Chapter 3 (sections 1-3)  

1. Writing and Balancing Chemical Reactions (section 3.1)
   1. Reactant(s) g Product(s).   Mass does not change during the reaction.  You start and end with the same exact mass.
   2. The # of atoms does not change during the reaction.  The identity of the atoms does not change during the reaction.  If you start with four carbon atoms, you must end with four carbon atoms.
   3. The charge stays the same.  If all the reactants are neutral overall, the products overall must be neutral.  Bonds do change.  Bonds break and form during a chemical reaction.
   4. Law of Conservation of Mass = mass cannot be created or destroyed.  This is why we balance reactions.
      1. Note that H₂O means there are 2 H atoms and 1 O atoms bonded together in the molecule while 2 H₂O means there are two separate water molecules
      2. Draw pictures to help you count atoms.  Balancing reactions is largely trial and error.  You put numbers in front of the compounds and check to see if that helped you get closer to having the same number of atoms on both sides.  
      3. The first step is to write the reaction with the correct compounds.  Remember that charges determine the formula of the compound so that the compound is neutral, such as NaCl, MgCl₂, etc...  Once you determine the correct formula you CAN NOT change the subscripts.  If you need two sodium atoms, don't change NaCl to Na₂Cl - there is no such thing.  
      4. We balance chemical reactions by adding coefficients - numbers in from the the compound.  If you need two sodiums, you write 2 NaCl which means you have 2 distinct and separate NaCl units.  
   5. Formation Reactions
      1. In general, element + element g compound where the elements are in their naturally occurring state (solid, liquid or gas) and diatomic or not.
      2. Write and balance the formation reaction between hydrogen and oxygen:  2 H₂(g)  +  O₂(g)  g  2 H₂O
      3. Write and balance the formation reaction between sodium and chlorine:  2 Na(s)  +  Cl₂(g)  g  2 NaCl
      4. Other examples
         1. 4 As(s) + 5 O₂(g) g 2 As₂O₅
         2. C(s) + O₂(g) g CO₂
         3. I₂(s) + 7 Cl₂(g) g 2 ICl₇
         4. 2 Li(s) + Br₂(l) g 2 LiBr
         5. 4 Fe(s) + 3 O₂(g) g 2 Fe₂O₃
   6. Combustion Reactions
      1. In general for hydrocarbons, C_xH_y + O₂(g) g CO₂(g) + H₂O(g)
      2. Write and balance the combustion reaction for C₂H₄:  C₂H₄ + 3 O₂(g) g 2 CO₂(g) + 2 H₂O(g)
      3. Write and balance the combustion reaction for C₃H₈:  C₃H₈ + 5 O₂(g) g 3 CO₂(g) + 4 H₂O(g)
2. Molar Mass (MM) (aka Formula Weight, Molecular Weight) (Section 3.3)
   1. Molar Mass is the sum of all the atomic masses
   2. Examples:  Check yourself.
      1. H₂O is 18.0 g/mol
      2. NaCl is 58.5 g/mol
      3. Ca₃(PO₄)₂ is 310.3 g/mol
3. Avogadro's Number - The Mole (3.3)
   1. The mole is just a number like a dozen is 12.
   2. A mole is a large number = 6.02 x 10²³.  This number is Avagadro's Number and is given the symbol N_A
   3. A mole of something will not weigh the same as a mole of something else, just like a dozen eggs do not weight the same as a dozen cars. 
   4. We measure things by weighing them or counting them.  One dozen = 12 anythings just like one mole = 6.02 x 10²³ anythings.  It is convenient to use dozens for cookies, donuts, and eggs.  It is convenient to use moles for atoms and molecules because they are so small.  We cannot actually count out atoms or molecules however.
      1. one dozen donuts = 12 donuts
      2. one mole of donuts = 6.02 x 10²³ donuts
      3. one mole of iodine = 6.02 x 10²³ atoms of iodine
      4. one mole of water = 6.02 x 10²³ molecules of water
      5. one mole of marbles = 6.02 x 10²³ marbles (which would fill up the Pacific ocean)
   5. The periodic table shows us the mass of one mole of atoms for each element. (MM)
      1. 6.02 x 10²³ carbons atoms = one mole = 12.0 grams thus 12.0 g/mol for carbon
      2. 6.02 x 10²³ oxygen atoms = one mole = 16.0 grams thus 16.0 g/mol for oxygen.
      3. 6.02 x 10²³ molecules of O₂ masses 32.0 grams.  So O₂ is 32.0 g/mol
      4. 6.02 x 10²³ water molecules = one mole = 18.0 grams thus 18.0 g/mol for water.
   6. Practice Questions.
      1. one mole of Neon has how many atoms?           answer: 6.02 x 10²³
      2. 23.0 grams of sodium is how many atoms?               answer:  this is one mole so 6.02 x 10²³ atoms
      3. 6.02 x 10²³ H atoms weigh how much?                answer: 1.0 gram
      4. one mole of carbon dioxide weighs how much?                answer: 44.0 grams (add up one C and two O)
   7. Molar ratios - Use molar ratios to go between moles of a molecule and moles of its atoms
      1. The ratio of atoms per molecule such as (2 H  / 1 H₂O)  is a molar ratio. 
      2. These ratios can be used in calculations such as:
         1. How many moles of H atoms are in 23.0 moles of NH₃?              Answer is 23.0 mol NH₃ ( 3 H  / 1 NH₃ ) = 69.0 mol H atoms
         2. How many moles of Cl atoms are in 4.2 moles of chlorine?             Answer is 4.2 moles Cl₂ ( 2 Cl  / 1 Cl₂ ) = 8.4 mol Cl atoms
         3. How many moles of Chloride ions are in 4.4 moles of table salt?          Answer is 4.4 mol NaCl (1 Cl^-  / 1 NaCl ) = 4.4 mol Cl^- ions
   8. Examples: Converting between moles and the number of actual atoms or molecules using N_A.
      1. How many atoms are in 2.50 moles of carbon?        Answer 2.50 mol C (6.02 x 10²³ atoms/mol) = 1.51 x 10²⁴ C atoms
      2. How many moles are in 5.00 x 10²³ N atoms?        Answer 5.00 x 10²³ N atoms ( mol / 6.02 x 10²³ atoms) = 0.831 mol N
      3. How many molecules are in 4.7 mol NH₃?        Answer 4.7 mol NH₃ (6.02 x 10²³ molecules/mol) = 2.8 x 10²⁴ molecules NH₃
   9. Consider this balanced reaction and what it means:  3 H₂ (g)  +  N₂ (g)  g  2 NH₃
      1. 3 molecules of hydrogen gas react with one molecule of nitrogen gas to make 2 molecules of ammonia
      2. 3 moles of hydrogen gas react with one mole of nitrogen gas to make 2 moles of ammonia
      3. 3(2.0 g/mol) + 28.0 g/mol = 2(17.0) g/mol  (Each side weighs 34 grams which follows the Law of Conservation of Mass)

   The End           Practice Problems Below

   Suggest problems from the text:  30, 38, 40, 42, 44, 50, 52, 54

Now we are ready to practice writing and balancing chemical reactions.  Remember, once you have written the reaction and have the correct formulas, you can NOT change the subscripts.  You can only add coefficients in front of the compounds to balance the reaction. 

1. Balance the following formation reaction:  Ca (s) + I₂(s) g CaI₂
2. Balance the following formation reaction:  Na (s) + Br₂(l) g NaBr
3. Write and balance the formation reaction for magnesium and chlorine.
4. Write and balance the formation reaction for potassium and oxygen.
5. Write and balance the formation reaction for calcium and oxygen.
6. Write and balance the formation reaction for phosphorus pentachloride.
7. Balance the following combustion reaction:  C₄H₈ + O₂(g) g CO₂(g) + H₂O(g)
8. Balance the following combustion reaction:  C₆H₁₄ + O₂(g) g CO₂(g) + H₂O(g)
9. Write and balance the combustion reaction for C₆H₆.
10. Write and balance the combustion reaction for C₃H₆.
11. Balance the following  reaction:  CaI₂ + Na₂SO₄ g CaSO₄ + NaI
12. Balance the following  reaction:  MgSO₄ + KNO₃ g Mg(NO₃)₂ + K₂SO₄

Answers

1. already balanced
2. 2 Na (s) + Br₂(l) g 2 NaBr
3. Mg (s) + Cl₂(g) g MgCl₂
4. 4 K (s) + O₂(g) g 2 K₂O
5. 2 Ca (s) + O₂(g) g 2 CaO
6. 2 P(s) + 5 Cl₂(g) g 2 PCl₅
7. C₄H₈ + 6 O₂(g) g 4 CO₂(g) + 4 H₂O(g)
8. 2 C₆H₁₄ + 19 O₂(g) g 12 CO₂(g) + 14 H₂O(g)
9. 2 C₆H₆ + 15 O₂(g) g 12 CO₂(g) + 6 H₂O(g)
10. 2 C₃H₆ + 9 O₂(g) g 6 CO₂(g) + 6 H₂O(g)
11. CaI₂ + Na₂SO₄ g CaSO₄ + 2 NaI
12. MgSO₄ + 2 KNO₃ g Mg(NO₃)₂ + K₂SO₄

Practice Problems

1. How many atoms are in 4.00 moles of carbon?
2. How many moles are in 1.25 x 10²⁵ atoms of neon?
3. How many molecules are in 2.99 moles of ammonia?
4. How many moles are in 5.25 x 10²⁴ units of lithium sulfide?
5. How many units are in 4.24 moles of sodium chloride?
6. How many H atoms are in 8.35 moles of water?
7. How many O atoms are in 1.44 moles of magnesium sulfate?
8. If I have 3.45 x 10²⁵ H atoms in a pure CH₄ sample, how many moles of CH₄ are present?
9. What is the molar mass of ReH₇, C₆H₆, KNO₃, CuCl₂, H₂O₂, Ne, CaSO₄, Mg₃(PO₄)₂, (NH₄)₂CO₃, Na?
10. 6.02 x 10²³ Cl atoms weigh how much? 
11. One mole of carbon dioxide contains how many molecules? 
12. How many moles are in 7.25 x 10²⁶ molecules oxygen gas?  

ANSWERS

1. 4.00 mol C (6.02 x 10²³ atoms / mol) = 2.41 x 10²⁴ C atoms
2. 1.25 x 10²⁵ atoms Ne ( mol / 6.02 x 10²³ atoms) = 20.8 mol Ne
3. 2.99 mol NH₃ (6.02 x 10²³ molecules / mol) = 1.80 x 10²⁴ molecules NH₃
4. 5.25 x 10²⁴ units Li₂S (mol / 6.02 x 10²³ units) = 8.72 mol Li₂S
5. 4.24 mol NaCl (6.02 x 10²³ units / mol) = 2.55 x 10²⁴ units NaCl
6. 8.35 mol H₂O ( 2 H / 1 H₂O) (6.02 x 10²³ atoms / mol) = 1.01 x 10²⁵ H atoms
7. 1.44 mol MgSO₄ ( 4 O / MgSO₄) (6.02 x 10²³ atoms / mol) = 3.47 x 10²⁴ O atoms
8. 3.45 x 10²⁵ H atoms ( 1 CH₄ molecule/ 4 H atoms) (mol / 6.02 x 10²³ molecules) = 14.3 mol CH₄
9. ReH₇-193.2 g/mol, C₆H₆-78.0 g/mol, KNO₃-101.1 g/mol, CuCl₂-134.5 g/mol, H₂O₂-34.0 g/mol,  Ne-20.2 g/mol, CaSO₄-136.2 g/mol, Mg₃(PO₄)₂-262.9 g/mol, (NH₄)₂CO₃-96.0 g/mol,  Na-23.0 g/mol
10. 35.5 grams - cause it is one mole - just look at the Periodic Table
11. 6.02 x 10²³
12. 7.25 x 10²⁶ molecules O₂ (mol/6.02 x 10²³ molecules) = 1.20 x 10³ mol O₂