General Chemistry: Chemical Reactions, Gases, and Stoichiometry – Practice Quiz Study Notes

Chemical Reactions

Overview of Chemical Reactions

Chemical reactions involve the transformation of one or more substances into new substances with different properties. Understanding how to classify, balance, and predict the products of reactions is fundamental in general chemistry.

• Reactants: Substances present before the reaction.

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

• Balancing Equations: Ensures the law of conservation of mass is obeyed; the number of atoms of each element is the same on both sides of the equation.

Example: Neutralization Reaction

• When an acid reacts with a base, water and a salt are formed.

• General equation: $\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$

• Example: $\mathrm{HNO_3(aq) + Ca(OH)_2(aq) \rightarrow Ca(NO_3)_2(aq) + 2H_2O(l)}$

Identifying Acid and Base Reactants

• Acid: Donates $\mathrm{H^+}$ ions (e.g., $\mathrm{HNO_3}$)

• Base: Donates $\mathrm{OH^-}$ ions (e.g., $\mathrm{Ca(OH)_2}$)

Precipitation Reactions

• Occur when two aqueous solutions combine to form an insoluble solid (precipitate).

• Example: $\mathrm{Na_2CO_3(aq) + Ba(NO_3)_2(aq) \rightarrow BaCO_3(s) + 2NaNO_3(aq)}$

Writing Net Ionic Equations

• Show only the species that actually participate in the reaction.

• Spectator ions are omitted.

• Example: $\mathrm{CO_3^{2-}(aq) + Ba^{2+}(aq) \rightarrow BaCO_3(s)}$

Chemical Quantities and Stoichiometry

Percent Composition

Percent composition expresses the mass percentage of each element in a compound.

• Formula: $\text{Percent by mass of element} = \frac{\text{mass of element in 1 mol of compound}}{\text{molar mass of compound}} \times 100\%$

• Example: To find the percent NaCl in a mixture, use the mass of NaCl and the total mass of the mixture.

Gases and Their Properties

Kinetic Molecular Theory and Gas Laws

Gases have unique properties described by several laws and the kinetic molecular theory.

• Root Mean Square Speed ($u_{rms}$): Average speed of gas molecules, increases with temperature and decreases with molar mass.

• Formula: $u_{rms} = \sqrt{\frac{3RT}{M}}$ where $R$ is the gas constant, $T$ is temperature in Kelvin, and $M$ is molar mass in kg/mol.

• Pressure: Force exerted by gas molecules colliding with container walls.

• Volume: Space occupied by the gas.

• Temperature: Directly proportional to average kinetic energy of gas molecules.

Ideal Gas Law

• Relates pressure, volume, temperature, and amount of gas.

• Equation: $PV = nRT$

• Where $P$ = pressure (atm), $V$ = volume (L), $n$ = moles, $R$ = 0.0821 L·atm/(mol·K), $T$ = temperature (K).

Partial Pressure and Dalton's Law

• In a mixture, each gas exerts a partial pressure; total pressure is the sum of partial pressures.

• Equation: $P_{total} = P_1 + P_2 + P_3 + \ldots$

• Partial pressure: $P_i = X_i P_{total}$, where $X_i$ is the mole fraction of gas $i$.

Gas Collection Over Water

• When collecting gas over water, subtract vapor pressure of water from total pressure to find the pressure of the dry gas.

Acid Rain and Atmospheric Chemistry

Formation of Acid Rain

Acid rain is caused by the reaction of sulfur dioxide and nitrogen oxides with water in the atmosphere, forming acids.

• Example reaction: $2SO_2(g) + 2H_2O(l) + O_2(g) \rightarrow 2H_2SO_4(aq)$

• Acid rain lowers the pH of rainwater, affecting ecosystems and structures.

Sample Table: Gas Law Relationships

Variable	Relationship (if others constant)	Equation
Pressure & Volume	Inversely proportional	$P_1V_1 = P_2V_2$ (Boyle's Law)
Volume & Temperature	Directly proportional	$\frac{V_1}{T_1} = \frac{V_2}{T_2}$ (Charles's Law)
Pressure & Temperature	Directly proportional	$\frac{P_1}{T_1} = \frac{P_2}{T_2}$ (Gay-Lussac's Law)

Key Concepts and Practice Applications

• Be able to classify reactions (acid-base, precipitation, redox).

• Calculate percent composition and empirical/molecular formulas.

• Apply gas laws to solve for unknowns (P, V, n, T).

• Use stoichiometry to relate quantities of reactants and products.

• Understand the environmental impact of chemical reactions (e.g., acid rain formation).

Additional info: These notes are based on a practice quiz covering chemical reactions, stoichiometry, gas laws, and atmospheric chemistry, all of which are core topics in General Chemistry.