Chapter 4

4.3 Reaction Stoichiometry

Introduction to Reaction Stoichiometry

Reaction stoichiometry is a fundamental concept in general chemistry that involves calculating the quantities of reactants and products involved in chemical reactions. It is based on the balanced chemical equation, which provides the mole ratios needed for these calculations.

• Predicting Product Amounts: The amount of products formed can be predicted based on the amount of reactants used.

• Predicting Reactant Amounts: The amount of reactants required to form a specific amount of product can be calculated using a balanced chemical equation.

Example: S'mores Analogy

• 2 crackers + 1 marshmallow + 3 rectangles chocolate = 1 s'more

• This analogy demonstrates how specific ratios of ingredients (reactants) yield a fixed amount of product.

Chemical Equation Example

• Given: $4 \text{HCl} + \text{O}_2 \rightarrow 2 \text{H}_2\text{O} + 2 \text{Cl}_2$

• Another example: $20 \text{HCl} + 5 \text{O}_2 \rightarrow 10 \text{H}_2\text{O} + 10 \text{Cl}_2$

Stoichiometric Calculations

• Given: 22 moles HCl. How many moles of chlorine gas (Cl2) are produced?

• From the balanced equation: $22 \text{moles HCl} \times \left(\frac{1 \text{mol Cl}_2}{2 \text{mol HCl}}\right) = 11 \text{moles Cl}_2$

4.4 Stoichiometry Relationships

Key Terms and Concepts

Stoichiometry relationships help chemists understand how reactants and products are related in a chemical reaction, especially when one reactant limits the amount of product formed.

• Limited Reactant: The reactant that is completely consumed first, limiting the amount of product formed. At the end of the reaction, there will be zero of the limiting reactant left.

• Excess Reactant: The reactant that is not completely used up; some amount remains after the reaction is complete.

• Theoretical Yield: The maximum amount of product that can be formed from the given amounts of reactants, calculated using stoichiometry and based on the limiting reactant.

• Actual Yield: The amount of product actually obtained from a reaction, usually measured in the laboratory.

• Percent Yield: A measure of the efficiency of a reaction, calculated as:

$\% \text{yield} = \frac{\text{actual yield}}{\text{theoretical yield}} \times 100$

Example: Limiting Reactant

• If you have 10 eggs and 8 slices of bread, and each sandwich requires 2 eggs and 2 slices of bread, the bread is the limiting reactant because you can only make 4 sandwiches before running out of bread.

Example: Percent Yield Calculation

• If the theoretical yield of a product is 50 grams, but only 45 grams are obtained in the lab, the percent yield is:

$\% \text{yield} = \frac{45}{50} \times 100 = 90\%$