Module 6 Intro
1. Module 6 Intro
1.5. Page 3
Module 6—Stoichiometry
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When you wrote chemical equations earlier in this course, you always ensured they were balanced. What does “balancing” an equation really mean? You may answer this question by stating that balancing an equation makes the quantity of each kind of atom equal with respect to the reactants and products sides of the equation.
law of conservation of mass: a law stating that in any physical or chemical system, the initial mass of the system will be identical to the final mass of the system
You may recall that balancing a chemical equation has a deeper significance—it describes how mass is conserved within a chemical system. The law of conservation of mass states that mass cannot be created nor destroyed. The atoms that exist today are essentially the same as those that existed millions of years ago; and over time, these atoms are recycled, breaking and forming bonds each time they react. The amount of matter at the beginning of a chemical reaction must be the same as the amount of matter at the end of the chemical reaction—all atoms and mass are accounted for.
To refresh your memory of how to balance chemical equations, read and work through the examples below. Additional information and examples appear on pages 52 and 53 in your textbook.
Example 1: Ammonia is produced by the combination of nitrogen and hydrogen gases. Balance the following chemical reaction for the formation of ammonia.
___ N2(g) + ___ H2(g) → ___ NH3(g)
Hydrogen is the atom with the largest subscript. Add a coefficient to the substances containing hydrogen on each side of the equation to balance the hydrogen atoms.
___ N2(g) + _3_ H2(g) → _2_ NH3(g)
Reactants |
Products |
3 × 2 = 6 H atoms |
2 × 3 = 6 H atoms |
Confirm that nitrogen atoms are balanced.
Reactants |
Products |
1 × 2 = 2 N atoms |
2 × 1 = 2 N atoms |
Recall that coefficients of 1 are not normally shown. Therefore, the balanced chemical equation is
N2(g) + 3 H2(g) → 2 NH3(g)
Example 2: Ethane is one component of natural gas. Balance the complete combustion reaction for ethane.
___ C2H6(g) + ___ O2(g) → ___ CO2(g) + ___ H2O(g)
Hydrogen is the atom with the largest subscript. Add a coefficient to balance the hydrogen atoms.
___ C2H6(g) + ___ O2(g) → ___ CO2(g) + _3_ H2O(g)
Add a coefficient to balance the carbon atoms.
___ C2H6(g) + ___ O2(g) → _2_ CO2(g) + _3_ H2O(g)
Add a coefficient to balance the oxygen atoms.
___ C2H6(g) + _3.5_ O2(g) → _2_ CO2(g) + _3_ H2O(g)
Recall that coefficients of 1 are not normally shown. Therefore, the balanced chemical equation is
C2H6(g) + 3.5 O2(g) → 2 CO2(g) + 3 H2O(g)
Note: If whole-number coefficients suit you better, multiply all coefficients by 2.
2 C2H6(g) + 7 O2(g) → 4 CO2(g) + 6 H2O(g)
Example 3: Solid silver forms as a precipitate when solid copper is placed in a solution of aqueous silver nitrate. Balance the reaction for this process.
___ Cu(s) + ___ AgNO3(aq) → ___ Ag(s) + ___ Cu(NO3)2(aq)
Treat polyatomic ions like a single unit. The nitrate ion is the group with the largest subscript. Add a coefficient to balance the atoms in the nitrate ions.
___ Cu(s) + _2_ AgNO3(aq) → ___ Ag(s) + ___ Cu(NO3)2(aq)
Add a coefficient to balance the silver atoms.
___ Cu(s) + _2_ AgNO3(aq) → _2_ Ag(s) + ___ Cu(NO3)2(aq)
Therefore, the balanced chemical equation is
Cu(s) + 2 AgNO3(aq) → 2 Ag(s) + Cu(NO3)2(aq)
Self-Check
SC 3. Referring to your answers to question SC 2, write a balanced chemical equation for each reaction in question 9 on pages 66 and 67 of the textbook.
Self-Check Answers
SC 3.
- 2 KCl(s) → 2 K(s) + Cl2(g)
- Cu(s) + Cl2(g) → CuCl2(s)
- C5H12(g) + 8 O2(g) → 5 CO2(g) + 6 H2O(g)
- AgNO3(aq) + NaCl(aq) → AgCl(s) + NaNO3(aq)
- 2 Al(s) + 3 Cu(NO3)2(aq) → 2 Al(NO3)3(aq) + 3 Cu(s)
- C8H18(g) + 12.5 O2(g) → 8 CO2(g) + 9 H2O(g)
- 2 Al2O3(s) → 4 Al(s) + 3 O2(g)
- 2 Fe(s) + 3 Br2(l) → 2 FeBr3(aq)
- Cu(NO3)2(aq) + 2 NaOH(aq) → Cu(OH)2(s) + 2 NaNO3(aq)
- 2 H3PO4(aq) + 3 Ca(OH)2(aq) → 6 H2O(l) + Ca3(PO4)2(s)