Week 21
Monday: Begin UNIT 5
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Write a recipe on the board.
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2 tablespoons olive oil
1 cup finely chopped onion
1 pound lentils, picked and rinsed
1 cup peeled and chopped tomatoes
2 quarts chicken or vegetable broth
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What is this? It is an everyday equation. It is a recipe. Recipes are equation we use every day.
This recipe: You ad each ingredient and it yields a final product  lentil soup.
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Chemical equations are essentially the SAME thing. We have many different “ingredients” that we
put together and it yields a final product. But this product is most likely NOT edible.
A recipe is also like a balanced equation because if one part is missing or you add ingredients in
different ratios, you are not going to get the same thing! It will throw everything off!
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So why is balancing equations important??
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Nearly everything we use is manufactured from chemicals: soaps, shampoos, CDs, cosmetics,
medicines, and clothes.
These products cannot cost more to make than to sell. They want to make a profit.
To do so, they use balanced equations to remain economical. To use ONLY what they need.
A balanced equation tells you what amounts of reactants to mix and what amounts of products to
expect.
Chemists use balanced chemical equations as a basis to calculate how much reactant is needed
or product is formed in a reaction.
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The calculation of quantities in chemical reactions is a subject of chemistry called
stoichiometry.
Interpreting Chemical Equations
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A balanced chemical equation can be interpreted in terms of different quantities, including
numbers of atoms, molecules, or moles; mass; and volume.
There are 5 quantities that can be described in a balanced chemical equation. Let’s use this example.
N2(g) + 3H2(g)  2NH3(g)
1. Number of Atoms: EQUAL number of each type of atoms on the reactants side and the
products side. Reactants: 2 atoms of N, 6 atoms of H. Products: 2 atoms of N, 6 atoms of H
2. Number of Molecules: We use ratios. The equation indicated that 1 molecule of N2 reacts
with 3 molecules of H2 to yield 2 molecules of NH3. The molecule is the whole thing.
Therefore, the ratio would be 1:3:2. What if I put a 2 in front of N 2? How would it change?
3. Number of Moles: THIS IS THE MOST IMPORTANT PIECE OF INFORMATION. The
coefficients of a balanced equation give us the number of moles of reactants and products
in a reaction. The total number of moles of reactants does NOT have to equal the number
of moles of the product. 1 mole of N2 reacts with 3 moles of H2 to yield 2 moles of NH3.
Therefore, 4 moles on the left and 2 moles on the right. This is OK.
4. Mass: Law of conservation of mass: Mass cannot be created or destroyed. The total mass
of the atoms does NOT change in a reaction. EQUAL mass on left side as the right side.
How do we calculate this?
a. Remember molar mass??? You need your periodic table. Let’s calculate mass!
N2(g) + 3H2(g)  2NH3(g)
b. 1 mol of N2 = 2x14=28.0g; 3 mol of H2= 3x 2.02= 6.0g; 2 mol NH3=
2x(14+1.01+1.01+1.01)= 34.0g
c. Reactants: 34.0g
d. Products: 34.0g
5. FOR HONORS CHEM: Volume: 1 mol of any gas= 22.4L at STP. The amount of liters on the
left side does NOT have to equal the amount of liters on the right side. 1 mol N 2= 22.4L N2;
3 mol H2= 3x22.4L= 67.2L; 2 mol NH3= 2x22.4L= 44.8L.
a. 89.6L DOES NOT EQUAL 44.8L. This is OK.
6. Have students get out notebook paper. Write NOTES and HOMEWORK. Do Conceptual Problem
12.1 on page 358.
a. 2H2S + 3O2  2SO2 + 2H2O
b. Interpret equation in terms of atoms, molecules, moles, mass.
c. Atoms: Left:2 atoms of N, 6 atoms of H. Right: 2 atoms of N, 6 atoms of H.
d. Molecules: Left: 2 Molec H2S, 3 Molec O2; Right: 2 molec NH3.
e. Moles: 2 Mol H2S, 3 Mol O2; Right: 2 mol NH3. Ratio: 2:3:2
f. Mass: Left: (2 x 34.1)+(3x32.0) = 164.2g;
Right: (2x64.1)+(2x18.0)= 164.2g
Classwork/ Homework: Page 358 Section 12.1 Assessment. #5-10. Writing Activity is EC 2 Points.
Tuesday/Wednesday
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Begin Section 12.2 Chemical Calculations
Thursday/Friday