Yield calculation experiments evaluate how successfully a reaction or experimental operation proceeded by comparing the amount of product actually obtained in a chemical reaction with the amount that should theoretically be obtainable.
Yield is an important result in many chemistry experiments, including organic synthesis, inorganic synthesis, precipitation reactions, crystallization, extraction, and purification.
Yield is not merely a calculated value, but also provides clues for discussing the extent of reaction progress, losses during operations, contamination with impurities, insufficient drying, and other factors.
In a discussion of yield calculations, it is not sufficient simply to write that “the yield was low” or “the amount was less than the theoretical value.”
It is necessary to explain which reagent was used as the basis for calculating the theoretical yield, whether the actual yield truly represents the amount of pure product, whether the reaction failed to proceed completely, and at which stage losses occurred during filtration, washing, recrystallization, or drying.
In addition, if the yield exceeds 100%, it is necessary to consider the possibility that moisture, solvent, or impurities remained rather than concluding that more product than expected was obtained.
This article clearly explains, as examples of discussions that can be used in laboratory reports on yield calculations, theoretical yield, actual yield, percent yield, limiting reagent, extent of reaction completion, side reactions, equilibrium reactions, purification loss, filtration loss, washing loss, insufficient drying, impurity contamination, causes of reduced yield, causes of yields exceeding 100%, and points for improvement.
Note:
This article is a reference intended to assist with discussions of yield-calculation results obtained in basic chemistry experiments, organic chemistry experiments, inorganic chemistry experiments, and analytical chemistry experiments at universities and similar institutions.
For the actual calculation method, reagent used as the basis, molecular weights, significant figures, drying conditions, purification conditions, and safety precautions, always follow the instructions in your university’s laboratory manual and those given by your instructor or TA.
- What Is Yield?
- Main Items to Include in the Results
- What Is Theoretical Yield?
- What Is Actual Yield?
- Discussion of the Limiting Reagent
- Main Causes of Reduced Yield
- When the Reaction Did Not Proceed Completely
- Reduced Yield Caused by Side Reactions
- Reduced Yield Caused by Equilibrium Reactions
- Loss During Filtration
- Loss During Washing
- Reduced Yield Caused by Recrystallization
- Loss During Extraction
- Loss During Transfer
- Apparent Increase in Yield Caused by Insufficient Drying
- Apparent Increase in Yield Caused by Impurity Contamination
- When the Yield Exceeds 100%
- Relationship Between Product Purity and Yield
- Relationship Between Melting Point and Yield
- Errors in Theoretical-Yield Calculations
- Significant Figures and Yield Calculations
- When the Results Can Be Considered Good
- Example Discussions When the Experiment Did Not Go Well
- How to Write Points for Improvement
- Difference Between a Superficial Discussion and a Good Discussion
- Examples of Expressions That Can Be Used in Reports
- Points to Check When Discussing Yield Calculations
- Summary
What Is Yield?
Yield is the percentage that expresses how much product was actually obtained compared with the amount of product that should theoretically be obtainable.
If the reaction proceeds completely, there are no losses during operations, and the product is obtained in pure form, the yield approaches 100%.
However, in actual experiments, the yield often differs from 100% because the reaction does not proceed completely, product is lost during operations, side reactions occur, or impurities contaminate the product.
Yield is an indicator used to judge the success of an experiment, but the quality of an experiment cannot be completely judged from yield alone.
Even if the yield is high, the result cannot be considered good if the product contains many impurities.
Conversely, even if the yield is low, the purification operation may have been appropriate if a highly pure product was obtained.
Yield must be discussed together with purity, reaction conditions, and the experimental procedure.
Yield = Actual yield / Theoretical yield × 100
Example Discussion:
Yield is a value indicating how much product was actually obtained compared with the amount theoretically obtainable.
Because the yield in this experiment was lower than 100%, the reaction may not have proceeded completely, or part of the product may have been lost during operations such as filtration, washing, or purification.
Therefore, yield must be discussed from both the reaction conditions and the experimental operations.
Main Items to Include in the Results
In the results of a yield calculation, organize the amounts of reagents used, number of moles, reaction equation, limiting reagent, theoretical yield, actual yield, percent yield, appearance of the product, and purity-confirmation results such as melting point or spectra.
It is important to clearly state which reagent was used as the basis for calculating the theoretical yield.
Main Items to Include in the Results
- Reaction equation
- Mass or volume of each reagent used
- Amount of substance of each reagent
- Molar ratio of each reagent
- Limiting reagent
- Theoretical amount of substance of the product
- Theoretical yield of the product
- Mass of product actually obtained
- Yield
- Color or crystal form of the product
- Drying conditions
- Purification method
- Results of purity confirmation
- Causes of reduced yield
- Points for improvement
Example of How to Write the Results:
From the stoichiometric ratio in the reaction equation, reagent A was determined to be the limiting reagent.
The theoretical yield of product B was calculated from the amount of substance of reagent A, and the yield was calculated by comparing it with the actual mass of the product obtained after drying.
As a result, the yield was lower than the theoretical value, and possible causes include incomplete reaction, loss during purification, and dissolution loss of the product.
What Is Theoretical Yield?
Theoretical yield is the maximum amount of product that can be obtained assuming that the reaction proceeds completely and there are no losses during experimental operations.
Theoretical yield is calculated from the stoichiometric ratio in the reaction equation and the amount of substance of the limiting reagent.
It is important to calculate it based not on the reagent used in the largest amount, but on the reagent that is consumed first.
The theoretical yield is not an amount that will necessarily be obtained in an actual experiment.
It is only the calculated maximum amount, and the actual yield becomes smaller because of incomplete reaction, side reactions, equilibrium, operational loss, purification loss, and other factors.
Therefore, correctly determining the theoretical yield is the starting point of yield calculations.
Example Discussion:
Theoretical yield is the maximum amount of product that can be obtained assuming that the reaction proceeds completely and that all of the product can be recovered.
In this experiment, the theoretical yield was calculated from the stoichiometric ratio in the reaction equation and the amount of substance of the limiting reagent.
Because the actual yield was smaller than the theoretical yield, factors causing deviation from the theoretical conditions were considered to have existed in the actual reaction or experimental operations.
What Is Actual Yield?
Actual yield is the amount of product actually obtained through the experimental operations.
Normally, the mass of the product after filtration, washing, drying, and purification is used as the actual yield.
However, if moisture, solvent, unreacted materials, or impurities remain in the product, the measured mass does not represent only the amount of pure product.
A larger actual yield is not always better.
If insufficient drying or impurity contamination causes the mass to increase, the apparent yield becomes higher, but the purity becomes lower.
When discussing actual yield, the appearance of the product, melting point, spectra, drying conditions, and purification conditions should also be checked.
Example Discussion:
Actual yield is the mass of product actually recovered.
However, if solvent, moisture, or impurities remain in the product, the measured mass becomes larger than the amount of pure product.
Therefore, when using actual yield in the yield calculation, it is necessary to confirm that the product has been sufficiently dried and purified.
Discussion of the Limiting Reagent
The limiting reagent is the reagent that is consumed first in the reaction.
The maximum amount of product that can be formed in a reaction is determined by the amount of the limiting reagent.
Therefore, the theoretical yield must always be calculated based on the limiting reagent.
It is incorrect to determine the limiting reagent simply as the reagent with the smaller mass without considering the stoichiometric ratio of the reaction equation.
The amount of substance must be calculated using the molecular weight and reaction coefficients and compared with the molar ratio required by the reaction.
If the limiting reagent is identified incorrectly, the theoretical yield is incorrect and the yield calculation is also incorrect.
Example Discussion:
The theoretical yield must be calculated based on the limiting reagent.
The limiting reagent is determined not by the magnitude of its mass but by the amount of substance that is insufficient relative to the stoichiometric ratio in the reaction equation.
If the limiting reagent is identified incorrectly, the theoretical yield is overestimated or underestimated and errors arise throughout the yield calculation.
Main Causes of Reduced Yield
Causes of a yield lower than the theoretical value can be divided into causes related to the reaction itself and causes related to losses during experimental operations.
Reaction-related causes include incomplete reaction, side reactions, equilibrium, and decomposition of reagents.
Operational causes include losses during filtration, washing, transfer, recrystallization, extraction, drying, and weighing.
When discussing reduced yield, do not simply write that “the yield was low because the experiment failed.”
Instead, specifically describe at which stage and by what mechanism loss could have occurred.
For example, for a crystalline product, possible causes include dissolution into the mother liquor, adhesion to filter paper, dissolution into the washing liquid, and scattering during drying.
In organic synthesis, possible causes include side reactions, partition loss during extraction, and dissolution loss during recrystallization.
| Type of Cause | Specific Example | Direction of Discussion |
|---|---|---|
| Incomplete reaction | Insufficient reaction time, insufficient temperature, insufficient mixing | Consider the possibility that unreacted material remained |
| Side reactions | Formation of other products | Consider that the amount of reagent available to form the desired product decreased |
| Operational loss | Filtration, washing, transfer, extraction | Consider the possibility that product remained on equipment or in solution |
| Purification loss | Recrystallization, column chromatography, distillation | Consider that the amount decreased in exchange for improved purity |
| Drying and weighing errors | Insufficient drying, scattering, moisture absorption | Consider errors in the estimation of actual yield |
Example Discussion:
Possible causes of the reduced yield include not only incomplete reaction but also loss of the product during experimental operations.
In particular, part of the product may have adhered to the filter paper or equipment during filtration, dissolved in the washing liquid during washing, or remained behind during transfer.
Therefore, reduced yield must be discussed from both the reaction conditions and the recovery operations.
When the Reaction Did Not Proceed Completely
If the reaction does not proceed completely, not all of the limiting reagent is converted into product, so the actual yield becomes smaller than the theoretical yield.
The reaction may stop partway because of insufficient reaction time, low reaction temperature, insufficient stirring, inadequate dissolution of reagents, insufficient catalyst amount, or other causes.
If unreacted materials remain in the solution or solid after the reaction, incomplete reaction can be discussed.
TLC, IR, NMR, melting point, color changes, and the presence or absence of precipitate may provide clues to unreacted materials.
In slow reaction systems, comparisons using different times or temperatures are important.
Example Discussion:
One possible reason for the low yield is that the reaction time was insufficient and the limiting reagent was not completely converted into the product.
If the reaction is incomplete, the actual yield becomes smaller than the theoretically obtainable amount.
To bring the reaction closer to completion, the reaction time, temperature, stirring conditions, and catalyst amount must be appropriately adjusted.
Reduced Yield Caused by Side Reactions
A side reaction is a reaction that forms a substance other than the desired product.
When side reactions occur, part of the limiting reagent is used to form by-products rather than the desired product, so the yield of the desired product decreases.
Particularly in organic synthesis, side reactions may occur under conditions such as excessively high temperature, excess reagent amounts, excessively long reaction times, or inappropriate pH.
The presence of side reactions may be inferred from the color or odor of the product, decreased melting point, increased numbers of TLC spots, or extra peaks in spectra.
If both yield and purity are low, side reactions should be considered.
Optimizing the reaction conditions may suppress side reactions.
Example Discussion:
One possible reason for the low yield of the desired product is that part of the reagents was converted into another product through side reactions.
When side reactions occur, the proportion of the limiting reagent used to form the desired product decreases, so the actual yield becomes smaller than the theoretical yield.
Side reactions may be suppressed by appropriately controlling the reaction temperature, reaction time, and order of reagent addition.
Reduced Yield Caused by Equilibrium Reactions
If a reaction is an equilibrium reaction, it does not necessarily proceed completely toward the products.
After a certain period, an equilibrium state may be reached in which reactants and products coexist, and the limiting reagent may not be completely consumed.
In this case, even though the theoretical yield represents the maximum amount, the actual yield becomes lower.
In reactions such as esterification in which water is produced, the equilibrium can be shifted toward the products by removing water, removing the product, or using one reactant in excess.
In equilibrium reactions, reduced yield can be discussed using Le Chatelier’s principle.
Example Discussion:
If this reaction is an equilibrium reaction, not all of the reactants are necessarily converted into products.
At equilibrium, reactants and products coexist, so the actual yield becomes lower than the theoretical yield.
To increase the yield, it is necessary to shift the equilibrium toward the products by removing a by-product, using one reactant in excess, or removing the product from the reaction system.
Loss During Filtration
In experiments where precipitates or crystals are recovered, product may be lost during filtration.
If the product consists of fine particles, it may pass through the filter paper or become difficult to recover because of clogging of the filter paper.
Product may also remain adhered to the filter paper, beaker, or glass rod.
Loss during filtration is a representative cause of reduced yield.
Particularly in small-scale synthesis, even a small amount adhering to equipment can have a large effect on yield.
The type of filter paper, strength of suction filtration, size of crystal particles, and thoroughness of rinsing are important.
Example Discussion:
One possible reason for the low yield is that part of the product adhered to the filter paper or equipment during filtration and could not be completely recovered.
If the product consisted of fine crystals, some may also have passed through the filter paper or remained dispersed in the mother liquor.
Therefore, sufficiently growing the product particles and using appropriate filter paper and suction conditions are important for improving yield.
Loss During Washing
Washing the product removes mother liquor and impurities.
However, if the product is slightly soluble in the washing liquid, washing reduces the amount recovered.
Dissolution loss may become large when the washing-liquid volume is too large, the number of washes is too high, or the temperature is high.
Washing is necessary to increase purity, but it can also cause reduced yield.
Therefore, it is important to select a washing liquid that poorly dissolves the product while readily dissolving impurities.
Using a cooled washing liquid may suppress dissolution of the product.
Example Discussion:
One possible reason for the reduced yield caused by washing is that part of the product dissolved in the washing liquid.
Washing is necessary to remove impurities, but if the washing-liquid volume is large or the product has high solubility, the desired product is also lost.
Therefore, it is important to appropriately select the type, amount, temperature, and number of washes.
Reduced Yield Caused by Recrystallization
Recrystallization is a purification method commonly used to increase product purity.
However, during recrystallization, part of the product remains dissolved in the mother liquor and the yield decreases.
Particularly when a solvent in which the product has high solubility is used or cooling is insufficient, the amount of crystals that can be recovered becomes small.
A low yield after recrystallization does not necessarily mean a poor result.
The amount may decrease because impurities were removed to improve purity.
Because yield and purity may have a trade-off relationship, the result should be discussed together with purity evaluations such as melting point and spectra.
Example Discussion:
The yield decreased after recrystallization because part of the product remained dissolved in the mother liquor and could not be recovered as crystals.
Recrystallization is an operation that increases purity but also causes dissolution loss and therefore reduces yield.
Accordingly, the result after recrystallization must be evaluated not only from yield but also together with improvement in purity based on melting point and appearance.
Loss During Extraction
When extraction is performed in organic synthesis or similar experiments, the product may not completely move into the desired layer and some may remain in the other layer.
Because of the partition coefficient, the product cannot be completely recovered in a single extraction.
Loss may also occur when an emulsion forms during layer separation or when the interfacial layer is discarded.
To reduce extraction loss, it is necessary to extract several times using small portions of solvent, correctly identify the layers, resolve emulsions, and rinse solutions remaining on the equipment.
In experiments involving extraction, partition loss can be discussed as a cause of reduced yield.
Example Discussion:
One possible reason for the reduced yield during extraction is that part of the product remained in the aqueous or organic layer rather than completely moving into the desired layer.
Because partitioning of the product is not complete, it is difficult to recover the entire amount with a single extraction.
Performing multiple extractions with small amounts of solvent may improve product recovery.
Loss During Transfer
In experiments, reaction solutions or products may be transferred many times between beakers, flasks, filtration apparatus, watch glasses, and other containers.
Each time, a small amount of product may adhere to the walls of the equipment, glass rods, filter paper, or weighing paper and be lost.
In experiments involving small amounts of product, such small losses can have a large effect on yield.
To reduce transfer loss, it is important to rinse the equipment with a small amount of an appropriate solvent and recover as much product as possible.
However, if the product dissolves in the solvent used for rinsing, the loss may instead increase.
Operations appropriate to the properties of the product are necessary.
Example Discussion:
One possible cause of reduced yield is that the product adhered to the walls of equipment or a glass rod and was lost when transferring the reaction solution or crystals to another container.
In small-scale synthesis, even such small losses can greatly affect yield.
Therefore, reducing the number of transfers and rinsing equipment when necessary can improve recovery.
Apparent Increase in Yield Caused by Insufficient Drying
If the product has not been sufficiently dried, moisture or solvent remains and the actual yield is measured as too large.
As a result, the yield is overestimated.
In some cases, the yield may exceed 100%, but this does not mean that more product than the theoretical amount was produced.
Insufficient drying is a common cause of error in yield calculations.
Causes include water adhering to crystal surfaces, solvent incorporated inside crystals, and moisture absorption by hygroscopic substances.
It is important to dry the product until the mass becomes constant and to cool it in a desiccator.
Example Discussion:
One possible reason why the yield was overestimated is that moisture or solvent remained in the product.
With insufficient drying, the actual yield includes mass other than that of the pure product, so the yield becomes higher than the actual value.
Therefore, to accurately determine the yield, the product must be sufficiently dried and weighed at or near constant mass.
Apparent Increase in Yield Caused by Impurity Contamination
If unreacted materials, by-products, salts, drying agents, pieces of filter paper, mother-liquor components, or other impurities contaminate the product, the actual yield becomes larger.
As a result, the yield may appear high.
If the yield is close to or exceeds 100%, the purity must be checked.
Impurity contamination occurs because of insufficient washing, insufficient purification, or inadequate separation.
If the melting point is low and broad, the crystal color differs from that expected, or extra peaks are present in spectra, impurities may be present.
Even a high yield requires improvement if the purity is low.
Example Discussion:
One possible reason for the high yield is that unreacted materials, by-products, or salts from the mother liquor contaminated the product.
When impurities remain, the actual yield increases and the apparent yield becomes high.
Therefore, even when the yield is high, the purity of the product must be confirmed by melting-point measurement or spectroscopic analysis.
When the Yield Exceeds 100%
Yield may exceed 100%, but this normally does not mean that more desired product than the theoretical amount was produced.
In most cases, the causes are that moisture, solvent, impurities, unreacted materials, or salts remained in the product, or that the theoretical yield was calculated incorrectly.
When the yield exceeds 100%, both the calculation and the experimental operations must be reviewed.
In calculating the theoretical yield, check the identification of the limiting reagent, molecular weights, reaction coefficients, and unit conversions.
In measuring the actual yield, check the drying condition, impurity contamination, and weighing errors.
In a report, it is important not to write that a yield above 100% means the experiment was successful, but to analyze the cause objectively.
Example Discussion:
If the yield exceeded 100%, it is highly likely that moisture, solvent, or impurities remained in the product rather than that more than the theoretical amount of desired product was formed.
In addition, if the limiting reagent, molecular weight, or reaction coefficient was calculated incorrectly, the theoretical yield may not have been obtained correctly and the yield may have been overestimated.
Therefore, the drying conditions, purification state, and theoretical-yield calculation must be rechecked.
Relationship Between Product Purity and Yield
Yield and purity are both important, but they do not mean the same thing.
Yield indicates the amount of product, while purity indicates how much of the product consists of the desired compound.
Even if the yield is high, the quality of the product is low if it contains many impurities.
Conversely, thorough purification can increase purity but may reduce yield.
Therefore, in synthesis experiments, the result is evaluated together with purity-confirmation methods such as melting point, TLC, IR, NMR, GC, and HPLC in addition to yield.
Even if the yield is low, if the melting point is close to the literature value and the spectra agree, a high-purity product may have been obtained.
It is important to consider the balance between yield and purity.
Example Discussion:
In evaluating the product, purity is important in addition to yield.
If the yield decreased after recrystallization but the melting point approached the literature value, impurities were considered to have been removed and the purity improved.
Therefore, reduced yield does not necessarily mean failure and must be evaluated together with improvement in purity.
Relationship Between Melting Point and Yield
When a solid product is obtained in organic synthesis, melting-point measurement is commonly used to check purity.
A pure substance melts over a relatively narrow temperature range.
If impurities are present, the melting point may decrease or the melting range may broaden.
If the yield is high but the melting point is low and broad, impurities may be present.
On the other hand, if the yield decreases after recrystallization but the melting point approaches the literature value, the purity can be considered to have improved through purification.
Considering yield and melting point together makes it possible to evaluate both the quantity and quality of the product.
Example Discussion:
If the yield was high but the melting-point range was broad, unreacted materials or by-products may have been present in the product.
When impurities are present, the melting point often decreases and the melting range broadens.
Therefore, even with a high yield, the purity may have been insufficient and the purification conditions must be reviewed.
Errors in Theoretical-Yield Calculations
One surprisingly common problem in yield calculations is an error in calculating the theoretical yield.
Causes include using the wrong molecular weight, overlooking the reaction coefficients, incorrectly identifying the limiting reagent, errors in unit conversion, and errors in converting volume to mass using density.
If the theoretical yield is incorrect, the yield cannot be calculated correctly.
Particularly when liquid reagents are used, it is necessary to convert volume to mass and mass to amount of substance.
When a solution of known concentration is used, the amount of substance is calculated from the molar concentration and volume.
Confirm the sequence of calculating the number of moles of product according to the stoichiometric ratio in the reaction equation and finally multiplying by the molecular weight of the product.
Example Discussion:
In yield calculations, an error in the theoretical-yield calculation can greatly affect the entire yield result.
The identification of the limiting reagent, stoichiometric ratio in the reaction equation, molecular weights, and unit conversions must be checked.
Particularly when using liquid reagents, accurate conversion among volume, density, mass, and amount of substance is important.
Significant Figures and Yield Calculations
Significant figures must also be considered in yield calculations.
The mass, volume, concentration, molecular weight of the reagents, and product mass each have their own measurement precision.
Writing the calculation result with many decimal places does not provide meaningful precision beyond that of the experimental values.
In a report, yield should be expressed with an appropriate number of digits according to the precision of the balance and volumetric equipment used in the experiment.
For example, if the actual yield is measured to the nearest 0.01 g, it may not be necessary to report the yield to many decimal places.
The treatment of significant figures is related to the reliability of the results.
Example Discussion:
Because yield is calculated from theoretical yield and actual yield, it is affected by the significant figures of each measured value.
Expressing the calculation result with excessively many digits has no meaning beyond the precision of the experimental values.
Therefore, the yield must be reported with appropriate significant figures considering the precision of weighing and volume measurements.
When the Results Can Be Considered Good
A yield-calculation result can be considered good when the theoretical yield has been calculated correctly, the actual yield is based on a sufficiently dried and purified product, and the yield is consistent with the nature of the reaction and the experimental operations.
Furthermore, if the melting point, spectra, appearance, and other properties of the product agree with those of the desired product, the result can be considered good in terms of both yield and purity.
A high yield is desirable, but it is not sufficient without purity confirmation.
Even if the yield is somewhat low, the purification operation may have been successful if a high-purity product was obtained.
The method of evaluation changes depending on whether the purpose of the experiment is to “obtain a large amount” or to “obtain a pure product.”
Example Discussion:
In this experiment, a certain amount of product was obtained relative to the theoretical yield, and the yield was within a reasonable range.
In addition, because the melting point of the product was close to the literature value and the appearance also matched the desired product, a relatively high-purity product was considered to have been obtained.
Therefore, from both the yield and purity perspectives, the reaction and purification operations were judged to have been generally appropriate.
Example Discussions When the Experiment Did Not Go Well
When a yield calculation does not go well, possible causes are considered from results such as an excessively low yield, a yield exceeding 100%, poor reproducibility, low product purity, or unstable actual yield.
Organizing the causes into reaction conditions, operational loss, purification loss, insufficient drying, and calculation errors makes the discussion easier.
Example Discussion:
Possible reasons for the low yield include incomplete reaction and loss of part of the product during filtration or washing.
Particularly if the product is slightly soluble in the washing liquid, the amount recovered decreases as the number of washes or washing-liquid volume increases.
Therefore, it is necessary to improve the reaction conditions and select washing conditions that take the solubility of the product into account.
Another Example Discussion:
One possible reason why the yield exceeded 100% is that the product was not sufficiently dried and moisture or solvent remained.
In addition, if unreacted materials, by-products, or salts from the mother liquor contaminated the product, the actual yield would also be measured as too large.
Therefore, it is necessary to dry the product to constant mass and, when necessary, perform recrystallization or washing to confirm purity.
Another Example Discussion:
One possible reason for poor reproducibility of the yield is that the reaction temperature, reaction time, stirring condition, or reagent-addition rate differed among experiments.
In addition, operations such as transfer, filtration, and extraction are easily affected by differences among experimenters and can influence the recovery rate.
To improve reproducibility, the reaction conditions and operating procedures must be kept as constant as possible.
How to Write Points for Improvement
In a discussion of yield calculations, writing not only the causes of reduced yield but also points for improvement makes the report easier to organize.
Points for improvement can be organized according to reaction conditions, separation operations, purification operations, drying and weighing, and confirmation of calculations.
Improvements to Reaction Conditions
- Allow sufficient reaction time
- Maintain an appropriate reaction temperature
- Stir sufficiently
- Adjust the reagent-addition rate
- Use an appropriate catalyst amount and pH
- Select conditions under which side reactions are less likely to occur
Improvements to Separation and Purification
- Rinse the product thoroughly during filtration
- Use appropriate filter paper or filters
- Select a washing liquid in which the product is poorly soluble
- Keep the washing-liquid volume to the minimum necessary
- Use an appropriate solvent volume during recrystallization
- Perform multiple extractions using small amounts of solvent
- Reduce the number of transfer operations
Improvements to Drying and Calculations
- Dry the product sufficiently
- Weigh the product at or near constant mass
- Store hygroscopic products in a desiccator
- Correctly identify the limiting reagent
- Confirm the stoichiometric ratio in the reaction equation
- Confirm molecular weights and unit conversions
- Use appropriate significant figures
Example of How to Write Points for Improvement:
To improve the yield, the reaction time and temperature must be set appropriately so that the reaction proceeds as close to completion as possible.
In addition, it is important to rinse products adhering to equipment during filtration, washing, and transfer and to keep the washing-liquid volume to the minimum necessary so that the product is not lost.
Furthermore, sufficiently drying the product before weighing and rechecking the limiting reagent and theoretical-yield calculations can improve the reliability of the yield calculation.
Difference Between a Superficial Discussion and a Good Discussion
In a discussion of yield calculations, simply writing that “the yield was low” or “the operation failed” results in a superficial discussion.
A good discussion specifically relates theoretical yield, actual yield, limiting reagent, incomplete reaction, side reactions, purification loss, insufficient drying, and impurity contamination.
| Superficial Discussion | Good Discussion |
|---|---|
| The yield was low. | In addition to incomplete reaction, part of the product may have been lost during filtration, washing, and transfer, causing the actual yield to become smaller than the theoretical yield. |
| The amount was less than the theoretical value. | The entire limiting reagent may not have been converted into the desired product, or part of it may have been converted into another product through side reactions, preventing the theoretical yield from being reached. |
| The amount decreased because it was washed. | Washing removed impurities, but because the product was slightly soluble in the washing liquid, the yield was considered to have decreased in exchange for improved purity. |
| The yield exceeded 100%, so the experiment went well. | If the yield exceeds 100%, it is highly likely that moisture, solvent, or impurities remained in the product or that the theoretical yield was calculated incorrectly. |
| The amount decreased during recrystallization. | During recrystallization, part of the product remains dissolved in the mother liquor, reducing the yield, but if the melting point approaches the literature value, the purity can be considered to have improved. |
Examples of Expressions That Can Be Used in Reports
The following expressions can be used when writing the results and discussion of yield calculations.
Adjust the necessary parts according to your own experimental results.
- The theoretical yield was calculated from the amount of substance of the limiting reagent and the stoichiometric ratio in the reaction equation.
- The mass of the product obtained after drying was used as the actual yield.
- The yield was calculated by dividing the actual yield by the theoretical yield and multiplying by 100.
- Possible causes of a yield lower than 100% include incomplete reaction and losses during experimental operations.
- When side reactions occur, part of the limiting reagent is consumed in pathways other than formation of the desired product, reducing the yield.
- During filtration or transfer, the product may adhere to equipment and be lost.
- During washing or recrystallization, part of the product may dissolve in the solvent and reduce the yield.
- If the yield exceeds 100%, insufficient drying or impurity contamination must be considered.
- Even with a high yield, the result cannot be considered good if the purity is low.
- The validity of the reaction and purification operations must be evaluated by considering both yield and purity.
Points to Check When Discussing Yield Calculations
Checking the following points before writing the report makes the discussion easier to write.
- Is the reaction equation written?
- Has the amount of substance of each reagent been calculated?
- Has the limiting reagent been correctly identified?
- Has the theoretical yield been calculated from the limiting reagent?
- Has it been confirmed that the actual yield is the mass after drying?
- Has the yield equation been used correctly?
- Has the possibility of incomplete reaction been considered?
- Have the effects of side reactions and equilibrium been considered?
- Have losses during filtration, washing, and transfer been considered?
- Have purification losses during recrystallization and extraction been considered?
- Has overestimation caused by insufficient drying or impurity contamination been considered?
- Are yield and purity evaluated separately?
Summary
Yield calculation is a basic concept used to evaluate the efficiency of reactions and experimental operations by comparing the amount of product theoretically obtainable with the amount actually obtained.
The theoretical yield is calculated from the stoichiometric ratio in the reaction equation and the limiting reagent, while the actual yield is based on the amount of product actually recovered after drying.
Yield is calculated by dividing the actual yield by the theoretical yield and multiplying by 100.
Causes of reduced yield include incomplete reaction, side reactions, equilibrium, filtration loss, washing loss, recrystallization loss, extraction loss, and transfer loss.
On the other hand, if the yield exceeds 100%, insufficient drying, impurity contamination, residual unreacted materials, or errors in calculating the theoretical yield must be considered.
It is important not to judge a high yield alone as a good result, but to evaluate it together with purity and product-confirmation results.
In a report, rather than simply writing that “the yield was low,” organize and discuss the theoretical yield, actual yield, limiting reagent, incomplete reaction, side reactions, losses during purification and separation operations, insufficient drying, impurity contamination, calculation errors, and points for improvement.
Yield calculation is an important indicator for quantitatively evaluating the results of chemical reactions and improving experimental operations.
