Balancing Chemical Equations

The balanced equation for the reaction between iron and chlorine, shown below, reflects the law of conservation of mass. 2Fe(s) + 3Cl₂(g) ® 2FeCl₃(s)

To balance an equation, you must find the correct coefficients for the chemical formulas in the skeleton equation. A coefficient in a chemical equation is the number written in front of a reactant or product. Coefficients are usually whole numbers, and are usually not written if the value is 1. A coefficient tells you the smallest number of particles of the substance involved in the reaction. That is, the coefficients in a balanced equation describe the lowest whole-number ratio of the amounts of all of the reactants and products.

Steps for balancing equations Most chemical equations can be balanced by following the steps given below. For example, you can use these steps to write the chemical equation for the reaction between hydrogen and chlorine that produces hydrogen chloride.

Step 1 Write the skeleton equation for the reaction. Make sure that the chemical formulas correctly represent the substances. An arrow separates the reactants from the products, and a plus sign separates multiple reactants and products. Show the physical states of all reactants and products.

H₂(g) + Cl₂(g) ® HCl(g)

Step 2 Count the atoms of the elements in the reactants. If a reaction involves identical polyatomic ions in the reactants and products, count the ions as if they are elements. This reaction does not involve any polyatomic ions. Two atoms of hydrogen and two atoms of chlorine are reacting.

H₂ + Cl₂ ®

2 atoms H 2 atoms Cl

Step 3 Count the atoms of the elements in the products. One atom of hydrogen and one atom of chlorine are produced.

HCl

1 atom H + 1 atom Cl

Step 4 Change the coefficients to make the number of atoms of each element equal on both sides of the equation. Never change a subscript in a chemical formula to balance an equation because doing so changes the identity of the substance.

Step 5 Write the coefficients in their lowest possible ratio. The coefficients should be the smallest possible whole numbers. The ratio 1 hydrogen to 1 chlorine to 2 hydrogen chloride (1:1:2) is the lowest possible ratio, because the coefficients cannot be reduced and still remain whole numbers.

Step 6 Check your work. Make sure that the chemical formulas are written correctly. Then, check that the number of atoms of each element is equal on both sides of the equation.

Figure 10-4 It is imperative to your study of chemistry to be able to balance chemical equations. Use this flowchart to help you master the skill.

Probably the most fundamental concept of chemistry is the law of conservation of mass that you first encountered in Chapter 3. All chemical reactions obey the law that matter is neither created nor destroyed. Therefore, it is also fundamental that the equations that represent chemical reactions include sufficient information to show that the reaction obeys the law of conservation of mass. You have learned how to show this relationship with balanced chemical equations. The flowchart shown in Figure 10-4 summarizes the steps for balancing equations. You will undoubtedly find that some chemical equations can be balanced easily, whereas others are more difficult to balance.

Поиск по сайту: