Lesson 4.02 Balancing Equations

Lesson Standards & Objectives SC.912.P.8.8—Apply the mole concept and the law of conservation of mass to calculate quantities of chemicals participating in reactions. Describe how mass and atoms are always conserved in a reaction. Demonstrate the use of coefficients to balance equations.

Vocabulary (Literacy Strategy) Chemical Equation: A representation, using formulas and symbols, of a chemical reaction. Reactants: The substances that are present at the beginning of a reaction. These are located on the left side of the arrow. Products: The new substances that are formed or produced in a chemical reaction. These are located on the right side of the arrow. Sample equation: A + B  AB “A” and “B” are both reactants. “AB” is the product You will learn about different types of reactions and in lessons 4.03 – 4.05

Law of Conservation of Mass: Matter is neither created nor destroyed in a chemical reaction. So, what does this have to do with equations? It is important that chemical equations apply this law by having the same number of atoms of each element on both sides of the equation; chemists call this a “balanced” equation.

So, why do we need to balance? Consider the following reaction between hydrogen gas and oxygen gas: N 2 + H 2  NH 3 Both hydrogen and nitrogen are diatomic elements. When they react, they will always produce ammonia, NH 3. Notice how it appears as though we lost a nitrogen atom and gained a hydrogen atom? We really didn’t. We just need to balance this equation! Let’s get to it.

Diatomic Elements Please make sure that you know the 7 diatomic elements. H 2, N 2, O 2, F 2, Br 2, Cl 2, I 2 When these elements are by themselves in a reaction, they will always have a subscript of a two. (ex: N 2 + H 2  NH 3 ) If these elements are in an ionic bond, you must look at their charges (Recall lesson 3.05/3.08)

What do we do to balance an equation? When we balance equations, we CANNOT change subscripts. Subscripts are what make the chemical. If we change the subscripts, we change the chemical and thus change the reaction. H 2 O is water H 2 O 2 is hydrogen peroxide. So, what do we do to balance an equation? –We add a coefficient in front of the chemical. –Let’s work through an example.

Basic Steps in Balancing Equations: 1.Make a table of total number of atoms of each element for the reactants and for the products. N 2 + H 2  NH 3 ElementReactantsProducts N H

Literacy strategy Why should we make a table? –Tables help organize challenging or complicated material. This will aid in understanding and mastering the concept. –In balancing equations, a table can help us quickly see how many atoms are on both sides of the equation. This will enable us to balance the equation and check our answer easier.

Basic Steps in Balancing Equations: 2. Determine which elements are not balanced. Let’s add a coefficient in front of the chemical to get them balanced. Update table with new TOTALS for each element. N 2 + H 2  NH 3 ElementReactantsProducts N 21 H 23

Basic Steps in Balancing Equations: 3. Determine if any other element needs to be balanced. If yes, place a coefficient to balance it. Update table with new TOTALS. Repeat for all remaining elements. N 2 + H 2  2NH 3 ElementReactantsProducts N22 H26

Basic Steps in Balancing Equations: 4. Check your final equation. Double check that you have not made an error. It is very easy to make silly mistakes in balancing. N 2 + 3H 2  2NH 3 ElementReactantsProducts N H

Summary of the steps: 1.Make a table with the total number of atoms of each element for the reactants and for the products. 2.Determine which elements are not balanced. Add a coefficient in front of the chemical to balance the element. Update table with new totals. 3.Continue for all elements until they are al balanced Update table with new totals. 4.Once all elements are balanced, double check your totals to catch any silly mistakes. Steps in Balancing Equations Summary

Let’s do Some Practice Now that we’ve seen a easy way to balance equations by using the table, let’s do some practice. There are 4 practice problems. Watch all, or stop if you feel comfortable balancing equations.

Practice #1 Fe 2 O 3 + H 2 → Fe + H 2 O ElementReactantsProducts Fe O H

Practice #2 H 2 S + O 2 → H 2 O + SO 2 ElementReactantsProducts H S O

Practice #3 C 8 H 18 + O 2  CO 2 + H 2 O ElementReactantsProducts C H O

Practice #4 Al 2 (SO 4 ) 3 + KOH → Al(OH) 3 + K 2 SO 4 ElementReactantsProducts Al S O K H