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Measurements of Chemical Reaction

In chemical reactions, reactants undergo changes to yield new molecules. When you begin a reaction, you should always measure the quantity of the reactants and products. These quantitative measurements are useful for assessing our results.

Measured Yield

A chemical reaction's actual yield is the amount of product made. We can measure it in mass, moles, or volume. It's unbelievably important to record this value! Why? Because various factors affect the product quantity. For example, the experimental conditions might change. Maybe the first time you do the experiment it's at STP. But, if you repeat the experiment, your temperature and pressure might be different. You might get very different yields. So, it's also important to record the conditions. Generally, the more product we make the better. If it is found that some conditions produce better product yield than others, then when the reaction is repeated it should be under those conditions.

Actual Yield: What is Actual Yield?

(Video)Actual Yield: How to Find Actual Yield?

By Conquer Chemistry

In this video, you will learn how to calculate the actual yield from percent yield and theoretical yield. You can multiply the percent yield by the theoretical yield to retrieve the actual yield.

We've been studying stoichiometry. It's the relationship between the quantity of the reactants and products. Honestly, all we're doing is converting between units of product and reactant. So, essentially it's dimensional analysis. So far, we've learned mole-to-mole, mass-to-mass, volume-to-volume, and stoichiometry using density. That's a lot of math! But why did we study all this?

When a chemist wants to perform a chemical synthesis, it's a huge process. It requires a lot of research. Part of that research is calculating the theoretical yield. That's the maximum amount of product our reaction could produce. In general chemistry, that's easy! We write our balanced chemical equation. Then, we take our reactant's mass, moles, or volume and calculate the product quantity. It's what we've been doing in stoichiometry.

Unfortunately, the real world is never as easy as general chemistry. In most chemical syntheses, you need multiple reactions to reach your final product. An unbelievable example is the cancer drug Halaven. It has a 62-step organic synthesis! Why do we care? Our synthesis often has a practical goal. For example, if we're making a medication, we want to find an efficient reaction mechanism. One criterion would be a high yield. It's a bad day if kilograms of reactant produce micrograms of product.

Theoretical vs. Actual Yield

In our last lesson, we studied the theoretical yield. It's the maximum amount of product that our reaction can produce. How did we find it? Stoichiometry! We went through each step. We started by writing and balancing our chemical equation. Then, we used the mole ratios to find our limiting reagent. Finally, we calculated how much product our reaction would produce.

But what would happen if we performed a chemical reaction in the lab? Unfortunately, we rarely reach our theoretical yield. Why? Whenever we perform an experiment, there are sources of error and uncertainty. These two factors chip away at our yield. At the end of the reaction, we measure what we actually produced. That's our actual yield.

Comparing Theoretical and Actual Yield

We'll discuss percent yield more in our next lesson. But, you may wonder what we do with our actual yield. We compare it to the theoretical yield! We write the comparison as a percentage. So, we creatively call it the percent yield. The formula is $\frac{actual}{theoretical}\times 100\%$. But makes sure your units match. Remember, we can use mass, moles, or volume.

A Quick Side Note About Staying Motivated

As a side note, if you end up struggling in the lab, that's okay! I remember doing an experiment in high school where my percent yield was 103%. I got more than the theoretical yield! Since the theoretical yield is the maximum, I clearly did something wrong. If I'm honest, my error was probably carelessness. I assume I received a poor grade for that lab. But it's okay! I went on to study chemistry in college and had success in many advanced classes. Don't be too hard on yourself. Sometimes, you'll make mistakes. I had many friends who had similar experiences. In response, they told themselves they were horrible at chemistry. Eventually, they got stuck in their response and dreaded chemistry class. If you're interested in chemistry or other sciences, don't get stuck on your mistakes!