General Chemistry: Chemical Reactions, Gas Laws, Thermochemistry, and Periodic Trends

Chemical Reactions and Stoichiometry

Balancing Chemical Equations and Reaction Types

Chemical reactions involve the transformation of reactants into products, often accompanied by observable changes such as gas evolution, color change, or temperature change. Balancing equations ensures the conservation of mass and atoms.

• Balanced Equation Example: $2E(s) + 2H^+(aq) \rightarrow 2E^{2+}(aq) + H_2(g)$

• Stoichiometry: Use mole ratios from balanced equations to relate quantities of reactants and products.

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

• Excess Reactant: The reactant that remains after the reaction is complete.

• Example Calculation: Determining the volume of gas produced from a given amount of reactant using stoichiometry and the ideal gas law.

Gas Laws and Collection of Gases Over Water

Gas Collection and Dalton's Law of Partial Pressures

Gases can be collected over water, and the total pressure measured includes both the gas and water vapor. Dalton's Law is used to determine the pressure of the collected gas.

• Dalton's Law: The total pressure of a mixture of gases is the sum of the partial pressures of each gas. $P_{\text{total}} = P_{\text{gas}} + P_{\text{water vapor}}$

• Example Calculation:

  • Measured total pressure: 746.6 mmHg

  • Water vapor pressure: 19.8 mmHg

  • Gas pressure: $746.6\ \text{mmHg} - 19.8\ \text{mmHg} = 726.8\ \text{mmHg}$

• Application: Used to determine the amount of gas produced in a reaction by measuring the volume and pressure of gas collected over water.

Thermochemistry

Heat, Temperature Change, and Calorimetry

Thermochemistry studies the heat changes that accompany chemical reactions and physical changes. Calorimetry is used to measure the heat absorbed or released.

• Heat Equation: $Q = mc\Delta T$

  • $Q$ = heat (Joules)

  • $m$ = mass (grams)

  • $c$ = specific heat capacity (J/g°C)

  • $\Delta T$ = change in temperature (°C)

• Example Calculation:

  • Given: $m = 1500\ g$, $c = 0.810\ J/g^\circ C$, $\Delta T = 180^\circ C$

  • $Q = 1500 \times 0.810 \times 180 = 218,700\ J$

• Application: Determining the amount of fuel needed to heat a substance by a certain temperature.

Endothermic and Exothermic Processes

Energy Changes in Solution

When substances dissolve, energy is either absorbed (endothermic) or released (exothermic). The temperature change of the solution can be used to determine the enthalpy change.

• Endothermic Process: The system absorbs heat, causing the temperature of the surroundings to decrease.

• Example: Dissolving $NH_4NO_3$ in water causes the temperature to drop, indicating an endothermic reaction.

• Calculation of $\Delta T$:

  • Initial temperature: $22.0^\circ C$

  • Final temperature: $24.6^\circ C$

  • $\Delta T = 22.0 - 24.6 = -2.6^\circ C$

• Explanation: The strength between solute-solute and solvent-solvent particles is stronger; in an endothermic reaction, particles gain energy to break these interactions.

Periodic Trends and Chemical Bonding

Atomic Radius, Ionization Energy, and Bonding

The periodic table reveals trends in atomic properties that influence chemical behavior and bonding.

• Atomic Radius: Increases down a group and decreases across a period.

• Ionization Energy: The energy required to remove an electron from an atom. Increases across a period and decreases down a group.

• Bond Types:

  • Ionic Bonds: Formed between metals and nonmetals via electron transfer.

  • Covalent Bonds: Formed between nonmetals via electron sharing.

• London Dispersion Forces (LDF): Weak intermolecular forces present in all molecules, stronger in larger, more polarizable atoms/molecules.

• Example: $Br_2$ has higher LDF than $Cl_2$ due to more electrons and greater polarizability, resulting in a higher boiling point.

Tables and Data Interpretation

Using Tables for Chemical Calculations

Tables are often used to organize experimental data, compare properties, and summarize results.

Trial	Initial Moles of HCl (mol)	Initial Moles of E (mol)	Volume of H2 (L)
1	0.0400	0.0200	0.408
2	0.0400	0.0400	0.781
3	0.0800	0.0400	0.781

Purpose: To determine the limiting reactant and the effect of reactant amounts on product formation.

Element	Atomic Radius (pm)
Li	152
Be	112
B	85

Purpose: To compare atomic radii and relate to periodic trends.