Quantities in Chemical Reactions: Stoichiometry, Limiting Reactants, and Percent Yield

Quantities in Chemical Reactions

Stoichiometry and Chemical Equations

Stoichiometry is the quantitative study of reactants and products in a chemical reaction. It allows chemists to predict the amounts of substances consumed and produced in a given reaction using balanced chemical equations.

• Balanced Chemical Equation: Represents the conservation of mass and atoms in a reaction. Each side of the equation must have the same number of each type of atom.

• Stoichiometric Coefficients: Numbers in front of chemical formulas that indicate the relative amounts of reactants and products.

• Example: $2Na + Cl_2 \rightarrow 2NaCl$

Limiting Reactant and Excess Reactant

In a chemical reaction, the limiting reactant is the substance that is completely consumed first, thus limiting the amount of product formed. The excess reactant is the substance that remains after the reaction is complete.

• Identifying the Limiting Reactant: Compare the mole ratio of reactants used to the ratio in the balanced equation.

• Example: If you have 2 cups of flour, 3 eggs, and 1 cup of oil to make muffins, and the recipe requires 1 cup flour + 1 egg + 1/3 cup oil per muffin, flour is the limiting reactant.

• Application: In the reaction $2Mg + O_2 \rightarrow 2MgO$, if you have 1.4 moles of Mg and excess O2, Mg is the limiting reactant.

Theoretical Yield, Actual Yield, and Percent Yield

The theoretical yield is the maximum amount of product that can be formed from the given amounts of reactants, calculated using stoichiometry. The actual yield is the amount of product actually obtained from the reaction. Percent yield measures the efficiency of a reaction.

• Percent Yield Formula:

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

• Example: If the theoretical yield is 75.0 g and the actual yield is 42.0 g, percent yield is $\left( \frac{42.0}{75.0} \right) \times 100 = 56\%$.

Calculating Quantities in Chemical Reactions

To determine the amount of reactants or products, use the molar relationships from the balanced equation and convert between grams and moles using molar mass.

• Steps:

  1. Convert given quantities to moles.

  2. Use stoichiometric coefficients to relate moles of reactants to moles of products.

  3. Convert moles of product to grams if needed.

• Example: How many grams of sodium metal are needed to make 29.3 grams of sodium chloride? Use $2Na + Cl_2 \rightarrow 2NaCl$ and molar masses.

Energy Changes in Chemical Reactions

Chemical reactions often involve energy changes, typically in the form of heat. The enthalpy change ($\Delta H$) indicates whether a reaction absorbs or releases energy.

• Exothermic Reaction: Releases heat ($\Delta H$ is negative).

• Endothermic Reaction: Absorbs heat ($\Delta H$ is positive).

• Example: $CO + 2H_2 \rightarrow CH_3OH \quad (\Delta H = -121 \text{ kJ})$

Reference Tables

Periodic Table of the Elements

The periodic table organizes elements by increasing atomic number and groups elements with similar chemical properties.

• Groups: Vertical columns with similar properties.

• Periods: Horizontal rows.

• Applications: Used to determine atomic masses, element symbols, and chemical behavior.

Common Polyatomic Ions

Polyatomic ions are ions composed of two or more atoms covalently bonded, carrying a net charge. They are important in writing chemical formulas and equations.

Ion	Formula	Name
Ammonium	NH4+	Ammonium
Nitrate	NO3-	Nitrate
Sulfate	SO42-	Sulfate
Carbonate	CO32-	Carbonate
Phosphate	PO43-	Phosphate
Hydroxide	OH-	Hydroxide
Acetate	C2H3O2-	Acetate
Chlorate	ClO3-	Chlorate
Permanganate	MnO4-	Permanganate
Additional info:	Other common ions include nitrite (NO2-), sulfite (SO32-), and dichromate (Cr2O72-).	Additional info: These ions are frequently encountered in chemical reactions and nomenclature.

Applications and Practice Problems

Sample Calculations

• Limiting Reactant: Given quantities of two reactants, determine which will run out first using mole ratios.

• Percent Yield: Calculate using actual and theoretical yields.

• Stoichiometry: Convert between grams and moles, use balanced equations to find quantities of products or reactants.

• Energy Calculations: Use $\Delta H$ to determine heat released or absorbed.

Practice with these concepts is essential for mastering chemical reaction calculations and understanding the quantitative relationships in chemistry.