Stoichiometry

(Stoo-eek-ee-aam-et-ry)

9.1 Using Chemical Equations

1 piece of bread + 3 tomato slices + 3 slices meat + 2 cheese slices 1 sandwich

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• How many sandwiches can you make? 20 pieces of bread, 44 slices of tomatoes, 332 slices of meat and 4 slices of cheese.

• F = frame, W = wheel• Wheels come in packages of 3• Frames come in packages of 2• A bike factory is manufacturing bikes. A bike is made

from one frame and two wheels. Write a balanced equation for producing bikes. (A bike, of course, is FW2).

• ___ F2 + ___ W3 ____ FW2

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•___ F2 + ___ W3 ____ FW2

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• If we order 12 cases of frames, how many cases of wheels will we need?– Use dimensional analysis! (Applause)

• The numbers in the parenthesis come from the balanced chemical equation

Using Chemical Equations

• The coefficients can represent

– the number of molecules and atoms OR

– The moles of molecules and atomsN2(g) + 3H2(g) 2NH3(g)

1 molecule N2 + 3 molecules H2 2 molecules NH3

1 mol N2 + 3 mol H2 2 mol NH3

• Mole Ratio: The ratio between two chemicals in a balanced chemical equation. It is used as a conversion factor.

____ Fe + ____ O2 ____ Fe2O3

• 4. The mole ratio for O2 to Fe2O3 is ___ mol O2 : ___ mol Fe2O3

• 5. How many moles of O2 are needed to form 21.0 moles of Fe2O3?

• (Convert 21.0 mol Fe2O3 to mol O2)

21.0 mol Fe2O3 ______________ =

• Stoichiometry (stoik ee ahm ih tree)Using mole ratios to convert from an amount of one substance to the amount of another substance

• If you know the amount of chemical A and you want to know the amount of chemical B, then it is just converting moles of A to moles of B using the mole ratio.

• 6. Questions may ask for the answer as a mass (in grams) rather than moles.

*Example: How many grams of Fe2O3 are

formed when 6.00 moles of O2 react?

• a. Convert ____________ to ____________• b. Still use the mole ratio, but then convert

mol Fe2O3 to g Fe2O3 a separate step.

• 6.00 mol O2 mol Fe2O3 __________

mol O2

• 7. Or, the question could start with the mass of the given chemical. – What mass (in grams) of Fe is needed to

react with 525 g of O2?

Limiting Reactant and Percent Yield

A new adventure in sales• We are going to sell hot dogs on the street (to raise

money for chemistry experiments).• Each hot dog we sell will require one bun and one

hotdog, so we order 200 packages of each.• But then we find that hot dogs are in packages of 10

and buns are in packages of 8.• So, which will we use up first? Hot dogs or buns?

So…• Since the hot dog business was to

unprofitable (400 extra hot dogs got moldy while in storage waiting for the state fair), we go back to bikes.

Bike stoichiometry• F = frame, W = wheel• Wheels come in packages of 3• Frames come in packages of 2• 3 packages of frames and 4 packages of

wheels make six bikes:• 3 F2 + 4 W3 6 FW2

• We have 21 cases of frames left in storage, but no wheels. How many packages of wheels must we buy?

• Oops. Actually, I just found 24 packages of wheels under my desk. So, do we use up the wheels, or do we have extra wheels?

• Mole Ratio: The ratio between two chemicals in a balanced chemical equation. It is used as a conversion factor.

• 4 Fe + 6 O2 2 Fe2O3

• The mole ratio for O2 to Fe2O3 is 6 mol O2 / 2 mol Fe2O3

9.2 Calculating mass

• You can think of a chemical equation as– Number of molecules, moles AND grams

N2(g) + 3H2(g) 2NH3(g)

1 molecule N2 + 3 molecules H2 2 molecules NH3

1 mol N2 + 3 mol H2 2 mol NH3

28 g N2 + 3 x 2.02 g H2 2 x 17g NH3

9.2 Calculating mass

• How many grams of NH3 can be made from 38.4 grams H2?

N2(g) + 3H2(g) 2NH3(g)

• The equation only gives the “mole” ratio, so we need to convert grams to moles.

• Plan:Grams H2 moles H2 moles NH3 grams NH3

38.4 g H2

2.02 g/ m

ol H2

moles H2

2 mol NH3/ 2 mol H2

moles NH3

17.0

4 g

/ m

ol N

H3

grams NH3

9.3 Limiting Reactants and Percent Yield

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• Imagine that we want to make sandwiches (and I love sandwiches).

• We have: 20 pieces of bread, 44 slices of tomatoes, 332 slices of meat and 4 slices of cheese.

• How many sandwiches can you make?• Yes, two. We need to buy more cheese if we

want to make more, and I want to watch the soccer game, so I can’t go to the store right now.

Determining the Limiting Reactant

• In the sandwich example, cheese was the limiting reactant (also called “limiting reagent).

• The limiting reactant is completely used up in a reaction.

• 3 F2 + 4 W3 6 FW2

• We have 21 cases of frames left in storage, but no wheels. How many packages of wheels must we buy? (Show work!)– Oops. Actually, I just found 24 packages of

wheels under my desk. So, do we use up the wheels, or do we have extra wheels?

N2(g) + 3H2(g) 2NH3(g)

• To use all 16.0 mol N2, you would need 48.0 mol H2.

• So what if you had 16.0 mol N2 and 19.0 mol H2. Which would you use up first?

Now, with chemicalsN2 + 3 H2 2 NH3

• You have 16.0 moles of N2. How many moles of H2 would be required?

Using the Limiting Reactant

• You have 16.0 mol N2 and 19.0 mol H2.

How many moles of NH3 will form?

– Since limiting reactant controls the amount of product formed, just convert from 19.0 mol H2 to mol NH3.

• How many moles of the excess reactant would remain?

Example: 25.0 kg N2 react with 5.00 kg H2. How much ammonia (NH3) will be made?

N2(g) + 3H2(g) 2NH3(g)

1. Calculate moles of H2 and N2.

2. Calculate how many moles N2 you need to react with H2. Use the mole ratio.

3. Ask yourself: Do you have enough H2?

Yes H2 is excess, N2 is limiting.

No H2 is limiting, N2 is excess

4. Use the limiting reactant to calculate grams NH3.

Tin(II) fluoride is used in some toothpastes.

It is made by reacting tin with hydrogen fluoride to produce tin(II) fluoride and hydrogen gas.

a.Write a balanced chemical equation for this reaction.

b.You have 3.15 mol of Sn and 4.96 mol HF.i. Which is the limiting reactant (show work!)?

ii. How many moles of tin(II) fluoride will form?

c.You have 21.1 g Sn and 84.5 g HF.i. Which will be limiting?

ii. What mass of tin(II) fluoride will form?

Percent Yield• In a real experiment, usually you get less product than

you would expect. Sometimes not all the chemicals react.

• Theoretical Yield – calculated amount of product• Actual Yield – amount of product actually collected in

lab• Knowing both of these, you can calculate percent yield.• % yield = actual yield 100

theoretical yield

Example

• In an experiment, Jenny collected 57.1 g of product. However, stiochiometrically she calculated that she should have gotten 66.8 g. Find the percent yield.

• 57.1 g 100% = 85.5% yield 66.8 g