Thermochemistry
(UNIT 2)
Grade 12 Chemistry SCH4U0
What is
THERMOCHEMISTRY?
THERMOCHEMISTRY is
the study of the energy
changes that accompany
physical or chemical
changes in matter.
Energy Transformations
Heat vs. Temperature


Heat (q) is the amount of energy
transferred between substances
(Units: Joules (J), kJ, kJ/mol)
Temperature (T) is the measure of
the average kinetic energy (energy
of motion) of the particles in a
sample of matter
(Units: oC, oK, oF)
Heat and Energy Changes


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What are the products when methane
combusts in air?
CH4 + 2O2  CO2 + 2H2O + energy
Energy that is released from this
CHEMICAL SYSTEM to the
SURROUNDINGS is called thermal
energy (heat).
Since the molecules have greater kinetic
energy, temp. of surroundings increases
System and Surroundings

The system is a well-defined part of the universe
singled out for study.

The surroundings is the remainder of the universe.

In a closed system energy, but not matter, can be
exchanged with the surroundings.

In an open system , both energy and matter can
flow into our out of the system.

In an isolated system (ideal system) neither matter
nor energy can move in or out.
Exothermic and Endothermic
Processes
Heat (q) is a form of energy transfer.
Units: 1 calorie (cal) = 4.184 J
q>0
Heat is transferred from surrounding to system.
Process is
endothermic.
q<0
Heat is transferred from system to
surroundings. Process is exothermic.
System q q
Surroundings
A basic calorimeter

)
Surroundings
everything else
System with
can as
boundary
Endothermic =
absorbing energy
Exothermic =
releasing energy
Law of conservation of energy = release and
absorption of energy must be equal
Heat and Temperature
Change
How is heat transferred related to the change in
temperature of a system with mass m?
q = specific heat (c)  m  T
The specific heat (c) of a substance is the
amount of heat required to raise the temperature
of 1 gram by 1 oC.
Al
0.90 J/gC
H2O
4.18 J/gC
Example Question 1
When 600 g of water in an electric kettle is heated
from 20 C to 85 C to a make a cup of Rupinder’s
favorite tea, how much heat flows into the water?
Given: m = 600 g, ΔT = 85C – 20 C = 65 C
c = 4.18 J/gC (from Table 1, p.301)
Required: q (amount of heat transferred)
Solution: q = mcΔT = (600)(4.18)(65) =
163 kJ
Let's try this question now

If 150.0 grams of iron at 95.0 °C, is
placed in an insulated container
containing 500.0 grams of water at
25.0 °C, and both are allowed to come
to the same temperature, what will
that temperature be? The specific heat
of water is 4.18 J/g °C and the specific
heat of iron is 0.444 J/g °C)
Energy and Enthalpy

Chemists give a special symbol, ΔH(delta H)
to the heat change in a reaction

This heat change is called ENTHAPLY


ΔH (enthalpy) = energy absorbed or
released to the surroundings when a
system changes from reactants to
products.
ΔHsystem = - Qsurrounding
HEAT
HEAT
HEAT
HEAT
Molar Enthalpy
ΔHx: the enthalpy change associated with
a physical, chemical or nuclear change
involving 1 mol of a substance
(x – is the letter used to indicate the type
of change that is occurring)
[Units: kJ/mol]
ΔHvap
ΔHsol
ΔHfr
ΔHcomb (See Table1 p.306)

Types of Molar Enthalpies
Example: COMBUSTION: (ΔHcomb )
 CH4(g) + 2 O2(g) CO2(g) + 2 H2O(l)
- 890kJ
ΔHcomb =
The amount of energy involved in a change depends
on the quantity of matter undergoing that change.
So, twice the mass of methane will release twice as
much energy into the surroundings.
Thus,
ΔH = n ΔHcomb
(ΔHcomb is obtained from a reference
source)
Enthalpies of Reaction
H is an extensive property, so H depends
on the amounts of reactants and products.
What is H for the combustion of 11.0 g of
CH4 in excess oxygen?
CH4(g) + 2 O2(g) CO2(g) + 2 H2O(g)
H = -
802 kJ
11.0 g CH4 
mol CH 4
16 . 04 g CH 4

 802 kJ
1 mol CH 4
= -550 kJ
Example Question (p.309)
In a calorimetry experiment, 7.46 g
of KCl is dissolved in 100.0 mL of
water at an initial T of 24.1 C. The
final T of the solution is 20C. What
is the molar enthalpy of solution of
KCl?
Calorimetry of Physical and
Chemical Changes
i)
ii)
iii)
Three simplifying assumptions often used in
calorimetry:
No heat is transferred between calorimeter
and the outside environment
Any heat absorbed or released by the
calorimeter material is negligible
A dilute aqueous solution is assumed to
have a density and specific heat equal to
that of water
Homework Questions
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p. 308 # 1 - 3
p. 310 # 4
p. 311 # 7, 9