 Let's start with the idea of a chemical reaction. Reactions occur when two or more molecules interact and the molecules change. Bonds between atoms are broken and created to form new molecules. That's it. What molecules are they? How do they interact? What happens? The possibilities are infinite. When you are trying to understand chemical reactions, imagine that you are working with the atoms. Imagine the building blocks are right in front of you on the table. Sometimes we use our chemistry toys to help us visualize the movement of the atoms. We plug and unplug the little connectors that represent chemical bonds. There are a few key points you should know about chemical reactions: 1. A chemical change must occur. You start with one molecule and turn it into another. Chemical bonds are made or broken in order to create a new molecule. One example of a chemical reaction is the rusting of a steel garbage can. That rusting happens because the iron (Fe) in the metal combines with oxygen (O2) in the atmosphere. Chemical bonds are created and destroyed to finally make iron oxide (Fe2O3).
2. A reaction could include atoms, ions, compounds, or molecules of a single element. You need to remember that a chemical reaction can happen with anything, just as long as a chemical change occurs. If you put pure hydrogen gas (H2) and pure oxygen gas in a room, they might be involved in a reaction to form water (H2O). However, it will be in very very small amounts. If you were to add a spark, those gases would be involved in a violent chemical reaction that would result in a huge explosion (exothermic). Another chemical reaction might include silver ions (Ag+). If you mix a solution with silver ions with a solution that has chloride (Cl-) ions, silver chloride (AgCl) precipitate will form and drop out of solution.
According to the modern view of chemical reactions, bonds between atoms in the reactants must be broken, and the atoms or pieces of molecules are reassembled into products by forming new bonds. Energy is absorbed to break bonds, and energy is evolved as bonds are made. In some reactions the energy required to break bonds is larger than the energy evolved in making new bonds, and the net result is the absorption of energy. Hence, different types of bonds may be formed in a reaction. A Lewis acid-base reaction, for example, involves the formation of a covalent bond between a Lewis base, a species that supplies an electron pair, and a Lewis acid, a species that can accept an electron pair. Ammonia is an example of a Lewis base. A pair of electrons located on a nitrogen atom may be used to form a chemical bond to a Lewis acid. Learn about the different types of chemical bonds. Chemists classify chemical reactions in a number of ways: by type of product, by types of reactants, by reaction outcome, and by reaction mechanism. Often a given reaction can be placed in two or even three categories, including gas-forming and precipitation reactions. Many reactions produce a gas such as carbon dioxide, hydrogen sulfide, ammonia, or sulfur dioxide. Cake batter rising is caused by a gas-forming reaction between an acid and baking soda (sodium hydrogen carbonate). Classification by types of reactants include acid-base reactions and oxidation-reduction reactions, which involve the transfer of one or more electrons from a reducing agent to an oxidizing agent. Examples of classification by reaction outcome include decomposition, polymerization, substitution, and elimination and addition reactions. Chain reactions and photolysis reactions are examples of classification by reaction mechanism, which provides details on how atoms are shuffled and reassembled in the formation of products. Read more below: Classifying chemical reactions Learn about acid-base reactions. Learn about chain, or self-sustaining, reactions. chemical reaction, a process in which one or more substances, the reactants, are converted to one or more different substances, the products. Substances are either chemical elements or compounds. A chemical reaction rearranges the constituent atoms of the reactants to create different substances as products. Chemical reactions are an integral part of technology, of culture, and indeed of life itself. Burning fuels, smelting iron, making glass and pottery, brewing beer, and making wine and cheese are among many examples of activities incorporating chemical reactions that have been known and used for thousands of years. Chemical reactions abound in the geology of Earth, in the atmosphere and oceans, and in a vast array of complicated processes that occur in all living systems. melting ice Chemical reactions must be distinguished from physical changes. Physical changes include changes of state, such as ice melting to water and water evaporating to vapour. If a physical change occurs, the physical properties of a substance will change, but its chemical identity will remain the same. No matter what its physical state, water (H2O) is the same compound, with each molecule composed of two atoms of hydrogen and one atom of oxygen. However, if water, as ice, liquid, or vapour, encounters sodium metal (Na), the atoms will be redistributed to give the new substances molecular hydrogen (H2) and sodium hydroxide (NaOH). By this, we know that a chemical change or reaction has occurred. The concept of a chemical reaction dates back about 250 years. It had its origins in early experiments that classified substances as elements and compounds and in theories that explained these processes. Development of the concept of a chemical reaction had a primary role in defining the science of chemistry as it is known today.
Put your science smarts under the microscope and see how much you know about bloodstones, biomes, buoyancy, and more! The first substantive 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 had particular significance. The influence of French chemist Antoine-Laurent Lavoisier was especially notable, in that his insights confirmed the importance of quantitative measurements of chemical processes. In his book Traité élémentaire de chimie (1789; Elementary Treatise on Chemistry), Lavoisier identified 33 “elements”—substances not broken down into simpler entities. Among his many discoveries, Lavoisier accurately measured the weight gained when elements were oxidized, and he ascribed the result to the combining of the element with oxygen. The concept of chemical reactions involving the combination of elements clearly emerged from his writing, and his approach led others to pursue experimental chemistry as a quantitative science. The other occurrence of historical significance concerning chemical reactions was the development of atomic theory. For this, much credit goes to English chemist John Dalton, who postulated his atomic theory early in the 19th century. Dalton maintained that matter is composed of small, indivisible particles, that the particles, or atoms, of each element were unique, and that chemical reactions were involved in rearranging atoms to form new substances. This view of chemical reactions accurately defines the current subject. Dalton’s theory provided a basis for understanding the results of earlier experimentalists, 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 identical elemental compositions). Get a Britannica Premium subscription and gain access to exclusive content. Subscribe Now Thus, experiment and theory, the two cornerstones of chemical science in the modern world, together defined the concept of chemical reactions. Today experimental chemistry provides innumerable examples, and theoretical chemistry allows an understanding of their meaning. When making a new substance from other substances, chemists say either that they carry out a synthesis or that they synthesize the new material. Reactants are converted to products, and the process is symbolized by a chemical equation. For example, iron (Fe) and sulfur (S) combine to form iron sulfide (FeS). Fe(s) + S(s) → FeS(s) 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 designated with the symbols (s) for solids, (l) for liquids, and (g) for gases. |
When a chemical reaction occurs, what happens to the atoms of the two substances?
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