MCAT Chemistry > Thermochemistry
Thermochemistry is the branch of chemistry related to heat and the transfer of energy.
Systems involve the total reactants and products being observed in a reaction
- Isolated Systems cannot exchange energy (heat and work), or matter with the surroundings (bomb calorimeter)
- Closed Systems can exchange energy (heat and work) but not matter with the surroundings (steam radiator)
- Open Systems can exchange both energy (heat and work) and matter with the surroundings (pot of boiling water)
Processes occur when a system changes on more of its properties such as the reactant & product concentration, temperature, or pressure
- Isothermal indicates that a systems temperature is constant
- Adiabatic indicates that no heat is exchanged between system and environment
- Isobaric indicates that pressure remains constant throughout system
States are described by macroscopic properties and useful for comparing equilibrium points.
Pathway between states is described as mechanical work (W) or heat (Q).
- Temperature (T)
- Pressure (P)
- Volume (V)
- Density (ρ)
- Internal Energy (E or U)
- Enthalpy (H)
- Entropy (S)
- Gibbs Free Energy (G)
Standard State is the most stable state of a molecule under standard conditions
Standard States & Conditions is 1 atm at 25°C (298k)
Standard Temperature & Pressure (STP) is 1 atm at 0°C (273K)
Temperature (T) is the average kinetic energy of molecules
Total Internal Energy is based on the first law of thermodynamics, which states the total internal energy of a system is equal to the amount of thermal energy (heat) transferred to the system minus the work done by the system on the surroundings
ΔU = Q – W
Thermodynamic vs. Kinetic Control
Describes how some reactions that reach the intermediate stage will either decide to take a kinetic route or a thermodynamic route based on variable factors. Some products are thermodynamically favored and some are kinetically favored.
Kinetically favored products form quicker than thermodynamically favored ones because they take a quicker time to form due to lower activation energy. The activation energy is the primary distinction between these two states. However, some products are thermodynamically favored because they reside at a lower, more stable final energy state.
At higher temperatures, molecules tend to form the thermodynamically favored product, but at low temperatures tend to favor the kinetic one