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Limiting Reactants & Percent Yield. Section 12.3. With this left over. Excess. If we have…how many sets can we make? Limiting reactant = the reactant that runs out first causing the reaction to stop Excess reactant = reactant that does not get completely used up during a reaction.

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Limiting Reactants & Percent Yield

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## Limiting Reactants & Percent Yield

Section 12.3

With this left over

Excess

If we have…how many sets can we make?

Limiting reactant = the reactant that runs out first causing the reaction to stop

Excess reactant = reactant that does not get completely used up during a reaction

### Steps to solve limiting reactants problems

• Split into 2 problems

• Solve each problem

• Pick smallest answer

• Smallest answer comes from limiting reactant

• Largest answer comes from excess reactant

### Calculating product when reactant is limiting

• If we had 12 moles of nitrogen and 18 moles of hydrogen, what is the maximum number of moles of NH3 that could be produced?

N2 + 3H2 2 NH3

### Calculating product when reactant is limiting

• If we had 112 grams of nitrogen and 18 grams of hydrogen, what is the maximum number of grams of NH3 that could be produced?

N2 + 3H2 2 NH3

2(14) 2(1) 14 + 3

28 2 17

Remember…

Nitrogen

Hydrogen

Pick lowest number

Therefore Hydrogen is your limiting reactant

### Calculating product when reactant is limiting

• With 48 grams of magnesium and 48 grams of oxygen available, how much product can be formed?

2Mg + O2 2 MgO

24.3 2(16) 24.3 + 16

32 40.3

Oxygen

Magnesium

Pick lowest number

Therefore Magnesium is your limiting reactant and oxygen is the excess reactant

### Limiting Reactants Practice III

• How many grams of sodium chloride can be produced in the following reaction with 150 grams of sodium and 200 grams of chlorine?

2Na + Cl2 2 NaCl

23 2(35.5) 23 + 35.5

71 58.5

Sodium

Chlorine

Pick lowest number

### Limiting Reactants Practice III

• In the above reaction, how many more grams of chlorine will be required to fully react with the remaining sodium?

• We need to figure out how many grams of chlorine is needed to react with 150 grams of sodium

2Na + Cl2 2 NaCl

23 2(35.5) 71

Sodium

Chlorine needed = 231 g

Chlorine given (from problem) = 200 g

Additional Chlorine needed = 31 g

### Percent Yield

• Actual yield = the actual amount of product formed during an experiment

• Theoretical yield = amount of product that could be produced according to calculations

• Percent yield = ratio of actual and theoretical yield

### Percent Yield Practice I

• A student uses stoichiometry to calculate the predicted yield of CO2 in a chemical reaction as 323 g. When the student mixes the chemicals to make the CO2, he finds that the reaction only produces 308 g of CO2. What is the percent yield?

• In the reaction, how many grams of potassium oxide can be produced with 78 grams of potassium and 140 grams of boron oxide?

6 K + B2O3 3 K2O + 2 B

39.1 2(10.8)+3(16) 2(39.1)+16

69.6 94.2

Potassium

Boron Oxide

Pick lower number

• Problem 2 continued

• If the actual yield from the above reaction was 81 grams, what is the percent yield?

• Actual yield = 81 grams

• Theoretical yield = 93.95 g (from last slide)

3. The actual yield in the above reaction was 39 grams of ammonium. Using 84 grams of nitrogen and 8 grams of hydrogen, what is the percent yield?

-first we must calculate the theoretical yield of product which means we need to find our limiting reactant

N2 + 3H2 2 NH3

2(14) 2(1) 14+ 3

28 2 17

Nitrogen

Hydrogen

Pick lower number

• Problem 3 continued

• Actual yield = 39 grams

• Theoretical yield = 45.3 g (from last slide)