http://www.ck12.org Chapter 11. Chemical Reactions
CH 4 (g) + O 2 (g)→CO 2 (g) + 2H 2 O(l)Now count the total number of oxygen atoms on the product side: two from the CO 2 and two from the 2H 2 O to give
a total of four. Place a 2 in front of the O 2.
CH 4 (g) + 2O 2 (g)→CO 2 (g) + 2H 2 O(l)Balancing difficult equations can be a trial-and-error process and is a skill that requires practice. If you find that one
particular strategy with a tough equation isn’t working, start over and balance a different element first. Persistence
will lead you to the correct balanced equation.
You can watch a video lecture about balancing chemical equations at http://www.khanacademy.org/science/chemi
stry/chemical-reactions-stoichiometry/v/balancing-chemical-equations.
If you want to practice and develop your skills at balancing chemical equations, you can download a simulation at
http://phet.colorado.edu/en/simulation/balancing-chemical-equations.
You can also play a game called "Balancing Act" at http://education.jlab.org/elementbalancing/index.html.
Lesson Summary
- Chemical reactions can be described as the process of one or more reactants being transformed into one or
more products. In a word equation, the names of the reactants are separated from the names of the products
by a yield arrow. - A skeleton equation shows the chemical formulas and physical states of the reactants and products, but it does
not account for their relative amounts. - Chemical reactions result solely from the rearrangement of atoms, so the law of conservation of mass must be
observed. Coefficients are placed in front of chemical formulas to balance a chemical equation, meaning that
the same quantities of each kind of atom are present on each side of the equation.
Lesson Review Questions
Reviewing Concepts
- Identify the reactants and products in each chemical reaction.
a. In photosynthesis, carbon dioxide and water react to form glucose and oxygen.
b. Magnesium oxide forms when magnesium is exposed to oxygen gas. - What is the relationship between an equal sign and a yield sign?
- Write sentences that completely describe the chemical reactions shown in the skeleton equations below.
a. H 2 O 2 (l)
MnO 2
→ H 2 O(l)+O 2 (g)
b. CuCO 3 (s)→∆CuO(s)+CO 2 (g)
c. Cs(s)+H 2 O(l)→CsOH(aq)+H 2 (g)- How many atoms of each element are represented by the following combinations of coefficients and chemical
formulas?
a. 5Br 2
b. 2NH 3