1. Stoichiometry Chapter 12

2. Stoichiometry
The process of using a balanced chemical equation to calculate the amounts of reactants needed or products formed

3. Chocolate Chip Cookies!!
1 cup butter
1/2 cup white sugar
1 cup packed brown sugar
1 teaspoon vanilla extract
2 eggs
2 1/2 cups all-purpose flour
1 teaspoon baking soda
1 teaspoon salt
2 cups semisweet chocolate chips
Makes 3 dozen
How many eggs are needed to make 3 dozen cookies?
How much butter is needed for the amount of chocolate chips used?
How many eggs would we need to make 9 dozen cookies?
How much brown sugar would I need if I had 1 ½ cups white sugar?

4. Cookies and Chemistry…Huh!?!?
Just like chocolate chip cookies have recipes, chemists have recipes as well
Instead of calling them recipes, we call them reaction equations
Furthermore, instead of using cups and teaspoons, we use moles
Lastly, instead of eggs, butter, sugar, etc. we use chemical compounds as ingredients

5. What does an equation represent?
2 Na + Cl2  2 NaCl or
2 Na + Cl2  2 NaCl

6. Chemistry Recipes
Looking at a reaction tells us how much of something you need to react with something else to get a product (like the cookie recipe)
Be sure you have a balanced reaction before you start!
Example: 2 Na + Cl2  2 NaCl
This reaction tells us that by mixing 2 moles of sodium with 1 mole of chlorine we will get 2 moles of sodium chloride
What if we wanted 4 moles of NaCl? 10 moles NaCl? 50 moles NaCl?

7. Practice
Write the balanced reaction for hydrogen gas reacting with oxygen gas.
How many moles of hydrogen are needed to make 2 moles of water?
What if we wanted 4 moles of water, how many moles of hydrogen would we need? How many moles of oxygen would we need?
What if we had 3 moles of oxygen, how much hydrogen would we need to react and how much water would we get?
What if we had 50 moles of hydrogen, how much oxygen would we need and how much water produced?

8. Use a “T” box with the mole ratios as the conversion factor
2H2 + O H20
Calculations are easy when you are working with whole numbers, but what if you produced 3.65moles of water, how many moles of oxygen would have been needed?
Use a “T” box with the mole ratios as the conversion factor

9. Mole Ratios
Mole ratios can be used to calculate the moles of one chemical from the given amount of a different chemical
Example: How many moles of chlorine is needed to react with 5 moles of sodium (without any sodium left over)?
2 Na + Cl2  2 NaCl
5 moles Na 1 mol Cl2
2 mol Na
= 2.5 moles Cl2

10. Stoichiometry STEP 1: You MUST start with a balanced equation
STEP 2:Start “T” box with known amount.
STEP 3: Use the mole ratio as a conversion factor in the “T” box

11. Mole-Mole Conversions
How many moles of sodium chloride will be produced if you react 2.6 moles of chlorine gas with sodium metal?

12. Practice
How many moles of oxygen are produced by the decomposition of 4.7moles of potassium chlorate?

13. How many moles of calcium carbonate decomposes to produce 8
How many moles of calcium carbonate decomposes to produce 8.3moles of carbon dioxide?

14. 2.9 moles of glucose (C6H12O6) reacts with oxygen to produce how many moles of carbon dioxide?

15. How many moles of lithium hydroxide decompose into 3.25moles of water?

16. Ws. Mole to Mole

17. Mole ↔Mass Conversions
Most of the time in chemistry, the amounts are given in grams instead of moles
We still go through moles and use the mole ratio, but now we also use molar mass to get to grams

18. MOLES moles ( use balanced equation)
MOLES  GRAMS GRAMS  MOLES
CALCULATE MOLAR MASS

19. Practice
Calculate how many moles of oxygen are required to make 10.0 grams of aluminum oxide

20. Practice
Ex. Calculate how many grams of ammonia are produced when you react 2.00grams of nitrogen with excess hydrogen.
N2 + 3 H2  2 NH3

21. Practice
How many grams of calcium nitride are produced when 2.00 grams of calcium reacts with an excess of nitrogen?

22. Ws. Mass- Mass Problems

23. Converting if Volume of Gas is given
Use “mole” chart
22.4 liters of a gas = 1 mole at STP

24. Ws. Mass-Mass with equation Writing

26. Thermochemistry A balanced equation shows if energy is….
required for the reaction to occur (endothermic)
Or given off when the reaction occurs (exothermic)
Shows energy to mole ratios in balanced equation

27. Ws. Thermochemistry

28. Back to the Chocolate Chip Cookies Recipe
1 cup butter
1/2 cup white sugar
1 cup packed brown sugar
1 teaspoon vanilla extract
2 eggs
2 1/2 cups all-purpose flour
1 teaspoon baking soda
1 teaspoon salt
2 cups semisweet chocolate chips
Makes 3 dozen
If I had 4 eggs and plenty of everything else and wanted to make 9 dozen cookies could I do it?

29. Limiting Reactant
Most of the time in chemistry we have more of one reactant than we need to completely use up other reactant.
That reactant is said to be in excess (there is too much).
The other reactant limits how much product we get. Once it runs out, the reaction s.
This is called the limiting reactant.

30. Limiting Reactant (Reagent)
Limiting reactant –runs out first
Excess reactant – some left over

31. Limiting Reactant
To find the limiting reactant, we have to try all of the reactants.
Calculate how much product we can get from each of the reactants to determine which reactant.
The reactant that makes the smallest amount of product is the limiting reactant.

32. Limiting Reactant: Example
10.0g of aluminum reacts with 35.0 grams of chlorine gas to produce aluminum chloride. Which reactant is limiting, which is in excess, and how much product is produced?
2 Al + 3 Cl2  2 AlCl3
Start with Al:
10.0 g Al 1 mol Al mol AlCl g AlCl3
27.0 g Al 2 mol Al mol AlCl3
= 49.4g AlCl3
Now Cl2
35.0g Cl mol Cl mol AlCl g AlCl3
71.0 g Cl mol Cl mol AlCl3
= 43.9g AlCl3

33. LR Example Continued
We get 49.4g of aluminum chloride from the given amount of aluminum, but only 43.9g of aluminum chloride from the given amount of chlorine. Therefore, chlorine is the limiting reactant. Once the 35.0g of chlorine is used up, the reaction comes to a complete

34. Limiting Reactant Practice
15.0 g of potassium reacts with 15.0 g of iodine. Calculate which reactant is limiting and how much product is made.

35. Finding the Amount of Excess
Start with the amount of LR and determine how much of the other reactant(s) is actually needed in the reaction.
Subtract that amount from the given amount to find the amount of excess.
Can we find the amount of excess potassium in the previous problem?

36. Finding Excess Practice
15.0 g of potassium reacts with 15.0 g of iodine. 2 K + I2  2 KI
We found that Iodine is the limiting reactant, and 19.6 g of potassium iodide are produced.
15.0 g I mol I mol K g K
254 g I mol I mol K
= 4.62 g K USED!
15.0 g K – 4.62 g K = g K EXCESS
Given amount of excess reactant
Note that we started with the limiting reactant! Once you determine the LR, you should only start with it!
Amount of excess reactant actually used

37. Limiting Reactant: Recap
You can recognize a limiting reactant problem because there is MORE THAN ONE GIVEN AMOUNT.
Convert ALL of the reactants to the SAME product (pick any product you choose.)
The lowest answer is the correct answer.
The reactant that gave you the lowest answer is the LIMITING REACTANT.
The other reactant(s) are in EXCESS.
To find the amount of excess, subtract the amount used from the given amount.
If you have to find more than one product, be sure to start with the limiting reactant. You don’t have to determine which is the LR over and over again!

38. Ws. Limiting and Excess Reactants

39. Limiting and Excess Reactants
Name ____________________________________ Period ________
Limiting and Excess Reactants
Limiting reagent – runs out first
Excess reagent – some left over
To determine the limiting reactant, calculate the amount of product (in grams) produced by each reactant. The limiting reactant is the one that makes the LEAST product.
Find the limiting reactant and excess reactant in #1-4.
Show all work !!!!!

40. Al Cl AlCl3
If you have 2 grams of each reactant.

41. C O CO2
If you have 10 grams of each reactant.

42. CO H CH3OH
If you have 10g CO and 30g H2

43. N H NH3
If you have .3 moles of N2 and .5 moles of H2

44. If you have more than one product, pick ONE and use with each reactant
If you have more than one product, pick ONE and use with each reactant. It doesn’t matter which product you choose, you’ll still find the limiting reactant. Try it and see. First, find the limiting reactant using one product, then find the limiting reactant using the other.
5C +2SO CS2 + 4CO
If you have 5 grams of each reactant

45. 6. N2H4 + 2H2O N H2O
If you have 10 grams of each reactant, which is the limiting reactant?

46. Ws. Finding Amount of Excess Reactant

47. Finding Amount of Excess Reactant
Once the amount (grams) of limiting reactant is used up, the reaction STOPS.
Use that amount to determine the amount of excess reactant needed to completely react with it.
Subtract the excess needed for the reaction from the amount of excess reactant you started with to determine the amount of excess unused.

48. In Problem #6 LR = H2O2
So… how much N2H4 is needed to completely react with the 10 grams of H2O2?
So…. If we started with 10grams of N2H4 and only 4.72g were needed to react with the 10grams of H2O2, then the amount of excess would be….
10g g = 5.28g N2H4 in excess.

49. Percent Yield -shows efficiency of reaction in lab
Actual yield x 100
Percent yield =
Theoretical yield

50. Percent Yield practice problem
CaCO3(s) CaO (s) + CO2(g)
What is the theoretical yield of CaO if 24.8g of CaCO3 is heated?

51. CaCO3(s) CaO (s) + CO2(g)
What is the percentage yield if 13.1g of CaO is actually produced when a 24.8g CaCO3 is heated?

52. Ws. Percent Yield

53. Review
Stoichiometry – using balanced equations to determine quantities of reactants and products
Limiting reagent- the reactant that runs out first.
Calc. Amt of product produced by each reactant.
Excess reactant – start with L.R., determine amt. of other reactant needed. Subtract needed from amt. started with
Thermochemistry – uses balanced equation to determine energy needed by reaction (endothermic) or energy given off as a product (exothermic)
% yield – determine amt. of product that should be produced based on balanced equation (theoretical)
Compare to actual product
actual x 100
theoretical