Stoichiometry

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Bellringer• Write the quantities of ingredients

you would use to make a bologna and cheese sandwich.

• Then determine how many sandwiches you could make from 24 slices of bread.

• Calculate how much of each ingredient is needed.

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Balanced Equations Show Proportions• Coefficients show the proportions of the

reactants and products.

2H2 + O2 2H2O

• Calculations involving chemical reactions use these proportions to find amounts of reactants and products.

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Relative amounts in equations can be expressed in moles

• Coefficients represent the moles of each substance.

2H2 + O2 2H2O

• 2 mol H2 +11 mol O2 form 2 mol H2O.

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Stoichiometry• You can determine:

• amount of reactant needed, or • amount of product formed

• This proportional relationship between chemical coefficients is called stoichiometry.

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OH, CHEMISTRY

(THE STOICHIOMETRY SONG)Sung to the tune of "Jingle Bells.“

Lyrics by: Brandi Abbott1992

• Verse 1 We started something new• called stoichiometry.• It's such a funny word.• It brings a smile to me.• The first step’s not so hard.• It's something that is old.• You take the grams that's given there• and convert them into moles.•

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• Chorus Ooooooh Chemistry, chemistry,

• How exciting it can be! • You have to really know your • stuff for stoichiometry!•

• (REPEAT)

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• Verse 2 The second step's not big.

• It really doesn’t boast.• You put the moles in a ratio• To show least over most.

• When grams are what you seek• a third step must be used.• The least known's weight per mole• will provide the final clue!

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Always, the same 3 steps

• Grams Moles

• Molar Ratio• Coeff. of what you want• Coeff. of what you have

• Only if ans. is to be in grams

• Molecular weight of what you want

• 1 mole9

Mass- Mass • What mass of hydrogen will be used

when 14 g of nitrogen gas is reacted to produce ammonia gas?

• N2 + 3H2 2NH3

• 14 g x

• 14 g N2 1mol N2 3 mol H2 2 g H2 = 3 g H2

28 g N2 1 mol N2 1 mol H2

• 1 2 3 • grams to moles least/most least known's weight

per mol

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Mole-Mass • How many grams of sodium will be

required to react with sufficient chlorine in order to produce 4.50 moles of NaCl?

• 2Na + Cl2 2NaCl

• x 4.50 mol

• 4.50 mol NaCl 2 mol Na 23 g Na = 103.5 g Na

2 mol NaCl 1 mol Na

• 1 2 3 • moles least/most least known's weight

(given) per mol

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Moles to Moles• 6.00 moles of water vapor are required to assist oxygen

delivery in surgical ventilators. How many moles of hydrogen and how many moles of oxygen must be introduced from the gas tanks in order to insure this amount?

2H2 + O2 2H2O x y 6.00 mol

6.00 mol H2O 2 mol H2 = 6.00 mol H2

2 mol H2O• 1 2 • moles least/most

6.00 mol H2O 1 mol O2 = 3.00 mol O2

2 mol H2O• 1 2 • moles least/most

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• Stoichiometry practice

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Limiting Reactants• According to the periodic table• 2Na + Cl2 2NaCl 46 g 71 g 117g

• Reactions do not always react perfectly.• Temperature• Age• Humidity

• 100% of expected product rarely achieved.

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Major PROBLEM in Industry • Companies must produce enough

product to supply their customers.

• They MUST use enough reactants to fill their orders WITHOUT producing excessive overage • stored ($ lost), or • thrown away ($ lost)

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• Industry adds an EXCESS of the cheapest reactant.

• Reactant that is completely consumed (used up) in the rxn.• LIMITING REACTANT

• The limiting reactant’s value is used for all stoichiometry calculations.

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Too Much Info!• How many moles of salt can be

produced from the amounts of reactants given in the following equation?

• 2Na + Cl2 2NaCl

46 g 91 g ?

Too much information• one number gives reliable product

information.• one number lies!

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• In our example

• 2Na + Cl2 2NaCl

46 g 91 g ?

• CALCULATE the amount of product from each amount…

• 46 g of sodium can produce 117 g of salt• 91 g of chlorine can produce 149.9 g of salt

• Therefore,• Sodium is the limiting reactant and 117 g is

the maximum amount of product that can be made.

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Check Both Numbers

• The calculation with the smaller answer is the correct product calculation and used the limiting reactant as its ‘have’ value.

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• How many grams of the excess reactant are ‘left over?’

• First determine how many grams of Cl2 were used.

• 2Na + Cl2 2NaCl• 46 g x

• 46 g Na 1 mol Na 1 mol Cl2 71 g Cl2 = 71 g Cl2 used

23 g Na 2 mol Na 1 mol Cl2

• Subtract the amount used from the total amount present.

• [91 g present] - [71 g used] = 20 g Cl2 left over Θ

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Percent Yield• The amount of product actually

produced as compared to the possible (calculated) amount.

• % = Actual Amount or Actual Amount

Possible Amount Stoich Answer

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Sample Calculation• When 45.8 g of potassium carbonate are reacted

completely w/ excess HCl, 46.3 g of potassium chloride are produced. Water and carbon dioxide are also produced. Calculate both the theoretical and % yield.

• K2CO3 + 2HCl 2KCl + H2O + CO2

45.8 g x

• Theoretical (Stoich)• 45.8 g K2CO3 1 mol K2CO3 2 mol KCl 74.5 g KCl = 49.1 g KCl

138 g K2CO3 1 mol K2CO3 1 mol KCl

• % Yield• 46.3 g KCl x 100 = 94.0 % actual yield stated in question

• 49.1 g KCl

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Mass - Energy Calculations • Balanced equations can be used to

determine the amt. of energy absorbed / released during the reaction. Treat energy as if it were a reactant or product.

• Heat of Reaction - qr

• If qr is (-) then products have less energy than the reactants - exothermic

• If qr is (+) then products have more energy than the reactants-endothermic.

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• Given:

2Na2O2 + 2H2O 4NaOH + O2 + 215.76kJ

• How much E is released when 1.99 g of Na2O2 is reacted?

• 1.99 Na2O2 1 mol 215.76 Kj = 2.75 Kj

.78.0 g 1 mol

• qr = negative, exothermic rxn

• qr = -2.75 Kj

Sample Calculation

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