Stoichiometry

Chemistry 6.0

The Mathematics of Chemical Reactions: STOICHIOMETRY

I. Balanced Chemical Equations A. Provide qualitative and quantitative information B. Supports the Law of Conservation of Matter

2H2 + O2 2H2O

The above equation is interpreted in terms of particles as follows:

1. 2 molecules of H2 react with 1 molecule of O2

to produce 2 molecules of water.

The ratio of H2 to O2 is 2:1.

or 2 moles of H2 react with 1 mole of O2 to produce

2 moles of water. The ratio of H2 to O2 is 2:1.

2. It is more convenient to interpret the coefficients as number of moles, because we measure amounts of substances by massing.

C. Stoichiometry 1. The study of the quantitative relationships that exist in a formula or a chemical reaction. 2. Importance

a. Provides for the safe, economical and reproducible manufacture of chemicals.b. Provides for the safe administration of pharmaceuticals.

D. Proof of the conservation of matter in a balanced equation 1. Convert all reactants and products to their mass equivalents. 2. Sum up the mass of reactants and compare the sum of the mass of products.

II. Stoichiometry Problems

A. Steps to Solve Problems

1. Write a balanced equation.

2. Identify the given ( ) and the unknown or required substance (?).3. Convert mass of given into moles.

4. Use the mole (molar) ratio to convert from given to required substance.5. If needed, convert moles of required into mass of required substance.

B. Examples

1. How many moles of oxygen are required to react with 16 moles of hydrogen in the production of water?

H2 + O2 H2O 2 2 ?

1 mole O2 2 moles H2

= 8.0 moles O2

Mole ratio links 1 substance to another in a reaction. Required in problem solving.

16 moles H2

2. Antimony reacts with water to produce antimony(III) oxide

and hydrogen. How many moles of hydrogen are produced

from 7.5 moles of antimony?

Sb + H2O Sb2O3 + H22 3 3

11 mol H2

3. What mass of aluminum oxide can be prepared by the reaction of 67.5 g of aluminum in a synthesis reaction?

Al + O2 Al2O34 3 2

128 mol Al2O3

4. Sodium bicarbonate, a.k.a. baking soda, can be used to extinguish a fire. When heated, it decomposes to give carbon dioxide gas which smothers the fire. It also produces sodium carbonate and water. If a sample contains 4.0 g of sodium bicarbonate, what mass of carbon dioxide is produced?

2 NaHCO3 Na2CO3 + H2O + CO2

III. Percent Yield

A. Expected Yield: the amount of product that should be produced (theoretical)

B. Actual Yield: the amount of product that is actually produced (experimental)

C. Percent Yield: percent of expected yield that was obtained

% Yield = (actual yield/expected yield) x 100

D. Steps to Solving Percent Yield Problems

1. Write a balanced equation2. Identify the given () which is the mass of

reactant, and identify the actual yield.3. Solve for the expected mass of product using

the given mass of reactant.4. Calculate the % yield.

% Yield = actual yield x 100 expected yield

E. Examples

1. A reaction between 2.80 g aluminum nitrate and excess sodium hydroxide produced 0.966 g of aluminum hydroxide in this double replacement reaction. Calculate the % yield.

Al(NO3)3 + 3NaOH Al(OH)3 + 3NaNO3

1.03 g Al(OH)3

2. Determine the % yield for the reaction between 3.74 g of

sodium and excess oxygen if 4.24 g of sodium oxide is recovered in the direct combination reaction.

4Na + O2 2Na2O

IV. Limiting Reactants

A. Definition: the reactant that determines, or limits, the amount of product(s) formed in a chemical reaction

B. Problem Solving Tips1. The limiting reactant is not necessarily the reactant

present in the smallest amount 2. When you are given the amounts of 2 or more

reactants, you should suspect that you are dealing with a limiting reactant problem.

C. Steps1. Write a balanced equation2. Calculate the number of moles of each

reactant3. Compare the mole ratios of the reactants as

available ratio (from the given masses) and the required ratio (from the coefficients)

4. Identify the limiting reactant, and use it to calculate the mass of product formed.

D. Examples

1. What mass of CO2 could be formed by the combustion of 16.0 g CH4 with 48.0 g O2?

CH4 + 2O2 CO2 + 2H2O

33.0 g CO2

Oxygen is the limiting reactant

2. What is the maximum mass of nickel(II) hydroxide that could be prepared by mixing 25.9 g nickel(II) chloride with 10.0 g sodium hydroxide?

NiCl2 + 2NaOH Ni(OH)2 + 2NaCl

11.6 g Ni(OH)2

Sodium Hydroxide is the LR

V. Solution StoichiometryA. Many reactants are introduced to a reaction chamber

as a solution.B. The most common solution concentration is molarity.

molarity = mol/liter

C. Examples1. Excess lead(II) carbonate reacts with 27.5 mL of 3.00M nitric

acid. Calculate the mass of lead(II) nitrate formed

PbCO3 + 2HNO3 Pb(NO3)2 + H2CO3

2. Calculate the volume, in mL, of a 0.324 molar solution of sulfuric acid required to react completely with 2.792 g of sodium carbonate according to the equation below.

H2SO4 + Na2CO3 Na2SO4 + CO2 + H2O

VI: Acid-Base Titration1. An acid-base titration is a carefully controlled

neutralization reaction or redox which can determine concentration of an unknown solution.

2. To determine the concentration of an unknown substance, a standard solution is needed. This solution has a known concentration.

3. Titration curve: graph that

shows how pH changes

during a titration.

4. An indicator, usually phenolphthalein, is used in a titration.

• Colorless in an acid, pink in a base.

5. The point at which enough standard solution is added to neutralize the unknown solution is called the equivalence point.

6. The point at which the indicator changes color is called the endpoint.

7. Therefore: [H+] = [OH-] at the equivalence point

22

Ex) Solutions of sodium hydroxide are used to unclog drains. A 43.0 mL volume of sodium hydroxide was titrated with 32.0 mL of 0.100 M HCl. What is the molarity of the sodium hydroxide solution?

Note: Mole ratio of the acid and base is 1:1

HCl (Acid) = NaOH (Base)

0.0744 M = Mb

HCl + NaOH NaCl + HOH

Ex) A volume of 25.0 mL of 0.120M sulfuric acid neutralizes 40.0mL of a sodium hydroxide solution. What is the concentration of the sodium hydroxide solution?

H2SO4 + NaOH Na2SO4 + HOHNote: Mole ratio between acid an base in not

1:1

2 2

Mb = 0.150M

VII. Stoichiometry and Heat Changes1. How much heat is released when 22.0g of

propane is burned? C3H8 + 5O2 → 3CO2 + 4H2O ΔH = -2.22x103 kJ

2. How much carbon dioxide is produced, in grams, when 2,500 kJ of energy is released?

-1.11x103 kJ (released)

150 g CO2

The standard enthalpy change of a reaction is equal to the sum of the standard molar enthalpies of formation of the products multiplied by its coefficient, n, in the balanced equation, minus the corresponding sum of standard molar enthalpies of formation of reactants.

Hrxn = ∑ nHf,products - ∑ nHf,reactants

VIII. Determining ∆H Using Heats of Formation

ΔH from ΔHf Problem:

Using the Heats of Formation Table, calculate the H for the following reaction:

SF6(g) + 3H2O(l) 6HF(g) + SO3(g)

Write the thermochemical equation for this reaction:

SF6 + 3H2O + 45.1kJ 6HF + SO3

45.1 kJ

ΔH from ΔHf Problem:

Using the Heats of Formation Table, calculate the standard heat of combustion for propane.C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g)

Write the thermochemical equation for this reaction:

C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(g) + 2043.9kJ

-2043.9 kJ