👩🏽‍🔬Honors Chemistry Unit 8 – Thermochemistry

Key Concepts and Definitions

• Thermochemistry studies the energy and heat associated with chemical reactions and physical transformations
• System refers to the specific part of the universe that is being studied or observed
• Surroundings include everything outside the system
• Energy is the capacity to do work or transfer heat
• Potential energy is stored energy an object possesses due to its position or composition
• Kinetic energy is the energy of motion
• Temperature measures the average kinetic energy of particles in a substance
• Heat is the transfer of thermal energy between substances due to a temperature difference

Energy in Chemical Reactions

• Chemical reactions involve the breaking and forming of chemical bonds
• Endothermic reactions absorb energy from the surroundings, resulting in a temperature decrease
  • Requires an input of energy to break bonds in the reactants (melting ice)
• Exothermic reactions release energy to the surroundings, resulting in a temperature increase
  • Energy is released when new bonds form in the products (combustion of fuel)
• Activation energy is the minimum energy required for a chemical reaction to occur
  • Represents the energy barrier that must be overcome for reactants to convert to products
• Catalysts lower the activation energy of a reaction without being consumed
  • Increase the rate of a chemical reaction by providing an alternative reaction pathway

First Law of Thermodynamics

• States that energy cannot be created or destroyed, only converted from one form to another
• In a closed system, the total energy remains constant
• $\Delta E_{system} = q + w$
  • $\Delta E_{system}$ is the change in the system's internal energy
  • $q$ is the heat exchanged with the surroundings
  • $w$ is the work done by or on the system
• Heat and work are two ways in which energy can be transferred between a system and its surroundings
• The change in internal energy depends only on the initial and final states, not the path taken

Enthalpy and Heat Transfer

• Enthalpy ($H$) is a state function that measures the total heat content of a system at constant pressure
• Change in enthalpy ($\Delta H$) is the heat transferred during a process at constant pressure
  • $\Delta H = H_{final} - H_{initial}$
• Endothermic processes have a positive $\Delta H$ (heat is absorbed by the system)
• Exothermic processes have a negative $\Delta H$ (heat is released by the system)
• Heat of reaction ($\Delta H_{rxn}$) is the enthalpy change for a chemical reaction
  • Depends on the stoichiometry and the physical states of the reactants and products

Calorimetry and Heat Capacity

• Calorimetry measures the heat transfer during a chemical or physical process
• Heat capacity ($C$) is the amount of heat required to raise the temperature of a substance by one degree Celsius
  • Specific heat capacity ($c$) is the heat capacity per unit mass of a substance
• The heat exchanged ($q$) is calculated using the equation: $q = mc\Delta T$
  • $m$ is the mass of the substance
  • $c$ is the specific heat capacity
  • $\Delta T$ is the change in temperature
• Bomb calorimeters measure the heat of combustion reactions at constant volume
• Coffee cup calorimeters measure the heat of reactions at constant pressure

Hess's Law and Enthalpy Calculations

• Hess's Law states that the total enthalpy change for a reaction is independent of the route taken
• Enthalpy changes are additive for a series of reactions
• Standard enthalpy of formation ($\Delta H_f^\circ$) is the enthalpy change when one mole of a compound is formed from its elements in their standard states at 1 atm and 25°C
• Standard enthalpy of reaction ($\Delta H_{rxn}^\circ$) can be calculated using the standard enthalpies of formation:
  • $\Delta H_{rxn}^\circ = \sum \Delta H_f^\circ (products) - \sum \Delta H_f^\circ (reactants)$
• Enthalpy changes for reactions can be calculated using Hess's Law and algebraic manipulation of known enthalpy values

Bond Energies and Enthalpies of Formation

• Bond energy is the energy required to break a specific bond in one mole of a substance
• Average bond enthalpies can be used to estimate the enthalpy change of a reaction
• Enthalpy of formation can be estimated using average bond enthalpies:
  • $\Delta H_f^\circ = \sum (bond$ energies_{bonds broken}) - \sum (bond energies_{bonds formed})
• Enthalpies of formation from bond energies are less accurate than experimental values due to variations in bond strengths within molecules

Real-World Applications and Examples

• Thermochemistry has numerous practical applications in everyday life and various industries
• Combustion of fuels (natural gas, gasoline) in engines and power plants releases energy for transportation and electricity generation
• Metabolism of food in the human body is an exothermic process that provides energy for biological functions
• Chemical hand warmers contain exothermic reactions (crystallization of supersaturated sodium acetate) to generate heat
• Endothermic reactions are used in cold packs for injury treatment (ammonium nitrate dissolving in water)
• Calorimetry is used in food science to determine the caloric content of food and optimize cooking processes
• Hess's Law is applied in the design and optimization of industrial chemical processes to minimize energy consumption and costs
• Understanding bond energies and enthalpies of formation helps in predicting the stability and reactivity of compounds, guiding the synthesis of new materials