Chapter 5: Chemical Reactions

Suggested Problems:

1-8, 20, 28, 30, 35, 44, 46, 50, 52, 56

Chemical Equations

• Reactants: Are the species that are combined together to give a chemical change

• Products: Are the species that are created as a result of the chemical change

Reactants Products

Chemical EquationsA + B C + D

• A and B are reactants• C and D are products• Equation is read as:

– A reacts with B to yield ( or give) products C and D

• Think of the arrow as an algebraic equals sign

• The equation is balanced when the atoms on the right equal the atoms on the left

Balancing Equations

Basic Rules• Balance elements other than oxygen and

hydrogen first• Balance polyatomic ions as units if on both

sides of the equation• Balance oxygen and hydrogen last• Never add subscripts add

COEFFICIENTS

Balancing Equations: Atom InventoryMg(OH)2 + HCl MgCl2 + H2O

1 Mg

O

H

Cl

Mg

O

H

Cl

2

3

1

1

1

2

2

Mg(OH)2 + 2HCl MgCl2 + H2O

1

2

4

2

Mg(OH)2 + 2HCl MgCl2 + 2H2O

1

2

4

2

Balance the Following Equation

Fe2(SO4)3 + BaCl2 Ba(SO4) + FeCl3

Types of Chemical Reactions

• Redox• Decomposition• Combination• Replacement

– Single and Double• Combustion• Acid Base

Helpful Phrases for REDOX Reactions

• Reduction: Gain of electrons by a species• Oxidation: Loss of electrons by a species

LEO the lion says GER!

GER!

OIL RIG

Helpful Phrases for REDOX Reactions

• Oxidizing Agent: This is the species that is reduced in a chemical reaction

• Reducing Agent: This is the species that is oxidized in a chemical reaction

• Reduction can’t happen without an oxidation to provide the electrons

Helpful Phrases for REDOX Reactions• Oxidation Number (ON): This is the charge of a

species in a chemical formulaRules

1. The ON in uncombined elements is zero2. The ON of simple ions equals its charge3. The ON of group 1A and 2A elements equals

+1 and +2 respectively4. The ON of H is +15. The ON of O is -2 except in peroxide it is -16. The sum of the ON’s of a chemical formula is

zero7. The sum of the ON’s of a polyatomic ion equal

the charge of the polyatomic ion

REDOX Reactions

• Simply boils down to a change in oxidation numbers of the products and reactants

• REDOX reaction is a broad classification that contains:– Combustion reactions– Decomposition reactions– Combination reactions– Single Replacements reactions

REDOX Example

• Balance the following equation. Then identify the ON’s of each species in the above reaction as well as identify the oxidizing agent and reducing agent.

KI + Cl2 KCl + I2

Decomposition Reactions

• A reaction in which one species is broken down into 2 or more simpler substances

A B + C

Combination Reactions• Known also as addition or synthesis

reactions: 2 or more substances react to form a single substance

A + B C

Single Replacement• Is a reaction in which one element reacts

with a compound and replaces another element

A + BX B + AX

Double Replacement

• A reaction in which the cations of two compound switch anion partners

AY + BX BY + AX

Combustion Reactions• An organic compound is heated in the

presence of oxygen gas, resulting in the products of H2O and CO2

• Balance this equation!

C6H12O6 + O2 CO2 + H2O

Endothermic and Exothermic Reactions

• Endothermic Reaction: is a reaction that requires energy as a reactant

• Exothermic Reaction: is a reaction that releases energy as a product

• Energy is most often in the form of heat• So it is common to think of endothermic

reaction as being cold and an exothermic reaction as being hot

Stoichiometry and Chemical Reactions

• The coefficients used to balance chemical reactions can now be used as conversion factors!!!

Mg(OH)2 + 2HCl MgCl2 + 2H2O

2

2

2

2

Mg(OH) mol 1

OH mol 2

HCl mol 2

MgCl mol 1

Mg(OH) mol 1

HCl mol 2

Stoichiometry Example

• If 25 ml of HCl, with a density of 1.079 g/mL reacts with Mg(OH)2, how many grams of water is produced?

Mg(OH)2 + 2HCl MgCl2 + 2H2O

Limiting Reagent

Limiting Reagent• Reaction mixtures do not always consists

of the perfect stoichiometric ratio of reactants

• Frequently on or more reagents are in excess; the other is called the limiting reactant (or limiting reagent)

• The limiting reagent is present in the smallest stoichiometric amount, but not necessarily the smallest molar amount

Limiting Reagent: Example

• Aluminum reacts with bromine to give aluminum bromide

• If 3.40 grams of Al are mixed with 26.2 grams of Br2, which is the limiting reagent?

Theoretical Yield

• Theoretical Yield is the maximum possible amount of product that can be produced

• The theoretical yield is determined by the limiting reagent

Theoretical Yield Example

• What mass of aluminum bromide can be produced from 3.40 grams of Al and 26.2 grams of Br2?

Percent Yield

• The percent yield:

100% yield ltheoretica

yield actual Yield %

Percent Yield: Example

• If 12.5 grams of AlBr3 are isolated from the reaction of 3.40 grams of Al and 26.2 grams of Br2, what is the percent yield?

Guideline for the Rest of the YEAR

Units of A given Moles of A

Moles of B

Units of B wanted