1. Ch 9 Stoichiometry “equal measure”
Used to quantitatively (mass or moles) to make predictions of chemical reactions

2. Calculate quantities in reactions
Balance equations: proportions (coeff.) indicate the amounts (moles) of reactants products
Relative amounts in an equation can be expressed in moles
Coefficients in a balanced equation represents the moles of each substance and is used to set up mole ratios of pairs of substances.
2H2(g) + O2(g) --> 2H2O(l)
2 moles H2 = 1 mole O2 = 2 moles H2O
Mole ratios?

3. Stoichiometry
The proportional relationship between two or more substances during a chemical reaction
Mole ratio is the key
==> allows the connection from known to unknown substances
Known moles ----> unknown moles
mole ratio from balance equation
Conversion factor

4. Steps 1. Determine what is known and unknown in the problem
2. Balance the reaction
3. Use stoichiometry to complete the conversion from known to unknown.
4. Answer with correct units and significant figures
Complete the following:
How many moles of H2CO3 can be formed when 2.57 mol CO2 reacts with excess water?
How many moles of O2 are necessary to completely burn 4.33 mol C3H8? Mol CO2 & H2O produced?
Steps

5. Convert: mass or volume
Mass <--> moles: (molar mass conversion factor)
Mass --> mol -->mol >mass
Molar mass mole ration molar mass
Volume<--> moles: (PV = nRT or molar volume)
Volume<-->mass(density)<-->moles (molar mass)
Steps are the same as before:
Known & unknown data
Balance equation
Use stoichiometry to make conversions
Answer with correct units and sig. Figs.
If particles are known: # part. --> mol (Avogadro's #)

6. Practice
What mass of water is produced if 65.2 g CaCO3 reacts with excess H3PO4?
What mass of ammonia is formed when 7.50g N2 reacts with excess H2?
85.5g pentane (C5H12) burned in excess oxygen. What volume of CO2 is produced? (DCO2 = g/L)
What mass of magnesium will burn in the presence of 189 mL oxygen to produce MgO? (DO2 = g/L)

7. Limiting reagents
One reactant limits the amount of product produced (theoretical yield predicted from stioch. cal.)
Limiting reactant forms the least amount of product.
Once used up the reaction cannot proceed further.
Substance that controls the quantity of products that can be formed in a chemical reaction
Excess reactant:
Substance that is not used up completely in a reaction.

8. Problem solving strategy
Additional step:
Predict the amount of the same product using the amount of reactant (stoich. X 2)
The reactant that produces the least amount of product is the limiting reagent and the amount of product is the theoretical yield.
Practice: g of FeS is mixed with 70.4 g HCl, what mass of H2S(g) is produced, what reactant is in excess and how much (mass)?

9. Limiting reactants/food you eat
Manufactures choose excess & limiting reactants based on cost to help maximize product and reduce production costs
Manufactures let the least costly reactant be in excess to ensure the more costly reactant(s) are (is) used up.

10. Actual yield & % yield Reactions do not always go to 100% completion
Actual yield AY):
the measured amount of product of a reaction
Less than the theoretical yield (TY) predicted from stiochiometrical calculations
Reasons:
Reversible reactions
Process of purify and separation involve product lose
Side reactions.

11. Determining % yield % yield = (AY/TY ) x 100
TY has to be determined with stiochiometrical calculations.
AY is measured in the lab and can be determined from % yield data.
Practice: A 15.0g sample of magnesium reacts with excess HCl to produce 46.6g of magnesium chloride. What is the %yield for this reaction.
30.0g of sodium reacts with excess chlorine. If the % yield for this reaction is 85.3% what is the actual yield of sodium chloride produced?

12. Stoichiometry and cars
Air bags:chemicals in correct stoichiometical amounts to ensure proper inflation(pg321figure 6).
2NaN3(s) (sodium azide)--> 2Na(s) + 3N2(g)
Na(s) is reduced by Fe2O3 (Na is very reactive)
6Na(s) + Fe2O3(s) --> 3Na2O(s) + 2Fe(s) + E

13. Stioch/cars cont.
The energy produced helps push NaN3 to completion and heats up gas to increase volume
Na2O(s) is corrosive so CO2 & H2O from air forms
Na2O(s) + 2CO2(g) + 2H2O(g) -> 2NaHCO3(s)
Temperature & density important to determine gas produced.

14. Practice
How many grams of NaN3 are needed to fill an air bag with 23.6 L of N2 (DN2 = 0.92 g/L)
How many grams of NaCO3 and acetic acid would be needed to inflate an air bag to a volume of 65.1 L when CO2 gas has a density of 2.68 g/L

15. Stoich.: engine efficiency
Gas is a mixture of hydrocarbons and determines octane rating
C8H18 has the molar mass of the weighted average of all cpds in gasoline
2C8H18(g) + 25O2(g) -->16CO2(g) + 18H2O(g)
Efficient combustion requires a 2 to 25 mole ratio
Incorrect ratio, engine may stall or flood.
Ratio changes when starting, idling and running, computer controls fuel injectors
How many liters of air are needed to burn 2.00 mol C8H18 (DC8H18 = g/mL, DO2 = 1.33g/L, air is 21% oxygen by volume)

16. Stoich. & pollution control
Automobiles are the primary source of air pollution
Fuel-air ratio influences pollutants formed
Too little O2 --> CO produced
N2 & O2 (in atm) at high temperature inside engine form:
N2 + O2 --> 2NO
2NO + O2 --> 2NO2
NO2 + O2 --> 2NO + O3 (photochemical smog)
Better gas mileage uses lass fuel per km resulting in less emissions per km.

17. Catalytic converters
Pt, Pd or rhodium used to increase the rate of the decomposition of NO and NO2 into N2 & O2, CO--> CO2 and unburned hydrocarbons into CO2 & H2O
What volume of oxygen is needed to react completely with 78.3 L of ethanol (C2H5OH)