Thermochemistry

Thermochemistry tracks energy transfer during physical and chemical change with clear system boundaries. Heat q is energy in transit caused by temperature difference, while temperature is a measure related to average particle kinetic energy. Sign conventions are essential: q is positive for a system that absorbs heat and negative for a system that releases heat. In coffee-cup calorimetry, a common AP assumption is that heat exchanged with the external environment is negligible, so heat lost by reaction equals heat gained by solution and cup combined. Most introductory calculations use q = mcDeltaT with mass in grams, specific heat in J g^-1 C^-1, and DeltaT in C. Keeping those units explicit prevents sign and scale mistakes. Conceptually, this chapter connects microscopic bond-energy changes to measurable temperature changes, which is why both arithmetic and interpretation are tested together. Define the system first to avoid sign errors in written explanations. Then label surroundings clearly before interpreting measured temperature changes.

Enthalpy change $\Delta H$ represents heat flow at constant pressure and allows reaction-level energy bookkeeping. Exothermic processes have negative $\Delta H$, meaning the system releases heat, while endothermic processes have positive $\Delta H$, meaning the system absorbs heat. Energy diagrams visualize these differences and help compare pathways with different activation barriers. Hess's law works because enthalpy is a state function, so total $\Delta H$ depends only on initial and final states, not on path. Practically, reversing an equation changes the sign of $\Delta H$, and multiplying an equation by a factor multiplies $\Delta H$ by the same factor. Formation-enthalpy calculations follow the same logic through products minus reactants sums weighted by coefficients. Reliable AP solutions state assumptions, track signs carefully, and include a reasonableness check such as whether temperature change direction matches endothermic or exothermic interpretation for the defined system and surroundings. This makes multi-equation energy synthesis much more reliable. It also simplifies error tracing when one intermediate equation is reversed incorrectly.