Thermochemistry deals with determining quantities of heat through measurement and calculation, allowing indirect quantification of heat that may be difficult to measure directly. Systems can be open, closed, or isolated depending on whether energy and matter can be transferred between the system and surroundings. Exothermic reactions release energy to the surroundings while endothermic reactions absorb energy from the surroundings, and these properties are illustrated through enthalpy diagrams.

1. TOPIC 8: THERMOCHEMISTRY
Thermochemistry is the branch of chemistry dealing
with determining quantities of heat by measurement
and calculation. Some of these calculations will allow
us to establish indirectly, a quantity of heat that would
be difficult to measure directly.

2. Some Main Concepts in Thermochemistry
Open
system
Closed
system
Isolated
system
System: The part of the universe
we choose to study.
Surroundings: The parts of the
universe with which the system
interacts.
Systems are classified into 3
groups:
Open system: Free transfer of
energy and matter between the
system and its surroundings.
Closed system: Only free transfer
of energy between the system and
its surroundings
Isolated system: No possible
transfer of energy and or matter
between the system and its
surroundings.

3. System and Surroundings
• System: The part of the universe we choose
to study.
• Surroundings: The parts of the universe
with which the system interacts.

4. Why applying a cold pack can relieve the
pain?

5. Heat transfer between the system and its
surroundings occurs as a result of temperature
difference.
•Heat moves from the hot environment to the
cold environment.
– Temperature is variable.
– Change of the state may occur
Heat

6. System and Surroundings
• System: The part of the universe we choose
to study.
• Surroundings: The parts of the universe
with which the system interacts.

7. Endothermic and exothermic reactions
Step 1: Energy must be
SUPPLIED to break
chemical bonds:
Step 2: Energy is RELEASED
when new chemical bonds are
made:
A reaction is EXOTHERMIC if more energy is RELEASED
than is SUPPLIED. A reaction is ENDOTHERMIC if more
energy is SUPPLIED than is RELEASED.

8. Endothermic reaction
• Endothermic reactions ABSORB
energy/heat from their surroundings.
• When endothermic reactions occur, it
makes the surroundings COLDER

9. Cools
Heat
energy
taken
in as the
mixture
returns
back to
room
temp.
Starts 25°C Cools to 5°C
Ammonium
nitrate
Water
Endothermic reactions cause a decrease in temperature.
Returns to 25°C
Endothermic Reactions

10. Endothermic reaction
Example of Endothermic Reactions
•Cooking an egg
•Melting ice
•Splitting a gas molecule apart
•Photosynthesis
•sunlight + 6CO2 + H2O = C6H12O6 + 6O2

11. Exothermic reaction
• Exothermic reactions RELEASE
energy/heat to their surroundings.
• When exothermic reactions occur, it makes
the surroundings HOTTER.

12. Exothermic reactions
• Exothermic reactions give out heat to the
surroundings e.g. burning, respiration
How can you tell a reaction is exothermic?
25o
C 45o
C
magnesium
Hydrochloric
acid
Gets hot Heat
energy
given
out

13. Exothermic reaction
Example of Exothermic Reactions
• Combustion of fireworks
• Burning of natural gas
2H2 + O2 → 2H2O + heat
• Rusting of iron
• Corrosion of metals

14. Guided Practice
Which change is exothermic?
a.freezing of water
b.melting of iron
c.vaporization of ethanol
d.sublimation of iodine
Why?
– Heat is released as molecules slow down going
from a liquid to a solid

15. Guided Practice
Which of the following changes is
endothermic?
a.Freezing of water
b.A candle flame
c.Dissolving ammonium nitrate
d.Baking bread
Why?
– The bread dough must absorb energy before it will
bake.

16. ENTHALPY (H)ENTHALPY (H)
Heat content of a substance
∆H = Hproducts - Hreactants
∆H difference of heat content of
products and reactants

17. First Law of Thermodynamics:
*the internal energy of an isolated system is
constant
*energy is always conservedenergy is always conserved
Signs (+/-) will tell you if energy is entering or
leaving a system
+ indicates energy entering a system
- indicates energy leaving a system

18. Enthalpy Diagrams
Enthalpy
Products
Products
Enthalpy
Reactants
Reactants
Endothermic
reactants
Exothermic
reactants

19. Energy changes in chemical reactions –Energy changes in chemical reactions –
Enthalpy diagramsEnthalpy diagrams
Exothermic reactionsExothermic reactions
The enthalpy of the reactants isThe enthalpy of the reactants is
higher than the products.higher than the products.
ReactantsReactants
ProductsProducts
Endothermic reactionsEndothermic reactions
The enthalpy of the reactants isThe enthalpy of the reactants is
lower than the products.lower than the products.
ProductsProducts
ReactantsReactants
Enthalpy(H)Enthalpy(H)
Enthalpy(H)Enthalpy(H)
∆∆ H is -ve ∆∆ H is +veHeat releasedHeat released Heat absorbedHeat absorbed
Reaction progressionReaction progression Reaction progressionReaction progression

20. ExamplesExamples
The combustion of methane isThe combustion of methane is
exothermic and releasesexothermic and releases
energy to the surroundingsenergy to the surroundings
The decomposition of calciumThe decomposition of calcium
carbonate is endothermic andcarbonate is endothermic and
absorbs energy from theabsorbs energy from the
surroundingssurroundings
Source diagrams:Source diagrams:
http://www.chemhume.co.uk/ASCHEM/Unit%203/13%20Enthalpy/13%20Enthalpyc.htmhttp://www.chemhume.co.uk/ASCHEM/Unit%203/13%20Enthalpy/13%20Enthalpyc.htm

21. Activation EnergyActivation Energy
Reaction progressReaction progress
ReactantsReactants
ProductsProducts
enthalpyenthalpy
ActivationActivation
energyenergy
∆∆ H
The activation energy EThe activation energy Eaa is the minimumis the minimum
amount of energy that is required by theamount of energy that is required by the
reactants for bonds to be broken and thereactants for bonds to be broken and the
reaction to proceed to the products.reaction to proceed to the products.
High EHigh Eaa means strongmeans strong
bonds in the reactantsbonds in the reactants

22. Chemical stabilityChemical stability
Source: http://www.teachmetuition.co.uk/Energetics/chemical_energetics1.htmSource: http://www.teachmetuition.co.uk/Energetics/chemical_energetics1.htm
Exothermic reactionsExothermic reactions
•Products are more stable thanProducts are more stable than
reactants due to lower enthalpyreactants due to lower enthalpy
contentcontent
•The lower the enthalpy the moreThe lower the enthalpy the more
stablestable
•The lower the enthalpy, theThe lower the enthalpy, the
stronger the bondsstronger the bonds
Endothermic reactionsEndothermic reactions
•Products are less stable thanProducts are less stable than
reactants due to a higher enthalpyreactants due to a higher enthalpy
contentcontent
•The higher the enthalpy the lessThe higher the enthalpy the less
stablestable
•The higher the enthalpy, theThe higher the enthalpy, the
weaker the bondsweaker the bonds

23. Enthalpy change summaryEnthalpy change summary
Thermochemical Reaction Exothermic Endothermic
Enthalpy Change
(ΔH = HP – HR)
HP < HR HP > HR
Sign ΔH Negative Positive
ΔT of surroundings Increases (warmer) Decreases (colder)
Enthalpy Diagram
Source diagrams:Source diagrams:
http://www.chemhume.co.uk/ASCHEM/Unit%203/13%20Enthalpy/13%20Enthalpyc.htmhttp://www.chemhume.co.uk/ASCHEM/Unit%203/13%20Enthalpy/13%20Enthalpyc.htm

24. Guided Practice
When 1 mol hydrocholric acid reacts with 1
mol of sodium hydroxide, 57.3 kJ of heat is
produced. Draw an energy level diagram for
this reaction.

25. Exercises
1. When 1 mol hydrogen reacts with 1 mol of
iodine to give 2 mol of hydrogen iodide ,
52 kJ of heat is absorbed . Draw an energy
level diagram for this reaction.
2. To decompose 100 g of calcium carbonate
into calcium oxide and carbon dioxide,
178 kJ of heat must be supplied. Write the
chemical equation.