🧪AP Chemistry Unit 6 – 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 being studied (reaction, physical process)
• Surroundings include everything outside the system
• Endothermic processes absorb energy from the surroundings, causing the system to increase in energy
• Exothermic processes release energy to the surroundings, causing the system to decrease in energy
• Heat ($q$) represents the transfer of energy between the system and surroundings due to a temperature difference
• Work ($w$) is the energy transfer due to a force acting over a distance
• Internal energy ($U$) is the sum of all kinetic and potential energies within a system

Energy and Heat Transfer Basics

• Energy can be transferred between the system and surroundings through heat, work, or matter exchange
• Temperature measures the average kinetic energy of particles in a substance
• Heat flows from regions of higher temperature to regions of lower temperature (second law of thermodynamics)
• Thermal equilibrium occurs when two objects in contact reach the same temperature and no net heat transfer takes place
• Specific heat capacity ($c$) is the amount of heat required to raise the temperature of 1 gram of a substance by 1°C
  • Varies depending on the substance (water has a high specific heat capacity of 4.18 J/g°C)
• Heat transfer can occur through conduction (direct contact), convection (fluid motion), or radiation (electromagnetic waves)
• The amount of heat transferred ($q$) is calculated using the formula: $q = mc\Delta T$, where $m$ is mass, $c$ is specific heat capacity, and $\Delta T$ is the change in temperature

First Law of Thermodynamics

• States that the total energy of an isolated system remains constant; energy cannot be created or destroyed, only converted from one form to another
• Mathematically expressed as $\Delta U = q + w$, where $\Delta U$ is the change in internal energy, $q$ is heat, and $w$ is work
• In a closed system, the change in internal energy is equal to the heat added to or removed from the system
• For an endothermic process, $q$ is positive, and for an exothermic process, $q$ is negative
• Work done by the system is negative ($w < 0$), while work done on the system is positive ($w > 0$)
• The first law of thermodynamics is a statement of the conservation of energy principle

Enthalpy and Enthalpy Changes

• Enthalpy ($H$) is a state function that represents the total heat content of a system at constant pressure
• Change in enthalpy ($\Delta H$) is the heat absorbed or released by a system during a process at constant pressure
• For an endothermic process, $\Delta H$ is positive, and for an exothermic process, $\Delta H$ is negative
• Standard enthalpy of formation ($\Delta H_f^°$) is the enthalpy change when one mole of a compound is formed from its constituent elements in their standard states at 1 atm pressure and 25°C
• Standard enthalpy of combustion ($\Delta H_c^°$) is the enthalpy change when one mole of a substance completely combusts in excess oxygen at standard conditions
• Enthalpy of reaction ($\Delta H_{rxn}$) is the enthalpy change associated with a chemical reaction
  • Can be calculated using Hess's Law or standard enthalpies of formation

Calorimetry and Heat Capacity

• Calorimetry measures the heat transferred during a chemical reaction or physical process
• A calorimeter is an insulated device used to measure heat transfer in a closed system
• The most common types of calorimeters are coffee cup calorimeters and bomb calorimeters
  • Coffee cup calorimeters are used for reactions at constant pressure (open to the atmosphere)
  • Bomb calorimeters are used for reactions at constant volume (sealed vessel)
• The heat capacity ($C$) of an object is the amount of heat required to raise its temperature by 1°C
  • Calculated using the formula: $C = \frac{q}{\Delta T}$, where $q$ is the heat added or removed and $\Delta T$ is the change in temperature
• Molar heat capacity ($C_m$) is the heat capacity per mole of a substance
• Specific heat capacity ($c$) is related to heat capacity by the formula: $C = mc$, where $m$ is the mass of the substance

Hess's Law and Enthalpy Calculations

• Hess's Law states that the total enthalpy change for a reaction is independent of the route taken from reactants to products
• Allows the calculation of enthalpy changes for reactions that cannot be directly measured or are difficult to carry out
• Based on the conservation of energy and the additivity of enthalpy changes
• To apply Hess's Law:
  1. Write the desired reaction equation
  2. Identify known reactions that can be combined to give the desired reaction
  3. Reverse any reactions as needed (reversing a reaction changes the sign of $\Delta H$)
  4. Multiply reactions by appropriate factors to ensure the desired reaction is obtained when adding the known reactions
  5. Sum the enthalpy changes of the known reactions to determine the enthalpy change of the desired reaction
• Enthalpy changes can also be calculated using standard enthalpies of formation ($\Delta H_f^°$)
  • $\Delta H_{rxn} = \sum \Delta H_f^°(products) - \sum \Delta H_f^°(reactants)$

Bond Energies and Formation Reactions

• Bond energy is the amount of energy required to break a specific bond in one mole of a substance
• Bond formation releases energy, while bond breaking requires energy
• The net enthalpy change of a reaction can be estimated using the difference between the bond energies of the reactants and products
  • $\Delta H_{rxn} = \sum(Bond\ energies\ of\ reactants) - \sum(Bond\ energies\ of\ products)$
• This method provides an approximate value for $\Delta H_{rxn}$ as it assumes that bond energies are independent of the specific molecule in which they occur
• Formation reactions are chemical reactions in which a compound is formed from its constituent elements in their standard states
• The enthalpy change associated with a formation reaction is the standard enthalpy of formation ($\Delta H_f^°$)
• Formation reactions are used as a reference point for calculating enthalpy changes of other reactions using Hess's Law or standard enthalpies of formation

Real-World Applications and Examples

• Thermochemistry has numerous real-world applications in areas such as energy production, materials science, and biochemistry
• Combustion reactions, such as the burning of fossil fuels, are exothermic and release heat energy that can be harnessed for power generation (coal, oil, natural gas)
• Metabolic processes in living organisms, such as cellular respiration and photosynthesis, involve energy transfer through chemical reactions
• Phase changes, like melting and vaporization, are endothermic processes that require heat input (melting of ice, boiling of water)
• Calorimetry is used in food science to determine the caloric content of foods and beverages
• Heat packs and cold packs utilize exothermic and endothermic reactions, respectively, to provide localized heating or cooling (instant hot packs, gel ice packs)
• Thermodynamic principles are applied in the design and optimization of industrial chemical processes, such as the Haber-Bosch process for ammonia synthesis
• Enthalpy changes associated with chemical reactions are crucial in understanding the stability and reactivity of compounds (formation of rust, combustion of propane)