Calorimetry
The Basics
What is calorimetry?
~ process by which we measure the flow of heat in a chemical reaction using a calorimeter.
Exothermic: If the chemicals release heat, qchemicals is negative; the calorimeter gains heat
(qcalorimeter is positive) and temperature rises.
Endothermic: If the chemicals absorb heat, qchemicals is positive; the calorimeter loses this heat
(qcalorimeter is negative) and temperature decreases.
How do you know if a process is exothermic or endothermic?
~ Does the temperature of the calorimeter increase?
Yes: Exothermic -- qcalorimeter is positive, qchemicals is negative
No: Endothermic -- qcalorimeter is negative, qchemicals is positive
Coffee-cup calorimetry
- constant-pressure
qcalorimeter = CcalΔT
Ccal = nwaterCwater
Important Formulas
qchemicals = - qcalorimeter
ΔE = q + w
qreaction = - qcalorimeter
ΔE = qv
qcalorimeter = CcalΔT
(for constant-volume process)
ΔEmolar = ΔE
n
ΔT= Tf -Ti
Ccal is the total heat capacity of the calorimeter, which means it is the amount of heat required to
raise the temperature of the entire calorimeter (water bath, container, & thermometer) by 1C.
If an electrical heater is used
qheater = - qcalorimeter
Ccal can be found by determining the temperature change resulting from a known amount of heat.
Ccal = qcalorimeter
ΔT
Ccal = - qheater
ΔT
Example: A calorimeter is calibrated with an electrical heater. The temperature of the
calorimeter is 22.5C before the heater is turned on. The heater releases 1.8x102 J of electrical
energy into the calorimeter and raises the temperature to 26.3C. What is the total heat capacity
of the calorimeter?
A coffee-cup calorimeter is calibrated using a small electrical heater. The addition of 3.45 kJ of
electrical energy raises the calorimeter temperature from 21.65C to 28.25C. Calculate the heat
capacity of the calorimeter.
Calculating Energy Changes
ΔE = q + w
ΔE = qv
(for constant-volume process)
qv = qreaction
ΔE = qreaction = - qcalorimeter
ΔEmolar = ΔE
n
(Molar Energy Change)
Rearranging and substituting these equations, we get
ΔEmolar = ΔE = - qv = - CcalΔT
n
n
n
Examples:
A 0.1250 g sample of octane (C8H18, MM=114.2 g/mol) is burned in excess O2 in a constant volume
calorimeter with a total heat capacity of 5.1x102 J/C. The temperature of the calorimeter rises
from 21.1C to 32.9C. What is the molar energy change for the combustion of octane?
When 0.100 g of graphite (elemental carbon) burns in a calorimeter with a total heat capacity of
5.1x102 J/C, the temperature rises from 23.5C to 29.9C. Determine the molar energy (ΔEmolar)
of combustion of graphite.
The combustion energy of glucose is 15.57 kJ/g. When a 1.7500 g sample of glucose burns in a
constant-volume calorimeter, the temperature increases from 21.45C to 23.34C. Find the total
heat capacity of the calorimeter.