General Chemistry: Chemical Reactions, Stoichiometry, Solutions, and Gas Laws Study Guide

Chemical Reactions and Stoichiometry

Types of Chemical Reactions

Chemical reactions can be classified based on the changes that occur to reactants and products. Understanding these types is essential for predicting products and balancing equations.

• Synthesis (Combination) Reaction: Two or more substances combine to form a single product. Example: $2H_2 + O_2 \rightarrow 2H_2O$

• Decomposition Reaction: A single compound breaks down into two or more simpler substances. Example: $2KClO_3 \rightarrow 2KCl + 3O_2$

• Single Replacement Reaction: An element replaces another in a compound. Example: $Zn + 2HCl \rightarrow ZnCl_2 + H_2$

• Double Replacement Reaction: Exchange of ions between two compounds. Example: $AgNO_3 + NaCl \rightarrow AgCl + NaNO_3$

Balancing Chemical Equations: The number of atoms of each element must be the same on both sides of the equation.

Predicting Products and Identifying Reaction Types

• Use activity series for single replacement reactions.

• Double replacement reactions often produce a precipitate, gas, or water.

• Decomposition reactions may require heat or a catalyst.

Stoichiometry

Stoichiometry involves quantitative relationships between reactants and products in a chemical reaction.

• Mole Ratio: Derived from balanced chemical equations.

• Example: For $2AlCl_3 + 3Na_2CO_3 \rightarrow 2Al_2(CO_3)_3 + 6NaCl$, the mole ratio of $AlCl_3$ to $NaCl$ is 2:6.

Solutions and Chemical Quantities

Concentration Units

Concentration expresses the amount of solute in a given amount of solvent or solution.

• Molarity (M): $M = \frac{\text{moles of solute}}{\text{liters of solution}}$

• Example Calculation: To find the molarity of a solution containing 3.2 g glucose ($C_6H_{12}O_6$) in 250 mL: $\text{Moles} = \frac{3.2\ g}{180\ g/mol} = 0.0178\ mol$ $M = \frac{0.0178\ mol}{0.250\ L} = 0.0712\ M$

Calculating Mass, Volume, and Moles

• Mass from Molarity: $\text{mass} = M \times V \times \text{molar mass}$

• Volume from Moles: $V = \frac{\text{moles}}{M}$

Acid-Base Equilibria

Brønsted-Lowry Acids and Bases

Acids are proton donors, and bases are proton acceptors.

• Example: $NH_4^+$ is a Brønsted-Lowry acid because it can donate a proton to form $NH_3$.

• Conjugate Acid-Base Pairs: $NH_4^+$ (acid) and $NH_3$ (base).

pH and pOH Calculations

• pH: $pH = -\log[H^+]$

• pOH: $pOH = -\log[OH^-]$

• Relationship: $pH + pOH = 14$

Gas Laws and Properties

Ideal Gas Law

The ideal gas law relates pressure, volume, temperature, and amount of gas.

• Equation: $PV = nRT$

• Variables: P = pressure (atm), V = volume (L), n = moles, R = gas constant ($0.0821\ L\cdot atm\cdot mol^{-1}\cdot K^{-1}$), T = temperature (K)

• Example: Calculate moles of gas in a 25 mL container at 1.5 atm and 298 K: $n = \frac{PV}{RT} = \frac{1.5 \times 0.025}{0.0821 \times 298} = 0.00153\ mol$

Gas Stoichiometry

• Use the ideal gas law to relate moles of gas produced or consumed in reactions.

• Example: Magnesium reacts with HCl to produce hydrogen gas. Calculate the volume of $H_2$ at given conditions using $PV = nRT$.

Temperature Conversion

• Celsius to Kelvin: $T(K) = T(^{\circ}C) + 273.15$

Thermochemistry

Heat Capacity and Calorimetry

Thermochemistry studies the heat changes in chemical reactions.

• Specific Heat Capacity ($C$): Amount of heat required to raise the temperature of 1 g of substance by 1°C.

• Equation: $q = mC\Delta T$

• Example: Calculate heat absorbed by a block using mass, specific heat, and temperature change.

Enthalpy Change ($\Delta H$)

• Equation: $\Delta H = \Delta E + P\Delta V$

• Standard enthalpy changes are measured at 1 atm and 25°C.

Additional info:

• Some problems involve conversion factors, such as mole ratios and molar masses.

• Universal gas constant $R$ is $0.0821\ L\cdot atm\cdot mol^{-1}\cdot K^{-1}$.

• All temperatures in gas law calculations must be in Kelvin.