Theoretical Yield vs Percent Yield
Learn the crucial difference between the maximum possible product (theoretical yield) and the actual efficiency achieved (percent yield) in chemical reactions.
Quick Comparison
| Aspect | Theoretical Yield | Percent Yield |
|---|---|---|
| Definition | Maximum amount of product possible from stoichiometry | Ratio of actual yield to theoretical yield × 100% |
| Formula | Calculated from balanced equation and limiting reactant | (actual yield / theoretical yield) × 100% |
| Units | Mass units (g, kg, etc.) or moles | Percentage (%) |
| Nature | Theoretical/calculated value | Measured/experimental efficiency |
| Value Range | Absolute amount based on stoichiometry | 0% to 100% (rarely exceeds 100%) |
| Typical Use | Predicting maximum product formation | Evaluating reaction efficiency and losses |
What Is Theoretical Yield?
Theoretical yield is the maximum amount of product that can be formed in a chemical reaction, calculated from stoichiometry and the balanced chemical equation. It assumes 100% reaction efficiency with no losses.
Theoretical yield is calculated by:
- Identifying the limiting reactant
- Converting limiting reactant to moles
- Using stoichiometric ratios from the balanced equation
- Converting product moles to desired units (grams, etc.)
What Is Percent Yield?
Percent yield measures the efficiency of a chemical reaction by comparing the actual amount of product obtained (actual yield) to the theoretical maximum. It's expressed as a percentage.
The formula is: Percent Yield = (Actual Yield / Theoretical Yield) × 100%
For example: If the theoretical yield is 50 g but you only obtain 40 g of product, the percent yield is (40 / 50) × 100% = 80%.
Key Differences Explained
Calculation vs Measurement
Theoretical yield is calculated from stoichiometry before the experiment. Percent yield is determined after the experiment by measuring the actual product obtained and comparing it to the theoretical maximum.
Ideal vs Real
Theoretical yield represents an ideal scenario with perfect conditions and no losses. Percent yield reflects real-world efficiency including all losses, side reactions, and imperfections.
Units and Expression
Theoretical yield is expressed in mass or mole units (grams, kilograms, moles). Percent yield is a dimensionless percentage that indicates efficiency.
Relationship with Actual Yield
Actual yield (what you measure) is always ≤ theoretical yield (what's possible). Percent yield = (actual / theoretical) × 100%, so it shows what percentage of the maximum was achieved.
When to Use Each
Use Theoretical Yield When:
- ✓Planning experiments and ordering reagents
- ✓Predicting maximum product formation
- ✓Calculating expected outcomes before reaction
- ✓Determining cost-effectiveness of reactions
- ✓Comparing different synthetic routes
Use Percent Yield When:
- ✓Evaluating reaction efficiency after completion
- ✓Comparing different experimental conditions
- ✓Identifying sources of product loss
- ✓Assessing quality of laboratory technique
- ✓Reporting experimental results
Common Confusions
❌ Mistake: Confusing Actual Yield with Theoretical Yield
Actual yield is what you measure in the lab. Theoretical yield is what stoichiometry predicts. They're rarely equal. Percent yield compares the two.
❌ Mistake: Expecting 100% Yield
Real reactions rarely achieve 100% yield due to side reactions, incomplete reactions, product losses during purification, and measurement errors. 70-95% is often considered good.
❌ Mistake: Using Excess Reactant for Theoretical Yield
Always base theoretical yield calculations on the limiting reactant, not the excess reactant. The limiting reactant determines the maximum product possible.
❌ Mistake: Percent Yield Over 100%
If percent yield exceeds 100%, there's an error—likely impure product, incorrect theoretical yield calculation, or measurement error. Product cannot exceed the theoretical maximum.
Frequently Asked Questions
What is the difference between theoretical yield and percent yield?
Theoretical yield is the calculated maximum product amount from stoichiometry. Percent yield is the efficiency ratio comparing actual product obtained to theoretical maximum, expressed as a percentage.
Can percent yield be over 100%?
No, percent yield should not exceed 100%. Values over 100% indicate errors such as impure product, incorrect calculations, or measurement mistakes. The actual yield cannot exceed the theoretical maximum.
Why is percent yield important?
Percent yield measures reaction efficiency, helps identify problems in procedure or technique, allows comparison of different methods, and is crucial for industrial cost-effectiveness and optimization.
How do you calculate theoretical yield?
Find the limiting reactant, convert it to moles, use stoichiometry from the balanced equation to find product moles, then convert to grams using the product's molar mass.
What is a good percent yield?
This depends on the reaction. For simple reactions, 80-95% is good. Complex synthesis may have 50-70%. Industrial processes aim for maximum yield for cost-efficiency. Below 70% often indicates problems worth investigating.
What causes low percent yield?
Common causes: incomplete reactions, side reactions, product losses during purification or transfer, measurement errors, impure reactants, non-optimal reaction conditions (temperature, time, etc.).
Is theoretical yield always higher than actual yield?
Yes, theoretical yield should always be equal to or greater than actual yield. It represents the maximum possible under perfect conditions. If actual exceeds theoretical, there's a calculation or measurement error.