General Chemistry: Chemical Reactions, Bonding, and Acids/Bases – Study Guide

Chemical Reactions and Changes

Physical vs. Chemical Changes

Chemical reactions are processes where substances are transformed into new substances with different properties. Understanding the distinction between physical and chemical changes is fundamental in chemistry.

• Physical Change: A change in the state or appearance of a substance without altering its chemical composition (e.g., melting, freezing, dissolving).

• Chemical Change: A process where one or more substances are converted into new substances with different chemical properties (e.g., burning, rusting, reacting acids with bases).

• Example: Ice melting is a physical change; iron rusting is a chemical change.

Where Do Chemical Reactions Occur?

• Chemical reactions can occur in various environments: in living organisms (biochemical reactions), in the atmosphere, in water, and in laboratories.

• They are essential for processes such as digestion, combustion, and corrosion.

Atoms, Elements, and Diatomic Molecules

The 7 Diatomic Elements

Certain elements naturally exist as molecules composed of two atoms. These are called diatomic elements.

• Diatomic Elements: H2, N2, O2, F2, Cl2, Br2, I2

• Mnemonic: "Have No Fear Of Ice Cold Beer"

• Example: Oxygen gas is O2, not just O.

Chemical Bonding

Ionic vs. Covalent Bonds

Atoms bond to achieve stable electron configurations. The two main types of bonds are ionic and covalent.

• Ionic Bonds: Formed by the transfer of electrons from a metal to a nonmetal, resulting in oppositely charged ions that attract each other.

• Covalent Bonds: Formed by the sharing of electrons between two nonmetals.

• Example: NaCl (ionic), H2O (covalent)

Electron Transfer and Sharing

• Ionic Bonds: The number of electrons transferred depends on the charges needed to balance the ions. For example, Na (1 valence electron) transfers one electron to Cl (needs 1 electron).

• Covalent Bonds: The number of shared electrons depends on the number of bonds. For example, in carbon monoxide (CO), three pairs (6 electrons) are shared (a triple bond).

Bohr Models of Bonds

• Ionic Bond Model: Shows complete transfer of electrons from one atom to another, resulting in ions.

• Covalent Bond Model: Shows shared pairs of electrons between atoms.

Energy and Bond Formation

• Bond Formation: Usually releases energy (exothermic process).

• Bond Breaking: Requires energy input (endothermic process).

Chemical Equations and States of Matter

Symbols for States of Matter

• (s): Solid

• (l): Liquid

• (g): Gas

• (aq): Aqueous (dissolved in water)

• Example: $\mathrm{NaCl} (aq)$ means sodium chloride dissolved in water.

Counting Atoms in a Compound

• Count the number of each type of atom by multiplying the subscript by the coefficient (if present).

• Example: In $2 \mathrm{H}_2\mathrm{O}$, there are 4 H atoms and 2 O atoms.

Balancing Chemical Equations

• Ensure the number of atoms of each element is the same on both sides of the equation.

• Use coefficients to balance equations, not subscripts.

• Example: $\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}$ (unbalanced), $2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}$ (balanced)

Writing and Balancing Equations from Sentences

• Identify reactants and products from the description.

• Write the correct formulas and balance the equation.

• Example: "Hydrogen reacts with oxygen to form water" becomes $2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}$

Reaction Types and Energy

Activation Energy

• Activation Energy: The minimum energy required for a reaction to occur.

• It is the energy barrier that must be overcome for reactants to transform into products.

Exothermic vs. Endothermic Reactions

• Exothermic Reaction: Releases energy (usually heat) to the surroundings. Products have lower energy than reactants.

• Endothermic Reaction: Absorbs energy from the surroundings. Products have higher energy than reactants.

• Example: Combustion is exothermic; photosynthesis is endothermic.

Energy Diagrams

• Exothermic: Reactants start at higher energy, products at lower energy; energy is released.

• Endothermic: Reactants start at lower energy, products at higher energy; energy is absorbed.

Types of Chemical Reactions

• Synthesis (Combination): Two or more substances combine to form one product. $A + B \rightarrow AB$

• Decomposition: One substance breaks down into two or more products. $AB \rightarrow A + B$

• Single Displacement: One element replaces another in a compound. $A + BC \rightarrow AC + B$

• Double Displacement: Ions in two compounds exchange places. $AB + CD \rightarrow AD + CB$

• Combustion: A substance reacts with oxygen, releasing energy, usually as heat and light.

Acids, Bases, and the pH Scale

Acids vs. Bases

• Acids: Substances that produce hydrogen ions (H+) in solution. Taste sour, turn blue litmus red.

• Bases: Substances that produce hydroxide ions (OH-) in solution. Taste bitter, feel slippery, turn red litmus blue.

• Example: HCl is an acid; NaOH is a base.

The pH Scale

• Measures the concentration of H+ ions in a solution.

• Scale ranges from 0 (most acidic) to 14 (most basic), with 7 being neutral.

• Each pH unit represents a tenfold change in H+ concentration.

• Formula: $\mathrm{pH} = -\log_{10}[\mathrm{H}^+]$

• Example: A solution with [H+] = $1 \times 10^{-3}$ M has pH 3 (acidic).

pH Value	Solution Type	[H+]
0-6	Acidic	High
7	Neutral	Equal to [OH-]
8-14	Basic	Low

Additional info: The above guide expands on the listed review points, providing definitions, examples, and equations for each topic relevant to a general chemistry course.