.8: hemical Equations onding and hemical Reactions Law of onservation of Mass 1 2 hemical reactions are occurring both inside of us and all around us every second of every day. s a matter of fact, there is never a time when some type of chemical reaction is not occurring. In these processes, the starting chemical compounds (reactants) are destroyed, and new chemical compounds (products) are created. However, during all of these trillions of chemical reactions that occur each and every day, matter is neither destroyed, nor is matter created. ut, what does this mean? idn t you just read that compounds are destroyed and new compounds are made during each chemical reaction? Yes, this is true, but during the chemical reactions, the masses of the chemicals that reacted will always equal the masses of the products created. This is the fundamental definition of a law that governs all chemical reactions, called the Law of onservation of Mass. Simply stated, the definition of the Law of onservation of Mass is mass is conserved and is neither created nor destroyed in a non-nuclear change. The total mass of the reactants equals the total mass of the products. The graphic on this page illustrates this point. In the balanced chemical equation, there are the same number of hydrogen and oxygen atoms on both sides of the equation. Matter was neither created nor destroyed during the reaction, it simply changed forms. 1
.8: hemical Equations onding and hemical Reactions 3 4 5 So, how did early chemists discover that mass was neither created nor destroyed? The earliest Greek natural philosophers postulated that nothing comes from nothing. They knew that matter did not spontaneously appear, but they could not yet fully explain how or why it changed forms. From these early times, scientists used experiments and their keen powers of observation to try to answer these questions. Many early scientists, known at the time as alchemists, worked with various elements and combined them to investigate what was produced. However, the conservation of mass was difficult for early scientists to conceptualize as many of the gases required for or produced during chemical reactions was difficult, if not impossible, to quantify and measure. The gases were invisible in most cases and would either come from or escape back into the atmosphere. classic example of this dilemma occurred with the burning of wood. s the product ash has less mass than the reactant wood, it seemed that mass was lost. It was not until experiments were conducted in sealed containers that scientists began understand that during chemical reactions, the masses of the starting and ending substances did not change. In other words, scientists began to understand that during chemical reactions, mass was conserved. This concept was pivotal in the process that changed the study of alchemy into what we know as modern chemistry. One of these early and influential alchemists was ntoine Lavoisier (1743 1794), and he is credited with being the first to discover the principle of mass conservation. His experiments were a continuation of the work of Henry avendish (1731-1810) who discovered the element hydrogen (though he did not name it at the time.) uring his experiments, avendish discovered what he called inflammable air (hydrogen) and found that this substance formed water as a byproduct of combustion. t the time, both water and air were considered elements. Lavoisier wanted to better understand what water and air were made of, so he conducted very meticulous quantitative experiments. He was careful to measure the masses of both the reactants and products of the chemical reactions that he was working with. ue to his precise measurements, he concluded that although the matter may have changed forms during the reaction, that no mass was lost. s a result of his experiments, Lavoisier identified and named both elements hydrogen and oxygen and subsequently became known as the forefather of modern science. 2
.8: hemical Equations onding and hemical Reactions 1 The Law of onservation of Mass tells us, in part, that matter is neither created nor destroyed. How can the Law of onservation of Mass for a chemical reaction be expressed? y using coefficients in a chemical equation y writing a balanced chemical equation y counting the atoms present on each side of the equation ll of the above 2 Early chemists, known at the time as alchemists, had a difficult time understanding the Law of onservation of Mass. Using the burning of wood as an example, what property of matter made this law difficult for early scientists to understand? Wood is heavier than ash. The fire was too bright to observe the reaction. The gases produced escaped to the atmosphere. ifferent types of wood burned differently. 3
.8: hemical Equations onding and hemical Reactions 3 Is there something wrong with the chemical equation shown on this page? Look carefully at this equation. H 4 + O 2 O 2 + H 2 O What would need to be done to this equation to make sure that it follows the Law of onservation of Mass? Remove 2 hydrogen atoms (reactant) and 1 oxygen atom (product) dd coefficients to each compound to balance the number of atoms dd 2 hydrogen atoms (product) and 1 oxygen atom (reactant) This equation already follows the Law of onservation of Mass 4
.8: hemical Equations onding and hemical Reactions 4 Early scientists performed experiments regarding mass and products for many years, but had difficulty demonstrating that mass was conserved. This changed when they conducted these experiments under specific conditions. What condition was this? Refer to Paragraph 4. They conducted experiments with specific gases. They conducted experiments with specific types of wood. They created scales that gave more precise measurements. The experiments were conducted in sealed containers 5 Many scientists worked to explain how matter and mass were connected in the process of chemical reactions. One scientist stands out with regards to the Law of onservation of Matter, often being credited as being the Father of Modern Science. Who is this scientist? lbert Einstein Henry avendish ntoine Lavoisier medeo vogadro 5
.8: hemical Equations onding and hemical Reactions 6 alanced chemical equations follow the Law of onservation of Mass as these types of reactions will contain the same number of atoms of each element on the reactant side and the product side. I. H 4 + O 2 O 2 + 2 H 2 O II. Na + 2 H 2 O H 2 + 2 NaOH III. ao 3 ao + O 2 IV. 2 l + 3 l 2 2 ll 3 V. P + 2 O 2 2 P 2 O 3 Which of the chemical reactions shown above is correctly balanced? I, II, V III, IV II, IV I, III, V 6