Chemical reaction
In 1667, Johann Joachim Becher proposed the phlogiston theory. This idea claimed a fire-like element called phlogiston existed inside combustible bodies. Scientists believed this substance released during combustion. The theory held sway for over a century until Antoine Lavoisier disproved it in 1785. Lavoisier found that combustion was actually a reaction with oxygen from the air. Earlier alchemists had tried to convert lead into gold using reactions of lead and lead-copper alloys with sulfur. They also produced mineral acids like sulfuric acid starting around 1300. In 1828, Friedrich Wöhler synthesized urea from inorganic precursors. This ended the belief that organic matter possessed a vital force. Joseph Proust developed the law of definite proportions based on work by John Dalton. These concepts eventually led to stoichiometry and chemical equations. By 1909, the Haber process allowed ammonia synthesis on an industrial scale.
A unimolecular reaction involves only one molecule transforming into other molecules. Such reactions require energy added as heat or light. A typical example is cis-trans isomerization where a compound converts forms. Dissociation splits a bond within a molecule resulting in two fragments. Homolytic splitting divides the bond so each product retains an electron. Heterolytic splitting leaves both electrons with one product creating charged ions. Bimolecular reactions occur when two molecules collide and react. Their merger is called chemical synthesis or addition. Redox reactions transfer electrons between species while acid-base reactions transfer protons. Elementary reactions are the smallest divisions of any chemical change. Most observed reactions build up from many elementary steps occurring sequentially. The actual sequence of these individual steps constitutes the reaction mechanism. Only one or two molecules participate because probability for several meeting is low.
Exothermic reactions release free energy and proceed spontaneously. Enthalpy changes determine if heat is released or consumed during the process. Endothermic reactions consume heat from the environment often forming gaseous products. Temperature influences which direction a reaction favors. The water gas shift reaction proceeds forward at low temperatures but reverse at high ones. Activation energy defines the amount required to start a reaction. Higher activation energy means reactants need more energy to begin. Catalysts lower this barrier without being destroyed themselves. Reaction rates increase as temperature rises due to more thermal energy available. Concentration affects speed by increasing collisions per unit time. Surface area matters for solid reactants in heterogeneous systems. Pressure decreases volume between molecules increasing collision frequency. Zero order reactions have rates independent of reactant concentrations. First-order reactions depend only on concentration and substance properties. The Arrhenius equation describes how rate constants vary with temperature.
Iron combines with sulfur to form iron(II) sulfide in synthesis reactions. Two or more simple substances combine to create complex ones. Electrolysis breaks water into oxygen and hydrogen gas via decomposition. A single displacement occurs when magnesium replaces hydrogen in water producing solid magnesium hydroxide. Double displacement swaps anions and cations between two compounds. Barium chloride reacts with magnesium sulfate switching SO42- ions with Cl- ions. Sodium metal reacts with chlorine gas to form sodium chloride through redox processes. Sodium loses one electron while chlorine gains that same electron. Elements with low electronegativity easily donate electrons acting as reducing agents. Oxides with high oxidation numbers can gain extra electrons serving as oxidizing agents. Combustion releases 5500 kJ from burning 1 mole of octane in oxygen. Photosynthesis converts carbon dioxide and water into glucose using solar energy. Batteries convert chemical energy back into electrical energy during discharge cycles.
Nucleophiles attack substrates forming new bonds while leaving groups depart. SN1 mechanisms proceed in two steps creating a carbocation intermediate. SN2 mechanisms involve simultaneous bond formation and cleavage without intermediates. Electrophilic substitution takes place almost exclusively in aromatic hydrocarbons. Radical substitution involves chain reactions where light disintegrates halogen molecules. E1 elimination ejects the leaving group first forming a carbocation. E2 eliminations require bases attacking simultaneously with leaving group departure. Markovnikov's rule predicts configuration when adding polar molecules to alkenes. Hydroboration-oxidation adds boron to less substituted carbon atoms. Michael reactions belong to conjugate additions forming C-C bonds mildly. The Diels-Alder reaction creates cyclohexene systems between dienes and alkenes. Cycloadditions form cyclic molecules requiring specific orbital overlaps. Woodward-Hoffmann rules describe potential for developing stereoisomeric products.
Biochemical reactions occur within active sites found in enzyme clefts. Induced fit models explain how molecular shape changes facilitate catalysis. Anabolism produces large molecules like proteins from smaller units using DNA control. Glucose and oxygen serve as important energy sources for organisms. All living things produce adenosine triphosphate to energize other reactions. Fungi decompose organic material breaking down complex structures into simpler ones. Enzymes increase rates of biochemical reactions impossible under ordinary conditions. They allow metabolic syntheses and decompositions to happen at cell temperatures. Protein enzymes catalyze consecutive series of chemical reactions called pathways. Bioenergetics studies these energy sources driving life processes forward.
Thermite reactions generate light and heat used in railway welding. Liquid iron flows into molds around rail gaps after sparks fly outwards. Chemical engineering synthesizes new compounds from natural raw materials like petroleum. Catalysts reduce energy required while increasing reaction speed significantly. Heterogeneous catalysts are solids powdered to maximize surface area. Platinum group metals hydrogenate commodity chemicals such as nitric acid. Homogeneous acids increase nucleophilicity allowing reactions with electrophiles. Real-time analysis measures pH and optical absorption spectra during slow processes. Radioactive isotopes track substance redistribution inside human bodies over time. Ultrafast laser spectroscopy monitors transition states scaled to femtoseconds. Safety monitoring techniques ensure efficient yield while minimizing waste output.
Common questions
When did Johann Joachim Becher propose the phlogiston theory?
Johann Joachim Becher proposed the phlogiston theory in 1667. This idea claimed a fire-like element called phlogiston existed inside combustible bodies.
Who disproved the phlogiston theory and when was it disproven?
Antoine Lavoisier disproved the phlogiston theory in 1785. He found that combustion was actually a reaction with oxygen from the air.
What year did Friedrich Wöhler synthesize urea from inorganic precursors?
Friedrich Wöhler synthesized urea from inorganic precursors in 1828. This synthesis ended the belief that organic matter possessed a vital force.
How does temperature influence the water gas shift reaction?
The water gas shift reaction proceeds forward at low temperatures but reverse at high ones. Temperature influences which direction a reaction favors.