Reversible, Irreversible, Thermal and Photochemical

• A reversible reaction can proceed in both directions, where reactants form products, but products also convert back into reactants, often reaching dynamic equilibrium.
• An example of a reversible reaction is the carbonic acid-bicarbonate system in blood.
• An irreversible reaction occurs in only one direction, where reactants fully transform into products without significant reversal.
• An example of an irreversible reaction is the combustion of glucose in respiration.

Decomposition (Thermal)

• A thermal decomposition reaction, also known as thermolysis, occurs when a single compound breaks down into two or more simpler substances when heated.
• Most thermal decompositions break down substances permanently, though some can be reversed under specific conditions.

Decomposition (Photochemical)

• A photochemical decomposition reaction, also known as photolysis or photodissociation, occurs when a chemical compound is broken down into other substances by absorbing energy from photos (light).
• Some photochemical reactions, like the decomposition of silver halides in photography, can be reversed under the right conditions.

Combination/Synthesis Reaction

• A synthesis reaction, also known as a direct combination or combination reaction, is a chemical process in which two or more simple elements or compounds combine to form a more complex product.
• Some synthesis reactions, like the formation of ammonia in the Haber process (N₂ + 3H₂ ⇌ 2NH₃), are reversible under specific conditions.

Replacement Reaction

• A replacement reaction occurs when elements switch places in compounds and involves ions.
• Generally, more reactive elements replace less reactive elements.
• A single replacement reaction occurs when one element replaces another element in one compound.
• A more reactive element replaces another, forming new products that don’t easily reverse.

Double Replacement Reaction

• Double replacement reactions—also called double displacement, exchange, or metathesis reactions—occur when parts of two ionic compounds are exchanged.
• The positive ions (cation) and the negative ions (anion) of the two reactants switch places, forming two new compounds or products.
• If a precipitate, gas, or weak electrolyte forms, the reaction is typically irreversible.

Displacement Reaction

• A displacement reaction is a chemical reaction in which a more reactive element displaces a less reactive element from its compound.
• Both metals and non-metals take part in displacement reactions.
• The replaced element does not easily revert to its original form.

Double Displacement Reaction

• In a double displacement reaction (also known as a double replacement or metathesis reaction), two compounds exchange ions, resulting in the formation of two new compounds.
• If a new stable compound forms, it is hard to reverse the reaction.

Redox Reaction (Oxidation)

• Oxidation is the gain of oxygen.
• For example, in the reaction H2 + CuO --> Cu + H2O, hydrogen is oxidized since it gained oxygen.
• Oxidation is the loss of electrons in a substance, increasing its oxidation number.
• The oxidized substance can sometimes be reduced again, as oxidation is part of a redox pair.

Redox Reaction (Reduction)

• Reduction is the loss of oxygen.
• For example, in the reaction H2 + CuO --> Cu + H2O, copper is reduced since it has lost oxygen.
• Reduction is the gain of electrons in a substance, decreasing its oxidation number.
• Reduction is the opposite of oxidation, and both occur together in a reversible redox system like in electrochemical cells.

Oxidation Number

• The oxidation number of an atom is defined as the charge that an atom appears to have on forming ionic bonds with other heteroatoms.
• An atom having higher electronegativity is assigned a negative oxidation state, even if it forms a covalent bond.

Electronegativity

• Electronegativity is an atom's ability to attract electrons when it's forming a covalent chemical bond.
• Electronegativity indicates how strongly an atom pulls on shared electrons.