Heat
High-Yield Summary
- Heat = energy transfer driven by a temperature difference, always flowing hot → cold. Temperature = average particle kinetic energy — related but not the same thing.
- Heat capacity: heat needed to raise a substance's temperature by 1°C. Specific heat = per gram; molar heat capacity = per mole.
- q = mcΔT calculates heat from mass, specific heat, and temperature change. Specific heat of water = 4.18 J/(g·K).
- Calorimetry measures heat changes; a bomb (constant-volume) calorimeter has ΔV = 0 → w = 0 → ΔE = q_v (heat measured directly equals internal energy change).
Key Terms
- Heat capacity
- The amount of heat required to raise a substance's temperature by 1°C.
- Specific heat capacity
- The heat capacity of 1 gram of a substance.
- Molar heat capacity
- Heat required to raise the temperature of 1 mole of a substance by 1°C.
- Calorimetry
- The technique used to measure the heat involved in a chemical or physical change.
- Bomb calorimeter
- A rigid, sealed, constant-volume container built to withstand high pressure, used to measure heat of combustion.
Heat from Temperature Change
q = m × c × ΔT
- q = heat
- m = mass
- c = specific heat capacity
- ΔT = temperature change
- Specific heat of water = 4.18 J/(g·K), equivalently J/(g·°C).
- Worked example: 250 g water, 20°C → 80°C: q = 250 × 4.18 × 60 = 62,700 J (62.7 kJ).
Bomb (Constant-Volume) Calorimetry
ΔE = q_v
- ΔE = change in internal energy
- q_v = heat measured at constant volume
- ΔV = 0 in a rigid bomb calorimeter → w = −PΔV = 0 → ΔE = q + w reduces to ΔE = q_v.
- Makes bomb calorimeters ideal for measuring heat of combustion directly, with no P-V work correction needed.
Must-Know Points
- Heat and temperature are NOT the same: temperature is a property a system has at any moment; heat only exists as energy in transit between systems.
- c can be expressed per °C or per K — ΔT is numerically identical in both since they're equal-sized temperature-DIFFERENCE units.
- Two calorimeter types exist: constant-volume (bomb) and constant-pressure. Bomb calorimetry gives ΔE directly; constant-pressure calorimetry gives ΔH (see Enthalpy).
Common MCAT Trap
- Don't confuse ΔE = q_v (bomb/constant-volume calorimetry) with ΔH = q_p (constant-pressure calorimetry, covered under Enthalpy) — the two calorimeter types measure different quantities directly.
- A large ΔT in °C converts to the same ΔT in K — don't add 273 when converting a temperature DIFFERENCE, only when converting an absolute temperature.
Quick Recall
How much heat is required to raise 100 g of water by 10°C?
In a bomb calorimeter, why does q_v equal the entire change in internal energy?
What's the difference between heat capacity and specific heat capacity?