1. Stoichiometry

2. Learning Goals By the end of this lesson I can Define Stoichiometry
Predict relative amounts of reactants and products needed for a chemical reaction

3. Ham Sandwiches: Can you adjust a recipe?
Suppose you want to make a ham sandwich according to the following recipe…
2 slices of bread + 3 slices of ham + 1 slice of cheese = 1 sandwich or…
2 B + 3 H + 1 C  1 S
How many complete sandwiches can be made according to the recipe, given…
6 B, 6 H, and 5 C ? How do you know? What is the limiting factor?
7 B, 15 H, and 4 C ? How do you know? What is the limiting factor?

4. Ham Sandwiches 2 B + 3 H + 1 C  1 S
Suppose you had more than enough cheese. You have 27 slices of ham, how many slices of bread should you get from the cupboard?

5. Ham Sandwiches

6. Stoichiometry?
This strange word comes from two Greek words for element and measure
στοιχεῖον stoicheion "element" and μέτρον metron "measure"
Stoichiometry is…
the calculation of relative quantities of reactants and products in chemical reactions.

7. Stoichiometry of a Chemical Reaction
Consider the following equation
Zn (s) + 2 HCl (aq)  ZnCl2(aq) + H2 (g)
Given adequate Zn(s)….
How many moles of H2 would be produced from 4 mol of HCl? How do you know?
How many moles of H2 would be produced from 1 mol of HCl? How do you know?
How many moles of HCl would be required to make 6 moles of H2? How do you know?

8. Demonstration Zn (s) + 2 HCl (aq)  ZnCl2(aq) + H2 (g)
2 H2(g) + O2(g)  2 H2O (g)

9. How many moles of methane CH4(g) would produce 8 moles of H2O(g)?
Formative Assessment: If you can do this, you’ve learned the basic idea of stoichiometry CH4 + 2 O2  CO2 + 2 H2O
How many moles of methane CH4(g) would produce 8 moles of H2O(g)?
How many moles of CO2(g) would be produced from12 moles of O2(g)?
What is Stoichiometry?
Determining the type of chemical reaction
Calculating relative amounts of reactants and products
Writing proper chemical formulas

10. The Arithmetic of Chemical Equations 12.1
A balanced chemical equation is like a recipe:
Quantities of reactants (ingredients)
Quantity of product
Balanced equations are used to calculate
how much reactant is needed
how much product will be produced (yield)
How much reactant is left over

11. Interpreting Chemical Equations
Chemical Equations contain information about quantities of
Atoms
Molecules
Moles
Mass
Volumes

12. Interpreting a Chemical Equation
N2 (g) + 3 H2 (g)→ 2 NH3 (g) @ STP
Chemical Equations contain information about quantities of N2 H2 NH3
Atoms
Molecules
Moles
Mass
Volumes

13. Interpret the Following Equations
2 SO2 (g) + O2 (g)  2 SO3 (g)
If you had 3 moles of SO2 and 2 moles of O2 , how many moles of SO3 would be produced?
2 moles of SO2 and 3 moles of O2 ?
6 moles SO2 and 2 moles O2 ?
4 FeS + 7O2  2Fe2O3 + 4 SO2
9 mol FeS and 5 mol O2 would yield how many mol Fe2O3?

14. Chemical Calculations 12.2
Mole ratios
conversion factors from balanced equation used to convert from moles of one substance to moles of another substance
1 N2 + 3 H2  2 NH3 ?Mole ratios of
N2 to NH3 ? NH3 to N2 ?
H2 to N2 ? N2 to H2 ?
NH3 to H2 ? H2 to NH3 ?

15. Practice 4 FeS + 7O2  2Fe2O3 + 4 SO2
How many moles of Fe2O3 can be produced by 16 moles of O2, given excess FeS?
What is the mass of Fe2O3 produced?

16. Using Mole Ratios N2 + 3 H2  2 NH3
If you had 6 moles of H2 and an unlimited supply of N2 , how many moles of NH3 would be formed?
3 moles N2 and excess H2?
If you wanted to make 16 moles NH3, how much H2 would you need?
Given 10 moles N2 and 27 mol H2 , how many moles NH3 would be produced? What is the limiting reactant?

17. Mass to Mass Calculations
In order to go from mass of one substance (A) to mass of another substance (B) you must convert to moles.
For the equation 2A  3B

18. Mass to Mass Calculations
N2 + 3 H2  2 NH3
Given 28 g N2 with excess H2 , how many grams of NH3 would be produced?
How many grams of H2 are needed to produce 136 g NH3 ?

19. Calculating molecules of a product
Moles to molecules conversions involve Avogadro’s number
Example: How many molecules of O2 are formed when 29.2 g of H2O decompose, given
2H2O(l) → 2H2(g) + O2(g)
Find moles O2 formed
Convert moles O2 to molecules O2
See sample 12.4 p. 364

20. Stoichiometry of Gases
At STP, 1 mole of any gas has a volume of 22.4 L
So, when gas problems are at STP, you can substitute liters for moles
i.e. the coefficients can represent liters
Example: How many liters of O2 (g) are needed to produce 20.4 L of SO3 at STP? Given…
2 SO2(g) + O2(g)  2 SO3 (g)

21. Stoichiometry of Gases
2 H2O2 (aq)  2 H2O (l) + O2 (g)
How many liters of O2 gas would be produced from the decomposition of g hydrogen peroxide at STP?
Convert…
mass of H2O2 to moles H2O mol H2O2
moles H2O2 to moles O mol O2
moles O2 to L of O L O2

22. 12.3 Limiting Reagent and Percent Yield
Quantities of reactants are not always present in exactly correct proportions.
Insufficient quantity of any reactant will limit the amount of product that forms
Limiting reagent
Limits the amount of product that can be formed
Is completely consumed by the reaction
Excess reagent
Is not completely consumed
Some is left over

23. Two Ways to Determine Limiting Reagent
By testing the yield of the reaction.
How much product can be made?
The reagent that produces the least product is the LR
By testing the amounts of reactants

24. Example of Limiting and Excess Reagent
N2 + 3 H2  2 NH3
Given 6 moles of N2 and 4 moles of H2, identify the limiting reagent
The limiting reagent produces the least amount of product
How many moles of NH3 would be produced by 6 moles of N2 ?
How many moles of NH3 would be produced by 4 moles of H2?
Which is the limiting reagent?

25. Alternative Way to Determine LR
N2 + 3 H2  2 NH3
Given 6 moles of N2 and 4 moles of H2, identify the limiting reagent
Rephrase the question: 6 moles of N2 would require how many moles of H2?
If you don’t have enough H2, then H2 is the LR
If you have enough H2, then N2 is the LR

26. Sample 12.7 2 Cu (s) + S (s)  Cu2S (s)
Given 80.0 g of Cu and 25.0 g S, which is the limiting reagent?

27. Percent Yield Is a measure of the efficiency of a reaction
Theoretical yield:
Amount of product you should get according to the equation (what we’ve been calculating)
Actual yield:
Amount of product you actually get
Determine experimentally by measuring the amount of product
Percent yield:
Ratio of actual to theoretical yield

28. Actual Yield
Is always less than theoretical yield, i.e. a reaction never produces as much product as predicted by the equation.
Why is this?
Impurities in the reactants
Side reactions, common in organic chemistry
Formation of unintended products
During the combustion of magnesium, a small amount of Mg reacts with N2 to form Mg3N2
2 Mg + O2  2 MgO

29. Practice
CaCO3  CaO + CO2
Find theoretical yield of CaO if 24.8 g CaCO3 is decomposed.
TY = 13.9 g CaO
If the measured product (AY) is 11.5 g, what is the percent yield?
82.7%