Reagent and Yields

Question 1

Limiting Reagent

Answer 1

The reactant that is COMPLETELY CONSUMED in a reaction.

Question 2

Excess Reagent

Answer 2

The reactant that is still PRESENT after the reaction is complete (leftover).

Ex. Determine the amount of Magnesium oxide produced when 2.77 mol of Magnesium reactions with 2.55 mol of Oxygen:

1. Pick one reactant and use it to find the theoretical moles of the other (given x / )
2. Use limiting to find moles (given x / ).

Question 3

Theoretical Yield

Answer 3

• The amount of mass of product predicted based on the stoichiometry (math) of the chemical equation.
• What’s supposed to happen.
• Math = Theory
• Calculated by assuming all of the limiting reactant is completely used up and can be calculated before a reactant occurs.
• In reality, theoretical yields are rarely achieved and instead we can talk about the actual yield.

Question 4

Actual Yield

Answer 4

• The amount or mass of product actually collected during an experiment or industrial process.

(Reality)
(Given in questions)

• We can examine 4 factors that can affect actual yield.

Question 5

The Nature of the Reaction (Actual Yield)

Answer 5

Some reactions go in both directions (two half arrows).

Question 6

The Experimental Procedure (Actual Yield)

Answer 6

Some product is lost through spillage, splattering, or transfer so it doesn’t all get into the experiment 100%.

Question 7

Impurities (Actual Yield)

Answer 7

Chemicals are rarely 100% pure, and may become more impure overtime.

Question 8

Completing Side Reactions (Actual Yield)

Answer 8

Sometimes completing reactions prevent the amount of product produced.

Question 9

Percentage Yield

Answer 9

• The ratio, expressed as a percentage, of the actual yield to the theoretical yield.

Percent Yield = Actual Yield (given) / Theoretical Yield (Math) x 100%

Question 10

Determining Percentage Yield

Answer 10

Use of stoichiometry (mole ratios) to determine theoretical yield of the product.

• Sometimes the problem tells you what the limiting reactant is. In this case, use its mass to find the number of moles of the product and calculate the theoretical yield.
• If you are given the mass for BOTH reactants, then find the limiting reactant before continuing.

Ex. During an investigation, 20.0 g of Bromine acid, HBrO3 is reacted with excess HBR. If 47.3 g of Bromine is produced, what is the percentage yield for this reaction?

1. Convert 20g to moles (n=m/M).
2. Use ratio to find moles (given x __ ).
3. Convert to mass (m=n x M).