heat transfer

Meaning: Heat (q) is energy in transit because of a temperature difference. It is not something an object contains. Heat flows spontaneously from the higher-temperature body to the lower-temperature one until thermal equilibrium is reached. There are three mechanisms: conduction (energy passed by collisions between particles in contact), convection (bulk motion of a fluid carrying energy), and radiation (electromagnetic waves, which can cross a vacuum).

Equation: q = mcΔT, where q is heat in joules (J), m is mass in kilograms (kg), c is specific heat capacity in J/(kg·K), and ΔT is the temperature change in kelvin (K). During a phase change, q = mL, where L is the latent heat in J/kg. Chemistry problems often use grams and J/(g·°C).

On the MCAT: Passages describe a coffee-cup calorimeter. A hot metal sample goes into water, and you find the final temperature or the metal's specific heat from qmetal = −qwater. Heating curves are also common. Sloped segments use mcΔT, and flat plateaus use mL. Water's high specific heat means it changes temperature slowly, so it buffers its surroundings.

Common confusion: Heat and temperature are different quantities. Temperature reflects average kinetic energy of the particles, while heat is energy transferred. During a phase change, heat is absorbed or released but the temperature stays constant, because the energy changes intermolecular attractions rather than kinetic energy. Also watch signs: q is positive when a system absorbs heat (endothermic) and negative when it releases heat.