General Chemistry I: Stoichiometry, Chemical Equations, and Gas Laws Study Guide

Stoichiometry and Balancing Chemical Equations

Introduction to Chemical Equations

Chemical equations represent the reactants and products in a chemical reaction. Balancing these equations obeys the law of conservation of mass, meaning the number of atoms of each element is the same on both sides.

• Reactants: Substances present before the reaction.

• Products: Substances formed as a result of the reaction.

• Balancing: Adjusting coefficients to ensure equal numbers of each atom on both sides.

Example: Balancing the reaction of nitrogen and hydrogen to form ammonia:

• Unbalanced: N₂ + H₂ → NH₃

• Balanced: $N_2 + 3H_2 \rightarrow 2NH_3$

Types of Chemical Reactions

Chemical reactions can be classified into several types based on the nature of the reactants and products:

• Synthesis (S): Two or more substances combine to form one product. $A + B \rightarrow AB$

• Decomposition (D): A single compound breaks down into two or more simpler substances. $AB \rightarrow A + B$

• Single Replacement (SR): An element replaces another in a compound. $A + BC \rightarrow AC + B$

• Double Replacement (DR): Two compounds exchange ions. $AB + CD \rightarrow AD + CB$

• Combustion (C): A substance reacts with oxygen, releasing energy. $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Writing and Balancing Equations

Steps to Balance Equations

1. Write the unbalanced equation with correct formulas.

2. Count the number of atoms of each element on both sides.

3. Add coefficients to balance the atoms.

4. Verify that all coefficients are in the lowest possible ratio.

Example: Balancing sodium and chlorine to form sodium chloride:

• Unbalanced: Na + Cl₂ → NaCl

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

Types of Equations in Solution Chemistry

Molecular, Complete Ionic, and Net Ionic Equations

In aqueous reactions, equations can be written in three forms:

• Molecular Equation: Shows all reactants and products as compounds.

• Complete Ionic Equation: Shows all strong electrolytes as ions.

• Net Ionic Equation: Shows only the species that actually change during the reaction.

Example: Reaction of potassium chromate and calcium chloride:

• Molecular: $K_2CrO_4(aq) + CaCl_2(aq) \rightarrow CaCrO_4(s) + 2KCl(aq)$

• Complete Ionic: $2K^+(aq) + CrO_4^{2-}(aq) + Ca^{2+}(aq) + 2Cl^-(aq) \rightarrow CaCrO_4(s) + 2K^+(aq) + 2Cl^-(aq)$

• Net Ionic: $Ca^{2+}(aq) + CrO_4^{2-}(aq) \rightarrow CaCrO_4(s)$

Stoichiometry: Quantitative Chemical Calculations

Introduction to Stoichiometry

Stoichiometry involves calculating the amounts of reactants and products in chemical reactions using balanced equations.

• Mole Ratio: Derived from coefficients in the balanced equation.

• Moles to Mass: Use molar mass to convert between moles and grams.

• Limiting Reactant: The reactant that is completely consumed first limits the amount of product formed.

Example: If 17 moles of NH₃ react with O₂:

• Balanced equation: $4NH_3 + 5O_2 \rightarrow 4NO + 6H_2O$

• The mole ratio NH₃ to O₂ is 4:5

Gas Laws and Calculations

Introduction to Gas Laws

Gas laws describe the relationships between pressure, volume, temperature, and amount of gas.

• Ideal Gas Law: $PV = nRT$

• Partial Pressure: The pressure exerted by each gas in a mixture. Dalton's Law: $P_{total} = P_1 + P_2 + ...$

Example: Calculating moles of gas in a balloon:

• Given: V = 30.1 L, T = 47°C (320 K), P = 73.1 kPa

• Use $PV = nRT$ to solve for n.

Concentration and Solution Calculations

Calculating Molarity

Molarity (M) is the concentration of a solution, defined as moles of solute per liter of solution.

• Formula: $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• This formula is used to determine the amount of solute or solvent required for reactions in a solution.

Example: What is the concentration of phosphoric acid after dilution?

• Use the dilution equation: $M_1V_1 = M_2V_2$

Summary Table: Types of Chemical Reactions

Type	General Form	Example
Synthesis (S)	$A + B \rightarrow AB$	$2H_2 + O_2 \rightarrow 2H_2O$
Decomposition (D)	$AB \rightarrow A + B$	$2H_2O \rightarrow 2H_2 + O_2$
Single Replacement (SR)	$A + BC \rightarrow AC + B$	$Zn + 2HCl \rightarrow ZnCl_2 + H_2$
Double Replacement (DR)	$AB + CD \rightarrow AD + CB$	$AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$
Combustion (C)	$Fuel + O_2 \rightarrow CO_2 + H_2O$	$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

Key Formulas and Equations

• Ideal Gas Law: $PV = nRT$

• Molarity: $M = \frac{\text{moles}}{\text{liters}}$

• Stoichiometry (mole-mass): $\text{mass} = \text{moles} \times \text{molar mass}$

• Dilution: $M_1V_1 = M_2V_2$

Additional info:

• Some questions require predicting products and writing balanced equations, which is essential for understanding reaction types and stoichiometry.

• Gas law problems involve calculations using the ideal gas law and Dalton's law of partial pressures.

• Solution chemistry questions focus on molarity and dilution, which are foundational for laboratory work.