Stoichiometry - Limiting & Excess Reactant, Theoretical & Percent Yield - Chemistry

Summary

This video explains key concepts in chemistry, focusing on limiting and excess reactants, theoretical yield, and percent yield. It demonstrates how to identify the limiting reactant through stoichiometry, calculate the maximum amount of product (theoretical yield), and compute the percent yield when actual results are measured. The video also includes a detailed example of a combustion reaction involving propane and oxygen, alongside a second example using benzene. The process of finding the amount of excess reactant remaining after the reaction is also covered, providing a comprehensive overview of these important topics in chemical reactions.

Takeaways

• 😀 Write a balanced chemical equation before proceeding with any calculations in chemical reactions.
• 😀 In a combustion reaction, balance carbon atoms first, then hydrogen atoms, and save oxygen atoms for last.
• 😀 To determine the limiting reactant, divide the moles of each reactant by its coefficient in the balanced equation.
• 😀 The limiting reactant is the one that produces the least amount of product, while the excess reactant remains after the reaction.
• 😀 Theoretical yield is the maximum amount of product that can be formed from the given reactants, calculated using stoichiometry.
• 😀 Percent yield is the ratio of actual yield (experimentally measured) to theoretical yield, multiplied by 100%.
• 😀 The percent error is found by subtracting the percent yield from 100%.
• 😀 To calculate the amount of excess reactant left over, subtract the amount of reactant that participates in the reaction from the total amount initially available.
• 😀 To find how much excess reactant is left, calculate how much of it actually reacts using stoichiometry, then subtract that from the total available.
• 😀 In some cases, you can calculate the theoretical yield from each reactant and compare to identify the limiting reactant (the one producing the smaller yield).
• 😀 The process for determining the excess reactant left over involves starting with the limiting reactant and using stoichiometry to calculate how much of the excess reactant is consumed.

Q & A

What is the first step in solving a combustion reaction problem?

The first step is to write a balanced chemical equation for the reaction.

How do you balance a combustion reaction?

Balance the carbon atoms first, then the hydrogen atoms, and save the oxygen atoms for last.

In the given example, how do we determine the limiting reactant between propane and oxygen?

The limiting reactant is the one that runs out first. One method to determine this is by dividing the moles of each reactant by their respective coefficients in the balanced equation.

What is the theoretical yield in a chemical reaction?

The theoretical yield is the maximum amount of product that can be produced from the given reactants in a reaction.

How do we calculate the theoretical yield using stoichiometry?

To calculate the theoretical yield, convert the moles of reactants to moles of the product using the stoichiometric ratios from the balanced equation.

In the given example, how many moles of CO2 can be formed from 2 moles of propane?

From 2 moles of propane, 6 moles of CO2 can be formed, according to the balanced equation (1:3 ratio between propane and CO2).

What is the percent yield in a reaction?

Percent yield is the ratio of the actual yield (amount obtained from the experiment) to the theoretical yield (maximum possible amount), multiplied by 100%.

How do we calculate the amount of excess reactant left over?

To calculate the excess reactant left over, subtract the amount of the reactant that actually participates in the reaction from the total amount initially present.

In the example involving propane and oxygen, how much propane is left over after the reaction?

After the reaction, 0.4 moles of propane are left over, as 1.6 moles of propane are consumed by the 8 moles of oxygen.

What does it mean if the percent yield is less than 100%?

If the percent yield is less than 100%, it indicates that the reaction was not perfect, and some of the product was lost or not formed due to inefficiencies or side reactions.