−(aq) + 2K+ (aq) + SO42− (aq) BaSO4(s) + 2Cl−(aq) + 2K+(aq)

Ba2+(aq) + SO42− (aq) BaSO4(s)

Fe(NO3)3(aq) + KOH(aq)

Fe(NO3)3(aq) Fe3+(aq) + 3NO3−(aq)

KOH(aq) K+ (aq) + OH− (aq)

Fe3+(aq) + 3NO3−(aq) +3K+ (aq) +3OH− (aq) Fe(OH)3(s) + 3NO3

−(aq) + 3K+(aq)

Fe3+(aq) + 3OH− (aq) Fe(OH)3(s)

3KOH(aq) 3K+ (aq) + 3OH− (aq)

Fe(NO3)3(aq) + 3KOH(aq) Fe(OH)3(s) + 3KNO3(aq)

BaCl2(aq) + KNO3(aq)

BaCl2(aq) Ba2+(aq) + 2Cl−(aq)

KNO3(aq) K+ (aq) + NO3− (aq)

BaCl2(aq) + 2KNO3(aq) Ba(NO3)2(aq) + 2KCl(aq)

Ba2+(aq) + 2Cl−(aq) + 2K+ (aq) + 2NO3− (aq)

Ba2+(aq) + 2NO3− (aq) + 2Cl−(aq) + 2K+(aq)

2KNO3(aq) 2K+ (aq) + 2NO3− (aq)

Types of reactions


Acid-base reactions

Acid: Substance that produces H+ ions in aqueous solution

Base: Substance that produces OH− ions in aqueous solution

NaOH(aq) Na+(aq) + OH−(aq)

H+(aq) + Cl−(aq) +Na+(aq) +OH−(aq) H2O(l) + Na+(aq) + Cl−(aq)

H+(aq) + OH−(aq) H2O(l)

HCl(aq) H+(aq) + Cl−(aq)

HCl(aq) + NaOH(aq) H2O(l) + NaCl(aq)

H+(aq) + OH−(aq) H2O(l)

acidic basic neutral

neutralization

HCl(aq) H+(aq) + Cl−(aq)

What is the molarity ofHCl(aq) or H+(aq)?

When reaction completes

nNaOH = nHCl

MNaOHVNaOH = MHClVHCl


prepared,known

measuredby buret, known

unknown measured bypippet, known


Read acid-base titration starting on page 158 and the online instructionfor next week’s titration

Example 4.14

The titration of a 10.00-mL sample of an HCl solution of unknown concentration requires 12.54 mL of a 0.100 M NaOH solution to reach the equivalence point. What is the concentration of the unknown HCl solution in M?



Types of reactions


Acid-base reactions

Oxidation-Reduction reactions

Reactions that involve electron transfer are called

oxidation-reduction reactions, or redox reactions.

2Mg(s) + O2(g) 2MgO(s)

Oxidation numbers (states)

1) For atoms in its elemental form, oxidation number = 0

A way to keep track of the electrons gained or lost

Na, Ag, Ar, O2, N2, P4

2) For monatomic ion, oxidation number = charge of the ion

Na+, Ca2+, Co2+, Co3+, Cl−, O2−

NaCl, Na2O, CaCl2, CaO, CoCl2, CoCl3, Co2O3, CoO

O

H H

H2O

3) In covalent compounds

O: −2 H: +1 F: −1

In a neutral compound, the sum of the oxidation number = 0

In a polyatomic ion, the sum of the oxidation number = ion charge

CO, CO2, SF6, SF4, H2S, NH3, P2O5, N2O3

NO3−, SO4

2−, NH4+, Cr2O7

2−, MnO4−

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Ch 4. Chemical Quantities and Aqueous Reactions