Chapter 7: Chemical Reactions and Quantities

Learning Objectives

• Recognize the parts of a chemical equation

• Balance chemical equations

• Understand the concept of the mole and Avogadro's Number

• Analyze chemical formulas to understand mole relationships of atoms in a compound

• Find the molar mass of elements and compounds

• Perform unit conversion problems using molar mass as a conversion factor

• Find mole-to-mole factors from a balanced chemical equation

• Define oxidation and reduction

Equations for Chemical Reactions

What is a Chemical Reaction?

A chemical reaction occurs when a substance is converted into one or more new substances with different formulas and properties. Chemical changes are fundamental to chemistry and are often observed by changes such as color, formation of a gas, formation of a solid, or heat production/absorption.

• Example: Iron nails react with oxygen to form rust (Fe2O3), changing color and properties.

Evidence of a Chemical Reaction

• Change in color

• Formation of a gas (bubbles)

• Formation of a solid (precipitate)

• Heat or flame produced or absorbed

Writing and Balancing Chemical Equations

Parts of a Chemical Equation

• Reactants: Substances present before the reaction (left side of the arrow)

• Products: Substances formed by the reaction (right side of the arrow)

• Arrow (→): Separates reactants from products; indicates the direction of the reaction

• Plus sign (+): Separates multiple reactants or products

• Physical states: Indicated in parentheses: (s) solid, (l) liquid, (g) gas, (aq) aqueous (dissolved in water)

• Delta (Δ): Indicates heat is used to start the reaction

Common Symbols in Chemical Equations

Balancing Chemical Equations

A balanced chemical equation has the same number of atoms of each element on both sides of the equation. This reflects the law of conservation of mass.

1. Count the atoms of each element in the reactants and products.

2. Use coefficients to balance each element, starting with the most complex formula.

3. Check the final equation to confirm it is balanced and coefficients are in the lowest ratio.

• Example:

Balancing Equations with Polyatomic Ions

Polyatomic ions that appear unchanged on both sides of the equation can be balanced as units.

• Example:

The Mole and Avogadro's Number

Definition of the Mole

The mole is a counting unit in chemistry, similar to a dozen, but much larger. One mole contains Avogadro's number of items:

• items (atoms, molecules, or ions)

This number is named after Amedeo Avogadro.

Examples of One Mole

Mole Relationships in Compounds

Subscripts in chemical formulas indicate the number of moles of each element in one mole of the compound.

• Example: In aspirin, C9H8O4:

  • 1 mole of C9H8O4 contains 9 moles C, 8 moles H, and 4 moles O

Molar Mass

Definition and Calculation

Molar mass is the mass in grams of one mole of a substance. For elements, it is the atomic mass in grams. For compounds, it is the sum of the atomic masses of all atoms in the formula.

• Example: 1 mole of sodium (Na) = 22.99 g

• Example: Molar mass of lithium carbonate, Li2CO3:

  • 2 Li: g

  • 1 C: g

  • 3 O: g

  • Total: g/mol

Using Molar Mass in Calculations

Molar mass is used as a conversion factor between grams and moles:

• Conversion factors: or

Example: How many moles are in 737 g of NaCl?

• Molar mass of NaCl = 58.44 g/mol

Law of Conservation of Mass

The law of conservation of mass states that matter cannot be created or destroyed in a chemical reaction. The total mass of reactants equals the total mass of products.

• Example:

Mole-Mole Relationships in Chemical Equations

Mole-Mole Ratios

Coefficients in a balanced chemical equation give the mole ratios of reactants and products.

• Example:

• 2 moles Fe react with 3 moles S to form 1 mole Fe2S3

These ratios are used as conversion factors in stoichiometric calculations.

Oxidation-Reduction (Redox) Reactions

Definition and Importance

Oxidation-reduction (redox) reactions involve the transfer of electrons from one substance to another. These reactions are essential for energy production in biological systems and in batteries, and are responsible for processes like rusting.

• Oxidation: Loss of electrons (may also be seen as gain of oxygen or loss of hydrogen)

• Reduction: Gain of electrons (may also be seen as loss of oxygen or gain of hydrogen)

Mnemonic: OIL RIG – Oxidation Is Loss, Reduction Is Gain (of electrons)

• Example: Rusting of iron:

Redox in Biological Systems

Coenzymes such as FAD (flavin adenine dinucleotide) participate in redox reactions by accepting or donating hydrogen atoms and electrons.

• Oxidized form: FAD

• Reduced form: FADH2 (after gaining 2H and 2e-)

Additional info: These notes are based on slides from "General, Organic, and Biological Chemistry: Structures of Life" (Timberlake, 6th Edition), Chapter 7, and are suitable for GOB Chemistry college students.