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Limiting Reactants & Percent YieldPowerPoint Presentation

Limiting Reactants & Percent Yield

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### Limiting Reactants & Percent Yield

Section 12.3

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

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

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 produced with 78 grams of potassium and 140 grams of boron oxide?
- 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)

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

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 ammonium. Using 84 grams of nitrogen and 8 grams of hydrogen, what is the percent yield?
- Actual yield = 39 grams
- Theoretical yield = 45.3 g (from last slide)

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