Balancing Chemical Reactions

Balancing Chemical Reactions

Chemical reactions occur when compounds or pure elements are brought into contact with each other and share or swap atoms to form new chemical compounds. In many cases the new compounds formed represent lower energy states for the particular atoms involved and are thus more stable than the original compounds. Remember that electron bonds represent stored energy in compounds. We write chemical equations to show a chemical reaction.

Chemicals that react with each other are called reactants. Chemicals that are produced from the reaction are called products. An arrow shows the direction of the reaction like this: reactants → products and some reactions go both ways: reactants ↔ products

Here is an example of a chemical reaction and its equation: H2 + 02 → H2O2

In this reaction 1 molecule of hydrogen gas combines with one molecule of oxygen gas to make 1 molecule of hydrogen peroxide, H2O2. Notice that the numbers of atoms of H and of O are equal on both sides of the equation, but not necessarily the number of molecules. In other words mass is conserved in every reaction. Where is the mass in an atom? ______. Finally, remember that energy is stored in chemical bonds. In any reaction where bonds are broken energy in the form of heat is released, and where bonds are made heat is absorbed. Reactions where energy is released in the form of heat are called exothermic reactions, and reactions where heat energy is absorbed are called endothermic reactions. Here’s another example: 1 C + 2Cl2 → CCl4

In this reaction 1 C atom combines with 2 molecules of Cl2 to make 1 molecule of CCl4. Notice again that the numbers (and types) of atoms must add up equally on both sides of the equation, but not necessarily the numbers of molecules so mass is conserved.

Balance the equation in the problems below and fill in words in problem #15. .

1. __2_Al2O3 → __4_Al + __3_O2

2. _2__Fe2O3 → __4_Fe + __3_O2

3. _1__Ca + __1_H2O → _1__H2 + _1__CaO

4. _1__Zn + __2_HCl →__1__H2 + __1_ZnCl2

5. __2_H2O →__2_H2 + __1_O2

6. __1_CaCO3 →_1__CaO + _1__CO2

7. __4_Li + __1_O2 → __2_Li2O

8. __1_N2 + __3_H2 →__2_NH3

9. __2_K + __2_H2O →_1__H2 + __2_KOH

10. _4__Fe + __3_O2 → ___ __2_Fe2O3

11. _6__Ag + _1__N2 →__2_Ag3N

12. __1_Fe + __1_H2SO4 → _1__H2 + __1_FeSO4

13. __2_CuS + _3__O2 → __2_CuO + _2__SO2

14. _1__SO3 + _1__H2O →__1_ H2SO4

15. Mass is conserved in the number of atoms on both sides of a chemical equation. Reactions that absorb heat are endothermic and that release heat are exothermic.