kelvin
Summary
The kelvin (K) is the SI unit of absolute temperature, with 0 K at absolute zero. One kelvin is the same size as one Celsius degree, and T(K) = t(°C) + 273.15. Gas laws and kinetic theory need temperature in kelvins, not Celsius.
Detail
Meaning: The kelvin (K) is the SI unit of thermodynamic temperature. The scale starts at absolute zero (0 K), so it is an absolute scale. Temperature in kelvins is directly proportional to the average translational kinetic energy of ideal gas particles, so doubling T doubles that energy. One kelvin is the same size as one Celsius degree; the scales differ only in where zero sits. The unit is written K, with no degree sign.
Equation: T = t + 273.15, where T is temperature in kelvins (K) and t is temperature in degrees Celsius (°C).
On the MCAT: Gas laws (PV = nRT, Charles's law), ΔG = ΔH − TΔS and the Arrhenius equation need T in kelvins; R = 8.314 J/(mol·K). Passages often give °C, so convert first (27 °C is about 300 K). Raising T from 300 K to 600 K at constant pressure doubles the volume. For a temperature change, as in q = mcΔT, ΔT in K equals ΔT in °C.
Common confusion: Doubling the Celsius temperature does not double the volume or the kinetic energy. Ratios are meaningful only on an absolute scale, so 20 °C to 40 °C is about 293 K to 313 K, a small increase.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Physics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.