STOICHIOMETRYSTOICHIOMETRYMass relationship of reactants and Mass relationship of reactants and

products in a chemical reactionproducts in a chemical reaction

Hydrogen gas + nitrogen gas ----> Hydrogen gas + nitrogen gas ----> ammonia gasammonia gas

Qualitative: Qualitative: describes the “nature” of describes the “nature” of the reactants and productsthe reactants and products

Quantitative: Quantitative: describes the amounts of describes the amounts of reactants and productsreactants and products

Steps for solving Stoichiometry problems:

1.1. Write a balanced equation for the Write a balanced equation for the question.question.

2.2. Understand the mole and molar ratioUnderstand the mole and molar ratio

3.3. Convert mass of reactant to moles of Convert mass of reactant to moles of reactant.reactant.

4.4. Convert moles of reactant to moles of Convert moles of reactant to moles of product.product.

5.5. Convert moles of product to grams of Convert moles of product to grams of product.product.

6.6. Pick up the calculator and do the math!!Pick up the calculator and do the math!!

How about something a little easier to follow????

Like………….

FOOD!!!!!

The Stoichiometry of a The Stoichiometry of a sandwich…..sandwich…..

The SMCHS famous sandwich The SMCHS famous sandwich consists of 2 slices of bread, 2 consists of 2 slices of bread, 2 slices of deli meat, 1 slice of slices of deli meat, 1 slice of cheese, and 3 slices of pickle.cheese, and 3 slices of pickle.Write an equation for the recipe…...Write an equation for the recipe…...

2 slices bread + 2 slices meat + 1 slice cheese + 3 slices pickle 2 slices bread + 2 slices meat + 1 slice cheese + 3 slices pickle 1 sandwich 1 sandwich

OR…..OR…..

2B + 2M + 1C + 3P2B + 2M + 1C + 3P 1B 1B22MM22CPCP33

How many SMCHS famous How many SMCHS famous sandwiches can be produced when sandwiches can be produced when 8 slices of bread, 8 slices of meat, 4 8 slices of bread, 8 slices of meat, 4

slices of cheese, and 12 slices of slices of cheese, and 12 slices of pickles are available?pickles are available?

Break it down…..

BREAD: 8B = 4 B2M2CP3

MEAT: 8M = 4 B2M2CP3

CHEESE: 4C = 4 B2M2CP3

PICKLES: 12P = 4 B2M2CP3

EVALUATION: WE CAN MAKE 4 COMPLETE SANDWICHES!!

How many of each How many of each ingredient is needed to ingredient is needed to make 10 SMCHS famous make 10 SMCHS famous sandwiches?sandwiches?

BREAK IT DOWN…..BREAD: 10B2M2CP3 x (2B / B2M2CP3 )= 20B

MEAT: 10B2M2CP3 x (2M / B2M2CP3 )= 20M

CHEESE: 10B2M2CP3 x (1C / B2M2CP3 )= 10C

PICKLES: 10B2M2CP3 x (3P / B2M2CP3 )= 30P

THUS: 10 Sandwiches require 20B, 20M, 10C, and 30P

But what if you ran out of pickles?

Pickles would be the LIMITING REAGENT for the sandwiches

In any reaction you will likely have a limiting reagent AND an excess reagent. Too little of one Too much of the

other

3 3 HH22 + + 11 NN2 2 22NHNH33

What do the coefficients mean?What do the coefficients mean? 33 molecules : molecules : 11 molecule : molecule : 22

moleculesmolecules

OR OR 33 moles : moles : 11 mole : mole : 22 moles moles

1N2 + 3H2 2NH3

1 molecules : 3 molecules : 2 molecules1 molecules : 3 molecules : 2 molecules

10 molecules : 30 molecules : 20 molecules10 molecules : 30 molecules : 20 molecules

100 molecules : 300 molecules : 200 molecules100 molecules : 300 molecules : 200 molecules

1 (1 (6x106x102323) : 3 () : 3 (6x106x102323) : 2 () : 2 (6x106x102323))

1 MOLE : 3 MOLE : 2 MOLE1 MOLE : 3 MOLE : 2 MOLE

11NN22 + + 33HH22 22NHNH33

1mole of : 3mole of : 2mole of

N2 molecules H2 molecules NH3 molecules

This is the MOLAR RATIOallowing you to solve problems dealing

with masses of substances!

1 N2 + 3 H2 2 NH3

If 75g of hydrogen reacts, A) How many g of N2 reacts?

Use the molar ratio!!First convert the grams to moles……

75g H2 x 1moleH2 / 2.02g H2

Next use molar ratio…..

75g H2 x 1mole x 1mole N2

2.02g H2 3mole H2

Now you’ve converted to moles of N2,

make the final conversion into grams!!!

75g H2 x 1mole x 1mole N2 x 28g N2

2.02g H2 3mole H2 1mole N2

75g H75g H22 x 1mole H x 1mole H2 2 x 1mole Nx 1mole N2 2 x x 28g N28g N22

2.02g H2.02g H22 3mole H 3mole H22 1mole N1mole N22

= 346.5g = 350g of N2

1 N1 N22 + 3 H + 3 H22 2 NH 2 NH33If 75g of hydrogen reacts,If 75g of hydrogen reacts,

B) How many g of NHB) How many g of NH33 are formed? are formed?

75g H75g H22 x 1mole H x 1mole H2 2 x x 2mole NH2mole NH 3 3 xx 17g NH17g NH33

2.02g H2.02g H2 2 3mole H3mole H22 1mole NH 1mole NH33

MOLAR RATIO:MOLAR RATIO:3mole3mole HH22 :: 1mole N1mole N22 : : 2mole NH2mole NH 3 3

= 420g NH= 420g NH33

Use the same system for Use the same system for thesethese

Mass - Volume StoichiometryMass - Volume Stoichiometry Instead of converting the moles of Instead of converting the moles of

unknown to grams you convert to unknown to grams you convert to volume (L)volume (L)

Volume - Volume - StoichiometryVolume - Volume - Stoichiometry Instead of using the molar ratio you Instead of using the molar ratio you

use volume ratio and solve for use volume ratio and solve for volumevolume