U7 AAC Test Review Notes

Energy Level Diagrams

• Be able to sketch endothermic and exothermic energy level diagrams.
  • Label reactants, products, and change in enthalpy.
  • If the diagram has a bump at the start, it represents an exothermic reaction.
  • If the diagram has a bump at the end, it represents an endothermic reaction.

Heat vs. Temperature

• Explain the difference between heat and temperature.
  • Heat is the measurement of thermal energy in a substance.
  • Temperature is the measurement of the average kinetic energy.

Endothermic vs. Exothermic Reactions

• Based on a scenario, decide if a reaction is endothermic or exothermic.
  • If a reaction releases cold (like ice), it is endothermic because heat is absorbed from the surroundings.
  • If a reaction releases heat (like fire), it is exothermic because heat is released to the surroundings.

Thermochemical Equations

• Decide if a thermochemical equation is endothermic or exothermic.
• Write \Delta H separately, including the correct sign and units.
  • If \Delta H is negative, the reaction is exothermic.
  • If \Delta H is positive, the reaction is endothermic.

Specific Heat Calculations

• Calculate the specific heat of an unknown substance given the energy change, the temperature change, and the mass.

Units for Temperature and Heat

• Be able to tell the correct units for temperature and heat.
  • Temperature: \degree C or K
  • Heat: J or kJ
  • Specific Heat: \frac{J}{g \cdot \degree C} (Joules per gram Celsius)
  • \frac{kJ}{mol}: Kilojoules per mole

Energy Absorption or Release

• Calculate the amount of energy absorbed or released as an item heats or cools if you know its mass and temperature change.

Enthalpy Change Using Bond Energies

• Calculate an enthalpy change using bond energies.
  • \Delta H = \sum{(\text{Bond energies of reactants})} - \sum{(\text{Bond energies of products})}
  • Example:
    • Reactants: \sum{\text{Bond energies of reactants}} = 4 \times 413 + 2 \times 498 = 2648 \, kJ
    • Products: \sum{\text{Bond energies of products}} = 2 \times 799 + 4 \times 463 = 3450 \, kJ
    • \Delta H = 2648 - 3450 = -802 \, kJ (Exothermic)

Enthalpy Change Using Heats of Formation

• Calculate an enthalpy change using heats of formation.
  • \Delta H{rxn}^{\circ} = \sum{\Delta H{f}^{\circ} \text{(products)}} - \sum{\Delta H_{f}^{\circ} \text{(reactants)}}

Enthalpy Change Using Calorimetry

• Calculate an enthalpy change of a reaction using lab data from calorimetry.
  • q = mc\Delta T
    • Where:
      • q is the heat absorbed or released
      • m is the mass
      • c is the specific heat capacity
      • \Delta T is the change in temperature
  • Example:
    • 100 mL HCl + 100 mL NaOH \rightarrow \Delta T = 6.8 \, \degree C
    • Mass = 200 g (assuming density = 1 g/mL)
    • q = 200 \times 4.18 \times 6.8 = 5684.8 \, J = 5.68 \, kJ
    • Moles of reaction = 0.1 mol \rightarrow \Delta H = \frac{-5.68}{0.1} = -56.8 \, kJ/mol

Enthalpy Change Using Hess’s Law

• Calculate an enthalpy change of a target reaction using Hess’s Law and the step reactions.
  • Key Idea: Sum the \Delta H of step reactions to get the \Delta H of the target reaction.
  • Example:
    • Target: C + \frac{1}{2}O_2 \rightarrow CO
    • Given:
      • (1) C + O2 \rightarrow CO2 (\Delta H = -393.5 \, kJ)
      • (2) CO + \frac{1}{2}O2 \rightarrow CO2 (\Delta H = -283.0 \, kJ)
    • Reverse (2): CO2 \rightarrow CO + \frac{1}{2}O2 (\Delta H = +283.0 \, kJ)
    • Add to (1): \Delta H = -393.5 + 283.0 = -110.5 \, kJ