Thermochemistry II
Heat capacity and Calorimetry
Calorie
 A calorie is a unit of
heat.
 It is defined as the
quantity of heat (q)
needed to raise one
gram of pure water
one degree C.
calorie versus Calorie
 Dietary units of
energy are
Calories.
 1000 calories =
 1 kilocalorie =
 1 Calorie
Joules
 Joule is the SI
unit for energy.
 1 joule=
0.2390 cal
 1 cal =
4.184 joule
Heat Capacity
 The amount of heat
needed to increase the
temperature of an object
exactly 1 degree C is the
heat capacity of an
object.
 Different objects have
different heat capacities.
 Which heats faster, Cu or
Al?
Heat Capacity
Specific heat capacity
 Specific heat
capacity or
Specific Heat, is
the amount of
heat it takes to
heat 1 gram of a
substance 1
degree C.
Examples
 If a chunk of silver has a heat capacity of
42.8 J/°C. If the silver has a mass of 181
grams, calculate the specific heat of
silver.
 Answer:
 0.236 J/(g °C)
Calorimetry
Enthalpy
 ∆H is the heat under constant
pressure.
 What pressure is constant?
 Atmospheric Pressure.
q= ∆H = m x C x ∆T
 ∆H is the heat under constant
pressure.
 C is the heat capacity.
 In calorimetry, C is the heat
capacity of water, the item
heated.(system or surrounding?)
 ∆T is the change in temperature.
 T final – T initial = ∆T
Bomb Calorimetry
∆H versus q
 In a bomb calorimeter, the
pressure is not constant.
 With combustion under a
constant volume, pressure will
increase.
 Therefore we must use q and not
∆H.
Example:
 Go to worksheet.