Chemical Reactions and Reaction StoichiometryChemistry, The Central Science, 13th edition

Theodore L. Brown, H. Eugene LeMay, Jr., Bruce E. Bursten, Murphy, Woodward, and Stoltzfus.

3.1 Chemical Equations

Chemical Equations

• Chemical equations represent chemical reactions

• Ex. 2 H2 + O2 2 H2O

• The “+” is read as reacts with while the “” reads as yields, produces, makes, forms, etc.

• Subscripts are used to tell how many of each element is in the molecule

• Coefficients are used to tell how many molecules/moles are needed for the reaction

• The reactants are on the right side of the equation

• The products are on the left side of the equation

Balancing Equations

• To construct a balanced chemical equation:

1. Write the reactants on the left

2. The products on the right

3. Balance the equation using the smallest possible coefficients

• NEVER change the subscripts of a molecular formula (or ionic formula)

• Subscripts are important to tell the reader what the chemical is

Example

• Methane reacts with oxygen to form carbon dioxide and water

• CH4 + O2 H2O + CO2 (unbalanced)

• Decide where you want to begin balancing and where you should balance last

• I suggest balancing diatomic elements last as a rule

• CH4 + 2 O2 2 H2O + CO2 (balanced)

Indicating the State of Reactant and Products

Symbols indicating the physical state of each reactant and product are often shown in the chemical equations.

The symbols are

Gas (g)

Liquid (l)

Solid (s)

Aqueous (aq)

Sometimes symbols that represent what helps the reaction along is written above the arrow

Δ is for heat

λ is for light

3.2 Simple Patterns of Chemical Reactivity

Simple Patterns of Chemical Reactivity

Combination Reactions

Two or more substances react to form one product

A + B AB

Decomposition Reaction

A single reactant breaks apart to form two of more substances

AB A + B or ABC AB + C etc…

carbonate decomposition

Usually forms carbon dioxide and the metal oxide

Bicarbonate decomposition

Forms carbonate, water, and carbon dioxide

Hydrate Decomposition

Usually the loss of water by heating

Hydrates…

Combustion Reactions

Combustion Reactions

Rapid reactions that produce a flame

Most reactions observed involve oxygen from air

The products are typically carbon dioxide and water

Single or Double Replacement

Single Replacement

A + BC AC + B

AB + C AC + B

Double Replacement

AB + CD AD + CB

Special types of Reactions

Acid/base, acid/carbonate, acid/metal,

3.3 Formula Weights

Formula Weights

The formula weight (FW) of a substance is the sum of the atomic weights (AW) of the atoms in the chemical formula of the substance

FW = Σ (type of element)(subscript of element)

If the chemical formula is that of a molecule, the formula weight is also called the molecular weight (MW)

Percentage Composition from Chemical Formulas

The percent composition or elemental composition comes straight from the chemical formula/molecular formula

The calculation depends on the formula weight of the substance, the atomic weight of the element of interest, and the number of atoms of that element in the chemical formula

Practice

What is the percent composition of carbon in C12H22o11

3.4 Avogadro’s Number and the Mole

Avogadro’s Number and the Mole

The mole is the amount of matter that contains as many objects as the number of atoms in exactly 12g of isotopically pure 12C

Scientists call this Avogadro’s number, N

Practice

Determine which of the following samples contains the fewest sodium atoms. A) 1 mol sodium oxide B) 45 g sodium fluoride C) 50 g sodium chloride D) 1 mole sodium nitrate

Calculate the number of H atoms in 0.350 mol of C6H12O6

How many moles of ions are found in a solution of sodium phosphate

Molar Mass

The molar mass of one mole of any element is the average mass of one 6.02 x 1023 atoms of the naturally occurring mixture

Units: g/mol

Calculate the molar mass of the following:

Sodium chloride

Ethane

A sample of an ionic compound containing iron and chlorine is analyzed and found to have a molar mass of 126.8 g/mol.

What is the charge of the iron?

Interconverting Masses and Moles

How many moles of sodium bicarbonate are in 508g of sodium bicarbonate?

How many moles of water are in 1.00 L of water, whose density is 99.3 g/mL?

Calculating Numbers of Molecules and Atoms from Mass

How many glucose molecules are in 5.23 grams of glucose? (C6H12O6)

How many oxygen atoms are in the sample?

How many chlorine atoms are in 12.2g of carbon tetrachloride?

3.5 Empirical Formulas from Analysis

Empirical Formulas from Analyses

The ratio of the numbers of moles of all elements in a compound gives the subscripts in the compound’s empirical formula

Practice Problems

Ascorbic acid (vitamin C) contains 40.92% C, 4.58% H, and the rest is oxygen by mass. What is the empirical formula of ascorbic acid

A 2.144g sample of phosgene, a compound used as a chemical warfare agent during WWI, contains 0.260 g C, 0.347g O and the rest is chlorine. What is the empirical formula?

Molecular Formulas from Empirical Formulas

We can obtain the molecular formula for any compound by calculating the empirical formula and knowing the molecular mass of the compound.

Whole-number multiple = (molecular weight)/(empirical formula weight)

Practice:

Mesitylene, a hydrocarbon found in crude oil, has an empirical formula of 3 carbons and 4 hydogens and an experimentally determined weight of 121 amu. What is its molecular formula?

Practice

Cyclohexane, a commonly used organic solvent, is 85.6% C and 14.4% H by mass with a molar mass of 84.2 g/mol. What is its molecular formula?

Side note: Cyclo- anything means that the linear chain hydrocarbon is now cyclic

Degree of Unsaturation

How unsaturated is cyclohexane?

Combustion Analysis

Combustion Analysis

Commonly used to determine the composition of hydrocarbons

When a hydrocarbon is fully combusted, the mass of water and carbon dioxide collected can be used directly to determine the amount of carbon and hydrogen in the original compound.

Practice

Isopropyl alcohol, sold as rubbing alcohol, is composed of C, H, and O. Combustion of 0.255 g of isopropyl alcohol produces 0.561g of carbon dioxide and 0.306g of water. Determine the empirical formula of isopropyl alcohol

Now solve using empirical formula calculations

Practice

The compound dioxane, which is used as a solvent in various industrial processes, is composed of C, H, and O atoms. Combustion of a 2.203g sample of this compound produces 4.401g carbon dioxide and 1.802g water. A separate experiment shows that it has a molar mass of 88.1 g/mol. What is the correct formula?

3.6 Quantitative Information from Balanced Equations

Quantitative Information from Balanced Equations

Coefficients in a chemical equation represents the relative numbers of molecules in a reaction.

The coefficients can be used to determine the amounts of grams, molecules, or moles of any substance needed or made in a reaction.

Practice

Determine how many grams of water are produced in the oxidation of 1.00g of glucose?

Sodium hydroxide reacts with carbon dioxide to form sodium carbonate and water. How many grams of sodium carbonate can be formed from 2.40g of sodium hydroxide?

3.7 Limiting Reactants

Limiting Reactants

When any reaction occurs there are always left over reagents in the reaction.

Using stoichiometry, one can determine the amount of excess reactant and which reactant stopped the overall reaction

Limiting reactant

It determines or limits the amount of product formed

Practice

The most important commercial process for converting nitrogen from the air into nitrogen-containing compounds is based on the reaction nitrogen gas reacting with hydrogen gas to form ammonia. How many moles of ammonia can be produced from 3 moles of nitrogen and 6 moles of hydrogen?

Theoretical and Percent Yield

Theoretical yield

The quantity of product calculated to form when all of a limiting reactant is consumed

Calculated from stoichiometry

Percent Yield

Calculates the amount of product retrieved from an experiment compared to the theoretical yield one should have obtained

Practice

Adipic acid, H2C6H8O4, used to produce nylon, is made commercially by a reaction between cyclohexane and oxygen. Liquid adipic acid is formed as well as water.

Assume that 25.0g of cyclohexane is used with excess oxygen present. What is the theoretical yield of adupic acid? If you obtain 33.5g of adipic acid, what is the percent yield for the reaction?

If 3.00g of Ti metal is reacted with 6.00g chlorine gas, to form 7.7f of titanium (IV) chloride in a combination reaction, what is the percent yield of the product?