Heating Curves

heat (q): a form of energy, usually measured in units of Joules (J). In chemistry, heat either:

· makes molecules move faster (increases temperature)

· gives molecules more freedom (e.g., changes state from solid à liquid or liquid à gas)

enthalpy (H): the energy contained within the molecules of a substance, usually measured in Joules/mole (J/mol). Think of enthalpy as the chemical energy that the molecules of a substance contain, and heat as the energy that you put in or get out.

specific heat (C_P): the amount of heat (q) that it takes to increase the temperature of 1 g of a substance by 1°C. For a given mass (m), specific heat (C_p), and temperature change (ΔT):

q = m C_p ΔT

The specific heat values for H₂O are 2.09  for ice, 4.184 for liquid water, and 1.97 for water vapor.

heat of fusion: the enthalpy change (ΔH) needed to melt a substance. ΔH_fus. for H₂O is 334 . The heat required by the process is:

q_melt = m ΔH_fus.

heat of vaporization: the enthalpy change (ΔH) needed to vaporize (boil) a substance. ΔH_vap. for H₂O is 2260 . The heat required by the process is:

q_boil = m ΔH_vap.

heating curve: a graph of the temperature of a substance vs. the amount of heat put in.

For example, the graph above shows the temperature profile when 1.0 g of H₂O is heated from −25°C to +125°C. Notice that the temperature remains constant during melting and boiling.

Example problem: How much heat would it take to raise the temperature of 1.0 g of H₂O from −25°C to +125°C?

q_ice = m C_p ΔT =

= 52.25 J

q_melt = m ΔH_fus. =
= 334 J

q_liq = m C_p ΔT =

= 104.6 J

q_boil = m ΔH_vap. =
= 2260 J

q_gas = m C_p ΔT =

= 49.25 J

q_total = q_ice + q_melt + q_liq + q_boil + q_gas
= 52.25 + 334 + 104.6 + 2260 + 49.25
= 2,800 J