ii. stoichiometry in the real world limiting reagents and % yield (p. 268-378) stoichiometry – ch....

II. Stoichiometry in the Real World

Limiting Reagents and % yield

(p. 268-378)

Stoichiometry – Ch. Stoichiometry – Ch. 1212

A. Limiting ReactantsA. Limiting ReactantsA. Limiting ReactantsA. Limiting Reactants

Available IngredientsAvailable Ingredients• 4 slices of bread• 1 jar of peanut butter• 1/2 jar of jelly

Limiting ReactantLimiting Reactant• bread

Excess ReactantsExcess Reactants• peanut butter and jelly

A. Limiting ReactantsA. Limiting ReactantsA. Limiting ReactantsA. Limiting Reactants

Limiting ReactantLimiting Reactant• used up in a reaction• determines the amount of product

Excess ReactantExcess Reactant• reactant left over… • cheaper & easier to recycle, added to

ensure that the other reactant is completely used up

A. Limiting ReactantsA. Limiting ReactantsA. Limiting ReactantsA. Limiting Reactants

1. Write a balanced equation.

2. For each reactant, calculate the

amount of product formed.

3. Smaller answer indicates:

• limiting reactant

• amount of product

A. Limiting ReactantsA. Limiting ReactantsA. Limiting ReactantsA. Limiting Reactants

79.1 g of zinc react with 0.90 L of 2.5M HCl. Identify the limiting and excess reactants. How many liters of hydrogen are formed at STP?

Zn + 2HCl ZnCl2 + H2 79.1 g ? L82.0g

Summary of Limiting Summary of Limiting ReactantReactantSummary of Limiting Summary of Limiting ReactantReactant

Do stoichiometry twice to go from each reactant to same product

Now you know LR and actual amount produced

Do stoichio once more from LR to ER to determine amount used…then subtract

A. Limiting ReactantsA. Limiting ReactantsA. Limiting ReactantsA. Limiting Reactants

79.1g Zn

1 molZn

65.39g Zn

= 27.1 L H2

1 molH2

1 molZn

22.4 LH2

1 molH2

Zn + 2HCl ZnCl2 + H2 79.1 g ? L82.0g

A. Limiting ReactantsA. Limiting ReactantsA. Limiting ReactantsA. Limiting Reactants

22.4L H2

1 molH2

82.0g

1 molHCl

36.5gHCl

= 25 L H2

1 molH2

2 molHCl

Zn + 2HCl ZnCl2 + H2 79.1 g ? L82.0

A. Limiting ReactantsA. Limiting ReactantsA. Limiting ReactantsA. Limiting Reactants

Zn 27.1 L H2 HCl 25 L H2

Limiting reactant: HCl

Excess reactant: Zn

Product Formed: 25 L H2

left over zinc

Choose smallest #

How many grams of excess How many grams of excess reactant remain?reactant remain?How many grams of excess How many grams of excess reactant remain?reactant remain?

Use stoichiometry to convert from L.R. (HCl) to amount of E.R. (Zinc) USED..getting…

Now subtract amount ER (73.5g) used from starting ER (79.1g) to get amount remaining (79.1 -73.5) =

73.5g Zn used

5.6grams of Zinc remaining

B. Percent YieldB. Percent YieldB. Percent YieldB. Percent Yield

100yield ltheoretica

yield actualyield %

calculated on paper

measured in lab

B. Percent YieldB. Percent YieldB. Percent YieldB. Percent Yield

When 45.8 g of K2CO3 react with excess

HCl, 46.3 g of KCl are formed. Calculate the theoretical and % yields of KCl.

K2CO3 + 2HCl 2KCl + H2O + CO2 45.8 g ? g

actual: 46.3 g

B. Percent YieldB. Percent YieldB. Percent YieldB. Percent Yield

45.8 gK2CO3

1 molK2CO3

138.21 gK2CO3

= 49.4g KCl

2 molKCl

1 molK2CO3

74.55g KCl

1 molKCl

K2CO3 + 2HCl 2KCl + H2O + CO2 45.8 g ? g

actual: 46.3 g

Theoretical Yield:

B. Percent YieldB. Percent YieldB. Percent YieldB. Percent Yield

Theoretical Yield = 49.4 g KCl

% Yield =46.3 g

49.4 g 100 =93.7%

K2CO3 + 2HCl 2KCl + H2O + CO2 45.8 g 49.4 g

actual: 46.3 g