Easy Examples to Help 9th Graders Understand Percent Yield and Theoretical Yield

Understanding percent yield and theoretical yield is important in chemistry, especially in stoichiometry. These concepts help you see how well a chemical reaction goes. Here are some simple examples to help 9th graders understand these ideas better.

1. Making Water

A simple example is how we make water from hydrogen and oxygen:

$$2H_2 + O_2 \rightarrow 2H_2O$$

Calculating Theoretical Yield:

• If you start with 4 moles of $H_2$ and 2 moles of $O_2$, we can figure out how much water we could make.
• Looking at our equation, we see that 2 moles of $H_2$ react with 1 mole of $O_2$ to produce 2 moles of water.
• Here, $O_2$ is the limiting reactant because it will produce less water than $H_2$.

So, the theoretical yield of water is:

$$\text{Theoretical Yield of } H_2O = 2 \, \text{moles of } H_2O$$

Calculating Percent Yield:

• If you actually make 1.5 moles of $H_2O$, we can use this to find the percent yield:

$$\text{Percent Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100$$ $$\text{Percent Yield} = \left( \frac{1.5}{2} \right) \times 100 = 75\%$$

2. Baking Soda and Vinegar

Another example is the reaction between baking soda ($NaHCO_3$) and vinegar ($CH_3COOH$):

$$NaHCO_3 + CH_3COOH \rightarrow CO_2 + H_2O + NaCH_3COO$$

• Imagine you use 0.1 moles of baking soda and 0.1 moles of vinegar. Both react in a 1:1 ratio.

Calculating Theoretical Yield:

• The theoretical yield of carbon dioxide ($CO_2$) is also 0.1 moles because they match up directly.

Calculating Percent Yield:

• If you collect 0.08 moles of $CO_2$, you can find the percent yield like this:

$$\text{Percent Yield} = \left( \frac{0.08}{0.1} \right) \times 100 = 80\%$$

3. Industrial Process - Making Ammonia

In the real world, chemical processes often don’t reach 100% yield because of side reactions or losing some materials. For example, in the process to make ammonia:

$$N_2 + 3H_2 \rightarrow 2NH_3$$

• Typically, the percent yield for making ammonia might be around 60% because of different inefficiencies.

Key Takeaways:

• Theoretical Yield is the maximum amount of product you can make based on the substances you started with.
• Percent Yield tells you how efficient a reaction was:

$$\text{Percent Yield} = \left( \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \right) \times 100$$

Using everyday examples, like simple chemical reactions or industrial processes, helps students understand these concepts. By seeing real-life situations, students can better grasp how percent yield and theoretical yield are used in chemistry.