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Chemistry 141 Fall 2003 Balancing chemical equations Balancing Chemical Equations What does a chemical equation tell us? The chemical formulas used in chemical equations can represent units of individual chemicals or represent moles of these chemicals. It is commonly understood that the coefficients in an equation represent moles of the reactants or products. There are several typical steps we have to follow to write a balanced chemical equation. Many equations can be balanced by trial and error while many require a lot of careful work. • • Equations can be balanced by adjusting the number of atoms of each element so that their number is the same on both sides of the equation. Do not change any formulas to balance an equation. The general procedure is as follows: 1). Identify the reaction: Write a description or word equation for the reaction. Consider the following example where mercury (II) oxide decomposes (breaks down) to mercury and oxygen when heated: Mercury (II) oxide mercury + oxygen 2). Write the unbalanced equation using the correct formulas, which should be determined from using the rules of naming. For our example: HgO Hg + O2 The oxygen has to written as O2 since it is a diatomic gas. 3). Balance the equation: Use the following steps as necessary: a). Count and compare the number of atoms of each element on each side of the equation and decide those that must be balanced. In our example Hg is balanced since there is 1 Hg atom on each side, the oxygen is unbalance since there are 2 atoms of oxygen on the products side while there is only 1 oxygen atom on the reactant side. b). Balance each element by placing coefficients (whole numbers that are always greater than zero, leave blank if 1) in front of the formulas containing the unbalanced element. It is usually best to balance in the order of metals, non-metals, hydrogen and then oxygen. In our example we need to place a 2 in front of the HgO to balance of oxygen: 2 HgO Hg + O2 1 Chemistry 141 Fall 2003 Balancing chemical equations c). A coefficient multiplies every atom in the formula by that number. Select the smallest coefficients that will give the same number of atoms of the elements on each side. d). Check all other elements, after each individual element is balanced to see whether, in balancing one element, other elements have become unbalanced. Make adjustments as needed. In our example the Hg is now unbalanced and we need to put a 2 in front of the Hg on the right. 2 HgO 2 Hg + O2 (balanced) 4). Do a final check to ensure that each element is balanced and the smallest possible set of whole-numbered coefficients is used. Thus: 2 HgO 4 HgO 2 Hg + O2 (correct form) 4 Hg + 2 O2 (INCORRECT) Consider the following equations for practice: 1. Write the balanced equation for the reaction when Magnesium metal (s) is burned in air to produce magnesium oxide (s). 2. Benzene (C6H6) an explosive liquid when burned in air produces carbon dioxide (g) and water (g). 3. Nitrogen (g) reacts with hydrogen (g) to produce ammonia (g). 4. Silver nitrate (aq) reacts with hydrogen sulfide (g) to produce silver sulfide (s) and nitric acid (HNO3 (aq)). 5. Sodium hydroxide (aq) when mixed with sulfuric acid (aq) produces sodium sulfate (aq) and water (l). 2