Chem H: Thermochemistry Formulas Flashcards

Thermochemistry Formulas

• Heat Capacity Formula: ( q = C \times \Delta t )

  • ( C ) represents heat capacity in Joules per degree Celsius (J/ºC).
  • ( \Delta t ) is the change in temperature computed as final temperature minus initial temperature.

• Heat Flow Formula: ( q = m \times c \times \Delta t )

  • ( m ) stands for mass, ( c ) is the specific heat, and ( \Delta t ) is the temperature change.

• Specific Heat of Water:

  • Water has a specific heat of 4.18 J/gºC, essential for calculations involving heat transfer.

• Heat Transfer in Reactions:

  • The relationship ( q_{\text{reaction}} = -q_{\text{calorimeter}} ) indicates that the heat exchanged by the reaction is equal in magnitude but opposite in sign to the heat measured in the calorimeter.

• Bomb Calorimeter Equation:

  • ( q_{\text{reaction}} = -C_{cal} \times \Delta t ) can determine the heat absorbed or released in a reaction using known calorimeter heat capacity and temperature change.

Thermodynamic Laws and Relationships

• Hess's Law:

  • ( \Delta H = \Delta H_1 + \Delta H_2 ) asserts that the total enthalpy change for a reaction is the sum of the changes for individual steps.

• Enthalpy Change at Constant Pressure:

  • ( q_{\text{reaction}} ) at constant pressure equals ( \Delta H ), where ( \Delta H ) is the difference in enthalpy between products and reactants.

• Standard Enthalpy Change:

  • Calculated using the formula ( \Delta H^\circ = \sum \Delta H^\circ_f \text{(products)} - \sum \Delta H^\circ_f \text{(reactants)} ).

Energy Conservation Principles

• Conservation of Energy:

  • The equation ( \Delta E_{\text{system}} = -\Delta E_{\text{surroundings}} ) embodies the principle that energy lost by one system is gained by another.

• First Law of Thermodynamics:

  • Expressed as ( \Delta E = q + w ) where total energy change equals the sum of heat flow and work done.

Enthalpy Relationships

• Heat Flow and Enthalpy:

  • At constant pressure, ( \Delta H = q_p ) shows that heat flow is directly related to changes in enthalpy.

• Constant Volume Heat Flow:

  • At constant volume, ( \Delta H = q_v ) still maintains a direct relationship between heat flow and enthalpy.

• Change in Enthalpy and Energy:

  • ( \Delta H = \Delta E + \Delta(PV) ) links changes in enthalpy to changes in energy and the product of pressure and volume.

• Relation between Enthalpy and Energy Change:

  • ( \Delta H = \Delta E + \Delta n_g RT ) describes how enthalpy change correlates to energy change plus adjustments for changes in gas moles at temperature ( T ), with R as the ideal gas constant (8.31 J/mol K).