Chemical equations do not come already balanced. This must be done before the equation can be used in a chemically meaningful way.
All chemical calculations to come must be done with a balanced equation.
A balanced equation has equal numbers of each type of atom on each side of the equation.
The The Law of Conservation of Mass is the rationale for balancing a chemical equation. The law was discovered by Antoine Laurent Lavoisier (1743-94) and this is his formulation of it, translated into English in 1790 from the Traité élémentaire de Chimie (which was published in 1789):
“We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements.”
A less wordy way to say it might be:
“Matter is neither created nor destroyed.”
Therefore, we must finish our chemical reaction with as many atoms of each element as when we started.
Here is the example equation for this lesson:
H2 + O2 —> H2O
It is an unbalanced equation (sometimes also called a skeleton equation). This means that there are UNEQUAL numbers at least one atom on each side of the arrow.
In the example equation, there are two atoms of hydrogen on each side, BUT there are two atoms of oxygen on the left side and only one on the right side.
Remember this: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow.
An equation is balanced by changing coefficients in a somewhat trial-and-error fashion. It is important to note that only the coefficients can be changed, NEVER a subscript.
The coefficient times the subscript gives the total number of atoms.
Three quick examples before balancing the equation.
(a) 2 H2 – there are 2 x 2 atoms of hydrogen (a total of 4).
(b) 2 H2O – there are 2 x 2 atoms of hydrogen (a total of 4) and 2 x 1 atoms of oxygen (a total of 2).
(c) 2 (NH4)2S – there are 2 x 1 x 2 atoms of nitrogen (a total of 4), there are 2 x 4 x 2 atoms of hydrogen (a total of 16), and 2 x 1 atoms of sulfur (a total of 2).
So, now to balancing the example equation:
H2 + O2 —> H2O
The hydrogen are balanced, but the oxygens are not. We have to get both balanced. We put a two in front of the water and this balances the oxygen.
H2 + O2 —> 2 H2
However, this causes the hydrogen to become unbalanced. To fix this, we place a two in front of the hydrogen on the left side.
2 H2 + O2 —> 2 H2O
This balances the equation.
Two things you CANNOT do when balancing an equation.
1) You cannot change a subscript.
You cannot change the oxygen’s subscript in water from one to two, as in:
H2 + O2 —> H2O2
True, this balances the equation, but you have changed the substances in it. H2O2 is a completely different substance from H2O.
2) You cannot place a coefficient in the middle of a formula.
The coefficient goes at the beginning of a formula, not in the middle, as in:
H2 + O2 —> H22O
Water only comes as H2O and you can only use whole formula units of it.
There is another thing you should avoid. Make sure that your final set of coefficients are all whole numbers with no common factors other than one. For example, this equation is balanced:
4 H2 + 2 O2 —> 4 H2O
However, all the coefficients have the common factor of two. Divide through to eliminate common factors like this.
The equation just above is correctly balanced, but it is not the BEST answer. The best answer has all common factors greater than one removed.
Balance this equation: H2 + Cl2 —> HCl
Remember that the rule is: A balanced equation MUST have EQUAL numbers of EACH type of atom on BOTH sides of the arrow.
The correctly balanced equation is:
H2 + Cl2 —> 2 HCl
Placement of a two in front of the HCl balances the hydrogen and chlorine at the same time.
Balance this equation: O2 —> O3
Hint: think about what the least common multiple is between 2 and 3. That’s right – six.
The LCM tells you how many of each atom will be needed. Your job is to pick coefficients that get you to the LCM.
The correctly balanced equation is:
3 O2 —> 2 O3
How many oxygens are indicated: 3 Ca(NO3)2
Balance these equations:
Zn + HCl —> ZnCl2 + H2
KClO3 —> KCl + O2
S8 + F2 —> SF6
Fe + O2 —> Fe2O3
Balancing Worksheet #1
Please note that several of these equations are already balanced as written. The answers are in this file and are several lines below the last problem. There are 50 problems in two columns.
1. H2 + O2 ---> H2O 26. N2 + H2 ---> NH3
2. S8 + O2 ---> SO3 27. N2 + O2 ---> N2O
3. HgO ---> Hg + O2 28. CO2 + H2O ---> C6H12O6 + O2
4. Zn + HCl ---> ZnCl2 + H2 29. SiCl4 + H2O ---> H4SiO4 + HCl
5. Na + H2O ---> NaOH + H2 30. H3PO4 ---> H4P2O7 + H2O
6. C10H16 + Cl2 ---> C + HCl 31. CO2 + NH3 ---> OC(NH2)2 + H2O
7. Si2H3 + O2 ---> SiO2 + H2O 32. Al(OH)3 + H2SO4 ---> Al2(SO4)3 + H2O
8. Fe + O2 ---> Fe2O3 33. Fe2(SO4)3 + KOH ---> K2SO4 + Fe(OH)3
9. C7H6O2 + O2 ---> CO2 + H2O 34. H2SO4 + HI ---> H2S + I2 + H2O
10. FeS2 + O2 ---> Fe2O3 + SO2 35. Al + FeO ---> Al2O3 + Fe
11. Fe2O3 + H2 ---> Fe + H2O 36. Na2CO3 + HCl ---> NaCl + H2O + CO2
12. K + Br2 ---> KBr 37. P4 + O2 ---> P2O5
13. C2H2 + O2 ---> CO2 + H2O 38. K2O + H2O ---> KOH
14. H2O2 ---> H2O + O2 39. Al + O2 ---> Al2O3
15. C7H16 + O2 ---> CO2 + H2O 40. Na2O2 + H2O ---> NaOH + O2
16. SiO2 + HF ---> SiF4 + H2O 41. C + H2O ---> CO + H2
17. KClO3 ---> KCl + O2 42. H3AsO4 ---> As2O5 + H2O
18. KClO3 ---> KClO4 + KCl 43. Al2(SO4)3 + Ca(OH)2 ---> Al(OH)3 + CaSO4
19. P4O10 + H2O ---> H3PO4 44. FeCl3 + NH4OH ---> Fe(OH)3 + NH4Cl
20. Sb + O2 ---> Sb4O6 45. Ca3(PO4)2 + 6 SiO2 ---> P4O10 + CaSiO3
21. C3H8 + O2 ---> CO2 + H2O 46. N2O5 + H2O ---> HNO3
22. Fe2O3 + CO ---> Fe + CO2 47. Al + HCl ---> AlCl3 + H2
23. PCl5 + H2O ---> HCl + H3PO4 48. H3BO3 ---> H4B6O11 + H2O
24. H2S + Cl2 ---> S8 + HCl 49. Mg + N2 ---> Mg3N2
25. Fe + H2O ---> Fe3O4 + H2 50. NaOH + Cl2 ---> NaCl + NaClO + H2O
Do Now: convert 56 mL to L. What are SI Units? What are SI prefixes?
Convert 6000 to scientific notation. Write the formula of percent
materials needed: Huge Periodic table chart in the classroom. Index cards. Data projector or overhead projector. Exothermic and Endothermic salt. 2 beakers. wood block. water. paper towels. (possible chemicals) Zn, HCl, Cu, water, vinegar, baking soda, calcium chloride, sodium hydroxide, copper sulfate
1.Devise an experiment that uses physical properties to identify
substances. List all materials needed. Use EOC reference tables. Pick
various physical properties from the table and list them on an index
card. List density, solubility, boiling points, and melting points.
Then give the list to another classmate. The classmate must figure out
what substance it is.
Activity 2. Science Writing Journal:
Write a paragraph that discusses indicators of a chemical change.
List specific examples. Devise an experiment to show 3 chemical changes. (see chemicals)
Activity 3. List properties of metals.p155. List properties of nonmetals. p158
A physical change results in the rearrangement of existing particles in
a substance. A chemical change results in the formation of different
substances with changed properties.
Chemical and physical changes can be exothermic or endothermic,(enrichment: potential diagrams)
(DEMO! add calcium chloride and salt b thermo. Show students immediate freeze of water. Also show immediate heat.)
structure and arrangement of particles and their interactions determine
the physical state of a substance at a given temperature and pressure.
(Have students draw a simple particle model to differentiate properties
of solids, liquids, and gases.
Physical properties of
substances can be explained in terms of chemical bonds and
intermolecular forces. These properties include conductivity,
malleability, solubility, hardness, melting point, and boiling point.
Summary: Have students come up to the board and make 2 columns, physical change, chemical change. They will list examples of each.
p65 #10, 11