Loading the lesson
Loading the lesson
Search AllRounder.ai
Search the courses, subjects, tracks, games and features, or jump straight to a page.
Search the courses, subjects, tracks, games and features, or jump straight to a page.
Learn it your way
A short class with Sarah and her students. The other ways are under More.
Sign up free to listen. Transcripts are free to read.
Today we are going to discuss limiting reactants. This concept is crucial because it helps us predict the maximum amount of product that can form in a reaction. Can anyone tell me what a limiting reactant is?
Isn’t it the reactant that gets used up first in a reaction?
Exactly! The limiting reagent is completely consumed first, and this determines how much product we can form. Now, how do we figure out which one it is?
We could do some calculations based on the mole ratios from the balanced equation!
Yes! First, we write the balanced equation. Then, we convert the available amounts of the reactants into moles. Who can give me an example?
In the thermite reaction, we use aluminum and iron(III) oxide, right?
Perfect! Let's say we have 10 grams of aluminum and 50 grams of iron(III) oxide. What would we do next?
We would convert those grams to moles!
Correct. So, after conversion, we can compare the mole ratio of the reactants needed to see which will run out first and thus is the limiting reagent.
In summary, the limiting reagent is identified through a series of steps: writing the balanced equation, converting to moles, and using mole ratios to find the one that produces less product.
Now that we know how to find the limiting reagent, let's talk about yields. Who can define theoretical yield?
Isn't it the maximum amount of product that could be produced if everything went perfectly?
That's right! Theoretical yield assumes complete conversion. What about actual yield?
Actual yield is what we really get from the reaction, right? It’s often less than the theoretical yield.
Exactly! Now, why do you think actual yield is usually less?
Because of side reactions, losses during transfer, or incomplete reactions.
Great points! Finally, how can we express the efficiency of our reaction?
Using percent yield, which is calculated by dividing actual yield by theoretical yield!
Correct! To summarize, theoretical yield is based on calculations, while actual yield is what we gain. The efficiency is expressed as percent yield.
Let's work through an example. If we have aluminum reacting with excess chlorine to produce aluminum chloride, and we get 12 grams of AlCl₃ from 5 grams of aluminum, how do we find the theoretical yield?
We need to start with the balanced equation! For this reaction, it's 2 Al + 3 Cl₂ → 2 AlCl₃.
Exactly! Now, what’s next?
Convert grams of aluminum to moles!
Right! So, if the molar mass of aluminum is 26.98 g/mol, how many moles do we have?
5 grams of aluminum divided by 26.98 g/mol is roughly 0.1853 moles.
Good job! Now, using the mole ratio, how many moles of AlCl₃ can this aluminum produce?
Since it's a 1:1 ratio, it would also be approximately 0.1853 moles of AlCl₃.
Correct! Next, convert that back to grams using the molar mass of AlCl₃. What is that?
It’s about 133.33 g/mol. So, calculating that gives around 24.72 grams.
Exactly! How do we calculate percent yield now?
We take the actual yield, which is 12 grams, over the theoretical yield of 24.72 grams, and multiply by 100.
Excellent! So what is the result?
The percent yield would be about 48.6%!
That's right. To recap, we can find both theoretical and percent yields through a set of calculations based on reactant amounts and balanced equations.
Limiting Reagents determine how much product can be formed in a chemical reaction.
Theoretical Yield represents the maximum possible yield based on stoichiometry.
Actual Yield is what is obtained experimentally, often less than theoretical yield.
Percent Yield indicates the efficiency of a reaction and is crucial for assessing reaction success.
In reactions that we create, one reactant is sure to abate, it’s the limiting reagent that’s fated to run out, this helps us predict what will sprout.
Imagine a baker with 10 bags of flour (reactant A) and only 5 bags of sugar (reactant B). He can make a maximum of 5 cakes, so sugar runs out first, making it the limiting ingredient.
LEAP: Limiting reagent, Excess in the background, Actual yield obtained, Percent yield for success!
LAP: Limiting Reagent, Actual Yield, Percent Yield to remember key concepts of yield calculations.
Term
What is the limiting reagent?
Click to flip
Term
How is percent yield calculated?
Click to flip
Term
Define actual yield.
Click to flip
Term
What does theoretical yield represent?
Click to flip
The reactant that is completely consumed first in a chemical reaction, determining the maximum amount of product formed.
A reactant present in greater quantity than needed to fully react with the limiting reagent.
The maximum amount of product that could form from the given amounts of reactants under ideal conditions.
The amount of product actually obtained from a reaction, usually less than the theoretical yield.
A measure of the efficiency of a reaction, calculated as (Actual Yield / Theoretical Yield) x 100%.
This section discusses the concept of limiting reactants in chemical reactions and how to calculate theoretical, actual, and percent yields.
The section outlines the process of identifying limiting reactants, explains the significance of theoretical and actual yields, and details the calculations involved in determining percent yield. It emphasizes the importance of understanding these concepts for predicting the outcomes of chemical reactions.
In real-world chemical reactions, reactants are seldom provided in stoichiometric proportions, which leads to one reactant being fully consumed before others. This key reactant is termed the limiting reagent and strictly governs how much product can be formed. Conversely, reactants that are present in excess do not limit the reaction.
In the thermite reaction:
2 Al(s) + Fe₂O₃(s) ⟶ 2 Fe(l) + Al₂O₃(s)
If 10.0 g of Al is mixed with 50.0 g of Fe₂O₃, first calculate moles for each:
Using the mole ratio (2:1), one finds that Al is the limiting reagent in this scenario.
Theoretical Yield: The highest quantity of product expected when all of the limiting reagent is converted to product, based on stoichiometry.
Actual Yield: The amount of product actually collected from the reaction, which is often less than the theoretical yield due to losses.
Percent Yield: A measure of efficiency calculated as:
For the reaction of aluminum with excess chlorine to produce aluminum chloride: