General Chemistry: Key Topics Overview (Chemical Reactions, Energy, Gases, Solutions, Acids & Bases)

Topic 5: Chemical Reactions

Introduction to Chemical Reactions

Chemical reactions are processes in which substances (reactants) are transformed into new substances (products) through the breaking and forming of chemical bonds.

• Definition: A chemical reaction involves the rearrangement of atoms to form new compounds.

• Types of Chemical Reactions:

  • Combination (Synthesis)

  • Decomposition

  • Single Replacement

  • Double Replacement

  • Combustion

• General Equation: $\text{Reactants} \rightarrow \text{Products}$

• Example: $2H_2 + O_2 \rightarrow 2H_2O$ (formation of water)

Topic 6: Energy, Reaction Rate & Equilibrium

Energy in Chemical Reactions

Chemical reactions involve changes in energy, typically in the form of heat (enthalpy).

• Exothermic Reactions: Release energy to the surroundings ($\Delta H < 0$).

• Endothermic Reactions: Absorb energy from the surroundings ($\Delta H > 0$).

• Activation Energy: The minimum energy required to initiate a reaction.

Reaction Rate

The rate of a chemical reaction is the speed at which reactants are converted to products.

• Factors Affecting Rate: Concentration, temperature, surface area, catalysts, and nature of reactants.

• Rate Law Example: $\text{Rate} = k[A]^m[B]^n$

Chemical Equilibrium

At equilibrium, the rates of the forward and reverse reactions are equal, and the concentrations of reactants and products remain constant.

• Equilibrium Constant ($K_{eq}$): $K_{eq} = \frac{[\text{Products}]}{[\text{Reactants}]}$

• Le Châtelier's Principle: If a system at equilibrium is disturbed, it will shift to counteract the disturbance.

Topic 7: Gases, Intermolecular Forces

Properties of Gases

Gases have unique properties such as compressibility, expansibility, and low density.

• Gas Laws:

  • Boyle's Law: $P_1V_1 = P_2V_2$ (at constant T, n)

  • Charles's Law: $\frac{V_1}{T_1} = \frac{V_2}{T_2}$ (at constant P, n)

  • Ideal Gas Law: $PV = nRT$

• Example: Calculate the volume of 1 mole of gas at STP using $PV = nRT$.

Intermolecular Forces

Intermolecular forces are attractions between molecules that affect physical properties like boiling and melting points.

• Types: London dispersion, dipole-dipole, hydrogen bonding.

• Example: Water has a high boiling point due to hydrogen bonding.

Topic 8: Solutions

Properties of Solutions

A solution is a homogeneous mixture of two or more substances.

• Components: Solvent (major component), solute (minor component).

• Concentration Units: Molarity ($M = \frac{\text{moles solute}}{\text{liters solution}}$), percent by mass, molality.

• Solubility: The maximum amount of solute that can dissolve in a solvent at a given temperature.

• Example: Preparing a 1.0 M NaCl solution by dissolving 58.44 g NaCl in 1.0 L water.

Topic 9: Acids, Bases and Buffers

Acids and Bases

Acids and bases are substances that donate or accept protons (H+), respectively.

• Arrhenius Definition: Acids produce H+ in water, bases produce OH-.

• Brønsted-Lowry Definition: Acids are proton donors, bases are proton acceptors.

• pH Scale: $\text{pH} = -\log[H^+]$

• Example: Hydrochloric acid (HCl) is a strong acid; sodium hydroxide (NaOH) is a strong base.

Buffers

Buffers are solutions that resist changes in pH when small amounts of acid or base are added.

• Composition: Typically a weak acid and its conjugate base, or a weak base and its conjugate acid.

• Buffer Equation (Henderson-Hasselbalch): $\text{pH} = \text{p}K_a + \log\left(\frac{[A^-]}{[HA]}\right)$

• Example: Acetic acid and sodium acetate buffer system.