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During a chemical reaction, atoms in reactants break their existing chemical bonds and form new bonds to create products. The atoms themselves remain unchanged—only their arrangement and bonding patterns differ. For example, methane and oxygen react to produce carbon dioxide and water through bond rearrangement, not atom creation or destruction.

Chemical equations must be balanced to satisfy the Law of Conservation of Matter, which states that matter cannot be created or destroyed in a reaction. Coefficients are added to ensure equal numbers of each atom type on both sides of the equation. For instance, balancing the hydrogen and oxygen equation requires 2H2 + O2 → 2H2O to account for all atoms.

Reversible reactions can proceed in both directions, with products converting back to reactants until equilibrium is reached. Irreversible reactions proceed only forward until reactants are consumed, such as combustion reactions that produce heat, light, carbon dioxide, and water. Reversible reactions show no net change once equilibrium is achieved.

Enzymes are macromolecules that act as catalysts, greatly speeding up chemical reactions in biological systems. Most biological reactions would take far too long to complete without enzyme assistance. Enzymes enable essential life processes like photosynthesis and cellular respiration to occur at rates compatible with organism survival.

Photosynthesis converts sunlight, carbon dioxide, and water into glucose and oxygen, storing solar energy in chemical bonds. Cellular respiration then breaks down glucose in the presence of oxygen to release usable energy for organism needs. These complementary chemical reactions power most life on Earth by cycling energy and matter.

In a balanced equation, reactants and products contain identical numbers and types of atoms, though arranged differently. Coefficients multiply all atoms in a compound, ensuring conservation of matter. For the ammonia synthesis reaction, one nitrogen molecule plus three hydrogen molecules yields two ammonia molecules, balancing all atoms on both sides.

Some reactions proceed only forward until reactants deplete, while others reach equilibrium where reactants and products coexist with no net concentration change. Reaction type, conditions, and reversibility determine the outcome. Under theoretical conditions, most reactions can be reversible, though practical factors like energy release or solid formation often drive reactions toward completion.