6.2 Enthalpy and Calorimetry

Enthalpy and Calorimetry (6.2)

A) Heat Exchange and Calorimetry

• Definition: Calorimetry is a laboratory method used for calculating heat exchange through the measurement of heat released or absorbed by an object, using a calorimeter.

Heat Exchange Principles

• The overall idea is that the heat released by an object is captured and measured by a calorimeter.
• Identifying key components in heat exchange:
  • Heat Released: The energy dissipated by an object.
  • Calorimeter: An instrument used to measure the amount of heat involved in a chemical reaction or physical change.

Heat Relationships

• Equation: The heat exchange can be expressed as: $q = C imes ext{AT}$
  • Where:
  • $q$ = Heat (energy) exchanged
  • $C$ = Heat Capacity of the object
  • $ext{AT}$ = Change in temperature

Total Heat Gained by an Object

• The total heat gained by an object depends on its heat capacity, the specific heat (s), and the change in temperature (ΔT).
• Specific heat (s): Specific heat is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C.
  • Units: J/°C

Factors Influencing Heat Capacity

• C: Refers to the physical and chemical characteristics of the object which include:
  • Chemical composition
  • Mass
  • Density
• Each material can have different heat capacities.

Examples of Heat Exchange with Different Objects

1. Complex Objects (Multiple Materials) Example:

   • $q = C imes ext{AT}$
   • Total heat will be calculated by considering all components.

2. Pure Substance Example:

   • Large pieces of ice will absorb more heat than smaller pieces.
   • Different samples may possess unique values of specific heat (C).
     • Example Equations:
     • $q_ ext{big} = C_{ ext{big}} imes ext{AT}$
     • $q_ ext{small} = C_{ ext{small}} imes ext{AT}$

3. Calculating Heat Capacity per Gram

   • For pure substances, since they have the same chemical composition, we can derive:
     • ext{Heat capacity} = C imes ext{#grams}
   • Where the mass of the object needs to be factored into the calculation.

4. Combining Components in a Calorimeter

   • Total heat absorbed includes the heat absorbed by the calorimeter and the heat absorbed by the water:
     • $q = ext{Heat (calorimeter)} + ext{Heat (water)}$

B) Calorimeter Complexities

Coffee Cup Calorimeter

1. Estimations:
   • Under typical conditions, disregard heat absorption by Styrofoam as the heat value depends solely on the water's mass in the calorimeter.
   • Example: If mass of water is given,
   • Formula used is:
     • $q = m<em>s imes C</em>{H_2O} imes ext{AT}$
   • Given values:
     • $m_s$ = mass of water (e.g., 5410 g)
     • Specific heat of water, $C<em>{H</em>2O}$ = 4.18 J/g°C or 1.0 cal/g°C
   • Relationship established:
     • $ext{Heat of object lost/gained} = ext{heat of water gained/lost} + ext{heat of Styrofoam cup gained/lost}$
     • Relationship can be simplified to:
     • $m<em>{s} imes C</em>{H<em>2O} imes ext{AT}</em>{object} = Q<em>{water} + Q</em>{styrofoam}$

Constant Pressure vs. Constant Volume Calorimetry

• Constant Pressure (Open Container)

  • Characteristics of an open container:
  • Pressure, P = 1 atm.
  • Commonly referred to as coffee cup calorimeter.
  • The formula used in constant pressure calorimetry is:
    • $q = m s imes A T$

• Constant Volume (Bomb Calorimeter)

  • A bomb calorimeter is a closed, heavy-duty container designed for measuring the heat of reaction at constant volume, useful in explosive reactions.
  • In this setup:
  • Work done (W) = 0 because the change in volume (ΔV) of the system is zero:
    • $W = P imes ext{ΔV}$
    • Therefore, $W = 0$
  • The change in internal energy (AE) in terms of heat is given by:
    • $ext{AE} = q$

Calculation Example (Bomb Calorimeter)

• In a hypothetical experiment:
  • If water starts at 25.0°C and reaches peak temperature of 28.4°C, the calculations for heat transfer may include the following:
  • Use of heat formula:
  • $q = C imes T = m (T<em>{final} - T</em>{initial})$
  • For an example where water has a mass of 30g and undergoes a temperature change:
    • $q = (4.18 ext{ J/g°C}) imes (30 g) imes (3.0 °C)$

C) Enthalpy (H)

• Definition: Enthalpy (H) is defined as the total heat content of a system at constant pressure (an open container where P = 1 atm).
• Enthalpy is a central concept in thermodynamics, representing the amount of thermal energy that can do work during a process at constant pressure. It is crucial for understanding how heat exchange occurs under varying conditions in chemical reactions.