Thermochemistry Unit Review - LTHS Chemistry

Exothermic and Endothermic Reactions

Exothermic Processes

• Characteristics:
  1. Energy is released as a product.
  2. Has a negative (-) enthalpy value.
  3. Heat energy is released from chemical bonds.
  4. R → E (Reactants to Products)
• Examples:
  • Combustion reactions.
  • Respiration.

Endothermic Processes

• Characteristics:
  1. Energy is absorbed as a reactant.
  2. Has a positive (+) enthalpy value.
  3. Heat energy is stored as chemical bonds.
• Examples:
  • Photosynthesis.
  • Melting of ice.

Specific Heat Capacity

Definition

• Specific heat capacity refers to the amount of energy required to raise the temperature of a substance by 1°C.

Importance

• A higher specific heat means it requires more energy to change temperature, implying the substance heats up slower.

Calculations Using Specific Heat

Example Calculation 1

• Problem: Determine the specific heat of aluminum when a 56.8 g sample increases in temperature by 9.2°C with 454 J of added heat.
• Solution:
  • Use formula: q = mcΔT
  • Given: q = 454 J, m = 56.8 g, ΔT = 9.2°C
  • Solve for specific heat (c).

Example Calculation 2

• Problem: Identify the specific heat of an unknown solid using calorimeter data.
• Data:
  • Mass of solid = 20.26 g, Final temperature = 33.3°C, Initial temperature of water = 30.7°C
  • Use q = mcΔT for both water and solid to find specific heat of solid.
• Potential Identifications:
  • Compare with specific heat table (e.g., Gold, Iron, Titanium).

Energy Change in Reactions

Hydrazine and Chlorine Reaction

• Equation: N₂H₄(l) + 2 Cl₂(g) → 4 HCl(g) + N₂(g)
• Enthalpy Change (ΔH): -420.0 kJ

Calculations

• Condition a: 25.4 g of hydrazine reacting:
  • Use molar mass to calculate energy change.
• Condition b: 1.45 mol HCl generated:
  • Calculate based on the stoichiometry of the reaction.

Magnesium Oxide Production

• Reaction: 2 Mg(s) + O₂(g) → 2 MgO(s)
• Enthalpy Change (ΔH): 288 kcal

Problem

• Calculate how much MgO is produced with 1,356 kcal.
• Use stoichiometry to determine the mass of MgO produced.