Updated March 23, 2018 By Lana Bandoim
If you spend time in a chemistry class, you have to learn how to balance equations. Although this may seem like a tedious task, it demonstrates a fundamental law of matter. Making sure both sides of an equation match up on an atomic level demonstrates the law of the conservation of mass.
Balancing equations demonstrates the fundamental law of the conservation of mass. It shows that you cannot create or destroy mass in a chemical reaction, so the mass stays constant.
The law of the conservation of mass states that the total weight of a reaction cannot change because matter cannot be destroyed or created. During a chemical reaction, the mass of the reactants and products must be the same. The total number of atoms stays equal. Elements cannot magically appear or disappear in a reaction, so you have to account for all of them.
In 1789, Antoine Lavoisier found that you cannot destroy or create matter, and the law of the conservation of mass was born. Although he gets most of the credit, he was not the first person to discover or notice this fundamental law in nature. During the fifth century, the Greek philosopher Anaxagoras said that you cannot create or destroy anything because everything is a rearrangement of prior ingredients.
To balance a chemical equation, you make sure the number of atoms for all the elements is the same on both sides – the number of atoms on the reactant side must match the amount on the product side. You cannot change the actual formula while balancing the equation.
Start the process by counting the number of elements on each side. Then, check if both sides are the same. If they are not, use coefficients, which are numbers in front of the formulas, to balance them.
For example, to balance the equation N2 + H2 -> NH3, you would have to make it N2 + 3H2 -> 2NH3, so all the atoms match up on both sides.
A balanced chemical reaction has the same number of atoms on the reactant and product sides. You can use coefficients to achieve this balance, such as multiplying by three and two as in the example.
In reactions under normal laboratory conditions, matter is neither created nor destroyed, and elements are not transformed into other elements. Therefore, equations depicting reactions must be balanced; that is, the same number of atoms of each kind must appear on opposite sides of the equation. 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. Chemists ordinarily work with weighable quantities of elements and compounds. For example, in the iron-sulfur equation the symbol Fe represents 55.845 grams of iron, S represents 32.066 grams of sulfur, and FeS represents 87.911 grams of iron sulfide. Because matter is not created or destroyed in a chemical reaction, the total mass of reactants is the same as the total mass of products. If some other amount of iron is used, say, one-tenth as much (5.585 grams), only one-tenth as much sulfur can be consumed (3.207 grams), and only one-tenth as much iron sulfide is produced (8.791 grams). If 32.066 grams of sulfur were initially present with 5.585 grams of iron, then 28.859 grams of sulfur would be left over when the reaction was complete. The reaction of methane (CH4, a major component of natural gas) with molecular oxygen (O2) to produce carbon dioxide (CO2) and water can be depicted by the chemical equation CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) Here another feature of chemical equations appears. The number 2 preceding O2 and H2O is a stoichiometric factor. (The number 1 preceding CH4 and CO2 is implied.) This 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). Analogously with the iron-sulfur example, we can say that 16 grams of methane and 64 grams of oxygen will produce 44 grams of carbon dioxide and 36 grams of water. That is, 80 grams of reactants will lead to 80 grams of products. 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, and 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. By international agreement, one mole of the most common isotope of carbon (carbon-12) has a mass of exactly 12 grams (this is called the molar mass) and 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; and one mole of water is 18.015 grams. Each of these masses represents 6.022140857 × 1023 molecules. Answer: Chemical reaction is just a rearrangement of atoms. It can neither create nor destroyed during the course of a chemical reaction. Chemical equations must be balanced to satisfy the law of conservation of matter, that states that matter cannot be produced or destroyed in a closed system. The law of conservation of mass governs the balancing of a chemical equation. According to this law, mass can neither be created nor be destroyed in a chemical reaction and obeying this law the total mass of the elements or molecules present on the reactant side should be equal to the total mass of elements or molecules present on the product side. If the chemical equation is not balanced the law of conservation is not applicable. The representation of a chemical reaction in the form of substances is known as a chemical equation. The equation in which the number of atoms of all the molecules is equal on both sides of the equation is known as a balanced chemical equation.
Take for example the combustion of methane (#"CH"_4"#): #"CH"_4"# + #"O"_2"# #rarr# #"CO"_2"# + #"H"_2"O"# If you count the number of atoms (subscripts) of carbon, hydrogen, and oxygen on both sides of the equation, you will see that on the reactant side (left side), there are one atom of carbon, four atoms of hydrogen, and two atoms of oxygen. On the product side (right side), there are one atom of carbon, two atoms of hydrogen, and three atoms of oxygen. Therefore, the equation does not satisfy the law of conservation of mass, and is not balanced. In order to balance the equation, we must change the amounts of the reactants and products, as necessary, by adding coefficients in front of the appropriate formula(s). When balancing an equation, NEVER change the subscripts, because that changes the substance. #"H"_2"O"# is NOT the same substance as #"H"_2"O"_2"#. To determine the number of atoms of each element, the coefficient is multiplied times the subscripts in each formula. If there is no coefficient or subscript, it is understood to be 1. The balanced equation for the combustion of methane is: #"CH"_4"# + #2"O"_2"# #rarr# #"CO"_2"# + #"2H"_2"O"# If you compare the unbalanced equation to the balanced equation, you will see that the chemical formulas of each reactant and product were not changed. The only change is the coefficient of 2 written in front of the formula for oxygen on the reactant side, and the coefficient of 2 written in front of the formula for water on the product side. So now there are one carbon atom, four hydrogen atoms, and four oxygen atoms on both sides of the equation, and the equation is balanced. Now the equation says that "One molecule of methane plus two molecules of oxygen produce one molecule of carbon dioxide and two molecules of water". When working with moles, the equation would be read as "One mole of methane plus two moles of oxygen produce one mole of carbon dioxide and two moles of water". Here is a video which discusses the importance of balancing a chemical equation. |
Why does a chemical equation need to be balanced to support the law of conservation of matter
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