Chapter 10

Chemical Reactions

3 types of formulas

• Molecular Formula: Identifies the actual number of atoms in a molecule. e.g. H2O, H2O2, C6 H12 O6

• Empirical Formula: The simplest whole number ratio of atoms in a compound. e.g. H2O, HO, CH2O.

• Structural Formula: Shows the arrangement of the atoms in the molecule, bonds are shown.

3 types of formulas for Covalent Compounds

• Molecular Formula: Identifies the actual number of atoms in a molecule. e.g. H2O, H2O2, C6 H12 O6

• Empirical Formula: The simplest whole number ratio of atoms in a compound. e.g. H2O, HO, CH2O.

• Structural Formula: Shows the arrangement of the atoms in the molecule, bonds are shown.

Molecular and Formula Weights

• The sum of the atomic weights of all the atoms in a chemical formula.

• The atomic weights are obtained from the periodic table.

• For covalent compounds it is called molecular weight, since the basic repeating unit is a molecule. e.g. H2O, H2O2, C6 H12 O6

• For ionic compounds it is called formula weight, since the basic repeating unit is a formula unit, the simplest ratio of ions in an ionic compound. e.g. NaCl, Ca(NO3)2.

Formula and molecular weights

Calculating Molecular and Formula Weights

• The molecular weight for C6 H12 O6 (glucose) is:

6C 6 x 12.01= 72.06 amu12H 12 x 1.01= 12.12 amu6O 6 x 16.00= 96.00 amu Total 180.18 amu

• The formula weight for Ca(NO3)2 (calcium nitrate is:

1 Ca 1 x 40.08 = 40.08 amu2 N 2 x 14.01 = 28.02 amu6O 6 x 16.00 = 96.00 amu Total 164.10 amu

Calculating Molecular Weight

Chemical Reactions

• Occur when bonds are formed and broken.• Involve changes in matter, creation of new

materials with new properties from existing materials, energy changes.

• A chemical equation represents a chemical reaction.

• Reactant(s) Product(s)• There can be one or more reactants and one or

more products.• The arrow means “yield”.

The reaction occurring here is:

C + O2 CO2 + energy

Balancing Chemical Equations• Combustion reaction:

C2H4 + O2 CO2 + H2O

Notice the numbers of oxygen, hydrogen, and carbon atoms on each side are not the same. To make them equal you can add a coefficient in front of one or more of the chemical formulas:

C2H4 + 3O2 2CO2 + 2H2O

Now you have 2 carbon atoms, 4 hydrogen atoms, and 6 oxygen atoms on each side of the equation.

Chemical equations

Reactants Products

Arrow means yield and can be drawn both ways

In chemical reactions, subscripts don’t change.

Chemical equations

Reactants Products

Arrow means yield and can be drawn both ways

Only coefficients change.

Rules for Balancing Equations:

• The same number of atoms of each element have to be on each side of the equation. Atoms are neither created nor destroyed in chemical reactions.

• Never alter the subscripts in the chemical formulas, since those determine the identity of the chemical compound. Only coefficients can be added.

• Leave the balancing of elements that appear by themselves for last, since they are easier to manipulate.

• Balance polyatomic ions as an entity rather than as individual atoms.

• Practice Sheet

Types of Reactions

• Combustion reactions occur when compounds which contain C, H, and/or O, which are organic compounds, react with oxygen (O2), to produce carbon dioxide (CO2) and water vapor.

C6H12O6 + 9O2 6CO2 + 6H2O + energy

This is the reaction which occurs in the human and animal body, respiration. This is the opposite of photosynthesis.

Photosynthesis is:6CO2 + 6 H2O + energy C6H12O6 + 9O2

Products that contain substances that undergo combustion

Propane is combusted (burns). This heats up the air inside of the balloon.Hot air is less dense than cold air and therefore it makes the balloon float.

Other examples of combustion

• C5H12 + 9O2 5CO2 + 6H2O + energy This is the reaction for the combustion of octane, the

active ingredient in gasoline.

• C8H18 + O2 8CO2 + 9H2O + energy • 2x(C8H18 + O2 8CO2 + 9H2O) + energy

• 2C8H18 + 25O2 16CO2 + 18H2O + energy

2

25

2

25

Combination Reactions

• Also called synthesis reactions

• X + Y XY

• X and Y can be atoms or molecules, or formula units.

e.g. 4Al + 3O2 2Al2O3

CO2 + H2O H2CO3

Decomposition Reactions

• Opposite of combination reactions.

• XY X + Y

Fe2O3 Fe + O2

K 2 CO3 K2O + CO2

Activity (Reactivity) of Metals and Hydrogen

Single Replacement Reactions• XY + Z ZY + X

• X and Z are metals or hydrogen.

• These reactions occur only if Z is more reactive than X.

• For instance, the alkali metals and alkaline earth metals can be “Z” in the reaction and the transition metals or group IIIA or IVA metals can be “X”, but not vice versa.

• Hydrogen is less reactive than Group IA, IIA, IIIA, and IVA metals, but more reactive than some of the transition metals.

e.g. 2K + CuCl2 2KCl + Cu 2Ag + CaCl2 No reaction

Mg + 2H2O H2 + Mg(OH)2

2(H-OH)

Reaction of Aluminum metal with copper (II) chloride (CuCl2)2Al(s) + 3CuCl2(aq) 3Cu(s) + 2AlCl3(aq)

Double Replacement Reactions• Also called ion exchange reactions.

• AX + BY BX + AY

• A and B are metal ions, hydrogen, or ammonium ion (positive ions). X and Y are nonmetal ions or polyatomic ions (negative ions).

• Only occur if one of the products is a solid which is not soluble in water, a liquid, or a gas.

e.g. CaCl2(aq) + Na2S(aq) CaS(s) + 2NaCl(aq)

aq = aqueous (dissolved in water)s = solid (not soluble in water) Also called a precipitate.l = liquidg = gas

Other examples of Double Replacement ReactionsLi2SO4(aq) + Mg(NO3)2(aq) MgSO4 (s)+ 2LiNO3(aq)

H3PO4(aq) + AlBr3(aq) AlPO4(s) + 3HBr(aq)

Also Double Replacement Reactions:Neutralization Reactions are reactions of an acid (starts with H),

with a base (ionic compound ends in OH)

HCl(aq) + KOH(aq) KCl(aq) + H2O(l) (H-OH)Acid Base Salt water (Ionic compound)

2HNO3(aq) + Ca(OH)2(aq) Ca(NO3)2(aq) + 2H2O(l) 2(H-OH)

The Law of Conservation of Mass

• The number of atoms of each element is the same on either side of a chemical equation. Therefore, if each atom is multiplied by its atomic weight, the total weight is the same on either side of a chemical equation.

• 2Na + O Na2O

2 atoms Na 2 atoms Na

1 atom O 1 atom O(2 x 22.99 amu) + (1 x 16.00 amu) (2 x 22.99 amu) + (1 x 16.00 amu)

61.98 amu total 61.98 amu total

Law of Conservation of Mass

• H2 + O2 2H2O

• If 4g of hydrogen react with oxygen to produce 36 g water, how many grams of oxygen were needed?

• 2H2 + O2 2H2O

4g Xg 36g

• 4g + X g = 36 g Xg=32 g O2

The Mole

• Since atoms and molecules are tiny, a grouping of atoms or molecules cannot be a dozen or a crate or a carton.

• There is a special grouping used for atoms and molecules which is called the mole, abbreviated mol.

• 1 mol is equal to 6.02 x 1023 atoms, molecules or formula units. This number is called Avogadro’s number.

Molar Mass

• The mass of 1 mol of an atom, molecule or a formula unit is equal to its formula mass with the units of grams instead of amu.

• This is called the molar mass. • Each mol consists of 6.02 x 1023 atoms or

molecules.• The mass of 1 mol of NaCl, which has an

formula weight of 58.44 amu is 58.44 g.• The mass of 1 mol of O2 is 32.00 g.

Exercises for Chapter 10

• p. 272-273 Applying Concepts:

# 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17.

p. 273-274 Parallel Exercises Group A:

# 1, 2, 4, 5, 6, 8, 9

New Book: p. 293-296 # 1, 2, 3, 4, 5, 6, 9, 10, 12, 13, 15, 16, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 30, 31, 32, 33, 34, 35, 36, 37, 39, 41, 42, 43, 45, 46, 47, 49, 50, 51, 52, 53.

p. 297-298 # 1, 2, 4, 5

Review• Empirical, molecular and structural formulas.• Empirical and formula weights.• Balancing chemical equations.• Combustion reactions (organic compound with C, H, and/or O

reacts with oxygen to produce carbon dioxide and water)• Combination and decomposition reactions.• Single replacement reactions. The metal by itself must be more

reactive than the metal in the compound.• Double replacement reactions. The product must be a solid, a liquid,

or a gas.• Neutralization reactions. A special type of double replacement

reaction. An acid reacts with a base to produce a salt (ionic compound) and water.

• The law of conservation of mass.• Moles, molar mass and Avogadro’s number.