Summary
Highlights
Thermochemistry is the study of heat absorbed or produced by chemical reactions. Reactions can be exothermic (releasing energy) or endothermic (absorbing energy). The change in energy is represented by Delta H, or the change in enthalpy. This can be understood as the difference in energy stored in the bonds of products versus reactants. A positive Delta H indicates an endothermic reaction (more energy in products), while a negative Delta H signifies an exothermic reaction (less energy in products).
Energy, the capacity to do work, exists in various forms and is conserved, meaning it changes forms but is not created or destroyed. Heat (q) is defined as the energy flow between a system and its surroundings due to a temperature difference, always moving from high to low temperature. A positive q means heat is absorbed by the system, while a negative q means heat is released. When temperature and pressure are constant, q equals Delta H.
Energy in thermochemistry is measured in joules or kilojoules. Another common unit is the calorie, equal to 4.184 joules, representing the energy needed to raise one gram of water by one degree Celsius. This is distinct from the food Calorie (capital C), which equals a kilocalorie or 1000 calories. A thermochemical equation reports the change in enthalpy (Delta H) for a given reaction based on the stoichiometric molar quantities. The enthalpy change is directly proportional to the amount of material reacting.