The world around us is a constant symphony of change, a whirlwind of transformations. From the rusting of iron to the burning of fuel, these changes are governed by the intricate dance of atoms and molecules, a dance we call chemical reactions. At the heart of this dance lies the rearrangement of atoms, a process that shapes our world in countless ways.
What is a Chemical Reaction?
A chemical reaction is a process where one or more substances, called reactants, are changed into one or more different substances, called products. Substances are either chemical elements or compounds. A chemical reaction rearranges the atoms in the reactants to create new substances as products.
Reactants and Products
Chemical reactions involve two key players: reactants and products. Reactants are the starting materials that undergo a chemical change, while products are the newly formed substances that emerge from the reaction. Imagine baking a cake; the flour, sugar, and eggs are the reactants, and the finished cake is the product.
Rearranging Atoms
Chemical reactions involve the breaking and forming of chemical bonds between atoms. Bonds hold atoms together in molecules, and their disruption and reformation allow for the creation of new arrangements. This rearrangement is not simply a physical shift of atoms but a fundamental change in their relationships, resulting in the formation of new substances with different properties.
Chemical Reactions vs. Physical Changes
While both chemical reactions and physical changes involve transformations, they differ in their fundamental nature. Physical changes alter the form or appearance of a substance without changing its chemical composition. For example, melting ice is a physical change; the water molecules remain intact, only their arrangement changes. During a physical change, the substance’s physical properties change, but its chemical identity stays the same. Water (H2O) is the same compound, with each molecule made of two hydrogen atoms and one oxygen atom.
However, burning wood is a chemical reaction; the wood’s molecules are broken down and rearranged, creating new substances like ash and carbon dioxide. When water, as ice, liquid, or vapor, encounters sodium metal (Na), the atoms will be rearranged to form new substances: molecular hydrogen (H2) and sodium hydroxide (NaOH).
The Importance of Chemical Reactions
Chemical reactions are the lifeblood of our world, driving countless processes essential to life, technology, and culture. Chemical reactions are an important part of technology, culture, and life itself.
- Life’s Symphony: From the intricate biochemical reactions within our bodies to the photosynthetic processes in plants, chemical reactions are the foundation of life itself.
- Technological Marvels: From the synthesis of plastics and pharmaceuticals to the combustion of fuels powering our cars, chemical reactions drive the technological advancements that shape our modern world.
- Cultural Influence: From the ancient art of pottery to the modern techniques of food preservation, chemical reactions have played a pivotal role in shaping human culture and society.
Tracing the Evolution of Chemical Reactions
The concept of chemical reactions, like many scientific concepts, emerged gradually over centuries, a culmination of countless observations and experiments. The concept of a chemical reaction dates back about 250 years. The concept of a chemical reaction had its origins in early experiments that classified substances as elements and compounds and in theories that explained these processes.
Early Explorations
Early civilizations, fueled by curiosity and a desire to understand the world around them, conducted experiments that laid the groundwork for understanding chemical reactions. Alchemists, driven by the pursuit of gold and other mystical substances, experimented with various materials and processes, unknowingly laying the foundation for modern chemistry.
The Pioneers of Chemical Change: Scheele, Priestley, and Lavoisier
Scientists like Carl Wilhelm Scheele, Joseph Priestley, and Antoine Lavoisier made significant contributions to our understanding of chemical reactions. The first important studies in this area were on gases. The identification of oxygen in the 18th century by Swedish chemist Carl Wilhelm Scheele and English clergyman Joseph Priestley was particularly significant. Scheele, known for his work with oxygen, discovered numerous elements and investigated the properties of various chemicals. Priestley, in his experiments with gases, discovered oxygen’s role in combustion. Lavoisier, considered the father of modern chemistry, revolutionized our understanding of chemical reactions through his meticulous experiments and the establishment of the law of conservation of mass. The influence of French chemist Antoine-Laurent Lavoisier was especially notable. Lavoisier’s insights confirmed the importance of measuring chemical processes carefully. Lavoisier identified 33 “elements” – substances that could not be broken down into simpler substances. Lavoisier accurately measured the weight gained when elements were oxidized. Lavoisier explained this result by saying that the element combined with oxygen. The idea of chemical reactions involving the combination of elements clearly emerged from Lavoisier’s writings. Lavoisier’s approach led others to study experimental chemistry as a science that uses measurements.
The Atomic Revolution: Dalton’s Theory
John Dalton’s atomic theory, proposed in the early 19th century, played a crucial role in defining the modern concept of chemical reactions. Another historically significant event concerning chemical reactions was the development of atomic theory. Much credit for developing atomic theory goes to English chemist John Dalton. Dalton proposed his atomic theory in the early 19th century. Dalton believed that matter is made of tiny, indivisible particles called atoms, that each element has unique atoms, and that chemical reactions involve rearranging atoms to create new substances. Dalton’s view of chemical reactions accurately describes the subject as we understand it today. Dalton’s theory provided a basis for understanding the results of earlier experiments, including the law of conservation of matter (matter is neither created nor destroyed) and the law of constant composition (all samples of a substance have the same elemental composition). His theory, based on the idea that matter is composed of indivisible particles called atoms, provided a framework for understanding how atoms rearrange during chemical reactions.
Chemical Equations
Chemical equations are a concise and powerful language used to represent chemical reactions. They provide a symbolic representation of the reactants, products, and the stoichiometric relationships between them. Experiment and theory are the two essential parts of chemical science in the modern world. Experiment and theory together defined the concept of chemical reactions. Chemists say that they either carry out a synthesis or that they synthesize the new material. Reactants are converted into products. The process of converting reactants to products is symbolized by a chemical equation. Iron (Fe) and sulfur (S) combine to form iron sulfide (FeS). The plus sign indicates that iron reacts with sulfur. The arrow signifies that the reaction “forms” or “yields” iron sulfide, the product. The state of matter of reactants and products is shown with the symbols (s) for solids, (l) for liquids, and (g) for gases.
A Fundamental Principle: Conservation of Matter
The law of conservation of mass, established by Lavoisier, is a fundamental principle governing chemical reactions. It states that in a closed system, the total mass of the reactants before a chemical reaction must equal the total mass of the products after the reaction. Matter is neither created nor destroyed in reactions under normal laboratory conditions. Elements are not transformed into other elements. Equations depicting reactions must be balanced. The balanced equation for the iron-sulfur reaction shows that one iron atom can react with one sulfur atom to give one formula unit of iron sulfide. The symbol Fe represents 55.845 grams of iron. The symbol S represents 32.066 grams of sulfur. The symbol FeS represents 87.911 grams of iron sulfide. The total mass of reactants is the same as the total mass of products. In other words, matter is neither created nor destroyed during a chemical reaction; it simply changes form.
Balancing the Equation: Ensuring Equality
Balancing chemical equations is essential for maintaining the law of conservation of mass. Balancing involves adjusting the coefficients in front of each chemical formula to ensure that the number of atoms of each element is equal on both sides of the equation.
Chemical Stoichiometry: Quantifying Change
Chemical stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It allows us to predict the amount of products formed from a given amount of reactants. The chemical reaction of methane (CH4, a major component of natural gas) with molecular oxygen (O2) to produce carbon dioxide (CO2) and water can be shown by the chemical equation CH4(g) + 2O2(g) → CO2(g) + 2H2O(l). The number 2 before O2 and H2O is a stoichiometric factor. The number 1 before CH4 and CO2 is implied. The stoichiometric factor indicates that one molecule of methane reacts with two molecules of oxygen to produce one molecule of carbon dioxide and two molecules of water. The equation is balanced because the same number of atoms of each element appears on both sides of the equation (here one carbon, four hydrogen, and four oxygen atoms). 16 grams of methane and 64 grams of oxygen will produce 44 grams of carbon dioxide and 36 grams of water. 80 grams of reactants will lead to 80 grams of products.
The Mole: A Unit of Measurement
The mole is the SI unit for measuring the amount of substance. It represents a specific number of particles, namely Avogadro’s number, which is approximately 6.022 × 10^23. The ratio of reactants and products in a chemical reaction is called chemical stoichiometry. Stoichiometry depends on the fact that matter is conserved in chemical processes. Calculations giving mass relationships are based on the concept of the mole. One mole of any element or compound contains the same number of atoms or molecules, respectively, as one mole of any other element or compound. One mole of the most common isotope of carbon (carbon-12) has a mass of exactly 12 grams. One mole of the most common isotope of carbon (carbon-12) is called the molar mass. One mole of the most common isotope of carbon (carbon-12) represents 6.022140857 × 1023 atoms (Avogadro’s number). One mole of iron contains 55.847 grams. One mole of methane contains 16.043 grams. One mole of molecular oxygen is equivalent to 31.999 grams. One mole of water is 18.015 grams. Each of these masses represents 6.022140857 × 1023 molecules.
Energy: The Driving Force of Change
Energy plays a crucial role in chemical reactions. Some reactions release energy, known as exothermic reactions, while others require energy input to proceed, known as endothermic reactions. The energy change in a chemical reaction is often expressed as enthalpy, a measure of the heat absorbed or released during the reaction. Energy plays a key role in chemical processes. In the modern view of chemical reactions, bonds between atoms in the reactants must be broken. In the modern view of chemical reactions, atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds. Energy is evolved as bonds are made. The energy required to break bonds is larger than the energy evolved on making new bonds.
Entropy: The Measure of Disorder
Entropy, a measure of disorder or randomness, also influences chemical reactions. Reactions tend to favor an increase in entropy, meaning that they spontaneously proceed towards a state of greater disorder. A reaction is said to be endothermic if the energy is in the form of heat. Endothermic is the opposite of exothermic. In an exothermic reaction, energy as heat is evolved. Exoergic is the more general term for energy evolved. Endoergic is the more general term for energy required. The formation of compounds from the constituent elements is almost always exothermic. The formation of water from molecular hydrogen and oxygen and the formation of a metal oxide such as calcium oxide (CaO) from calcium metal and oxygen gas are examples of exothermic reactions. The combustion of fuels (such as the reaction of methane with oxygen mentioned previously) is an example of a widely recognizable exothermic reaction. Entropy is important in determining the favourability of a reaction. Entropy is a measure of the number of ways in which energy can be distributed in any system. Entropy accounts for the fact that not all energy available in a process can be manipulated to do work. A chemical reaction will favor the formation of products if the sum of the changes in entropy for the reaction system and its surroundings is positive. Wood has a low entropy. Burning wood produces ash as well as the high-entropy substances carbon dioxide gas and water vapor. The entropy of the reacting system increases during combustion. Heat energy transferred by the combustion to its surroundings increases the entropy in the surroundings. The total of entropy changes for the substances in the reaction and the surroundings is positive. The combustion of hydrogen is product-favoured.
Initiating Change: The Role of Energy Input
For many chemical reactions, energy input is necessary to overcome the activation energy barrier, the minimum energy required for the reaction to proceed. This energy input can be provided in various ways, such as heat, light, or electrical energy. Chemical reactions commonly need an initial input of energy to begin the process. The combustion of wood, paper, or methane is an exothermic process. A burning match or a spark is needed to initiate this reaction. The energy supplied by a match arises from an exothermic chemical reaction that is itself initiated by the frictional heat generated by rubbing the match on a suitable surface. The energy to initiate a reaction can be provided by light. Numerous reactions in Earth’s atmosphere are photochemical, or light-driven, reactions initiated by solar radiation. The transformation of ozone (O3) into oxygen (O2) in the troposphere is an example of a photochemical reaction. The absorption of ultraviolet light (hν) from the Sun to initiate this reaction prevents potentially harmful high-energy radiation from reaching Earth’s surface.
Influencing the Rate of Change: Factors Affecting Reaction Rates
Several factors can influence the rate at which chemical reactions occur. These factors include:
- Temperature: Increasing temperature generally increases reaction rates, as molecules have more kinetic energy and thus a greater chance of colliding with sufficient energy to break bonds.
- Concentration: Increasing the concentration of reactants increases the frequency of collisions between molecules, leading to a faster reaction rate.
- Surface Area: For reactions involving solids, increasing the surface area exposed to the reactants can increase the reaction rate.
- Catalysts: Catalysts are substances that speed up the rate of a chemical reaction without being consumed in the process. They provide an alternative pathway with a lower activation energy barrier. A reaction must also occur at an observable rate. Reaction rates are influenced by the concentrations of reactants, the temperature, and the presence of catalysts. Concentration affects the rate at which reacting molecules collide. Temperature is influential because reactions occur only if collisions between reactant molecules are sufficiently energetic. The proportion of molecules with sufficient energy to react is related to the temperature. Catalysts affect rates by providing a lower energy pathway by which a reaction can occur. Precious metal compounds used in automotive exhaust systems accelerate the breakdown of pollutants such as nitrogen dioxide into harmless nitrogen and oxygen. Chlorophyll in plants facilitates the reaction by which atmospheric carbon dioxide is converted to complex organic molecules such as glucose.
Biological Catalysts: The Power of Enzymes
Enzymes are biological catalysts, proteins that speed up specific biochemical reactions within living organisms. They are essential for life, allowing reactions to occur at rates compatible with life processes. Biochemical catalysts are also known, including chlorophyll in plants and many biochemical catalysts called enzymes. The enzyme pepsin assists in the breakup of large protein molecules during digestion.
Classifying Chemical Reactions
Chemical reactions can be classified based on various criteria, providing a framework for understanding their diverse nature and behavior. Chemists classify reactions in a number of ways: by the type of product, by the types of reactants, by reaction outcome, and by reaction mechanism.
Classifying by Product Type
- Gas-forming Reactions: These reactions produce gaseous products, often accompanied by visible bubbling or fizzing. An example is the reaction of an acid with a carbonate, producing carbon dioxide gas. Gas-forming reactions produce a gas such as carbon dioxide, hydrogen sulfide (H2S), ammonia (NH3), or sulfur dioxide (SO2). When a metal carbonate such as calcium carbonate (CaCO3, the chief component of limestone, seashells, and marble) is mixed with hydrochloric acid (HCl) it produces carbon dioxide. Cake-batter rising is caused by a gas-forming reaction between an acid and baking soda, sodium hydrogen carbonate (sodium bicarbonate, NaHCO3). Tartaric acid (C4H6O6), an acid found in many foods, is often the acidic reactant. Most baking powders contain both tartaric acid and sodium hydrogen carbonate. Tartaric acid and sodium hydrogen carbonate are kept apart by using starch as a filler. Baking powder is mixed into the moist batter. The acid and sodium hydrogen carbonate dissolve slightly, which allows them to come into contact and react. Carbon dioxide is produced and the batter rises.
- Precipitation Reactions: These reactions involve the formation of an insoluble solid precipitate from the reaction of two soluble reactants. An example is the reaction of silver nitrate with sodium chloride, forming a white precipitate of silver chloride. The formation of an insoluble compound will sometimes occur when a solution containing a particular cation (a positively charged ion) is mixed with another solution containing a particular anion (a negatively charged ion). The solid that separates is called a precipitate. Compounds having anions such as sulfide (S2−), hydroxide (OH−), carbonate (CO32−), and phosphate (PO43−) are often insoluble in water. A precipitate will form if a solution containing one of these anions is added to a solution containing a metal cation such as Fe2+, Cu2+, or Al3+. Minerals are water-insoluble compounds. Precipitation reactions in nature can account for mineral formation in many cases, such as with undersea vents called “black smokers” that form metal sulfides.
Classifying by Reactant Type
- Oxidation-Reduction (Redox) Reactions: These reactions involve the transfer of electrons between reactants. One reactant undergoes oxidation (loss of electrons) while the other undergoes reduction (gain of electrons). An example is the reaction of iron with oxygen, where iron is oxidized to iron oxide (rust). Oxidation-reduction (redox) reactions involve the transfer of one or more electrons from a reducing agent to an oxidizing agent. The transfer of one or more electrons from a reducing agent to an oxidizing agent has the effect of reducing the real or apparent electric charge on an atom in the substance being reduced and of increasing the real or apparent electric charge on an atom in the substance being oxidized. Magnesium burns in oxygen to form magnesium oxide (MgO). The product of magnesium burning in oxygen to form magnesium oxide (MgO) is an ionic compound. Magnesium burning in oxygen to form magnesium oxide (MgO) occurs with each magnesium atom giving up two electrons and being oxidized and each oxygen atom accepting two electrons and being reduced. Another common redox reaction is one step in the rusting of iron in damp air. Iron metal is oxidized to iron dihydroxide (Fe(OH)2). Elemental oxygen (O2) is the oxidizing agent. Redox reactions are the source of the energy of batteries. The electric current generated by a battery arises because electrons are transferred from a reducing agent to an oxidizing agent through the external circuitry. In the common dry cell and in alkaline batteries, two electrons per zinc atom are transferred to the oxidizing agent. The transfer of two electrons per zinc atom to the oxidizing agent converts zinc metal to the Zn2+ ion. In dry-cell batteries, the electrons given up by zinc are taken up by ammonium ions (NH4+) present in the battery as ammonium chloride (NH4Cl). In alkaline batteries, electrons are transferred to a metal oxide such as silver oxide (AgO). The transfer of electrons to a metal oxide such as silver oxide (AgO) reduces silver oxide (AgO) to silver metal in the process.
- Acid-Base Reactions: These reactions involve the transfer of protons (H+) between reactants. Acids donate protons, while bases accept protons. An example is the reaction of hydrochloric acid with sodium hydroxide, forming water and salt. Acids and bases are important compounds in the natural world. The chemistry of acids and bases is central to any discussion of chemical reactions. The Arrhenius theory views an acid as a substance that increases the concentration of the hydronium ion (H3O+) in an aqueous solution. The Arrhenius theory views a base as a substance that increases the hydroxide ion (OH−) concentration. Well-known acids include hydrochloric acid (HCl), sulfuric acid (H2SO4), nitric acid (HNO3), and acetic acid (CH3COOH). Bases include such common substances as caustic soda (sodium hydroxide, NaOH) and slaked lime (calcium hydroxide, Ca(OH)2). Ammonia (NH3) reacts with water to give a basic solution. The reaction of ammonia (NH3) with water to give a basic solution occurs to a very small extent. The hydroxide ion concentration is small but measurable. Natural bases are known, including morphine, cocaine, nicotine, and caffeine. Synthetic drugs are also bases. Amino acids are able to function both as acids and as bases. Amino acid molecules contain both acidic (―COOH) and basic (―NH2) sites. Amino acids exist in both the molecular form and the so-called “zwitterionic” form, H3N + CH2CO2−. In the Arrhenius theory, acid-base reactions involve the combination of the hydrogen ion (H+) and the hydroxide ion to form water. The reaction of aqueous solutions of sodium hydroxide and hydrochloric acid is an example of an acid-base reaction. The Brønsted-Lowry theory defines an acid as a proton donor. The Brønsted-Lowry theory defines a base as a proton acceptor. In the Brønsted-Lowry theory, the reaction of an acid and base is represented as an equilibrium reaction. Acid (1) and base (1) are called a conjugate acid-base pair. Acid (2) and base (2) are called a conjugate acid-base pair. The Brønsted-Lowry theory has the advantage of its predictive capacity. The equilibrium lies toward the reactants (reactant-favoured) or the products (product-favoured). The equilibrium is determined by the relative strengths of the acids and bases. The Brønsted-Lowry theory is often closely associated with the solvent water. Dissolving an acid in water to form the hydronium ion and the anion of the acid is an acid-base reaction. Acids are classified as strong or weak. A strong acid ionizes completely in water to form the hydronium and chlorine (Cl−) ions in a product-favoured reaction. In the Brønsted-Lowry theory, the reaction of ammonia and hydrochloric acid in water is represented by the following equation: NH3(aq) + HCl(aq) → NH4+(aq) + Cl−(aq). Hydrochloric acid and the chlorine ion are one conjugate acid-base pair. The ammonium ion and ammonia are the other conjugate acid-base pair. The acid-base reaction is the transfer of the hydrogen ion from the acid (HCl) to the base (NH3). The equilibrium favors the weaker acid and base. The Lewis theory defines bases as electron-pair donors. The Lewis theory defines acids as electron-pair acceptors. Acid-base reactions involve the combination of the Lewis acid and base through sharing of the base’s electron pair. Ammonia is an example of a Lewis base. The pair of electrons located on the nitrogen atom may be used to form a chemical bond to a Lewis acid such as boron trifluoride (BF3). Ammonia, water, and many other Lewis bases react with metal ions to form a group of species known as coordination compounds. The reaction to form coordination compounds is another example of a Lewis acid-base reaction. The light blue color of a solution of Cu2+ ions in water is due to the [Cu(H2O)6]2+ ion. Ammonia is added to this solution. The water molecules attached to copper are replaced by ammonia molecules. The beautiful deep blue ion [Cu(NH3)4]2+ is formed when ammonia is added to a solution of Cu2+ ions in water.
Classifying by Reaction Outcome
- Decomposition Reactions: These reactions involve the breakdown of a single compound into two or more simpler substances. An example is the decomposition of hydrogen peroxide into water and oxygen. Chemists often classify reactions on the basis of the overall result. Decomposition reactions are processes in which chemical species break up into simpler parts. Decomposition reactions usually require energy input. A common method of producing oxygen gas in the laboratory is the decomposition of potassium chlorate (KClO3) by heat. Another decomposition reaction is the production of sodium (Na) and chlorine (Cl2) by electrolysis of molten sodium chloride (NaCl) at high temperature. A decomposition reaction that was very important in the history of chemistry is the decomposition of mercury oxide (HgO) with heat to give mercury metal (Hg) and oxygen gas. The decomposition of mercury oxide (HgO) with heat to give mercury metal (Hg) and oxygen gas is the reaction used by 18th-century chemists Carl Wilhelm Scheele, Joseph Priestley, and Antoine-Laurent Lavoisier in their experiments on oxygen.
- Substitution, Elimination, and Addition Reactions: These reactions are common in organic chemistry, involving the replacement of atoms or groups of atoms, the removal of atoms or groups of atoms, or the addition of atoms or groups of atoms to a molecule, respectively. In a substitution reaction, an atom or group of atoms in a molecule is replaced by another atom or group of atoms. Methane (CH4) reacts with chlorine (Cl2) to produce chloromethane (CH3Cl). Chloromethane (CH3Cl) is a compound used as a topical anesthetic. Substitution reactions are widely used in industrial chemistry. Substituting two of the chlorine atoms on chloroform (CHCl3) with fluorine atoms produces chlorodifluoromethane (CHClF2). Chlorodifluoromethane (CHClF2) undergoes a further reaction when heated strongly. In an elimination reaction, a hydrogen atom and a chlorine atom are eliminated from the starting material as hydrochloric acid (HCl). The other product of the reaction of chlorodifluoromethane (CHClF2) when heated strongly is tetrafluoroethylene. Tetrafluoroethylene is a precursor to the polymer known commercially as Teflon. Addition reactions are the opposite of elimination reactions. The common industrial preparation of ethanol (CH3CH2OH) is an example of an addition reaction. Ethanol (CH3CH2OH) was historically made by fermentation. Ethanol (CH3CH2OH) has been made commercially by the addition of water to ethylene.
- Polymerization Reactions: These reactions involve the joining of small molecules (monomers) into long chains (polymers). Examples include the formation of polyethylene from ethylene monomers. Polymers are high-molecular-weight compounds fashioned by the aggregation of many smaller molecules called monomers. The plastics that have so changed society and the natural and synthetic fibres used in clothing are polymers. Two basic ways to form polymers are linking small molecules together, a type of addition reaction, and combining two molecules (of the same or different type) with the elimination of a stable small molecule such as water. The latter type of polymerization combines addition and elimination reactions and is called a condensation reaction. The union of thousands of ethylene molecules gives polyethylene. Polypropylene is made by polymerizing H2C=CHCH3. Polystyrene is made from H2C=CH C6H5. Polyvinyl chloride is made from H2C=CHCl. Starch and cellulose are examples of the second type of polymer. Starch and cellulose are members of a class of compounds called carbohydrates. Carbohydrates are substances with formulas that are multiples of the simple formula CH2O. Starch and cellulose are polymers of glucose. Glucose is a sugar with the formula C6H12O6. In starch and cellulose, molecules of glucose are joined together with concomitant elimination of a molecule of water for every linkage formed. The synthetic material nylon is another example of this type of polymer. Water and a polymer (nylon-6,6) are formed when an organic acid and an amine (a compound derived from ammonia) combine. The natural fibres of proteins such as hair, wool, and silk are also polymers that contain the repeating unit (-CHRCONH-), where R is a group of atoms attached to the main polymer. The natural fibres of proteins such as hair, wool, and silk form by joining amino acids with the elimination of a water molecule for each CONH or peptide linkage formed. A tripeptide chain is formed from three units of the amino acid glycine (NH2CH2CO2H).
- Solvolysis and Hydrolysis Reactions: These reactions involve the cleavage of a chemical bond by a solvent molecule, such as water. Hydrolysis specifically involves the cleavage of a bond by water molecules. A solvolysis reaction is one in which the solvent is also a reactant. Solvolysis reactions are generally named after the specific solvent. The term hydrolysis is used when water is involved. A hydrolysis reaction may be represented by the reversible chemical reaction AB + HOH ⇌ AH + BOH. The hydrolysis of an organic compound is illustrated by the reaction of water with esters. Esters have the general formula RCOOR′, R and R′ being combining groups (such as CH3). The hydrolysis of an ester produces an acid and an alcohol. Hydrolysis reactions play an important role in chemical processes that occur in living organisms. Proteins are hydrolyzed to amino acids. Fats are hydrolyzed to fatty acids and glycerol. Starches and complex sugars are hydrolyzed to simple sugars. The rates of these processes are enhanced by the presence of enzymes, biological catalysts. Hydrolysis reactions are also important to acid-base behaviour. Anions of weak acids dissolve in water to give basic solutions. The hydrolysis of the acetate ion, CH3COO−, is an example of the hydrolysis of weak acids. A solution containing the acetate ion exhibits basic properties. Hydrolysis reactions account for the basic character of many common substances. Salts of the borate, phosphate, and carbonate ions give basic solutions that have long been used for cleaning purposes. Many food products also contain basic anions such as tartrate and citrate ions.
Classifying by Reaction Mechanism
- Chain Reactions: These reactions involve a series of steps where a product of one step acts as a reactant in the next step, creating a chain reaction. An example is the combustion of methane. Reaction mechanisms provide details on how atoms are shuffled and reassembled in the formation of products from reactants. Chain reactions occur in a sequence of steps in which the product of each step is a reagent for the next. Chain reactions generally involve three distinct processes: an initiation step that begins the reaction, a series of chain-propagation steps, and, eventually, a termination step. Polymerization reactions are chain reactions. The formation of Teflon from tetrafluoroethylene is one example of a chain reaction. A peroxide is a compound in which two oxygen atoms are joined together by a single covalent bond. Peroxides readily form highly reactive free-radical species. Free-radical species have an unpaired electron. Free-radical species initiate the reaction. Teflon is formed from tetrafluoroethylene. Peroxides decompose to form radicals. The chlorine molecule (Cl2) undergoes an endothermic reaction to give chlorine atoms. Chlorine atoms are formed under ultraviolet irradiation. Some chlorine atoms recombine to form chlorine molecules. When a chlorine atom collides with a methane molecule, a two-step chain propagation occurs. The first propagation step produces the methyl radical (CH3). The methyl radical (CH3) reacts with a chlorine molecule to give the product and a chlorine atom. The chlorine atom continues the chain reaction for many additional steps. Possible chain-termination steps include combination of two methyl radicals to form ethane (CH3CH3) and a combination of methyl and chlorine radicals to give chloromethane.
- Photolysis Reactions: These reactions involve the breaking of chemical bonds by light energy. An example is the splitting of water molecules into hydrogen and oxygen gas by sunlight during photosynthesis. Photolysis reactions are initiated or sustained by the absorption of electromagnetic radiation. The decomposition of ozone to oxygen in the atmosphere is mentioned above in the section Kinetic considerations. The synthesis of chloromethane from methane and chlorine is initiated by light. The overall reaction of the synthesis of chloromethane from methane and chlorine is CH4(g) + Cl2(g) + hυ → CH3Cl(g) + HCl(g). hυ represents light. The synthesis of chloromethane from methane and chlorine is also a chain reaction. The symbol (aq) signifies that a compound is in an aqueous, or water, solution.
Chemical Reactions vs. Physical Changes
Chemical reactions fundamentally differ from physical changes in that they involve the formation of new substances with distinct chemical compositions. Physical changes only alter the form or appearance of a substance, not its chemical makeup.
Chemical Reactions in Our Lives
Chemical reactions are ubiquitous in our daily lives, shaping our experiences and influencing our world in countless ways. A chemical reaction is a process where bonds within reactant molecules are broken and new bonds are formed within product molecules to create a new substance. Chemical reactions are all around us, from the metabolism of food in our body to how the light we get from the sun is the result of chemical reactions. A burning candle is the best example of physical and chemical change. The burning of the candle is a chemical change. The conversion of the candle to wax is a physical change. A physical change is basically a change of state of the substance. A chemical change is mostly where a new substance is formed in which either energy is given off or absorbed. Chemical changes are accompanied by certain physical changes.
- Cooking: Cooking is a symphony of chemical reactions, from the browning of meat through the Maillard reaction to the rising of bread through the fermentation of yeast.
- Rusting: The rusting of iron is a classic example of a chemical reaction where iron reacts with oxygen and water to form iron oxide, a process known as corrosion.
- Burning: Burning fuels like wood, gas, or coal is a combustion reaction involving the rapid reaction of these fuels with oxygen, releasing energy in the form of heat and light.
- Digestion: Our bodies rely on a complex series of chemical reactions to break down food into smaller molecules that can be absorbed and utilized for energy and growth.
Chemical Equations
Chemical equations are like recipes for chemical reactions, providing a symbolic representation of the reactants, products, and the stoichiometric relationships involved. A chemical reaction is a process that occurs when two or more molecules interact to form a new product(s). The compounds that interact to produce new compounds are called reactants. The newly formed compounds are called products. Chemical reactions play an integral role in different industries, customs and even in our daily life. Chemical reactions are continuously happening in our general surroundings. Rusting of iron, pottery, fermentation of wine are examples of chemical reactions happening in our general surroundings. In a chemical reaction, a chemical change must occur, which is generally observed with physical changes like precipitation, heat production, color change etc. A reaction can take place between two atoms or ions or molecules. In a reaction, they form a new bond. In a reaction, no atom is destroyed or created. In a reaction, a new product is formed from reactants. The rate of reaction depends on and is affected by factors like pressure, temperature, the concentration of reactants. A chemical equation is a mathematical statement which symbolizes the product formation from reactants while stating certain condition for which how the reaction has been conducted.
Balancing Chemical Equations
Balancing chemical equations is crucial for ensuring that the law of conservation of mass is upheld. It involves adjusting the coefficients in front of each chemical formula to ensure that the number of atoms of each element is equal on both sides of the equation. The reactants are on the left-hand side of a chemical equation. The products formed are on the right-hand side of a chemical equation. Reactants and products are connected by a one-headed or two-headed arrows in a chemical equation. A and B are the reactants. C and D are the products. Reactants are denoted by their chemical formula in a chemical equation. A chemical equation must be balanced. A chemical equation must assure the law of conservation of mass. Balancing of the equation is ensuring the number of atoms on both sides must be equal.
The Importance of Balancing
Balancing chemical equations is essential for accurately representing the quantitative relationships between reactants and products. It allows us to predict the amount of reactants required for a given amount of product, and vice versa, crucial information for chemical synthesis and analysis.
The Spectrum of Chemical Reactions
Chemical reactions are a diverse and essential aspect of our world, driving countless processes that shape our lives, our technologies, and our understanding of the universe. From the simple act of burning a candle to the complex biochemical reactions within our bodies, chemical reactions are the driving force of change, constantly rearranging the building blocks of our world. A combustion reaction is a reaction with a combustible material with an oxidizer to give an oxidized product. An oxidizer is a chemical a fuel requires to burn, generally oxygen. A decomposition reaction is a reaction in which a single component breaks down into multiple products. A decomposition reaction requires certain changes in energy in the environment like heat, light or electricity breaking bonds of the compound. A neutralization reaction is basically the reaction between an acid and a base giving salt and water as the products. A water molecule is formed by the combination of OH– ions and H+ ions. The overall pH of the products when a strong acid and a strong base undergo a neutralization reaction will be 7. A REDuction-OXidation reaction is a reaction in which there is a transfer of electrons between chemical species. A Precipitation or Double-Displacement Reaction is a type of displacement reaction in which two compounds react and consequently, their anions and cations switch places forming two new products. A synthesis reaction is one of the most basic types of reaction wherein multiple simple compounds combine under certain physical conditions giving out a complex product. The product of a synthesis reaction will always be a compound. In a chemical change, a new compound is formed but in a physical change, the substance changes its state of existence. Atoms or ions or molecules which react to form a new substance are called reactants. New atoms or molecules formed are called products.