Percentage Yieldand Energy

Lesson 3

Sometimes reactions do not go to completion. Reaction can have yields from 1% to 100%.

1. How many grams of Fe are produced by the reaction of 100. g of Fe2O3, if the percentage yield is 75.0%?

= 52.4 gx 0.750x 55.8 g1 mole

x 4 mole Fe2 mole Fe2O3

x 1 mole159.6 g

100. g Fe2O3

100. g ? g

Fe2O3 + 3 C Fe + 3 CO2 42

Percentage Yield = Actual Yield x 100%Theoretical Yield

 Actual Yield is what is experimentally measured. Theoretical Yield is what is calculated using stoichiometry.

2. In an experiment 152. g of AgNO3 is used to make 75.1 g of

Ag2SO4(s). Calculate the percentage yield.

 

AgNO3(aq) + Na2SO4(aq) Ag2SO4(s) + 2NaNO3(aq)

75.1 g actual yield

? g 152 g

= 139.5 gx 311.9 g1 mole

x 1 Ag2SO4

2 mole AgNO3

x 1 mole169.9 g

152. g AgNO3

= 53.8 %x 100 %= 75.1139.5

% yield

12

Energy CalculationsThe energy term in a balanced equation can be used to calculate the amount of energy consumed or produced in a reaction.

3.How much energy is required to produce 25.4 g of H2?

+ 2H2O H2 + O2

25.4 g? kJ

= 1340 kJ2 mole H2

x 213 kJ 2.02 g

x 1 mole25.4 g H2

2213 kJ

4. How many molecules of H2 can be produced when 452 kJ of

energy if consumed?

? Molecules452 kJ

= 2.55 x 1024 molecsx 6.02 x 1023 molecs1 mole213 kJ

x 2 moles H2452 kJ

2213 kJ2 H2O + H2 + O2

5. How much energy is produced by an explosion of a 5.2 L balloon full of hydrogen at STP?

  H2 + O2 2H2O +

? kJ5.2 L

= 25 kJx 213 kJ 2 moles H222.4 L

x 1 mole 5.2 L

213 kJ2