Understanding Chemical Reactions

Chemical Reactions

• A chemical reaction occurs when the atoms of reactants rearrange to form new substances with different properties.
• Reactants are initial substances in a chemical reaction.
• Products are substances formed after the reaction.

Physical Change vs. Chemical Change

• A physical change doesn't alter the composition of a substance (e.g., melting ice).
• A chemical change alters the composition of a substance (e.g., burning wood).

Writing Chemical Equations

• A chemical equation represents the reactants and products, 2H2 + O2 → 2H2O.

Types of Chemical Reactions

• Synthesis: Two or more substances combine to form a new one (A + B → AB).
• Decomposition: A compound breaks down into simpler substances(AB → A + B).
• Single Substitution: One element replaces another in a compound (A + BC → AC + B).
• Double Substitution: Elements are exchanged between two compounds (AB + CD → AD + CB).

Energy in Chemical Reactions

• Chemical reactions can release or absorb energy.
• Exothermic Reactions: Release energy in the form of heat (e.g., combustion).
• Endothermic Reactions: Absorb energy (e.g., photosynthesis).
• An exothermic reaction releases heat and increases the surrounding temperature.
• An endothermic reaction requires heat and decreases surrounding temperature.
• Examples of exothermic reactions include gasoline combustion and cellular respiration.
• Examples of endothermic reactions include cooking an egg and water evaporation.

Atomic Structure

• An atom has a nucleus with protons (+) and neutrons (0), surrounded by electrons (-) in various energy levels.
• Atomic number (Z) is the number of protons in an atom.
• Mass number (A) is the sum of protons and neutrons.
• The number of neutrons equals the atomic mass (A) - the atomic number (Z).
• EI N. Atomico, is the same and is placed in protons and electrons.

Periodic Table

• Groups are the vertical columns, numbered 1 to 18.
• Group 1: Alkali metals
• Group 2: Alkaline earth metals
• Groups 3-12: Transition metals
• Group 17: Halogens
• Group 18: Noble gases
• Periods are horizontal rows, numbered 1 to 7.
• Periods indicate the number of energy levels in the atoms of the elements in that row.
• Metals are on the left and in the center like Sodium (Na), Iron (Fe), Gold (Au).
• Nonmetals are on the right like Oxygen (O), Chlorine (Cl), Neon (Ne).
• Metalloids are diagonally between metals and nonmetals like Boron (B), Silicon (Si), Arsenic (As).

Periodic Properties

• Electronegativity is an atom's ability to attract electrons in a bond.
• Fluorine (F) has the highest.
• Francium (Fr) has the lowest, increasing left to right across a period and decreasing top to bottom in a group.
• Atomic Radius is the distance from the nucleus to the outermost electron.
• It increases from top to bottom and decreases from left to right within a period.
• Ionization Energy is the energy needed to remove an electron from a gaseous atom.
• It increases from left to right across a period and decreases from top to bottom in a group.
• Electron Affinity is the energy released when an atom gains an electron and follows a similar trend to electronegativity.
• Halogens have high electron affinity values.