The Percentage Yield Revision Notes for Leaving Cert Chemistry

The Percentage Yield

What is percentage yield?

In chemistry, we often find that reactions don't produce as much product as we expect them to. Percentage yield is a way of measuring how efficient a chemical reaction is by comparing what we actually get to what we should theoretically get.

When we carry out chemical reactions, there are two important types of yield to understand:

Theoretical yield: This is the maximum amount of product that could be formed if the reaction went perfectly to completion with no losses
Actual yield: This is the amount of product we actually obtain when we carry out the experiment in practice

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The percentage yield tells us what proportion of the theoretical yield we actually achieved in our experiment. This measurement is crucial for understanding the efficiency of chemical processes and predicting real-world production outcomes.

The percentage yield formula

The formula for calculating percentage yield is straightforward:

$\text{Percentage yield} = \frac{\text{Actual yield of product}}{\text{Theoretical yield of product}} \times 100$

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This formula can be used whether you're working with masses (in grammes) or volumes (in litres or cm³). The key requirement is that both the actual and theoretical yields must be in the same units.

Why is actual yield usually less than theoretical yield?

In real laboratory conditions, we rarely achieve 100% conversion of reactants to products. This happens because:

Incomplete reactions: Some reactions don't go all the way to completion
Side reactions: Competing reactions may produce unwanted products
Physical losses: Some product may be lost during handling, filtering, or transferring
Practical limitations: Equipment limitations and human error can reduce yield

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Understanding that actual yields are typically lower than theoretical yields helps explain why percentage yields are usually less than 100%. In industrial processes, achieving yields of 80-90% is often considered very good.

Step-by-step approach to percentage yield calculations

When solving percentage yield problems, follow this systematic approach:

Step 1: Write the balanced equation

Start with the balanced chemical equation for the reaction. This shows the mole ratios between reactants and products.

Step 2: Calculate moles of reactants

Convert the given masses to moles using the formula:

$\text{Number of moles} = \frac{\text{Mass}}{\text{Relative molecular mass}}$

Step 3: Identify the limiting reactant (if necessary)

If multiple reactants are given, determine which one limits the amount of product that can be formed.

Step 4: Calculate theoretical yield

Use stoicheiometry and the balanced equation to find the maximum possible amount of product that could be formed.

Step 5: Apply the percentage yield formula

Use the actual yield given in the question along with your calculated theoretical yield to find the percentage yield.

Worked example: Ethene production

Let's look at a practical example involving the dehydration of ethanol to produce ethene:

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Worked Example: Calculating Percentage Yield for Ethene Production

Given reaction: $\text{C}_2\text{H}_5\text{OH} \rightarrow \text{C}_2\text{H}_4 + \text{H}_2\text{O}$

Starting with 20 g of ethanol, a student obtained 8 g of ethene. Calculate the percentage yield.

Step 1: Calculate moles of ethanol used

Molar mass of ethanol (C₂H₅OH) = 46.068 g/mol
Moles of ethanol = $\frac{20 \text{ g}}{46.068 \text{ g/mol}} = 0.434 \text{ mol}$

Step 2: Calculate theoretical yield of ethene

From the balanced equation: 1 mol ethanol → 1 mol ethene
Therefore: 0.434 mol ethanol → 0.434 mol ethene
Molar mass of ethene (C₂H₄) = 28.052 g/mol
Theoretical yield = $0.434 \text{ mol} \times 28.052 \text{ g/mol} = 12.18 \text{ g}$

Step 3: Calculate percentage yield

Percentage yield = $\frac{8.0 \text{ g}}{12.18 \text{ g}} \times 100 = 65.68\%$

Important calculation tips

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Critical Points for Percentage Yield Calculations:

Units matter: Always ensure your actual and theoretical yields are in the same units
Show your working: Break down calculations into clear steps
Check your answer: Percentage yields should typically be less than 100%
Round appropriately: Follow significant figure rules based on your given data

The percentage yield gives us valuable information about the efficiency of our chemical process and helps us understand how much product we can expect in real-world conditions.

Summary

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Key Points to Remember:

Percentage yield formula: $\frac{\text{Actual yield}}{\text{Theoretical yield}} \times 100$ - this is the key formula you need to memorise
Theoretical yield is the maximum possible amount of product, while actual yield is what you actually get in practice
Actual yields are usually lower than theoretical yields due to incomplete reactions, side reactions, and practical losses
Follow the systematic approach: balanced equation → moles → limiting reactant → theoretical yield → percentage yield calculation
Always check units
- make sure actual and theoretical yields are in the same units before calculating percentage yield

The Percentage Yield (Leaving Cert Chemistry): Revision Notes