1. Exothermic and Endothermic Reactions
Key Differences Between the Two Types of Reactions

2. Systems & Surroundings
In thermodynamics, the world is divided into a system and its surroundings.
A system is the part of the world we want to study (e.g. a reaction mixture in a flask).
The surroundings consist of everything else outside the system.
SYSTEM
SURROUNDINGS
SYSTEM
SURROUNDINGS

3. this relation is only valid at constant pressure.
The heat supplied/removed is equal to the change in a thermodynamic property called enthalpy (H), which comes from a Greek word meaning “heat inside.”
H = q
this relation is only valid at constant pressure.
ENTHALPY (H)
Since most reactions in chemistry take place at constant pressure, we can say that:
A change in enthalpy = A change in the heat supplied/released

4. 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.

5. Exothermic
The sum of the bond energies of products is greater than the sum of the bond energies of reactants.
The enthalpy value for the products is lower than the enthalpy value of the reactants.
Negative ΔH value.
Heat flows to the surroundings.
Typically, the reaction vessel feels warm.
The temperature of the water in the calorimeter rises.

6. Burning fossil fuels is an exothermic reaction.
EXOTHERMIC REACTIONS
Exothermic process: a change (e.g. a chemical reaction) that releases heat.
A release of heat corresponds to a decrease in enthalpy.
Exothermic process: H < 0 (at constant pressure)
Burning fossil fuels is an exothermic reaction.

7. The PE diagram goes downhill.
EXOTHERMIC REACTIONS
The PE diagram goes downhill.

8. Endothermic
The sum of the bond energies of products is less than the sum of the bond energies of reactants.
The enthalpy value for the products is higher than the enthalpy value of the reactants.
Positive ΔH value.
Heat flows into the system from the surroundings.
Typically, the reaction vessel feels cold.
But, can be very hot from an added heat source!!
Temperature of the water in the calorimeter falls.

9. ENDOTHERMIC REACTIONS
Endothermic process: a change (e.g. a chemical reaction) that requires (or absorbs) heat.
An input of heat corresponds to an increase in enthalpy.
Endothermic process: H > 0 (at constant pressure)
Forming Na+ and Cl- ions from NaCl is an endothermic process
Photosynthesis is an endothermic reaction (requires energy input from sun)

10. ENDOTHERMIC REACTIONS
The PE diagram goes uphill.

11. ENDOTHERMIC REACTIONS
The PE diagram goes uphill.

12. Measuring Heat
reaction
Exothermic reaction: heat given off & temperature of water rises.
reaction
Endothermic reaction: heat taken in & temperature of water drops.

13. Vaporization Melting Freezing
Energy has to be supplied to a liquid to enable it to overcome forces that hold molecules together.
endothermic process (H positive)
Melting
Energy is supplied to a solid to enable it to vibrate more vigorously until molecules can move past each other and flow as a liquid.
endothermic process (H positive)
Freezing
Liquid releases energy and allows molecules to settle into a lower energy state and form a solid.
exothermic process (H negative)
(we remove heat from water when making ice in freezer)

14. All chemical reactions either release or absorb heat.
Reaction Enthalpies
All chemical reactions either release or absorb heat.
Exothermic reactions:
Reactants products energy as heat (H -ve)
e.g. burning fossil fuels
Endothermic reactions:
Reactants + energy as heat products (H +ve)
e.g. photosynthesis

15. Exothermic reactions: Reactants products + energy as heat (H -ve)
Reaction Examples
Exothermic reactions:
Reactants products energy as heat (H -ve)

16. Exothermic reactions: Reactants products + energy as heat (H -ve)
Reaction Examples
Exothermic reactions:
Reactants products energy as heat (H -ve)

17. Exothermic reactions: Reactants products + energy as heat (H -ve)
Reaction Examples
Exothermic reactions:
Reactants products energy as heat (H -ve)

18. Endothermic reactions: Reactants + energy as heat products (H +ve)
Reaction Examples
Endothermic reactions:
Reactants + energy as heat products (H +ve)

19. Endothermic reactions: Reactants + energy as heat products (H +ve)
Reaction Examples
Endothermic reactions:
Reactants + energy as heat products (H +ve)
The contents of the crucible are the “system.” The Bunsen burner that is providing the heat is the “surroundings.”

20. Endothermic reactions: Reactants + energy as heat products (H +ve)
Reaction Examples
Endothermic reactions:
Reactants + energy as heat products (H +ve)
The contents of the test tube are the “system.” The Bunsen burner that is providing the heat is the “surroundings.”

21. Exothermic/Endothermic
For exothermic reactions, heat is released.
For endothermic reactions, heat is absorbed.