AP Chemistry Chapter 3

Stoichiometry

Stoichiometry

• Is the study of the quantities of substances consumed and produced in chemical reactions

• Derived from the Greek words stoicheion meaning “element” and metron meaning “measure”

Law of Conservation of Mass

• Discovered by Antoine Lavoisier in the late 1700’s– Found that the masses of substances after a

chemical reaction are equal to the masses of substances before the reaction

• Atoms are neither created nor destroyed in a chemical reaction– Reactions rearrange atoms in order to form new

substances

Chemical Equations

2H2 + O2 2H2O

products

coefficient“reacts with”

“produces”

reactants

subscript

Indicates relative numbersof molecules/atoms

Indicates number of atomsin a molecule

Balancing Equations

• To satisfy the law of conservation of mass

• Done by adding or changing coefficients

• NEVER balance an equation by changing subscripts!

Balancing Equations Helpful Hints:

1. Start balancing with the elements that appear in the fewest chemical formulas on each side of the equation

2. Balance oxygen last3. Balance hydrogen next to last

Sample Exercise 3.1The following diagram represents a chemical reaction in which the red spheres are oxygen atoms and the blue spheres are nitrogen atoms. (a) Write the chemical formula for the reactants and the products. (b) Write a balanced equation for the reaction. (c) Is the diagram consistent with the law of conservation of mass?

State Symbols

• Used to indicate the states of reactants and products in chemical reactions– (g)– (l)– (s)– (aq)

• ∆ above the arrow indicates addition of heat to the reaction

Sample Exercise 3.2Balance this equation: Na(s) + H2O(l) NaOH(aq) + H2(g)

Simple Patterns in ReactionsCombination (Synthesis) Decomposition

2 or more substances react to form one product

one substance undergoes a reaction to produce 2 or more other substances

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

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

EXAMPLES:2NaN3(s) 2Na(s) + 3N2(g)

CaCO3(s) CaO(s) + CO2(g)

EXAMPLES:

Decomposition Reactions

• Many compounds undergo decomposition reactions when heated.

• Metal carbonates often decompose to form a metal oxide and carbon dioxide gas.

Sample Problem 3.3Write balanced equations for the following reactions: (a) The combination reaction that occurs when lithium metal and fluorine gas react. (b) The decomposition reaction that occurs when solid barium carbonate is heated.(Two products form: a solid and a gas.)

Combustion in Air

• Reaction with oxygen

• Rapid reaction that produces a flame

• Combustion of hydrocarbons produce CO2 and H2O– Not enough oxygen available? CO formed– Severe lack of oxygen? Solid carbon (soot) formed

Oxidation vs. Combustion

• Conversion of glucose in the body happens at body temperature and without the production of a flame…this is oxidation rather than combustion.

C6H12O6 + 6O2 6CO2 + 6H2O

Sample Exercise 3.4Write the balanced equation for the reaction that occurs when methanol, CH3OH(l), is burned in air.

Formula Weights

• Sum of atomic weights of each atom in the chemical formula of a compound

• Example: H2SO4 has a formula weight of 98.1 amu

• Atomic weight (for atoms) and molecular weight (for molecular compounds)

Sample Exercise 3.5Calculate the formula weight of (a) sucrose, C12H22O11, and (b) calcium nitrate, Ca(NO3)2

Percentage Composition

• Percent by mass contributed by each element in the substance

Sample Exercise 3.6Calculate the percentage of carbon, hydrogen, and oxygen in C12H22O11.

Avogadro’s Number & the Mole

6.022 x 1023

Indicates the number of particles (atoms, ions, or molecules) in a mole

FYI: mole comes from Latin and means “a mass”

molecule is the diminutive form of the word and means “a small mass”

Sample Exercise 3.7Without using a calculator, arrange the following samples in order of increasing numbers of carbon atoms: 12 g 12C, 1 mol C2H2, 9 x1023 molecules of CO2.

Molar Mass

• Mass in grams of one mole of a substance

• Units are g/mol

• Numerically equivalent to formula weight

Sample Exercise 3.9What is the mass in grams of 1.00 moles of glucose, C6H12O6?

Sample Exercise 3.10Calculate the number of moles of glucose, C6H12O6, in 5.30 g of C6H12O6.

Sample Exercise 3.11Calculate the mass, in grams, of 0.433 mol of calcium nitrate.

Converting between grams and atoms/molecules

Conversion factors:

1 mole = 6.022 x 1023 particles

1 mole = ________ grams

molar mass

Sample Exercise 3.12(a) How many glucose molecules are in 5.23 g of C6H12O6? (b) How many oxygen atoms are in this sample?

Empirical Formulas

• Gives the relative numbers of atoms of each element in a substance

• Based on experiments that give the # of moles of each element in a sample of the compound

• Empirical means based on observation & experiment

Finding Empirical FormulasMass % of elements

Grams of each

element

Moles of each

elementMole ratio*

assume 100 grams

use molar mass

divide by the smallest# of moles

multiply untilall #’s are

whole

Empirical formula

* Due to experimental errors, results may not lead to exact integers Ex. 1.99 2

Molecular Formulas

• Subscripts in a molecular formula are always a whole number multiple of the corresponding subscripts in the corresponding empirical formula

Whole # multiple = molecular weight

empirical formula weight

Sample Exercise 3.13Ascorbic acid (vitamin C) contains 40.92% C, 4.58% H, and 54.50% O by mass. What is the empirical formula of ascorbic acid?

Sample Exercise 3.14Mesitylene, a hydrocarbon that occurs in small amounts in crude oil, has an empirical formula of C3H4. The experimentally determined molecular weight of the substance is 121 amu. What is the molecular formula of mesitylene?

Sample Exercise 3.15Isopropyl alcohol, a substance sold as rubbing alcohol, is composed of C, H, and O. Combustion of 0.255 g of isopropyl alcohol produces 0.561 g of CO2 and 0.306 g of H2O. Determine the empirical formula of isopropyl alcohol.

Quantitative Information from Equations

• Coefficients indicate relative numbers of molecules involved in a reaction (also relative # of moles)

• Stoichiometrically equivalent to…use coefficients to make mole conversion factors between different substances involved in a reaction

Sample Exercise 3.16How many grams of water are produced in the oxidation of 1.00 g of glucose, C6H12O6?

Sample Exercise 3.17Solid lithium hydroxide is used in space vehicles to remove exhaled carbon dioxide. The lithium hydroxide reacts with gaseous carbon dioxide to form solid lithium carbonate and liquid water. How many grams of carbon dioxide can be absorbed by 1.00 gram of lithium hydroxide?

Limiting Reactants

Let’s make a sandwich!

2 Bd + Ch Bd2Ch10 slices 7 slices

can make5 sandwiches

can make7 sandwiches

In this reaction, bread is the limiting reactant and will becompletely used up.

Cheese is the excess reactant. We will have 2 slices of cheese left over once the reaction has completed.

Limiting Reactants

• Also called limiting reagents

• In a chemical reaction, the reaction stops when one reactant is totally used up. This is the limiting reactant.

Limiting Reactant Example

2H2(g) + O2(g) 2H2O(l)Initially we have 10 mol hydrogen gas and 7 moles oxygen gas. Which is limiting and how much excess remains unreacted?

Sample Exercise 3.18The most important commercial process for converting N2 from the air into nitrogen-containing compounds is based on the reaction of N2 and H2 to form ammonia (NH3):

N2(g) + 3H2(g) 2NH3(g)

How many moles of NH3 can be formed from 3.0 mol of N2 and 6.0 mol of H2?

Sample Exercise 3.19Consider the following reaction:

2 Na3PO4 (aq) + 3 Ba(NO3)2(aq) Ba3(PO4)2 (s) + NaNO3 (aq)

Suppose a solution containing 3.50 g of Na3PO4 is

mixed with a solution containing 6.40 g of Ba(NO3)2. How many grams of Ba3(PO4)2 can be formed?

Sample Exercise 3.20Adipic acid, H2C6H8O4 , is used to produce nylon. The acid is made commercially by a controlled reaction between cyclohexane (C6H12) and O2:

2 C6H12 (l) + 5 O2(g) 2 H2C6H8O4 (l) + 2H2O (g)

(a) Assume that you carry out this reaction starting with 25.0 g of cyclohexane and that cyclohexane is the limiting reactant. What is the theoretical yield of adipic acid? (b) If you obtain 33.5 g of adipic acid from your reaction, what is the percent yield of adipic acid?