Lesson, Chemistry: Thermochemistry

1. Thermochemistry

2. Recall
Law of Conservation of Mass and Energy states that in a
chemical reaction mass is neither created nor destroyed
Energy in chemical reactions may be measured in
terms of the amount of heat involved.

3. Thermochemistry is the study of
the heat energy which is
associated with chemical reactions
and/or physical transformations. A
reaction may release or absorb
energy, and a phase change may do
the same, such as in melting and
boiling.

4. Energy
is the ability to do work, thus work may also be defined
as the process of expanding energy.
Energy is not only utilized in Physical activities, but
also in chemical reactions
Each chemical reactions require sometimes releases
energy for the reaction to proceed

5. Energy can be classified into two types:
Kinetic Energy - “energy in motion
Potential Energy - “energy at rest”

6. Law of Conservation of Energy states that energy is
neither created nor destroyed, but it is converted from
another form of energy

8. Energy and Chemistry

9. Energy Changes in Chemical Reactions
Chemical reactions show conversion of reactants into products
In stoichiometry, reactants are related to product in terms of their
masses while the Law of Conservation of Mass states that the Mass
of the reactants is equal to the mass of the products
However, in some cases, there is a very small, hardly measurable
difference between the mass of the reactants and products when an
experiment is performed on chemical reactions

10. Energy Changes in Chemical Reactions
Most chemical reactions involved the absorption or release of energy
in the transfer of heat
Heat is define as the transfer of thermal energy
If heat is being absorbed or
released, it means that there
are two objects involved, the
donor and the receiver of the
heat

11. Energy Changes in Chemical Reactions
For a much specific example, the chemical reagents inside a test tube
are considered as the system while what are outside the test tube
are the surroundings
Allow the transfer of
mass and energy
Allow the flow of energy
but not mass
Do not allow the transfer
of mass and energy

12. Energy Changes in Chemical Reactions
The transfer of energy is either Exothermic or Endothermic
The wood is part of the system, as
it burns, a warm sensation is felt
while sitting beside it. The person’s
body sitting beside the burning
wood is part of the surroundings
and the heat produced by the
burning wood is transferred to the
body

13. The process of transferring heat
from the system to the
surroundings is exothermic. If the
flow of heat is from the
surroundings to the system, it is
endothermic

14. Exothermic Vs. Endothermic

15. Task:
Classify the following processes as endothermic or exothermic
1. Melting of Rubber
2. Rubbing the hands with alcohol
3. Burning of Gasoline
4. Mixing HCL with water
5. Subliming napthalene
6. Perspiring
7. Making of ice
8. Ice or snow forming in clouds
9. Inflating a bicycle tire
10.Breaking down of food

16. Thermodynamics
Thermodynamics, science of the relationship between heat,
work, temperature and energy.
In broad terms, thermodynamics deals with the transfer of
energy from one place to another and from one form to
another. The key concept is that heat is a form of energy
corresponding to a definite amount of mechanical work.

17. Thermodynamics
Thermodynamics provides a better view
on the energetics of a chemical reaction.
For example:
Ice will not melt unless heat is provided.
Knowing energy of a reaction dictates
the spontaneity and direction of a
reaction

18. Thermodynamics
The study of thermodynamics also focuses on the changes in
the state of the system, which pertains to the total or over-
all properties of the system including temperature, pressure
and volume
Energy, such as gravitational potential is also a state function
State function the degree of their change depends only on
the initial and final states of the system

19. First Law of Thermodynamics
The energy is neither
created nor destroyed but
converted to another form
The second law focuses on
explaining why a chemical
process tends to favor one
direction while the Third law is
an extension of the second law.

21. In solving problems, the following sign convention
must be used
Energy entering a system is positive
Heat absorbed by the system is positive
Work done on the system is positive
Energy leaving a system is negative
Heat given off the system is negative
Work done by the system is negative

22. Sample Problems
What is the change in internal energy of a system that
absorbs 523 J of heat and does 452 J of work as a
result?

23. Sample Problems
The internal energy of a system increases by 20 J and
the quantity of work done on a system is 50 J. Is the
heat absorbed or given off? By how much?

24. Enthalpy of a Chemical Reactions

26. Enthalpy

27. Thermochemical Equations
The heat of a reaction is the heat associated with the
transformation of the reactants to products. It is the
amount of heat or released or absorbed when specified
amounts of substances react. The balanced equation showing
the associated heat of reaction is called thermochemical
equation

31. Sample Problem

32. Sample Problem

33. Thermochemical Equations

35. Hess’s Law and Heats of Reaction
Hess’s law states that H is independent of the number of
steps involved in a reaction, It is the same whether the
reaction occurs in one step or in a series of steps.
Another way of looking at it is to remember that H is a
state function, hence it is independent of the route of the
reaction is carried out.