Problem 84 - Chemistry Matter and Change

Question

How are actual yield and theoretical yield determined?

Step-by-Step Solution

Answer

Actual yield and theoretical yield are determined using a balanced chemical equation, initial amounts of reactants and the amount of products formed. First, write the balanced chemical equation, then determine the limiting reactant by calculating the moles of each reactant and comparing their mole ratios to their stoichiometric ratios. Calculate the theoretical yield using the limiting reactant and the stoichiometry from the balanced chemical equation by converting moles of product to mass. The actual yield is the amount of product obtained experimentally. Finally, if necessary, calculate the percent yield using the formula Percent Yield = \(\frac{Actual \, Yield}{Theoretical \, Yield}\) × 100.

Step 1: Write the balanced chemical equation

In this step, write down the balanced chemical equation for the reaction, which gives the stoichiometric ratio between reactants and products.

Step 2: Determine the limiting reactant

Calculate the moles of each reactant by using their masses and molar masses. Then, compare the mole ratios of the reactants to their stoichiometric ratios in the balanced chemical equation to determine the limiting reactant. The limiting reactant is the reactant that is completely consumed during the reaction and determines the maximum amount of product that can be formed.

Step 3: Calculate the theoretical yield

Using the limiting reactant, calculate the moles of the desired product, using the stoichiometry from the balanced chemical equation. Convert the moles of product to mass using its molar mass. The mass you obtain is the theoretical yield, which is the maximum amount of product that can be formed from the given reactants.

Step 4: Determine the actual yield

Actual yield is the amount of product obtained from an experiment or reaction, which is usually given or should be measured. It is often less than the theoretical yield due to side reactions, losses during the process, or incomplete reactions.

Step 5: Calculate the percent yield

Finally, if necessary, find the percent yield, which is a measure of the efficiency of a chemical reaction. Use the following formula: Percent Yield = \(\frac{Actual \, Yield}{Theoretical \, Yield}\) × 100 This will give a percentage value that represents how efficiently the reaction proceeded, comparing the actual yield (measured) to the theoretical yield (predicted).

Key Concepts

Limiting Reactant

In stoichiometry, the limiting reactant is a fundamental concept critical to understanding how chemical reactions proceed. When you mix chemicals for a reaction, not all reactants are always entirely used up. The limiting reactant is the one that gets used up first, stopping the entire reaction. It's like running out of milk while you're trying to make as many cakes as possible with your ingredients. Once the milk is gone, you can’t make more cakes, even if you have enough flour and sugar.
To identify the limiting reactant, follow these steps:

• Calculate the moles of each reactant using their masses and molar masses.
• Look at the balanced chemical equation to find the stoichiometric ratio.
• Compare the moles of each reactant to their required stoichiometric ratio.
• The reactant that falls short is your limiting reactant.

By finding the limiting reactant, you can predict the maximum amount of product that the reaction can produce.

Theoretical Yield

Theoretical yield is what we call the maximum amount of product that can be produced from a given amount of limiting reactant, calculated based on the balanced chemical equation of the reaction. Think of it as the absolute perfect scenario where everything goes right—no spills, perfect conditions, everything in the chemical reaction turning into product.

• Use the molar amount of the limiting reactant to find the equivalent moles of product based on the stoichiometric ratios.
• Convert these moles of product to mass using the molar mass of the product substance.
• This calculated mass is your theoretical yield.

Theoretical yield helps chemists understand the potential of a reaction and plan accordingly, though real-life conditions make hitting this mark challenging.

Percent Yield

Percent yield is a useful measurement for gauging the success of a chemical reaction in practice. It compares the theoretical yield to the actual yield, which is the amount of product you actually manage to collect after conducting the experiment. Reactions often result in less product than we predict due to various factors like incomplete reactions or losses in procedure.
Here's how you calculate percent yield:

• Measure the actual yield of the product from the experiment (typically on a scale).
• Use the theoretical yield you've calculated.
• Plug these values into the formula:
  • Percent Yield = \( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \) × 100

The result is a percentage that tells you the efficiency of your reaction. A higher percent yield indicates a more efficient reaction, while lower values suggest some inefficiencies that might be worth investigating.