Download Thermodynamics (Thermochemistry) – Ch

Thermodynamics (Thermochemistry) – Ch. 5 and pages 494 – 511
The energy of a system is made up of a combination of the _____________ and the
___________________.
Kinetic Molecular Theory (KMT)
Matter can exist in three states. All matter, if there is enough energy for a physical
change to occur, will change state and exist in all three states.
Kinetic energy is the _______________________.
At the molecular level it is a combination of:
Potential energy is _______________________.
In the case of molecules, it is related to:
We are primarily concerned with changes in potential energy which result from the
making and breaking of chemical bonds.
While kinetic energy can be determined in absolute terms, potential energy is more
difficult to measure.
Measuring Energy Changes - Calorimetry
Define Calorimetry
q = mc ΔT
q = quantity of heat transferred
m = mass of water or substance
T = change in temperature
c = specific heat capacity
 Specific heat capacity
o
Ex. Water = 4.18 J/g C
o
Aluminum = 0.90 J/g C
o
o
Ice (-10 C) = 2.05 J/g C
Ex.
A drinking glass and a bathtub are both filled with water at room temperature (20oC).
The glass contains 250 g of water and the bathtub contains 400 kg of water. Which
would require more heat, raising the temperature of the water in the drinking glass 80oC
or raising the temperature of the water in the tub, 1oC? (The tub by 20x)
Ex. 2
A 1 kg sample of water whose temperature was 50oC, lost 33 600 J or heat over a 5
minute period. What was the temperature of the water at the end of the 5 minute period?
(42oC)
Ex. 3
When 0.510 g of ethanol (C2H5OH) was burned in a calorimeter containing 1200 g of
water, the temperature of the water rose from 22.46oC to 25.52 oC. What is the amount of
energy produced by the combustion of ethanol?
Calorimetry Problems
1. What quantity of heat would have to be added to 5 kg of water to change its
temperature from 20oC to 80 oC?
2. If 500 grams of water at 25 oC loses 1.05 x 104 J of heat, what will be the final
temperature of the water?
3. A 100 g sample of gold is warmed from 18 oC to 32 oC. The specific heat of gold
is 0.129 J/g oC. How much heat is required to make this change?
4.
Do questions p. 302 # 8, 9, 10
Enthalpy Change
The term used to describe chemical potential energy, or the heat content is called
___________________.
It is given the symbol ΔH. We focus on measuring changes in enthalpy by comparing
initial and final states to each other.
Enthalpy of a system is the heat transferred / flows to its surroundings
It follows the Law of Conservation of Energy
Therefore:
H = +/- q (surroundings)
The change in E (ΔH), is the difference between the final amount of E and the initial
amount of E.
For example:
When you melt ice, it changes from solid to liquid. In order for this to occur, energy
must be added or taken from the surroundings. Therefore, the change has an increase in
energy for the products. The reaction amount is 335 kJ of heat. This would be an
example of an ________________________.
We focus on measuring changes in enthalpy by comparing initial and final states to each
other.
We can diagram the enthalpy change, using a potential energy diagram.
Enthalpy Change – Potential Energy Diagrams
The diagram shows the initial energy of the reactants, goes through a reaction, and
creates products that also have energy.
a = initial energy of the reactants
b = energy of the products
c = change in energy ( ΔH)
An exothermic reaction is one in which
the products have less potential energy
than the reactants.
Enthalpy is decreasing, so ΔH is
negative.
Exothermic reactions release energy to
the environment, so the kinetic energy
will increase. In the diagram, the
enthalpy change is represented by (c)
An endothermic reaction is one in
which the products have more potential
energy than the reactants.
Enthalpy is increasing, so ΔH is
positive.
Endothermic reactions absorb energy
from the environment, so the overall
kinetic energy will decrease. In the
diagram below, the enthalpy change is
represented by (c)
Diagram – figure 6 – p. 303
Enthalpy is an example of a state property.
State properties are:
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
An important fact about state properties is that changes in those properties are
independent of the pathway of a reaction and depend only on the initial and final
conditions.
In other words, the net energy change is the same regardless of the pathway.
Since temperature and pressure can affect the enthalpy change of a system, we must
establish a common set of conditions to compare different reactions.

Standard state conditions are defined as 25°C (298 K) and 1 atmospheric
pressure (101.3 kPa).
Therefore, the First Law of Thermodynamics can be re-written:
Questions
 The process of forming water is an exothermic process, in which the ΔH = - 285
kJ. Draw the potential energy diagram