CHAPTER 6 – THERMOCHEMISTRY
THERMOCHEMISTRY: study of interconversion between
thermal energy (heat) and chemical energy.
Law of Conservation of Energy: Energy cannot be created or
destroyed, it can only be converted from one form to another.
1st Law of Thermodynamics: The total amount of energy of
the universe is constant.
 A chemical system exchanges energy with its surroundings
through heat.
System – the reaction or process you’re interested in.
i.e. CH4
+ 2 O2
CO2 + H2O
+
heat
Surroundings
Surroundings – everything else
System
Exothermic Process: heat produced in process (heat released to
surroundings)
Surroundings
Endothermic Process: heat put into process
System
UNITS OF ENERGY
 Calorie (C): nutritional energy unit
Calorie = 1000 calories (kilocalorie)
 calorie (c): amount of energy required to raise temperature
of 1.00 g of water by 1 oC
 Joules (J): amount of energy required to move a 1 kg mass
a distance of 1 meter (kg * m2/s2)
*1 calorie = 4.184 joules (J)
A baked potato has a nutritional value of 120.0 Calories. How
many kilojoules (kJ) is this?
ENTHALPY (H): heat added or lost in a chemical reaction at
constant pressure.
Change in Enthalpy (H): change in heat between reactants and
products.
H = Hfinal – Hinitial
H = Hproducts – Hreactants
Endothermic Reactions
 H is positive (+)
 heat added to system (system absorbs heat from
surroundings)
 example: cold packs
What is H for the following endothermic reaction?
i.e.
H2O(l) +
286 kJ
H2(g)
+
½ O2(g)
H = ________ k J
Exothermic Reactions
 H is negative (-)
 heat leaves system (surroundings absorb heat from system)
 i.e. hot packs
What is H for the following reaction? Is it endo or exothermic?
i.e.
H2O(g)
H2O(l)
+
44 kJ
H = _______kJ
Is sweat evaporating from your skin “the cooling effect” an
endothermic or exothermic process? What is H?
SPECIFIC HEAT (Cs)
 Amount of heat energy required to raise the temperature of
one gram of a substance 1oC (units: J/goC).
 Sensitivity of a substance to heat. (easier to change temp of
substance lower specific heat, i.e. copper)
Substance
Specific Heat
Water
Copper
4.184 J/goC
0.385 J/goC
Why is Santa Monica cooler than Woodland Hills?
HEAT CAPACITY (C) –
 Amount of heat required to raise a substance’s temperature
by 1 oC. (units: J/oC)
 Dependent on mass of substance.
Calculating the heat (q) that goes into a pure substance
q = m X Cs X T
q = amount of heat (J)
m = mass of substance (g)
Cs (specific heat) = J/g oC
T = TempFinal – TempInitial
Calculate the amount of heat transferred to a 2.00 gram copper
sample where the temperature rose from 293K to 617 K
(specific heat of Cu = 0.385 J/g K)?
q = m X Cs X T
What is the specific heat of silicon if it takes 192 J to raise the
temperature of 45.0 g of Si by 6.0 oC?
Calorimetry: Measures heat evolved in a chemical reaction,
where heat exchanged between reaction (system) and
surroundings is measured by the change in temp. of
surroundings.
 Amount of heat gained by calorimeter equals amount of
heat released by system.
qcalorimeter = - qrxn
Bomb Calorimeter: A device to measure the heat released
during a combustion reaction.
Heat transferred to water and calorimeter.
Calculate the amount of heat (kJ/g) produced and the Hrxn for
the combustion reaction of 1.00 g of octane in a bomb
calorimeter. The calorimeter contains 1.20 kg of water, has a
heat capacity of 837 J/K and the temperature rises from 25.00 oC
to 33.20 oC during combustion.
Energy to heat water:
q H2O = mass H2O X Cs H2O X T H2O
Energy to heat calorimeter:
q calorimeter = C calorimeter X T H2O
Total energy produced:
q surroundings = q H2O + q calorimeter
q surroundings = -q system
Hrxn: q system / gram
Calculate the amount of heat (kJ/mol) transferred to the
surroundings from 500.0 mg of benzoic acid, C7H6O2, that
undergoes combustion in a bomb calorimeter, when the
temperature rises from 20.00oC to 23.01oC. The calorimeter
contains 950.0 grams of water and has a heat capacity of 420.0
J/oC.
Energy to heat water:
q H2O = massH2O X Cs H2O X TH2O
Energy to heat calorimeter:
q calorimeter = Ccalorimeter X TH2O
Total energy transferred:
q surroundings = q H2O + q calorimeter
q surroundings = -q system
Energy transferred per mole of benzoic acid:
Coffee Cup Calorimeter: A device to measure the heat released
by a substance or from a chemical reaction.
Determine the specific heat of a 55.0 g metal sample that was
heated to 99.8 oC and placed into a coffee cup calorimeter. The
calorimeter contained 225 mLs of water and temperature rose
from 21.0 oC to 23.1 oC after the metal was added. (assume
calorimeter did not absorb heat)
Heat gained by calorimeter = heat lost by metal
q H2O = -q metal
Calculate the enthalpy change, Hrxn, kJ per mole of
magnesium, for the reaction between 0.500 g of Mg and 100.0
mLs of a 1.00 M HCl solution in a coffee cup calorimeter. The
temperature of the solution increases from 22.2 oC to 44.8 oC.
(assume specific heat sol’n = 4.20 J/gK; density =1.0 g/mL)
2 HCl(aq) + Mg(s)
Determine limiting reactant:
H2(g)
+
MgCl2(aq)
Hess’s Law:
The change in enthalpy for a stepwise process is the sum of the
enthalpy changes of the steps.
Hrxn = H1 + H2 + ….
Example: Calculate Hrxn (kJ) for the following combustion
reaction.
C6H6(l) + 15/2 O2 (g)
6 C(s) + 3 H2 (g)
H2(g) + ½ O2 (g)
C (s) + O2 (g)
3 H2O(l) +
C6H6(l)
H2O(l)
CO2(g)
6 CO2(g)
H1 = +49.0 kJ
H2 = -285.8 kJ
H3 = -393.5 kJ
Standard Enthalpy Change (Ho): The change in enthalpy
when reactants and products are in standard state.
Standard State Conditions:
P = 1 atm and T= 25 oC
Standard Enthalpy of Formation (Hof): the energy
associated (H) from the formation of one mole of a compound
from its elements.
 Hof for pure compounds value obtained from data table
 Hof = 0 for pure elements
H2(g) + ½ O2 (g)
H2O(l)
Hof = -285.8 kJ/mol
Write an equation for the formation of MgCO3 from its elements
in their standard states and include Hof.
Engineer’s Shortcut
Horxn =  n Hof (products) –  n Hof (reactants)
Calculate Horxn for the following reaction:
C6H6(l) + 15/2 O2 (g)
Compound
Hof (kJ/mol)
C6H6 (l)
CO2 (g)
H2O (l)
+49.0
-393.5
-285.8
3 H2O(l) +
6 CO2(g)