Chem 162 - Ch 9 Thermochemistry (specific heat, heat capacity, heats of fusion & vaporization)

Heat Capacity

• Definition: Heat capacity refers to the amount of heat required to raise the temperature of a substance.

• Specific Heat Capacity: Often abbreviated as Cₚ, it is the quantity of heat needed to raise the temperature of one gram of a substance by one degree Celsius at constant pressure.

  • Specific to Substance: Each substance has its own specific heat capacity (e.g., iron vs. aluminum vs. water).

• Units: Specific heat capacity is expressed in joules per gram degree Celsius (J/g°C).

Calculating Heat Changes

• Formula: The formula for calculating the heat (Q) involved is:

  Q = mass × specific heat × change in temperature

• Where:

  • Q = heat (in joules)

  • mass = mass of the substance (in grams)

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

  • change in temperature = final temperature - initial temperature (in °C)

Example: Calculating Heat for Zinc

• Scenario: A piece of zinc weighing 35.8 grams is heated from 20°C to 28°C.

• Given Information:

  • Specific heat of zinc = 0.388 J/g°C

  • Mass = 35.8 g

  • Temperature change = 28°C - 20°C = 8.00°C (This is a positive change, indicating heat is added.)

• Endothermic Process: Energy is absorbed when heating, leading to a positive enthalpy change.

• Calculation:Q = 35.8 g × 0.388 J/g°C × 8.00°C

  • Result: Q = 111 joules of heat required.

Heat Capacity vs. Molar Heat Capacity

• Heat Capacity uses the units of joules per mole degree Celsius (J/mol°C).

• The version used in calculations depends on the specific heat given (either in grams or moles).

Heats of Fusion and Vaporization

• These concepts relate to energy changes during state changes rather than temperature changes.

  • Heat of Fusion: Energy required to change a substance from solid to liquid and vice versa.

  • Heat of Vaporization: Energy required to change a substance from liquid to gas and vice versa.

Key Points

• Specific heat capacity is essential in calorimetry for calculating energy transfers during reactions.

• Knowing unit conversions and understanding whether to use grams or moles is critical for accurate calculations.